Article

# Subsurface Oceans and Deep Interiors of Medium-Sized Outer Planet Satellites and Large Trans-Neptunian Objects

Authors:
• International Space Science Institute
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## Abstract

The detection of induced magnetic fields in the vicinity of the jovian satellites Europa, Ganymede, and Callisto is one of the most surprising findings of the Galileo mission to Jupiter. The observed magnetic signature cannot be generated in solid ice or in silicate rock. It rather suggests the existence of electrically conducting reservoirs of liquid water beneath the satellites’ outermost icy shells that may contain even more water than all terrestrial oceans combined. The maintenance of liquid water layers is closely related to the internal structure, composition, and thermal state of the corresponding satellite interior. In this study we investigate the possibility of subsurface oceans in the medium-sized icy satellites and the largest trans-neptunian objects (TNO’s). Controlling parameters for subsurface ocean formation are the radiogenic heating rate of the silicate component and the effectiveness of the heat transfer to the surface. Furthermore, the melting temperature of ice will be significantly reduced by small amounts of salts and/or incorporated volatiles such as methane and ammonia that are highly abundant in the outer Solar System. Based on the assumption that the satellites are differentiated and using an equilibrium condition between the heat production rate in the rocky cores and the heat loss through the ice shell, we find that subsurface oceans are possible on Rhea, Titania, Oberon, Triton, and Pluto and on the largest TNO’s 2003 UB313, Sedna, and 2004 DW. Subsurface oceans can even exist if only small amounts of ammonia are available. The liquid subsurface reservoirs are located deeply underneath an ice-I shell of more than 100 km thickness. However, they may be indirectly detectable by their interaction with the surrounding magnetic fields and charged particles and by the magnitude of a satellite’s response to tides exerted by the primary. The latter is strongly dependent on the occurrence of a subsurface ocean which provides greater flexibility to a satellite’s rigid outer ice shell.

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... These satellites show evidence of cryovolcanic resurfacing (e.g., Croft and Soderblom, 1991) and tectonic activity in differing proportions; for example, Oberon is heavily cratered and less faulted than Titania. Hussmann et al. (2006) constructed detailed models of the thermal and mechanical equilibrium of a range of icy bodies in the outer solar system, including those at Uranus. They considered two-and three-layer models consisting of a rocky core surrounded by two water layers and calculated the heat flux, the resulting density, pressure and temperature profile, and hence an indication of whether the subsurface water layer could be in a liquid state. ...
... Two solutions were found for Oberon (R = 481.0 km) with oceans between 16 and 39.3 km thick, overlaid by an ice (I) shell of thickness between 264.4 and 241.1 km respectively (Hussmann et al., 2006). Fig. 1 shows a schematic of these models. ...
... We first construct a model for the magnetospheric magnetic field at Uranus and use it to calculate the primary field at each of Uranus' main satellites over a Uranus orbit (Section 2). The amplitude of the primary field harmonics are evaluated in section 3 and applied to compute the induced response using the internal structure models of Hussmann et al. (2006). Finally, we estimate the field at Titania during potential satellite flybys and conclude that the calculated subsurface oceans from Hussmann et al. (2006) would result in a detectable induced signature from a flyby of this satellite. ...
Article
The discovery of subsurface oceans in the outer solar system has transformed our perspective of ice worlds and has led to consideration of their potential habitability. The detection and detailed characterisation of induced magnetic fields due to these subsurface oceans provides a unique ability to passively sound the conducting interior of such planetary bodies. In this paper we consider the potential detectability of subsurface oceans via induced magnetic fields at the main satellites of Uranus. We construct a simple model for Uranus' magnetospheric magnetic field and use it to generate synthetic time series which are analysed to determine the significant amplitudes and periods of the inducing field. The spectra not only contain main driving periods at the synodic and orbital periods of the satellites, but also a rich spectrum from the mixing of signals due to asymmetries in the uranian planetary system. We use an induction model to determine the amplitude of the response from subsurface oceans and find weak but potentially-detectable ocean responses at Miranda, Oberon and Titania, but did not explore this in detail for Ariel and Umbriel. Detection of an ocean at Oberon is complicated by intervals that Oberon will spend outside the magnetosphere at equinox but we find that flybys of Titania with a closest approach altitude of 200 km would enable the detection of subsurface oceans. We comment on the implications for future mission and instrument design.
... Only the southern hemispheres of these moons have ever been seen, due to the southern-summer season in 1986. A major outstanding question is whether tidal interactions could lead to the presence of liquid water oceans beneath their icy crusts [40], therefore extending the zone of potential habitability out to solar distances that were previously unimaginable. In this special issue, Schenk and Moore explore the diverse geology of these satellites. ...
... In contrast to the Uranian system, the Neptunian system appears to have been substantially altered by the presence of an interloper from the Kuiper Belt: the large moon Triton, with its own atmosphere, active geology and plumes of nitrogen gas and dust, providing an intriguing connection between a future Neptune mission and the recent New Horizons exploration of Pluto and 486958 Arrokoth. Triton is considered as a key "Ocean World" for future exploration [41], and may also harbour a subsurface ocean [40]. Beyond the major moons, almost nothing is known about the minor satellites of Uranus and Neptune, as they were too small and distant for even Voyager's cameras. ...
Article
The international planetary science community met in London in January 2020, united in the goal of realizing the first dedicated robotic mission to the distant ice giants, Uranus and Neptune, as the only major class of solar system planet yet to be comprehensively explored. Ice-giant-sized worlds appear to be a common outcome of the planet formation process, and pose unique and extreme tests to our understanding of exotic water-rich planetary interiors, dynamic and frigid atmospheres, complex magnetospheric configurations, geologically-rich icy satellites (both natural and captured), and delicate planetary rings. This article introduces a special issue on ice giant system exploration at the start of the 2020s. We review the scientific potential and existing mission design concepts for an ambitious international partnership for exploring Uranus and/or Neptune in the coming decades. This article is part of a discussion meeting issue ‘Future exploration of ice giant systems’.
... These observations are consistent with sub-surface liquid water oceans within Miranda and Ariel, perhaps even persisting to present day. Furthermore, thermal evolution modeling results suggest that the outermost moons Titania and Oberon could have retained subsurface oceans if they contain just a few percent by total volume of ammonia (Hussmann et al., 2006). Thus, the Uranian system represents a compelling target in the search for Ocean Worlds and potentially habitable environments beyond Earth, with Miranda and Ariel highlighted as priority targets for future exploration by the recent NASA Roadmap to Ocean Worlds (Hendrix et al., 2019). ...
... Although models assuming simple compositions of silicates and water predict that an ocean within Miranda would have frozen out by the present era (Hussmann et al. 2006), the presence of volatile clathrates or other insulating materials could support the persistent presence of sub-surface liquid by providing thermal insulation and adding rigidity to the ice, both of which lower the Rayleigh number and thus inhibit solid-state convection (Croft 1987, Kamata et al. 2019. Within Saturn's moon Titan and the dwarf planets Ceres and Pluto, methane clathrates may impede solid state convection in ice, leading to inefficient cooling that slows the freeze-out of subsurface oceans. ...
Preprint
Full-text available
The Galileo mission to Jupiter discovered magnetic signatures associated with hidden sub-surface oceans at the moons Europa and Callisto using the phenomenon of magnetic induction. These induced magnetic fields originate from electrically conductive layers within the moons and are driven by Jupiter's strong time-varying magnetic field. The ice giants and their moons are also ideal laboratories for magnetic induction studies. Both Uranus and Neptune have a strongly tilted magnetic axis with respect to their spin axis, creating a dynamic and strongly variable magnetic field environment at the orbits of their major moons. Although Voyager 2 visited the ice giants in the 1980s, it did not pass close enough to any of the moons to detect magnetic induction signatures. However, Voyager 2 revealed that some of these moons exhibit surface features that hint at recent geologically activity, possibly associated with sub-surface oceans. Future missions to the ice giants may therefore be capable of discovering sub-surface oceans, thereby adding to the family of known ocean worlds in our solar system. Here, we assess magnetic induction as a technique for investigating sub-surface oceans within the major moons of Uranus. Furthermore, we establish the ability to distinguish induction responses created by different interior characteristics that tie into the induction response: ocean thickness, conductivity, and depth, and ionospheric conductance. The results reported here demonstrate the possibility of single-pass ocean detection and constrained characterization within the moons of Miranda, Ariel, and Umbriel, and provide guidance for magnetometer selection and trajectory design for future missions to Uranus.
... Magnetic induction at Triton has not yet been studied in detail; we have chosen the same composition for Triton's ocean as for Callisto, and a slightly thicker ice shell of 112 km. Our Triton interior model is based on the moment of inertia and surface temperatures assumed for Pluto by Hussmann et al. (2006). Figures 8 and 9 show the differences in the component of the induced magnetic field resulting from asymmetry in the ionospheres of Callisto and Triton, respectively. ...
... Uniform conductivity, an average ionospheric thickness of 200 km, a lower bound for the ionosphere of 250 km altitude, and a total height-integrated conductivity of 20 kS are assumed, based on the structure inferred by Tyler et al. (1989) from Voyager measurements. The interior structure we suppose for Triton is based on a moment of inertia inferred for Pluto by Hussmann et al. (2006) and geophysical modeling using the PlanetProfile framework. An ocean with dissolved MgSO 4 and a 112 km-thick ice shell are assumed. ...
Preprint
Full-text available
Magnetic investigations of icy moons have provided some of the most compelling evidence available confirming the presence of subsurface, liquid water oceans. In the exploration of ocean moons, especially Europa, there is a need for mathematical models capable of predicting the magnetic fields induced under a variety of conditions, including in the case of asymmetric oceans. Existing models are limited to either spherical symmetry or assume an ocean with infinite conductivity. In this work, we derive an analytic result capable of determining the induced magnetic moments for an arbitrary, layered body. Crucially, we find that degree-2 tidal deformation results in changes to the induced dipole moments. We demonstrate application of our results to models of plausible asymmetry from the literature within the oceans of Europa and Miranda and the ionospheres of Callisto and Triton. For the models we consider, we find that in the asymmetric case, the induced magnetic field differs by more than 2 nT near the surface of Europa, 0.25$-$0.5 nT at 1 $R$ above Miranda and Triton, and is essentially unchanged for Callisto. For Miranda and Triton, this difference is as much as 20$-$30% of the induced field magnitude. If measurements near the moons can be made precisely to better than a few tenths of a nT, these values may be used by future spacecraft investigations to characterize asymmetry within the interior of icy moons.
... Strong tidal heating anticipated from an ancient capture event would not explain the currently observed young surface age, while more recent capture is unlikely (Noguiera et al. 2011). Theoretical models suggest that a subsurface liquid layer could be present today; thus Triton is a candidate ocean world (Hussmann et al. 2006;Nimmo & Spencer 2015). ...
... Based on the extreme degree of tidal heating following capture, we expect that Triton has differentiated into a rocky core with a water-ice mantle. Analysis of bodies with substantial rock content in their bulk composition led Hussmann et al. (2006) to propose that formation of liquid layers on icy moons could be common from radiogenic heating of water laced with ammonia, which would depress the melting point. Although Triton's orbit is now circular, a finite eccentricity immediately after capture would have resulted in tidal dissipation of heat, which could have been retained in the interior over geological history, maintaining a liquid layer in the subsurface (Gaeman et al. 2012). ...
... Neptune's large satellite Triton (Figure 1) was revealed by Voyager 2 in 1989 to be a diverse, youthful, and active body, and is now a major focus of planetary exploration [1,2] and has been designated as the top priority target for future exploration of icy ocean worlds (e.g., [3,4]). Triton's very young surface (perhaps as young at~10 myr; [5][6][7][8]), outer ice-rich shell, geologic complexity [6,9], and active atmospheric plumes [10] related to either solar or geothermal heating [11,12] all suggest that Triton, most likely a captured Kuiper belt dwarf planet, has been subject to high levels of internal heat, may be currently active, and likely has an internal ocean today (e.g., [3,4,7,13,14]). The known surface ices on Triton include water, methane, CO and CO 2 ices [15][16][17]. Unlike Cassini or New Horizons, Voyager did not carry a mapping infrared spectrometer and did not map out the distribution or geologic correlations of these materials. ...
... Remote Sens. 2021,13, 3476 ...
Article
Full-text available
The topography of Neptune’s large icy moon Triton could reveal important clues to its internal evolution, but has been difficult to determine. New global digital color maps for Triton have been produced as well as topographic data for <40% of the surface using stereogrammetry and photoclinometry. Triton is most likely a captured Kuiper Belt dwarf planet, similar though slightly larger in size and density to Pluto, and a likely ocean moon that exhibited plume activity during Voyager 2′s visit in 1989. No surface features or regional deviations of greater than ±1 km amplitude are found. Volatile ices in the southern terrains may take the form of extended lobate deposits 300–500 km across as well as dispersed bright materials that appear to embay local topography. Limb hazes may correlate with these deposits, indicating possible surface–atmosphere exchange. Triton’s topography contrasts with high relief up to 6 km observed by New Horizons on Pluto. Low relief of (cryo)volcanic features on Triton contrasts with high-standing massifs on Pluto, implying different viscosity materials. Solid-state convection occurs on both and at similar horizontal scales but in very different materials. Triton’s low relief is consistent with evolution of an ice shell subjected to high heat flow levels and may strengthen the case of an internal ocean on this active body.
... It ranges from 10 4 for Oberon (the farthest from Uranus) to 10 6 for Miranda (closest moon), suggesting a wide range of internal properties across the Uranian moons system. Considering the moon's k 2 may range from 10 −3 to 10 −2 (Hussmann et al., 2006), we infer dissipation factors in excess of 1000 in Miranda, and potentially also Oberon, suggesting that the moons are not dissipative at present and may be near frozen. Interestingly, the 2014)). ...
... e.g.,Hussmann et al. (2006)): (a) the heating associated with accretion during planet formation, (b) the gravitational energy released during planetary differentiation, (c) radiogenic heating in the silicate component due to the decay of long-lived radioactive isotopes (U, Th, and K), and (d) tidal heating due to viscoelastic dissipation. Of these sources, only (c) and (d) are of relevance for the long-term evolution of the planet while the two first sources are mostly linked to the early stages of planetary accretion. ...
Preprint
Full-text available
Solid body tides provide key information on the interior structure, evolution, and origin of the planetary bodies. Our Solar system harbours a very diverse population of planetary bodies, including those composed of rock, ice, gas, or a mixture of all. While a rich arsenal of geophysical methods has been developed over several years to infer knowledge about the interior of the Earth, the inventory of tools to investigate the interiors of other Solar-system bodies remains limited. With seismic data only available for the Earth, the Moon, and Mars, geodetic measurements, including the observation of the tidal response, have become especially valuable and therefore, has played an important role in understanding the interior and history of several Solar system bodies. To use tidal response measurements as a means to obtain constraints on the interior structure of planetary bodies, appropriate understanding of the viscoelastic reaction of the materials from which the planets are formed is needed. Here, we review the fundamental aspects of the tidal modeling and the information on the present-day interior properties and evolution of several planets and moons based on studying their tidal response. We begin with an outline of the theory of viscoelasticity and tidal response. Next, we proceed by discussing the information on the tidal response and the inferred structure of Mercury, Venus, Mars and its moons, the Moon, and the largest satellites of giant planets, obtained from the analysis of the data that has been provided by space missions. We also summarise the upcoming possibilities offered by the currently planned missions.
... The bulk composition concerns the ratios between rock, water ice, non-water volatiles and organic compounds, while the thermal history affects the freezing-thawing cycles, degree of ice melting, extent and duration of chemical interaction between rock and liquid water, degassing of the deep interior, and secondary precipitation of organic and inorganic phases. The wide possible variation in these factors leads to a diverse range of possible evolutionary pathways among the ocean-bearing bodies of the Solar System [see reviews by Hussmann et al. (2006), Nimmo & Pappalardo (2016), Lunine (2017) and Mann (2017) for known, probable and plausible ocean-bearing Solar System bodies]. Since Europa is subject to ongoing tidal heating, this long-lived heat source means that it is likely to experience extensive and prolonged water-rock interactions somewhat akin to hydrothermal systems on Earth, as previously mentioned. ...
... Many of the icy objects in the outer Solar System are believed to harbour liquid oceans, as suggested variously by observation, modelling or plausible circumstance [see e.g. reviews by Hussmann et al. (2006), Massé et al. (2014) and Nimmo & Pappalardo (2016); see also Table 1 of Thompson et al. (2018)]. Some of these will be in direct contact with their rocky cores. ...
Article
Full-text available
The precipitation of hydrated phases from a chondrite-like Na–Mg–Ca–SO 4 –Cl solution is studied using in situ synchrotron X-ray powder diffraction, under rapid- (360 K h ⁻¹ , T = 250–80 K, t = 3 h) and ultra-slow-freezing (0.3 K day ⁻¹ , T = 273–245 K, t = 242 days) conditions. The precipitation sequence under slow cooling initially follows the predictions of equilibrium thermodynamics models. However, after ∼50 days at 245 K, the formation of the highly hydrated sulfate phase Na 2 Mg(SO 4 ) 2 ·16H 2 O, a relatively recent discovery in the Na 2 Mg(SO 4 ) 2 –H 2 O system, was observed. Rapid freezing, on the other hand, produced an assemblage of multiple phases which formed within a very short timescale (≤4 min, Δ T = 2 K) and, although remaining present throughout, varied in their relative proportions with decreasing temperature. Mirabilite and meridianiite were the major phases, with pentahydrite, epsomite, hydrohalite, gypsum, blödite, konyaite and loweite also observed. Na 2 Mg(SO 4 ) 2 ·16H 2 O was again found to be present and increased in proportion relative to other phases as the temperature decreased. The results are discussed in relation to possible implications for life on Europa and application to other icy ocean worlds.
... Gas giants are hostile to life 9 , but their icy satellites offer environments that may be more conducive to it. The icy moons Europa and Triton probably have water ice crusts several tens of kilometres thick and even thicker oceans 14,15 , whereas tiny Enceladus may have an ocean with a much thinner crust (~5 km) 16 . Hydrothermal systems supply reducing nutrients to support chemoautotrophy on Earth, and subsurface oceans could contain similar environments if in contact with rocky mantles. ...
... Aided by radiogenic heating, the larger TNOs ≥800 km in radius (for example, Sedna) may maintain liquid water oceans over geological timescales despite surface temperatures of ~40 K (ref. 14 ). Estimates show that of the ~10 9 Kuiper belt objects of size ≥1 km, at least 40 may be Sedna-like 17 . ...
Article
The question of the possibility of life beyond Earth, as framed by the Drake equation, can be quantified to show that habitable environments for life as we know it are commonplace in the Galaxy.
... Furthermore, thermal evolution modeling results suggest that the outermost moons Titania and Oberon could have retained subsurface oceans if they contain just a few percent by total volume of ammonia (Hussmann et al., 2006). Thus, the Uranian system represents a compelling target in the search for Ocean Worlds and potentially habitable environments beyond Earth, with Miranda and Ariel highlighted as priority targets for future exploration by the recent NASA Roadmap to Ocean Worlds (Hendrix et al., 2019). ...
... Although models assuming simple compositions of silicates and water predict that an ocean within Miranda would have frozen out by the present era (Hussmann et al., 2006), the presence of volatile clathrates or other insulating materials could support the persistent presence of subsurface liquid by providing thermal insulation and adding rigidity to the ice, both of which lower the Rayleigh number and thus inhibit solid-state convection Croft, 1987;Kamata et al., 2019). Within Saturn's moon Titan and the dwarf planets Ceres and Pluto, methane clathrates may impede solid-state convection in ice, leading to inefficient cooling that slows the freeze-out of subsurface oceans. ...
Article
Full-text available
The Galileo mission to Jupiter discovered magnetic signatures associated with hidden subsurface oceans at the moons Europa and Callisto using the phenomenon of magnetic induction. These induced magnetic fields originate from electrically conductive layers within the moons and are driven by Jupiter's strong time‐varying magnetic field. The ice giants and their moons are also ideal laboratories for magnetic induction studies. Both Uranus and Neptune have a strongly tilted magnetic axis with respect to their spin axis, creating a dynamic and strongly variable magnetic field environment at the orbits of their major moons. Although Voyager 2 visited the ice giants in the 1980s, it did not pass close enough to any of the moons to detect magnetic induction signatures. However, Voyager 2 revealed that some of these moons exhibit surface features that hint at recent geologically activity, possibly associated with subsurface oceans. Future missions to the ice giants may therefore be capable of discovering subsurface oceans, thereby adding to the family of known “ocean worlds” in our Solar System. Here, we assess magnetic induction as a technique for investigating subsurface oceans within the major moons of Uranus. Furthermore, we establish the ability to distinguish induction responses created by different interior characteristics that tie into the induction response: ocean thickness, conductivity and depth, and ionospheric conductance. The results reported here demonstrate the possibility of single‐pass ocean detection and constrained characterization within the moons of Miranda, Ariel, and Umbriel, and provide guidance for magnetometer selection and trajectory design for future missions to Uranus.
... Still, such an atmosphere may exist when Sedna moves near the Sun, i.e. ~200 yrs out of a 10,500-year orbital period. Paper [20] refers to the possible existence of a subsurface ocean inside Sedna. This theory is explained by the presence of ammonia in concentrations up to 1.4% and the estimated size of the object is from 650 to 1500 km [9,10,20,21]. ...
... Paper [20] refers to the possible existence of a subsurface ocean inside Sedna. This theory is explained by the presence of ammonia in concentrations up to 1.4% and the estimated size of the object is from 650 to 1500 km [9,10,20,21]. The presence of ammonia makes it possible to reduce the melting point of water ice inside Sedna. ...
Article
Full-text available
Current research focuses on designing fast trajectories to the trans-Neptunian object (TNO) (90377) Sedna to study the surface and composition from a close range. Studying Sedna from a close distance may provide unique data about the earliest stages of the Solar System evolution, including the protoplanetary disc stage and related mechanisms. The trajectories to Sedna are determined considering flight time and the total characteristic velocity (ΔV) constraints. The time of flight for the analysis was limited to 20 years. The direct flight, the use of gravity assist manoeuvres near Venus, the Earth and the giant planets Jupiter and Neptune, and the flight with the Oberth manoeuvre near the Sun are considered. It is demonstrated that the use of flight scheme with ΔVEGA (ΔV and Earth Gravity Assist manoeuvre) and Jupiter-Neptune gravity assist leads to the lowest cost of ΔV≈6.13 km/s for launch in 2041. The maximum payload for schemes with ΔVEGA manoeuvre is 500 kg using Soyuz 2.1.b, 2000 kg using Proton-M and Delta IV Heavy and exceeds 12,000 kg using SLS. For schemes with only Jupiter gravity assist, payload mass is twice less than for ones with ΔVEGA manoeuvre. As a possible expansion of the mission to Sedna, it is proposed to send a small spacecraft to another TNO during the primary flight to Sedna. Five TNOs suitable for this scenario are found, three extreme TNOs 2012 VP113, (541132) Leleākūhonua (former 2015 TG387), 2013 SY99) and two classical KBOs: (90482) Orcus, (20000) Varuna.
... Still, such an atmosphere may exist when Sedna moves near the Sun, i.e. ⁓200 yrs out of a 10,500-year orbital period. Paper [20] refers to the possible existence of a subsurface ocean inside Sedna. This theory is explained by the presence of ammonia in concentrations up to 1.4% and the estimated size of the object is from 650 to 1,500 km [9,10,20,21]. ...
... Paper [20] refers to the possible existence of a subsurface ocean inside Sedna. This theory is explained by the presence of ammonia in concentrations up to 1.4% and the estimated size of the object is from 650 to 1,500 km [9,10,20,21]. The presence of ammonia makes it possible to reduce the melting point of water ice inside Sedna. ...
Preprint
Full-text available
Current research focuses on designing fast trajectories to the trans-Neptunian object (TNO) (90377) Sedna to study the surface and composition from a close range. Studying Sedna from a close distance can provide unique data about the Solar System evolution process including protoplanetary disc and related mechanisms. The trajectories to Sedna are determined considering flight time and the total characteristic velocity (${\Delta}V$) constraints. The time of flight for the analysis was limited to 20 years. The direct flight, the use of gravity assist manoeuvres near Venus, the Earth and the giant planets Jupiter and Neptune, and the flight with the Oberth manoeuvre near the Sun are considered. It is demonstrated that the use of flight scheme with ${\Delta}VEGA$ (${\Delta}V$ and Earth Gravity Assist manoeuvre) and Jupiter-Neptune gravity assist leads to the lowest cost of ${\Delta}V$=6.13 km/s for launch in 2041. The maximum payload for schemes with ${\Delta}$VEGA manoeuvre is 500 kg using Soyuz 2.1.b, 2,000 kg using Proton-M and Delta IV Heavy and exceeds $12,000$ kg using SLS. For schemes with only Jupiter gravity assist, payload mass is twice less than for ones with ${\Delta}$VEGA manoeuvre. As a possible expansion of the mission to Sedna, it is proposed to send a small spacecraft to another TNO during the primary flight to Sedna. Five TNOs suitable for this scenario are found, three extreme TNOs 2012 VP113, (541132) Lele\=ak\=uhonua (former 2015 TG387), 2013 SY99) and two classical Kuiper Belt objects: (90482) Orcus, (20000) Varuna.
... Strong tidal heating anticipated from an ancient capture event would not explain the currently observed young surface age, while more recent capture is unlikely (Noguiera et al. 2011). Theoretical models suggest that a subsurface liquid layer could be present today; thus Triton is a candidate ocean world (Hussmann et al. 2006;Nimmo & Spencer 2015). ...
... Based on the extreme degree of tidal heating following capture, we expect that Triton has differentiated into a rocky core with a water-ice mantle. Analysis of bodies with substantial rock content in their bulk composition led Hussmann et al. (2006) to propose that formation of liquid layers on icy moons could be common from radiogenic heating of water laced with ammonia, which would depress the melting point. Although Triton's orbit is now circular, a finite eccentricity immediately after capture would have resulted in tidal dissipation of heat, which could have been retained in the interior over geological history, maintaining a liquid layer in the subsurface (Gaeman et al. 2012). ...
... The densities of the Uranian moons suggest their bulk composition to be more rock-rich than Saturn's mid-sized moons (e.g., Johnson 2003, and references therein), but they do not exhibit a clear trend with orbital distance, such as observed among the Galilean satellites. Although the size of the largest Uranian moons, Oberon and Titania, is comparable to Saturn's moon Rhea (764 km radius) whose gravitational moments suggest a largely homogeneous interior (Anderson and Schubert 2007; Iess et al. 2007), their higher rock content and thus larger radiogenic heat reservoir might have allowed their differentiation, and these moons could still harbor liquid oceans in their interior to the present day (Hussmann et al. 2006). The sustainability of such oceans is dependent upon the presence of volatiles that can lower the melting temperature of water ice, such as ammonia, in the interior of the moons. ...
... Owing to its slightly higher density, and therefore larger fraction of rocks and radiogenic sources, the presence of a subsurface ocean at Triton is likely. But, in the absence of data on composition and gravitational moments, it is unclear whether this ocean would be in direct contact with the rocky core beneath, or instead with a layer of high-pressure ices (Hussmann et al. 2006;McKinnon and Kirk 2014). From the perspective of habitability conditions, the ability of the ocean to interact with the rocky core is of crucial importance. ...
Article
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The comparative study of planetary systems is a unique source of new scientific insight: following the six “key science questions” of the “Planetary Exploration, Horizon 2061” long-term foresight exercise, it can reveal to us the diversity of their objects (Question 1) and of their architectures (Question 2), help us better understand their origins (Question 3) and how they work (Question 4), find and characterize habitable worlds (Question 5), and ultimately, search for alien life (Question 6). But a huge “knowledge gap” exists which limits the applicability of this approach in the solar system itself: two of its secondary planetary systems, the ice giant systems of Uranus and Neptune, remain poorly explored. Starting from an analysis of our current limited knowledge of solar system ice giants and their systems in the light of these six key science questions, we show that a long-term plan for the space exploration of ice giants and their systems will greatly contribute to answer these questions. To do so, we identify the key measurements needed to address each of these questions, the destinations to choose (Uranus, Neptune, Triton or a subset of them), the combinations of space platform(s) and the types of flight sequences needed. We then examine the different launch windows available until 2061, using a Jupiter fly-by, to send a mission to Uranus or Neptune, and find that: (1) an optimized choice of platforms and flight sequences makes it possible to address a broad range of the key science questions with one mission at one of the planets. Combining an atmospheric entry probe with an orbiter tour starting on a high-inclination, low periapse orbit, followed by a sequence of lower inclination orbits (or the other way around) appears to be an optimal choice. (2) a combination of two missions to each of the ice giant systems, to be flown in parallel or in sequence, will address five out of the six key questions and establish the prerequisites to address the sixth one: searching for life at one of the most promising Ice Giant moons. (3) The 2032 Jupiter fly-by window, which offers a unique opportunity to implement this plan, should be considered in priority; if this window cannot be met, using the 2036 Jupiter fly-by window to send a mission to Uranus first, and then the 2045 window for a mission to Neptune, will allow one to achieve the same objectives; as a back-up option, one should consider an orbiter + probe mission to one of the planets and a close fly-by of the other planet to deliver a probe into its atmosphere, using the opportunity of a future mission on its way to Kuiper Belt Objects or the interstellar medium; (4) based on the examination of the habitability of the different moons by the first two missions, a third one can be properly designed to search for life at the most promising moon, likely Triton, or one of the active moons of Uranus. Thus, by 2061 the first two missions of this plan can be implemented and a third mission focusing on the search for life can be designed. Given that such a plan may be out of reach of a single national agency, international collaboration is the most promising way to implement it.
... 4. Ocean Worlds: What can Uranus' natural satellite system and the captured Kuiper Belt Object Triton reveal about the drivers of active geology, subsurface oceans, and habitability in the outer Solar System? Exploring the geological and surface composition diversity [30,31] of the five largest moons of Uranus (Miranda, Ariel, Umbriel, Titania, Oberon) would reveal new insights into the formation and continued evolution (e.g., tidal interactions, internal melting, and the potential for subsurface water oceans [32]) of primordial satellite systems, in contrast with the Saturnian moons of similar size. Only the southern hemispheres of these moons have ever been seen, due to the southern-summer season on Uranus in 1986. ...
Article
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Of all the myriad environments in our Solar System, the least explored are the distant Ice Giants Uranus and Neptune, and their diverse satellite and ring systems. These ‘intermediate-sized’ worlds are the last remaining class of Solar System planet to be characterised by a dedicated robotic mission, and may shape the paradigm for the most common outcome of planetary formation throughout our galaxy. In response to the 2019 European Space Agency call for scientific themes in the 2030s and 2040s (known as Voyage 2050 ), we advocated that an international partnership mission to explore an Ice Giant should be a cornerstone of ESA’s science planning in the coming decade, targeting launch opportunities in the early 2030s. This article summarises the inter-disciplinary science opportunities presented in that White Paper [1], and briefly describes developments since 2019.
... Remnant heat stemming from these interactions may have persisted to the present day, as suggested by recent studies showing that the heat flow on Triton should be around 10-100 mW m -2 (Ruiz, 2003, Martin-Herrero et al., 2018. These values are much larger than those obtained by assuming only radiogenic production and tidal dissipation for fixed orbital eccentricities (2-4 mW m -2 , Gaeman et al. 2012, Brown et al. 1991, Hussmann et al. 2006. The large heat flow could also be produced by internal ocean tidal heating due to Triton's orbit obliquity (Chen et al., 2014;Nimmo and Spencer, 2015;Dubois et al., 2017). ...
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Neptune's moon Triton shares many similarities with Pluto, including volatile cycles of N2, CH4 and CO, and represents a benchmark case for the study of surface-atmosphere interactions on volatile-rich Kuiper Belt objects. The observations of Pluto by New Horizons acquired during the 2015 flyby and their analysis with volatile transport models (VTMs) shed light on how volatile sublimation-condensation cycles control the climate and shape the surface of such objects. Within the context of New Horizons observations as well as recent Earth-based observations of Triton, we adapt a Plutonian VTM to Triton, and test its ability to simulate its volatile cycles, thereby aiding our understanding of its climate. Here we present numerical VTM simulations exploring the volatile cycles of N2, CH4 and CO on Triton over long-term and seasonal timescales (cap extent, surface temperatures, surface pressure, sublimation rates) for varying model parameters (including the surface ice reservoir, albedo, thermal inertia, and the internal heat flux). We explore what scenarios and model parameters allow for a best match of the available observations. In particular, our set of observational constraints include Voyager 2 observations (surface pressure and cap extent), ground-based near-infrared (0.8–2.4 μm) disk-integrated spectra (the relative surface area of volatile vs. non-volatile ice) and the evolution of surface pressure as retrieved from stellar occultations. Our results show that Triton's poles act as cold traps for volatile ices and favor the formation of polar caps extending to lower latitudes through glacial flow or through the formation of thinner seasonal deposits. As previously evidenced by other VTMs, North-South asymmetries in surface properties can favor the development of one cap over the other. Our best-case simulations are obtained for a bedrock surface albedo of 0.6–0.7, a global reservoir of N2 ice thicker than 200 m, and a bedrock thermal inertia larger than 500 SI (or smaller but with a large internal heat flux). The large N2 ice reservoir implies a permanent N2 southern cap (several 100 m thick) extending to the equatorial regions with higher amounts of volatile ice at the south pole, which is not inconsistent with Voyager 2 images but does not fit well with observed full-disk near-infrared spectra. Our results also suggest that a small permanent polar cap exists in the northern (currently winter) hemisphere if the internal heat flux remains relatively low (e.g. radiogenic, < 3 mW m⁻²). A non-permanent northern polar cap was only obtained in some of our simulations with high internal heat flux (30 mW m⁻²). The northern cap will possibly extend to 30°N in the next decade, thus becoming visible by Earth-based telescopes. On the basis of our model results, we also discuss the composition of several surface units seen by Voyager 2 in 1989, including the bright equatorial fringe and dark surface patches. Finally, we provide predictions for the evolution of ice distribution, surface pressure and CO and CH4 atmospheric mixing ratios in the next decades. According to our model, the surface pressure should slowly decrease but remain larger than 0.5 Pa by 2060. We also model the thermal lightcurves of Triton for different climate scenarios in 2022, which serve as predictions for future James Webb Space Telescope observations.
... The origin of such a shade may be associated with the presence of a layer of tholine or hydrocarbon sediment on the surface of this object [5]. Some researches give data, according to which there may be an ocean under the surface of Sedna, heated by the internal heat of this object, generated by radioactive decay [6]. The orbital period of Sedna is more than 10 thousand years. ...
Conference Paper
This research is devoted to the determination and analysis of trajectories to the trans-Neptunian object Sedna in launch windows within the 2029-2037 period. Brief analysis has been carried out for the launch within 2029-2034 and a more detailed one was performed for 2036 and 2037 launch years. The gravity assist maneuvers considered in the research can reduce the required value of ∆V and flight duration. Gravity assists of Venus, Earth and giant planets are considered. It is shown that for launch windows in the considered time interval, the use of the Venus, Earth, and Jupiter gravity assists, can significantly reduce the value of the total ∆V required to reach Sedna, with the time of flight limited by 50 years. On the other hand, possibility of reducing the ∆V with help of the Neptune gravity field, does happen on the launch in 2034 and 2036 with the time of flight of about 27.5 years or more. The inclusion of Saturn gravity assist into the flight schemes reduce total ∆V for launch in 2036 and 2037.
... In our Solar System alone, there are several icy moons that with certainty are ocean worlds: Europa, Ganymede, Enceladus, and Titan (Carr et al., 1998;Iess et al., 2012Iess et al., , 2014Saur et al., 2015). Other icy worlds may also harbor oceans beneath thick icy crusts, for example, Callisto, Dione, Triton, and Pluto (Carr et al., 1998;Hussmann et al., 2006;Zannoni et al., 2020). As we continue to study these worlds, refining our knowledge of Earth's FBY can help guide Solar System exploration. ...
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The physical processes active during the first billion years (FBY) of Earth's history, such as accretion, differentiation, and impact cratering, provide constraints on the initial conditions that were conducive to the formation and establishment of life on Earth. This motivated the Lunar and Planetary Institute's FBY topical initiative, which was a four-part conference series intended to look at each of these physical processes to study the basic structure and composition of our Solar System that was set during the FBY. The FBY Habitability conference, held in September 2019, was the last in this series and was intended to synthesize the initiative; specifically, to further our understanding of the origins of life, planetary and environmental habitability, and the search for life beyond Earth. The conference included discussions of planetary habitability and the potential emergence of life on bodies within our Solar System, as well as extrasolar systems by applying our knowledge of the Solar System's FBY, and in particular Earth's early history. To introduce this Special Collection, which resulted from work discussed at the conference, we provide a review of the main themes and a synopsis of the FBY Habitability conference.
... Pluto hosts five known satellites named Charon, Kerberos, Hydra, Nix, and Styx, of which Charon is by far the largest. The Pluto system is, because of Pluto's size and relative brightness, presently the beststudied of all of the Trans-Neptunian objects (TNOs) (Hussmann et al., 2006). This is a consequence of a protracted history of Earth-based remote sensing (Malhotra and Williams, 1997;Dobrovolskis et al., 1997;Olkin et al., 2003) and not least the flyby of the New Horizons spacecraft in 2015 . ...
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Existence of subsurface oceans on the satellites of the giant planets and Trans-Neptunian objects has been predicted for some time. Oceans on icy worlds exert a considerable influence on the dynamics of the ice-ocean system and, because of the astrobiological potential, represent an important objective for future missions. The Pluto-Charon system is representative of an icy moon orbiting a dwarf planet formed from the remnants of a giant impact. Evolution of icy moons is primarily controlled by the mode and efficiency of heat transfer through the outer ice shell, which is influenced by the presence of impurities, by tidal dissipation in the ice shell, and the radioactive element budget in the core. Previous studies on the evolution of the Pluto-Charon system considered either only the thermal or the tidal evolution, and in the cases where both were considered, the important effect of the presence of impurities in the liquid oceans was not addressed. We consider the joint tidal-thermal evolution of the system by combining a comprehensive tidal model that incorporates a viscoelastic tidal response with a parameterized convection model developed for icy worlds. This approach enables an extensive analysis of the conditions required for formation and maintenance of subsurface liquid oceans to the present. Our results show that because of fast circularization and synchronization of the orbits, tidal heating is only important during the early stages of evolution (<1 Myr). We test the sensitivity of our results to the initial orbital and thermal parameters. In all the cases, oceans on Pluto are always predicted to remain liquid to the present, ranging from 40 km to 150-km thick, whereas oceans on Charon have solidified. This is supported by New Horizons observations of extensional faults on Pluto and both extensional and compressional faults on Charon.
... Besides their dimensions and masses, those icy worlds are interesting because they could be hosting subsurface oceans [21]. Furthermore, the possibility to study those dwarf planets for many years by following them during their path around the Sun could reveal important information about the origins of the solar system, its evolution, as well as about the nature of the Kuiper Belt itself. ...
Article
The Direct Fusion Drive (DFD) is a nuclear fusion engine that will provide thrust and electrical power for any spacecraft. It is a compact engine, based on the D -³He aneutronic fusion reaction that uses the Princeton field reversed configuration for the plasma confinement and an odd parity rotating magnetic field as heating method to achieve nuclear fusion (Cohen et al., 2019), which will heat the deuterium, also used as propellant. In this work we present possibilities to explore the solar system outer border using the DFD. The objective is to reach some trans-Neptunian object, such as the dwarf planets Makemake, Eris and Haumea in less than 10 years with a payload mass of at least of 1500 kg, so that it would enable all kind of missions, from scientific observation to in-situ operations. For each mission a thrust-coast-thrust profile is considered. For this reason, each mission is divided into 3 phases: i. the spiral trajectory to escape Earth gravity; ii. the interplanetary travel, from the exit of Earth sphere of influence to the end of the coasting phase; iii. maneuvers to rendezvous with the dwarf planet. Propellant mass consumption, initial and final masses, velocities and ΔV for each maneuver are presented. Calculations to reach a vicinity at 125 AU for the study of Sun magnetosphere as well as Eris via flyby are also presented, with interest on the influence of different acceleration phases. Our calculations show that a spacecraft propelled by DFD will open unprecedented possibilities to explore the border of the solar system, in a limited amount of time and with a very high payload to propellant masses ratio.
... Remnant heat stemming from these interactions may have persisted to the present day, as suggested by recent studies showing that the heat flow on Triton should be around 10-100 mW m -2 (Ruiz, 2003, Martin-Herrero et al., 2018. These values are much larger than those obtained by assuming only radiogenic production and tidal dissipation for fixed orbital eccentricities (2-4 mW m -2 , Gaeman et al. 2012, Brown et al. 1991, Hussmann et al. 2006. The large heat flow could also be produced by internal ocean tidal heating due to Triton's orbit obliquity (Chen et al., 2014;Nimmo and Spencer, 2015;Dubois et al., 2017). ...
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Neptune's moon Triton shares many similarities with Pluto, including volatile cycles of N2, CH4 and CO, and represents a benchmark case for the study of surface-atmosphere interactions on volatile-rich KBOs. Within the context of New Horizons observations of Pluto as well as recent Earth-based observations of Triton, we adapt a Plutonian VTM to Triton, and test its ability to simulate its volatile cycles, thereby aiding our understanding of its climate. We present VTM simulations exploring the volatile cycles on Triton over long-term and seasonal timescales for varying model parameters. We explore what scenarios and model parameters allow for a best match of the available observations. In particular, our set of observational constraints include Voyager 2 observations, ground-based NIR (0.8 to 2.4 {\mu}m) disk-integrated spectra and the evolution of surface pressure as retrieved from stellar occultations. Our results show that Triton's poles act as cold traps for volatile ices and favor the formation of polar caps extending to lower latitudes through glacial flow. As previously evidenced by other VTMs, North-South asymmetries in surface properties can favor the development of one cap over the other. Our best-case simulations are obtained for a global reservoir of N2 ice thicker than 200 m and a bedrock thermal inertia larger than 500 SI. The large N2 ice reservoir implies a permanent N2 southern cap extending to the equator. Our results also suggest that a small permanent polar cap exists in the northern (currently winter) hemisphere if the internal heat flux remains radiogenic (< 3 mW m-2). Finally, we provide predictions for the evolution of ice distribution, surface pressure, CO and CH4 atmospheric mixing ratios in the next decades. We also model the thermal lightcurves of Triton in 2022, which serve as predictions for future JWST observations.
... Some special requirements also need to be considered to sample extraterrestrial ice, lakes, and oceans, which may provide a completely different perspective to discover the universe. For example, some extraterrestrial bodies are expected to harbor liquid oceans beneath their surface icy shells, such as Jupiter's moon Europa [244]. Scientists and engineers have been considering critical technologies to penetrate the thick ice shell with an ice-penetrating probe to determine the possibility of existence of life [245]. ...
Article
Regolith penetration for placing instruments into ground and regolith-sampling for re-entry or in-situ analysis play an extremely critical role in searching for alien life and revealing the geological information of extraterrestrial bodies. The planetary regolith sampler (PRS), a type of device that can penetrate, collect, transfer, and stow regolith samples, is commonly equipped on a lander or rover in planetary exploration, and is regarded as essential to broaden the application scenarios of planetary robots. Because of their extensive application prospects in deep space exploration, scientists and engineers worldwide have shown great interest in the design and development of such multifunctional devices. However, owing to the significant environmental differences among extraterrestrial bodies, it is challenging to create a full-featured PRS. To date, a large number of PRSs have been designed and developed for terrestrial scientific research or extraterrestrial regolith exploration. The PRSs utilized in previous extraterrestrial regolith-sampling missions and the latest advancements are reviewed in detail. Next, this work classifies the current PRSs into six categories according their sampling methods, namely drilling, excavating/grabbing, projecting, ultrasonic/sonic, pneumatic, and bio-inspired samplers, and summarizes their general characteristics. The challenges and constraints in sampling extraterrestrial bodies, including terrestrial technology, planetary environment, and remote distance, are analyzed and discussed in depth. The critical technologies for PRSs to change from a conceptual stage to a practical prototype are detailed, including design and fabrication, tool–regolith interaction, terrestrial validation, autonomous control, and sample fidelity. Finally, the critical trends of PRS are presented, covering near-term robotic exploration to cover the full geography to long-term human settlements.
... Rhea, they would depress the freezing point of water-ice increasing the probability of an aqueous layer. Chlorine-based salts (e.g., NaCl) have been detected in plumes of Enceladus (20), which provide evidence for an internal ocean; however, it is not likely that chlorine compounds could migrate to the surface of Rhea through cracks in the ice shell (assuming Rhea is partially differentiated) owing to the great depth of such a liquid layer as determined through numerical models (21). The only other possible source of chlorine is via exogenic delivery by chondritic asteroids over the history of Rhea. ...
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We present the first analysis of far-ultraviolet reflectance spectra of regions on Rhea’s leading and trailing hemispheres collected by the Cassini Ultraviolet Imaging Spectrograph during targeted flybys. In particular, we aim to explain the unidentified broad absorption feature centred near 184 nm. We have used laboratory measurements of the UV spectroscopy of a set of candidate molecules and found a good fit to Rhea’s spectra with both hydrazine monohydrate and several chlorine-containing molecules. Given the radiation-dominated chemistry on the surface of icy satellites embedded within their planets’ magnetospheres, hydrazine monohydrate is argued to be the most plausible candidate for explaining the absorption feature at 184 nm. Hydrazine was also used as a propellant in Cassini’s thrusters, but the thrusters were not used during icy satellite flybys and thus the signal is believed to not arise from spacecraft fuel. We discuss how hydrazine monohydrate may be chemically produced on icy surfaces.
... At a planetary scale, thermal conductivity is a key component controlling the transport of heat from the core to the surface via conductive and convective processes. On many icy bodies, thermal convection processes (e.g., plate tectonics, thermal plumes) are limited and conduction becomes the predominant mechanism in heat transport through the icy mantle (Guillot, 1995;Hussmann et al., 2006;McKinnon, 1998;Robuchon & Nimmo, 2011). These systems host a number of intriguing phenomena, such as subsurface oceans on icy satellites (e.g., Pluto, Enceladus) (Nimmo & Pappalardo, 2016;Robuchon & Nimmo, 2011) and anomalous magnetic fields around gas-ice giants (e.g., Neptune, Uranus) (Stanley & Bloxham, 2006). ...
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Plain Language Summary Neptune, Pluto, and Uranus all belong to an important subclass of objects in our solar system, and beyond, known as icy planetary bodies. These systems commonly have an internal “ice” layer, consisting of water, methane, and ammonia, that controls the outward flow of heat from the core. The dynamics within this layer contribute to the intriguing behaviors, such as supercritical subsurface oceans and odd magnetic fields, present in these systems. To understand these dynamics, we collected unique measurements under extreme pressure on methane's thermal conductivity, the fundamental property defining the rate of energy transport through a material. We quantified the rate of conductivity increase with pressure up to 25 gigapascals (GPa; 1 GPa ≈ 10⁴ atm) and captured an anomalous spike at a phase boundary that has never been previously observed. We combined this information with similar measurements on the other major “ice” components to constrain the composition and bulk physical properties of these layers. As our understanding of icy planetary bodies improves, primarily through remote‐sensing, our research provides a framework to link surficial measurements to empirically‐derived interior properties. This insight is critical for comprehending both the external and internal phenomena exhibited by icy bodies.
... Remnant heat stemming from these interactions may have persisted to the present day, as suggested by recent studies showing that the heat flow on Triton should be around 10-100 mW m -2 (Ruiz, 2003, Martin-Herrero et al., 2018. These values are much larger than those obtained by assuming only radiogenic production and tidal dissipation for fixed orbital eccentricities (2-4 mW m -2 , Gaeman et al. 2012, Brown et al. 1991, Hussmann et al. 2006. The large heat flow could also be produced by internal ocean tidal heating due to Triton's orbit obliquity (Chen et al., 2014;Nimmo and Spencer, 2015;Dubois et al., 2017). ...
Thesis
The stellar occultation technique is a very powerful tool to observe distant and/or small objects in the Solar System. In particular, ground-based occultations can detect tenuous atmospheres, down to a pressure of ~ 10 nbar. Triton’s atmosphere can be detected by this technique. A stellar occultation by Triton was observed on 5 October 2017. 90 positive detections were obtained, 42 of them featuring a central flash detection. Using the Abel inversion and ray-tracing methods, I was able to obtain the density, pressure, and temperature profiles in the altitude range ~ 8 km to ~ 190 km, where a pressure of 1.18 ± 0.03 µ bar was found at a reference radius of 1400 km (47 km altitude). A novel analysis of the Voyager 2 data was then performed, to directly compare its results to those obtained in 2017. It shows that they are consistent with each other, implying that Triton’s atmospheric pressure obtained in 2017 is at its Voyager 2 epoch levels. A survey of stellar occultations obtained between 1989 and 2017 suggests an increase in the atmospheric pressure. This is, however, debatable, due to very few high signal-to-noise ratio light curves and data accessible for reanalysis. Volatile Transport Models examined suggest that any increase during this time frame should be modest, as they do not support a strong increase in the surface pressure. A central flash analysis allowed the study of the lower atmosphere’s shape. It shows that there is no evidence of atmospheric distortion. An upper limit of 0.0011 for the apparent oblateness of the atmosphere near the 8 km altitude is found.
... The origin of such a shade may be associated with the presence of a layer of tholine or hydrocarbon sediment on the surface of this object [5]. Some researches give data, according to which there may be an ocean under the surface of Sedna, heated by the internal heat of this object, generated by radioactive decay [6]. The origin of Sedna is the subject of many discussions. ...
Conference Paper
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The Oort Cloud is a hypothetical outer part of the Solar System and is considered a source of long-periodic comets that can occasionally be visible while approaching the Sun. Other bodies belonging to the Oort Cloud may stay at a very significant distance from the Sun and be considered asteroids or planetoids, depending on their sizes. Orbits of these objects are expected to be at the boundary of stability, and as the result of different perturbations, the objects may approach the inner part of the Solar System and be observed. The trans-Neptunian object (90377) Sedna discovered in 2003 is probably related to the Oort Cloud since its orbit's aphelion is estimated at 1 thousand AU with an orbital period of more than 10 thousand years. In 2074-75, Sedna will be in the vicinity of its perihelion at a distance of about 75 AU from the Sun. That provides a unique chance to study this object from a close distance. The current paper focuses on analyzing flight trajectories to Sedna for launch in 2029-2037. A flight using gravity assist maneuvers is considered. Use of these maneuvers reduces the required ∆V with a significant decrease of flight duration. Gravity assists of Venus, Earth, Jupiter, Saturn, and Neptune are discussed. Analysis of the spacecraft mass which can be delivered to Sedna by various launch vehicles also is considered in the article. Nomenclature ΔV = characteristic velocity, km/s; ΔV0 = launch ΔV from Earth, km/s; ΔVΣ = total ΔV required for flight to Sedna, km/s; LEO = low Earth orbit M0 = initial mass, kg; IUpS = upper stage specific impulse, s; MUpS = dry mass of the upper stage, kg; МSC0 = spacecraft mass (including the deep space buster), kg; Мk, = final mass, kg; Is is specific impulse, s; g0 = 9,80665, m/s 2. Мtank capacity = the capacity of tanks, kg; ΔVi = impulse for each maneuver in deep space or near a planet; МSCi = spacecraft mass after maneuver, kg; MSB = dry mass of booster, kg; SF SC М = the payload mass delivered by Soyuz 2.1.a/Fregat, kg; PB SC М = the payload mass delivered by Proton-M/Briz-M, kg; PD SC М = the payload mass delivered by Proton-M/DM-03, kg; AV SC М = the payload mass delivered by Atlas V 551, kg; DIV SC М = the payload mass delivered by Delta IV Heavy, kg; T = the time of flight to Sedna, yrs; C3 = double energy, km 2 /s 2. ESed = direct flight Earth-Sedna EJSed = Earth-Jupiter-Sedna EVEEJSed = Earth-Venus-Earth-Earth-Jupiter-Sedna EVEEJSSed = Earth-Venus-Earth-Earth-Jupiter-Saturn-Sedna EVEEJNSed = Earth-Venus-Earth-Earth-Jupiter-Neptune-Sedna EVEEJSNSed = Earth-Venus-Earth-Earth-Jupiter-Saturn-Neptune-Sedna VEGA = Venus-Earth Gravity Assists maneuver VEEGA = Venus-Earth-Earth Gravity Assists.
... Many studies of bodies made of ice and rock are based on solving the equilibrium structure equations for a given mass (e.g., Léger et al. 2004;Hussmann et al. 2006;Noack et al. 2016;Van Hoolst et al. 2019, and the refrences therein), on dividing the evolution into separate phases (e.g., Travis et al. 2012), or on following the thermal evolution and inferring structural changes based on it (O'Rourke & Stevenson 2014;Zeng & Sasselov 2014;Bhatia & Sahijpal 2017). However, in order to understand the present structure of ice-rich objects, continuous self-consistent evolution models are essential. ...
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The interest in the structure of ice-rich planetary bodies, in particular the differentiation between ice and rock, has grown due to the discovery of Kuiper belt objects and exoplanets. We thus carry out a parameter study for a range of planetary masses $M$, yielding radii $50 \aplt R \aplt 3000$~km, and for rock/ice mass ratios between 0.25 and 4, evolving them for 4.5~Gyr in a cold environment, to obtain the present structure. We use a thermal evolution model that allows for liquid and vapor flow in a porous medium, solving mass and energy conservation equations under hydrostatic equilibrium for a spherical body in orbit around a central star. The model includes the effect of pressure on porosity and on the melting temperature, heating by long-lived radioactive isotopes, and temperature-dependent serpentinization and dehydration. We obtain the boundary in parameter space [size, rock-content] between bodies that differentiate, forming a rocky core, and those which remain undifferentiated: small bodies, bodies with a low rock content, and the largest bodies considered, which develop high internal pressures and barely attain the melting temperature. The final differentiated structure comprises a rocky core, an ice-rich mantle, and a thin dense crust below the surface. We obtain and discuss the bulk density-radius relationship. The effect of a very cold environment is investigated and we find that at an ambient temperature of $\sim$20~K, small bodies preserve the ice in amorphous form to the present.
Conference Paper
Article
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The article focuses on trajectory design to the trans-Neptunian object (90377) Sedna for launch in 2029–2034. Sedna is currently moving to the perihelion at a distance of around 74 au from the Sun. The perihelion passage is estimated to be in 2073-74. That opens up of opportunities to study such a distant object. Known for its orbit and 10 thousand year period, Sedna is an exciting object for deep space exploration. The current research provides two possible scenarios of transfer to Sedna. A direct flight and flights including gravity assist manoeuvres are considered. The present study showed that a direct flight would be practically unrealistic due to the high total characteristic velocity and the time of flight value. Promising scenarios include gravity assist manoeuvres near Venus, Earth, Jupiter, Saturn and Neptune. The analysis of the close approach to asteroids during the flight to Sedna had been performed. Results of the research presented in this article show that the launch in 2029 provides the best transfer conditions in terms of minimum total characteristic velocity. The analysis shows that with a small additional impulses flybys of the large main-belt asteroids (16) Psyche for launch in 2034 and (20) Massalia for launch in 2029 are possible. Full text available at link: https://authors.elsevier.com/a/1dICw%7E6OilWeB.
Chapter
In this chapter we suggest that possible habitats for extremophiles in the Solar System can be tested with sulfur isotopes as biomarkers. This search is feasible with miniaturized mass spectrometry that has already reached a significant degree of development. This instrument is suitable for the forthcoming exploration of the Moon, Mars and the icy surface of the Jovian moon Europa. We suggest that these biomarkers may be relevant on the icy moons of giant and ice planets. We conclude by raising the question, “Are there environments for extremophiles on exoplanets?”
Article
Science goals for icy ocean worlds missions include characterizing the chemical composition of the surface and interior using remote sensing and in situ techniques. The next class of flight mass spectrometers for these missions will obtain compositional identifications and isotope ratio measurements of volatiles evolved from the ice surface (exosphere) and plumes. These mass spectra will be combined with data from other flight instruments to infer the composition of the interior from volatiles. To ensure accurate interpretation of these measurements, it is critical to verify whether the fundamental assumption that volatiles observed in icy ocean world exospheres or plumes will be a direct reflection of the sub-ice ocean. The present study evaluates whether isotopologues from an initial CO2 gas fractionate by interacting with seawater (brine) of varying salt composition and concentration. δ¹³CCO2 are affected by the pH of the brine and subsequent speciation of {CO2}, where {CO2} represents the combination of CO2, H2CO3, HCO3⁻, and CO3²⁻. {CO2} for low pH brines hypothesized for Europa will preferentially speciate as CO2. Analyzed δ¹³CCO2 for low pH brines are within error of the original δ¹³CCO2, demonstrating that volatile CO2 in a low pH system will be a direct reflection of the original CO2. However, Europa’s radiative environment and rapid depressurization due to plume ejection may impose fractionation effects. In contrast, high pH systems relevant to Enceladus or a more alkaline Europa are expected to form all carbonate species, while favoring speciation as HCO3⁻. High pH brines in these experiments include both an original CO2 gas and an isotopically distinct HCO3⁻ (from NaHCO3 salt). These alkaline experiments demonstrate that δ¹³CCO2 values are highly variable, and depend on the concentration of the dominant carbonate species, (Na)HCO3⁻. Mass balance estimates indicate that measured δ¹³CCO2 is a thermodynamically predictable mixture of both carbon sources, suggesting that measurements of δ¹³C at Enceladus would directly reflect of the sources of CO2 and carbonate buffering in the ocean. δ¹⁸O measurements for CO2 interacting with KCl and MgCl2 follow established models for δ¹⁸O-ionic strength. CO2 interacting with MgSO4, Na2SO4, and NaCl demonstrate an offset from established δ¹⁸O-ionic strength models depending on the concentration of initial CO2. These results suggest that current predictive models for δ¹⁸O in brines need to be resolved for changing concentrations of CO2.
Chapter
“Moon–magnetosphere interaction” refers to the interaction of magnetospheric plasma with a moon orbiting within the host planet's magnetosphere. Observations and modeling of moon–magnetosphere interactions is a highly interesting area of space physics because it helps to better understand the basic physics of plasma flows in the universe and provides geophysical information about the interior of the moons. Moon–magnetosphere interaction is caused by the flow of magnetospheric plasma relative to the orbital motions of the moons. The relative velocity is usually slower than the Alfvén velocity of the plasma around the moons. Thus the interaction generally forms Alfvén wings instead of bow shocks in front of the moons. The local interaction, i.e., the interaction within several moon radii, is controlled by properties of the atmospheres, ionospheres, surfaces, nearby dust populations, the interiors of the moons as well as the properties of the magnetospheric plasma around the moons. The far‐field interaction, i.e., the interaction further away than a few moon radii, is dominated by the magnetospheric plasma and the fields but it still carries information about the properties of the moons. In this chapter the basic physics of moon–magnetosphere interaction is reviewed. We also give a short tour through the solar system highlighting the important findings at the major moons.
Chapter
Volcanism based on melting rocks (silicate volcanism) is long known on Earth and has also been found on Jupiter’s moon Io. Remnants of this type of volcanism have been identified also on other bodies in the solar system. Energy sources powered by accretion and the decay of radioactive isotopes seem to be dominant mainly inside larger bodies, which have enough volume to accumulate and retain this energy in significant amounts. On the other hand, the impact of tidal forces allows even tiny bodies to melt up and pass into the stage of cryovolcanism. The dependence of tidal heating on the size of the object is minor, but the masses of and the distances to accompanying bodies as well as the inner compositions of the heated body are central factors. Even though Io as an example of a body supporting silicate volcanism is striking, the physics of tidal forces might suggest a relatively high probability for cryovolcanism. This chapter aims at considering the parameters known and objects found so far in our solar system to give insights into where in our system and other planetary systems cryovolcanism might be expected.
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This review of miniaturised instrumentation is motivated by the ongoing and forthcoming exploration of the confirmed, or candidate ocean worlds of the Solar System. It begins with a section on the evolution of instrumentation itself, ranging from the early efforts up to the current rich-heritage miniaturised mass spectrometers approved for missions to the Jovian system. The geochemistry of sulphur stable isotopes was introduced for life detection at the beginning of the present century. Miniaturised instruments allow the measurement of geochemical biosignatures with their underlying biogenic coding, which are more robust after death than cellular organic molecules. The role of known stable sulphur isotope fractionation by sulphate-reducing bacteria is discussed. Habitable ocean worlds are discussed, beginning with analogies from the first ocean world known in the Solar System that has always being available for scientific exploration, our own. Instrumentation can allow the search for biosignatures, not only on the icy Galilean moons, but also beyond. Observed sulphur fractionation on Earth suggests a testable “Sulphur Hypothesis”, namely throughout the Solar System chemoautotrophy, past or present, has left, or are leaving biosignatures codified in sulphur fractionations. A preliminary feasible test is provided with a discussion of a previously formulated “Sulphur Dilemma”: It was the Galileo mission that forced it upon us, when the Europan sulphur patches of non-ice surficial elements were discovered. Biogenic fractionations up to and beyond δ34S = −70‰ denote biogenic, rather than inorganic processes, which are measurable with the available high sensitivity miniaturised mass spectrometers. Finally, we comment on the long-term exploration of ocean worlds in the neighbourhood of the gas and ice giants.
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Volcanism powered by tidal forces inside celestial bodies can provide enough energy to keep important solvents for living systems in the liquid phase. A prerequisite to calculate such tidal interactions and consequences is depending on simulations for tidal accelerations in a multi-body system. Unfortunately, from measurements in many extrasolar planetary systems, only few physical and orbital parameters are well-known enough for investigated celestial bodies. For calculating tidal acceleration vectors under missing most orbital parameter exactly, a simulation method is developed that is only based on a few basic parameters, easily measurable even in extrasolar planetary systems. Such a method as the one presented here allows finding a relation between the tidal acceleration vectors and potential heating inside celestial objects. Using the values and results of our model approach to our solar system as a “gold standard” for feasibility allowed us to classify this heating in relation to different forms of volcanism. This “gold standard” approach gave us a classification measure for the relevance of tidal heating in other extrasolar systems with a reduced availability of exact physical parameters. We help to estimate conditions for the identification of potential candidates for further sophisticated investigations by more complex established methods such as viscoelastic multi-body theories. As a first example, we applied the procedures developed here to the extrasolar planetary system TRAPPIST-1 as an example to check our working hypothesis.
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We use an existing thermal evolution model to assess the plausibility of subsurface oceans on the moons of Uranus. We adopt a conservative approach neglecting tidal heating and find that Titania and Oberon could support a subsurface ocean to the present day if conductive heat loss through the ice shell is limited. This could occur due to porosity, an ammonia-rich ocean, or clathrates at the base of the ice shell. This result emphasizes the importance of designing future missions with the capability to detect an ocean at all five of the large Uranian satellites.
Chapter
Cryovolcanism has been either observed on or suspected for numerous icy bodies across the Solar System, most notably Saturn’s tiny moon Enceladus, where jets of water vapor and other constituents are spewed into space from giant fractures near the South Pole. In this chapter, we review cryomagmatism and cryovolcanism, which are the subsurface and surface processes, respectively, resulting from the mobilization and migration of fluids generated in the interiors of icy bodies. Although these phenomena have no direct equivalents on Earth, they are important processes in the icy Solar System, and we can draw inferences as to how they operate from silicate volcanism in the inner Solar System. We discuss mechanisms of cryomagmatism and cryovolcanism, the possible compositions of cryomagmas, and the observational evidence found so far on extraterrestrial bodies, which range from plumes to geological features interpreted as cryovolcanic in origin.
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The article focuses on trajectory design to the trans-Neptunian object (90377) Sedna for launch in 2029-2034. Sedna is currently moving to the perihelion at a distance of around 74 au from the Sun. The perihelion passage is estimated to be in 2073-74. That opens up of opportunities to study such a distant object. Known for its orbit and 10 thousand year period, Sedna is an exciting object for deep space exploration. The current research provides two possible scenarios of transfer to Sedna. A direct flight and flights including gravity assist manoeuvres are considered. The present study showed that a direct flight would be practically unrealistic due to the high total characteristic velocity and the time of flight value. Promising scenarios include gravity assist manoeuvres near Venus, Earth, Jupiter, Saturn and Neptune. The analysis of the close approach to asteroids during the flight to Sedna had been performed. Results of the research presented in this article show that the launch in 2029 provides the best transfer conditions in terms of minimum total characteristic velocity. The analysis shows that with a small additional impulses flybys of the large main-belt asteroids (16) Psyche for launch in 2034 and (20) Massalia for launch in 2029 are possible.
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Cassini measurements of Mimas' physical libration can be explained by either a non-hydrostatic core or a global, liquid water ocean beneath a 24–31 km thick ice shell. An ocean within Mimas would be surprising, given the lack of comparable geologic activity to that observed on other ocean-bearing moons like Europa and Enceladus, and thus has important implications for the prevalence and identification of ocean worlds. Here, we calculate the tidal heating that would be generated within an ocean-bearing Mimas and determine the ice shell thicknesses that would result. Our model accounts for tidal heating caused by Mimas' eccentricity and libration and uses a depth-dependent rheological profile in the ice. We find that the ability of Mimas to host a 24–31 km ice shell over an ocean depends on the rheology of the ice, the surface temperature, and the basal heat flux. We find that, using the most reasonable assumptions, Mimas would have the suggested ocean and ice shell thicknesses today. We report corresponding average surface heat fluxes for these cases and discuss observational data that could further assess whether Mimas is, indeed, a present-day ocean world.
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The existence of subsurface oceans on the satellites of the giant planets and Trans-Neptunian objects has been predicted for some time. Liquid oceans on icy worlds, if present, exert a considerable influence on the dynamics of the ice-ocean system and, because of the astrobiological potential, represent an important objective for future missions to the outer solar system. The Pluto-Charon system is representative of an icy moon orbiting a dwarf planet that is believed to have formed from the remnants of a giant impact. The evolution of icy moons is primarily controlled by the mode and efficiency of heat transfer through the outer ice shell, which is influenced by the presence of impurities, by tidal dissipation in the ice shell, and by the radioactive element budget in the silicate core. Previous studies on the evolution of the Pluto–Charon system generally considered either only the thermal or the tidal evolution, and in the cases where both were considered, the important effect of the presence of impurities in the liquid oceans was not addressed. Here, we consider the joint tidal-thermal evolution of the Pluto-Charon system by combining a comprehensive tidal model that incorporates a viscoelastic description of the tidal response with a parameterized thermal convection model developed for icy worlds. This approach enables an extensive analysis of the conditions required for the formation and maintenance of subsurface liquid oceans up to the present. Our results show that because of relatively fast circularization and synchronization of the orbits of Pluto and Charon, tidal heating is only important during the early stages of evolution (<1 Myr). As part of our study, we test the sensitivity of our results to a number of parameters, including initial orbital and thermal parameters. In all the studied cases, oceans on Pluto are always predicted to remain liquid to the present, ranging in thickness from 40 km to 150-km, whereas oceans on Charon, while in-place for approximately 4 Gyr, have solidified. This is supported by New Horizons observations of primarily extensional faults on Pluto and both extensional and compressional faults on Charon.
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Here we present an improved algorithm to model the serpentinization process in planetesimals in the early solar system. Although it is hypothesized that serpentinization-like reactions played an important role in the thermal evolution of planetesimals, few and restricted models are available in this topic. These processes may be important, as the materials involved were abundant in these objects. Our model is based on the model by Góbi & Kereszturi and contains improvements in the consideration of heat capacities and lithospheric pressure and in the calculation of the amount of interfacial water. Comparison of our results with previous calculations shows that there are significant differences in, e.g., the serpentinization time—the time necessary to consume most of the reactants at specific initial conditions—or the amount of heat produced by this process. In a simple application we show that in icy bodies, under some realistic conditions, below the melting point of water ice, serpentinization reaction using interfacial water may be able to proceed and eventually push the local temperature above the melting point to start a “runaway” serpentinization. According to our calculations in objects with radii R ≳ 200 km, serpentinization might have quickly reformed nearly the whole interior of these bodies in the early solar system.
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The interest in the structure of ice-rich planetary bodies, in particular the differentiation between ice and rock, has grown due to the discovery of Kuiper Belt objects and exoplanets. We thus carry out a parameter study for a range of planetary masses M , yielding radii 50 ≲ R ≲ 3000 km, and for rock to ice mass ratios between 0.25 and 4, evolving them for 4.5 Gyr in a cold environment, to obtain the present structure. We use a thermal evolution model that allows for liquid and vapor flow in a porous medium, solving mass and energy conservation equations under hydrostatic equilibrium for a spherical body in orbit around a central star. The model includes the effect of pressure on porosity and on the melting temperature, heating by long-lived radioactive isotopes, and temperature-dependent serpentinization and dehydration. We obtain the boundary in parameter space (size, rock content) between bodies that differentiate, forming a rocky core, and those which remain undifferentiated: small bodies, bodies with a low rock content, and the largest bodies considered, which develop high internal pressures and barely attain the melting temperature. The final differentiated structure comprises a rocky core, an ice-rich mantle, and a thin dense crust below the surface. We obtain and discuss the bulk density–radius relationship. The effect of a very cold environment is investigated, and we find that at an ambient temperature of ∼20 K, small bodies preserve the ice in amorphous form to the present.
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Dawn revealed that Ceres is a compelling target whose exploration pertains to many science themes. Ceres is a large ice- and organic-rich body, potentially representative of the population of objects that brought water and organics to the inner solar system, as well as a brine-rich body whose study can contribute to ocean world science. The Dawn observations have led to a renewed focus on planetary brine physics and chemistry based on the detection of many landforms built from brines or suspected to be emplaced via brine effusion. Ceres’ relative proximity to Earth and direct access to its surface of evaporites that evolved from a deep brine reservoir make this dwarf planet an appealing target for follow-up exploration. Future exploration, as described here, would address science questions pertinent to the evolution of ocean worlds and the origin of volatiles and organics in the inner solar system.
Chapter
The NASA Dawn mission, launched in 2007, aimed to visit two of the most massive protoplanets of the main asteroid belt: Vesta and Ceres. The aim was to further our understanding of the earliest days of the Solar System, and compare the two bodies to better understand their formation and evolution. This book summarises state-of-the-art results from the mission, and discusses the implications for our understanding not only of the asteroid belt but the entire Solar System. It comprises of three parts: Part 1 provides an overview of the main belt asteroids and provides an introduction to the Dawn mission; Part 2 presents key findings from the mission; and Part 3 discusses how these findings provide insights into the formation and evolution of the Solar System. This is a definitive reference for academic researchers and professionals of planetary science, asteroid science and space exploration.
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The icy bodies of the outer solar system have a different chemistry than rocky planets. The main composition of the upper layers is water ice with potentially salts and volatiles. The presence of aqueous ammonia and methanol, if their concentrations at the interior are above trace abundances, could contribute to form liquid layers within the icy shells at temperatures much lower than the melting point of pure water, or to the maintenance of oceans under the icy crusts, due to their strong antifreeze capacity. However, they can also interact in different ways with other compounds, such as other volatiles and salts. Specifically, their interactions with carbon-bearing molecules may lead to an interesting organic chemistry. In this research, by means of Raman spectroscopy, we focus on the study of the kinetics and pH evolution of the moderate low-temperature (down to 240 K) and high-pressure (up to 500 bar) chemistry of ammonia–methanol aqueous solutions in the presence of different types of carbon sources, that is, carbon dioxide and sodium carbonate, and determine the disparities in chemical interactions in each case. At the pressures of work, the experiments mimicked cryomagmas located relatively close to the surface, which underwent a primary chemical evolution during their journey from the ocean. Previous fractional precipitations along this path may result in aqueous solutions concentrated in volatiles. Thus, aqueous solutions with NH3 and CH3OH at concentrations around 15 wt % were mixed with CO2 or 5 wt % Na2CO3. The systems H2O–NH3–CO2 and H2O–NH3–CH3OH–CO2 led to the fast formation (less than 300 min at 280 K, and 1400 min at 251 K) of ammonium bicarbonate ((NH4)HCO3). In the system with CH3OH, when the bicarbonate precipitated, the Raman signatures of CH3OH also suffered changes that could be attributed to its adhesion to the surface of the mineral or the formation of a metastable co-crystal. Regarding the pH changes throughout the experiments, it was observed that the presence of CH3OH reversed the trends. In the ternary system, the pH decreased from values around 10–7 after the mineral formation. However, when CH3OH was also present, the pH increased from 7 to 9 after the mineral precipitation. Once the bicarbonate was formed, we checked its stability in water at the highest pressure over time, observing that the mineral kept stable, probably due to slow kinetics. When CO2 was substituted by Na2CO3, the Na+–NH4+ cation exchange was not favorable, and then the mineral, which stabilized upon cooling in the ternary system H2O–NH3–Na2CO3, was the hydrated sodium carbonate (Na2CO3·10H2O). The pH decreased from 12 to 10 after precipitation. The addition of CH3OH to the system altered the redox equilibrium to the acidic form, transforming the aqueous carbonate (CO32–) in bicarbonate (HCO3–), due to a pH change from 12 to 7.2, and no precipitation of any solid was observed at the temperatures of work. The results represent extreme cases of highly evolved, volatile-rich cryomagmas that provide broad chemical insights into expected trends during their evolution at near-surface locations.
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Plain Language Summary The icy moons of Uranus may harbor subsurface oceans. These oceans may be habitable environments and tell us about the moons' formation and the evolution of their orbits. Here we explore the possibility that these oceans could be detected and characterized by magnetic field measurements from a spacecraft. In particular, the time‐varying magnetic field of Uranus may generate currents in a salty ocean by the process of induction. These currents could then generate a another magnetic field detectable by a spacecraft flyby. We explore this possibility by calculating the induced magnetic fields around the five largest moons of Uranus: Miranda, Ariel, Umbriel, Titania, and Oberon. We find that the spin of Uranus and the orbital motion of the moons lead to strong time‐varying fields at each moon. If these moons harbor thick (several to tens of km) oceans with salinities like that of Earth's oceans, their induced fields would likely be detectable by a close spacecraft flyby. Longer term measurements from a Uranus orbiter and/or dedicated icy moon orbiter could likely constrain the ocean and ice thickness and ocean salinity.
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We have obtained visible and near infrared spectra of the Trans Neptunian object 2004 DW, a few days after its discovery, at the Telescopio Nazionale Galileo (TNG). 2004 DW belongs to the plutino dynamical class and has an estimated diameter of about 1600 km, that makes it the largest known object, except Pluto, in the plutino and classical TNO populations. Our data clearly show the 1.5 and 2 mum bands associated to water ice, while the visible spectrum is nearly neutral and featureless. To interpret the available data we modelled the surface composition of 2004 DW with two different mixtures of organics (Titan tholin and kerogen), amorphous carbon and water ice. Bases on observations obtained at the Telescopio Nazionale Galileo, La Palma, Spain.
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We present a study of the 3.1 micron absorption band, attributed to the OH stretching mode of the water molecule in the form of solid ice, observed in many protostellar lines of sight by the SWS instrument on board ISO. Using ice optical constants and Mie theory, we obtain reasonable fits to the peculiar band shapes observed in twelve sources. The fits clearly show that some scattering effects arise in this absorption band, as the grain sizes used in the calculations are of the order of a few tenths of a micron. In the fit residuals, we search for the nu 1 and nu 3 vibrations of the ammonia molecule which fall in the same spectral region, leading only to upper limits for the NH3 content of a few percents of the total water ice content in these different lines of sight. We also discuss the occurrence of a 3.47 mu m absorption band which could be related to the formation of an ammonia hydrate in the ice mantles. On the assumption that this band is due merely to this hydrate and with the help of relevant laboratory experiments, we show that ammonia represents therefore at most 5% in abundance relative to water ice in these interstellar grain mantles. Finally, this study sheds light on the 3.1 mu m/6 mu m column density paradox" obtained when comparing the water ice absorption band at 3.1 (stretching mode) and 6 mu m (bending mode). We show that this paradox is resolved by considering different extinction regimes, in which scattering affects only the 3.1 mu m band whereas pure absorption dominates at 6 mu m.
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The composition of cometary ices provides key information on the chemical and physical proper- ties of the outer solar nebula where comets formed, 4.6 Gyr ago. This chapter summarizes our current knowledge of the volatile composition of cometary nuclei, based on spectroscopic observations and in situ measurements of parent molecules and noble gases in cometary comae. The processes that govern the excitation and emission of parent molecules in the radio, infrared (IR), and ultraviolet (UV) wavelength regions are reviewed. The techniques used to convert line or band fluxes into molecular production rates are described. More than two dozen parent molecules have been identified, and we describe how each is investigated. The spatial distribution of some of these molecules has been studied by in situ measure- ments, long-slit IR and UV spectroscopy, and millimeter wave mapping, including interferometry. The spatial distributions of CO, H CO, and OCS differ from that expected during direct sublimation from the nucleus, which suggests that these species are produced, at least partly, from extended sources in the coma. Abundance determinations for parent molecules are reviewed, and the evidence for chemical diversity among comets is discussed.
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Interior models of a differentiated Titan with an internal ammonia-water ocean and chondritic radiogenic heat production in an undifferentiated rock + iron core have been calculated. We assume thermal and mechanical equilibrium and calculate the structure of the interior as a function of the thickness of an ice I layer on top of the ocean as well as the moment of inertia factor and the tidal Love numbers for comparison with Cassini gravity data. The Love numbers are linearly dependent on the thickness of the ice I shell at constant rheology parameters but decrease by one order of magnitude in the absence of an internal ocean. Ice shell thicknesses are between 90 and 105 km for models with 5 wt.% ammonia and for core densities between 3500 and 4500 kg m-3. For 15 wt.% ammonia, the shell is 65 to 70 km thick. We use a strongly temperature-dependent viscosity parameterization of convective heat transport and find that the stagnant lid comprises most of the ice I shell. Tidal heating in the warm convective sublayer is of minor importance. The internal ocean is several hundred kilometers thick, and its thickness decreases with increasing thickness of the ice shell. Core sizes vary from 1500 to 1800 km radius with associated moment of inertia factors of 0.30 +/- 0.01.
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Evolution of large icy satellites is controlled by heat transfer across the outer ice I layer. After the core overturn a possible structure consists of a silicate core and a shell of molten ices. As the satellite cools down, the primordial ocean crystallizes. If the outer layer is thick enough, convection is very likely to occur in it. We have used the results of a recent two-dimensional numerical model of convection including variable viscosity to estimate the vigor and the efficiency of convection in this layer. Viscosity variations induce the apparition of a stagnant lid at the top of the fluid, which reduces the efficiency of heat transfer. In the present study, the Rayleigh number Ra and the heat flux Phi are computed as a function of the thickness of the layer, assuming that the ice flow is Newtonian. Calculations are first made for a generic satellite of radius R=2500km and mean density =1.9g/cm3. It is then shown that variations of +/-500 km on the radius and +/-0.5 g/cm3 on the mean density do not induce significant differences in the values of Ra and Phi. On the other hand, variations of the reference viscosity mu0, and of the activation energy E induce major differences. The reference viscosity is equal to the viscosity close to the melting point, and its possible value yields around mu0=5×1013Pas. A possible value of E is 60 kJ/mol. For these values of the rheological parameters we find that the initial ocean may crystallize completely in ~3.6 Gyr. Higher values of mu0 and/or E reduce significantly the vigor and the efficiency of convection. The influence of the composition of the initial ocean is also investigated. The presence of ammonia reduces the convective strength and the heat flux. The upper structure of icy satellites is discussed as a function of the rheological and compositional parameters. The presence of a sub-surface ocean could be explained by either the presence of volatiles in the initial ocean or the presence of additional heat sources, such as tidal dissipation.
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The characteristics of thermal convection in a fluid whose viscosity varies strongly with temperature are studied in the laboratory. At the start of an experiment, the upper boundary of an isothermal layer of Golden Syrup is cooled rapidly and maintained at a fixed temperature. The fluid layer is insulated at the bottom and cools continuously. Rayleigh numbers calculated with the viscosity of the well-mixed interior are between 106 and 108 and viscosity contrasts are up to 106. Thermal convection develops only in the lower part of the thermal boundary layer, and the upper part remains stagnant. Vertical profiles of temperature are measured with precision, allowing deduction of the thickness of the stagnant lid and the convective heat flux. At the onset of convection, the viscosity contrast across the unstable boundary layer has a value of about 3. In fully developed convection, this viscosity contrast is higher, with a typical value of 10. The heat flux through the top of the layer depends solely on local conditions in the unstable boundary layer and may be written $Q_{\rm s} = - CK_{\rm m} (\alpha g/\kappa \nu_{\rm m})^{\frac{1}{3}} \Delta T^{\frac{4}{3}}_{\rm v}$, where km and νm are thermal conductivity and kinematic viscosity at the temperature of the well-mixed interior, κ thermal diffusivity, α the coefficient of thermal expansion, g the acceleration due to gravity. ΔTv, is the ‘viscous’ temperature scale defined by $\Delta T_{\rm v} = - \frac{\mu (T_{\rm m})}{({\rm d}\mu /{\rm d}T)(T_{\rm m})}$ where μ(T) is the fluid viscosity and Tm the temperature of the well-mixed interior. Constant C takes a value of 0.47 ± 0.03. Using these relations, the magnitude of temperature fluctuations and the thickness of the stagnant lid are calculated to be in excellent agreement with the experimental data. One condition for the existence of a stagnant lid is that the applied temperature difference exceeds a threshold value equal to (2ΔTv).
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Gravitational fiels of the satellites: flyby determination of C22, gravity results; Io, Europa, Ganymede, Callisto - interior models; Thermal considerations in the maintenance of intra-ice oceans on the icy Galilean satellites: melting relations, energy balances and equilibrium models, the probability of oceans and their thicknesses; Formation of the Galilean satellites.
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The deformation of Io, the tidal dissipation rate, and its interior spatial distribution are investigated by means of numerical simulations based on (1) a three-layer model (with dissipation in the mantle) or (2) a four-layer model (with dissipation in the asthenosphere). The mathematical derivation of the models is outlined; the selection of the input-parameter values is explained; the results are presented in extensive graphs and contour maps; and the constraints imposed on the models by observational data on the hot-spot distribution, tidal deformation, and gravity field are discussed in detail. It is found that both dissipation mechanisms may play a role on Io: model (2) is better able to explain the concentration of hot spots near the equator, while the presence of a large hot spot near the south pole (if confirmed by observations) would favor model (1).
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Data from the recent gravity measurements by the Galileo mission are used to construct wide ranges of interior structure and composition models for the Galilean satellites of Jupiter. These models show that mantle densities of Io and Europa are consistent with an olivine-dominated mineralogy with the ratios of Mg to Fe components depending on mantle temperature for Io and on ice shell thickness for Europa. The mantle density and composition depend relatively little on core composition. The size of the core is largely determined by the core's composition with core radius increasing with the concentration of a light component such as sulfur. For Io, the range of possible core sizes is between 38 and 53% of the satellite's radius. For Europa, there is also a substantial effect of the thickness of the ice layer which is varied between 120 and 170 km on the core size. Core sizes are between 10 and 45% of Europa's radius. The core size of Ganymede ranges between one-quarter and one-third of the surface radius depending on its sulfur content and the thickness of the ice shell. A subset of the Ganymede models is consistent with an olivine-dominated mantle mineralogy. The thickness of the silicate mantle above the core varies between 900 and 1100 km. The outermost ice shell is about 900 km in thickness and is further subdivided by pressure-induced phase transitions into ice I, ice III, ice V, and ice VI layers. Callisto should be differentiated, albeit incompletely. It is proposed that this satellite was never molten at a large scale but differentiated through the convective gradual unmixing of the ice and the metal/rock component. Bulk iron-to-silicon ratios Fe/Si calculated for the inner pair of satellites, Io and Europa, are less than the CI carbonaceous chondrite value of 1.7±0.1, whereas ratios for the outer pair, Ganymede and Callisto, cover a broad range above the chondritic value. Although the ratios are uncertain, in particular for Ganymede and Callisto, the values are sufficiently distinct to suggest a difference in composition between these two pairs of satellites. This may indicate a difference in iron–silicon fractionation during the formation of both classes of satellites in the protojovian nebula.
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The physical mechanisms driving thermal and chemical evolution of large icy satellites depend on the thermodynamic properties of the high pressure phases of ice. After reviewing some important aspects of the internal dynamics of the large icy satellites of Jupiter and Saturn, this paper describes the phase diagram of water ice and its variations due to the presence of other chemical compounds. Values of the elastic parameters and other thermodynamic constants are given. To illustrate how the experimental data strongly influence models of the internal structure and evolution of icy satellites, the effect of ammonia on the internal structure of Europa and Titan is described.
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▪ Abstract The icy moons of the outer solar system have not been quiescent bodies, in part because many have a substantial water component and have experienced significant internal heating. We can begin to understand the thermal evolution of the moons and the rate of viscous relaxation of surface topography because we now have good constraints on how ice (in several of its polymorphic forms) flows under deviatoric stress at planetary conditions. Details of laboratory-derived flow laws for pure, polycrystalline ice are reviewed in detail. One of the more important questions at hand is the role of ice grain size. Grain size may be a dynamic quantity within the icy moons, and it may (or may not) significantly affect rheology. One recent beneficiary of revelations about grain-size-sensitive flow is the calculation of the rheological structure of Europa's outer ice shell, which may be no thicker than 20 km.
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The brittle and ductile rheology of ices of water, ammonia, methane, and other volatiles, in combination with rock particles and each other, have a primary influence of the evolution and ongoing tectonics of icy moons of the outer solar system. Laboratory experiments help constrain the rheology of solar system ices. Standard experimental techniques can be used because the physical conditions under which most solar system ices exist are within reach of conventional rock mechanics testing machines, adapted to the low subsolidus temperatures of the materials in question. The purpose of this review is to summarize the results of a decade-long experimental deformation program and to provide some background in deformation physics in order to lend some appreciation to the application of these measurements to the planetary setting.
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Mantle Convection in the Earth and Planets is a comprehensive synthesis of all aspects of mantle convection within the Earth, the terrestrial planets, the Moon, and the Galilean satellites of Jupiter. The authors include up-to-date discussions of the latest research developments that have revolutionized our understanding of the Earth and the planets. The book features a comprehensive index, an extensive reference list, numerous illustrations (many in color) and major questions that focus the discussion and suggest avenues of future research. It is suitable as a text for graduate courses in geophysics and planetary physics, and as a supplementary reference for use at the undergraduate level. It is also an invaluable review for researchers in the broad fields of the Earth and planetary sciences.
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In an ice-rock body, it is possible for a subsurface ocean to exist as long as there is a sufficient heat source in the rocky core to maintain melting temperatures in the ice layer. Since the melting point of ice I decreases with pressure, it is only necessary for temperatures to reach ˜ 251 K for liquid water to be present in the ice layer. If ammonia is present, the minimum necessary temperature decreases further, to around 176 K. Heat loss in differentiated ice-rock bodies occurs primarily through thermal convection in the outer layer of ice, as long as the body is not too small (larger than ˜ 200 km radius). To model convection in the ice layer, we used scaling laws for stagnant lid convection with Newtonian rheology (Solomatov, 1995), which relate heat flux to the internal Rayleigh number of the convecting layer. Newtonian rheology is appropriate for the low stresses under consideration. Since the viscosity of ice I is strongly temperature-dependent, convection in the ice layer occurs in the stagnant lid regime, which allows for higher temperatures at depth than constant viscosity convection. For ice-rock bodies with a given size, composition, and heat source, we calculated the interior temperature and compared it to the ice I solidus to determine whether an ocean could be present. Since both the heat flux at the base of the ice layer and the gravitational acceleration are proportional to the radius of the body, it is effectively much harder for oceans to exist in small bodies. Using plausible choices of parameters for pure ice I, it is possible for oceans to exist in bodies as small as ˜ 1000 km radius, meaning that candidates for subsurface oceans include not only the icy Galilean satellites and Titan but also Triton and Pluto. If ammonia is present, oceans can exist in bodies as small as the largest Saturnian and Uranian moons ( ˜ 750 km radius). In all of these cases, whether oceans can be present depends strongly on the rheological parameters of ice I, which are not well known at planetary conditions. Another important parameter is the radiogenic heating rate of the rock, which may be greater than typical chondritic values if the rock is undepleted in potassium.
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Parameterized models of the thermal evolution of planets are usually based on the assumption that the lithosphere-convecting mantle boundary can be defined by an isotherm at a temperature below which viscosity is infinite on geologic timescales. Recent experimental results argue against this assumption. We have investigated both the definition of the lithosphere-convecting mantle boundary and the power law relation describing convecting heat transfer, based on numerical experiments of thermal convection in a volumetrically heated fluid with temperature-dependent viscosity. Other recent studies have treated only the heating from below, but volumetric heating is likely to be the dominant mode of heating in planetary mantles, either as a consequence of radioactive heating or as a proxy for secular cooling. Convection can occur either in the whole box or be located under a stagnant lid. In the lid regime, convection is driven by a temperature contrast depending on the rheology of the fluid and the interior temperature. This result, in agreement with experimental studies, indicates that boundary between the stagnant lid and the convecting layer (similar to the lithosphere-convecting mantle boundary) cannot be defined as a fixed isotherm. During thermal evolution of planets, the viscosity contrast in the convecting mantle remains constant, not the temperature at the bottom of the lithosphere. We present an example showing that the evolution of planets is strongly dependent on the criterion chosen to define the lithosphere-convecting mantle boundary. For reasonable values of the activation energy for thermally activated creep, the temperature defining the lithosphere-convecting mantle boundary, the mantle temperature, and the thickness of the lithosphere could be larger than expected from previous models which treat the base of the lithosphere as a fixed isotherm.
Article
Tidal dissipation may constrain the nature of the surface and interior of Titan. Provided the surface of Titan is covered by satellite-wide hydrocarbon seas and oceans, the solid body of Titan would have to respond not only to the external tidal disturbance potential but also to the loading and shifting weights of liquids on the surface. To estimate the tidal response of Titan's interior which is assumed to be differentiated into a crust-mantle-core structure, several endmember-type models have been considered, each of them constrained by Titan's mass and radius and consistent with either a volatile-rich or a volatile-poor evolution of the satellite. The dissipation rates as a consequence of the inelastic response of Titan's interior to body and loading tides have been computed from the imaginary parts of complex Love numbers and mass load coefficients. Additionally, a new analytical model of bottom frictional dissipation as a consequence of tidal currents in a global ocean is presented and is shown to compare well with a numerical model for both the radial and libration components of the ocean tide. In contrast to the solid volatile-poor interior structure, the volatile-rich interior contains an internal liquid ammonia-water region overlain by an icy shell, thereby mechanically decoupled from the deep interior. While the latter produces higher tidal dissipation rates than the former due to greater shell flexibility, the greater motion of the ocean floor accordingly induces smaller ocean currents, and hence much lower ocean dissipation rates for the volatile-rich scenario. The resultant total dissipation rates have been compared to those required to damp Titan's orbital eccentricity over the age of the Solar System. While the highly dissipative volatile-rich interior requires a time constant of orbit circularization of only about half this time period, the volatile-poor interior structure would allow restrictions on the minimum ocean depth and on ocean composition. Consequently, the volatile-rich scenario seems inconsistent with a primordial origin of the orbit eccentricity. This suggests Titan's interior may be rigid and that there is no global ocean. Alternatively, the eccentricity of the orbit may have a more recent origin. It is, however, also conceivable that the thermal history could have been governed by more rapid interior cooling in the past, thereby notably diminishing the amount of solid dissipation in the deep interior.
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Limb profiles from Voyager 2 images of Triton show it to be an ellipsoid of axes 1354.6, 1352.8, and 1352.4 km (+/-0.9 km). Topography on the limbs suggests no large provinces such as continents'' with relief over a kilometer. The shape solution is not accurate enough to constrain Triton's moments of inertia. The apparently relaxed shape is consistent with the youthful geologic features and supports interpretations that atmospheric distortion suggested by occultation data is due to atmospheric phenomena rather than physical body distortions.
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Presented in this Resource Letter is an annotated, selected bibliography of the literature on the various measurements of Newtonian gravitation that have been made over the past 2 centuries. The focus is on the unusual variety of effects for which Newtonian gravitation has been tested, as well as on the care that has gone into the determination of the absolute value of G. The letter E after an item indicates elementary level or material of general interest to persons becoming informed in the field. The letter I, for intermediate level, indicates material of somewhat more specialized nature; and the letter A indicates rather specialized or advanced material. An asterisk (*) indicates those articles to be included in an accompanying Reprint Book.
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The Nearby Supernova Factory reports that spectra (range 320-1000 nm) of supernovae 2005gn and 2005hb (IAUC #8616), obtained October 18.6 UT with the Supernova Integral Field Spectrograph on the University of Hawaii 2.2-meter telescope, indicate that both supernovae are of Type II. SN2005gn is a Type IIn supernova at a redshift of z = 0.0397, with prominent H- alpha (FWHM 4000 km/s) and Ca II IR triplet emission.
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Astronomical as well as palaeontological evidence suggests a secular retardation of the Earth’s rotation, which is attributed to tidal friction, i.e., mainly to the nonequilibrium and imperfectly fluid response of the Earth’s oceans, as well as to the imperfectly elastic response of the solid Earth to tidal forces. Estimates of the rotational energy dissipated in the oceans show that the oceanic term probably accounts for most of the dissipated energy (Pekeris and Accad, 1969; Pariiskii et al. , 1972; Kuznetsov, 1972; Brosche and Siindermann, 1972; Hendershott, 1972), although the exact share between both, the oceanic dissipation and the dissipation within the solid Earth, is not known. This is attributed to insufficiencies in the knowledge of the marine tides in the open oceans, and to the fact that nothing is known about the rheological mechanism of tidal dissipation within the solid Earth. Measurements of tidal gravity variations at the Earth’s surface, as well as precise observations of the tidal effect on satellite orbits have not yet revealed reliable results on imperfectly elastic body tides of the Earth. Model calculations give also only rough estimations of the tidal energy dissipated within the Earth, mainly because no information is available on the specific tidal dissipation function, i.e., the quality factor Q within the Earth.
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Prior to the Cassini Tour of the Saturnian system Jacobson (2004 AJ 128, 492) determined the orbits of the Saturnian satellites and a revised Saturnian system gravity field from Earth-based astrometry and observations acquired with the Pioneer 11 and Voyager spacecraft. We have been extending that work to include additional Earth-based astrometry, Cassini Doppler tracking data, radiometric range, and optical navigation imaging. Results based on data taken through March 2005 appeared Jacobson et al. (2005 BAAS 37(2), 524). The data set contained Cassini data acquired during the Cassini close flyby of Phoebe, the January 2005 encounter with Iapetus, the February and March encounters with Enceladus, and four flybys of Titan. In this paper we report on the current status of the gravity field and orbits. We have incorporated the results from two additional Titan flybys and another close encounter with Enceladus. More importantly, however, we have included the tracking data from a series of 4 consecutive Saturn periapsis passages by Cassini. As a consequence we have significantly improved our knowledge of the Saturnian system GM and the zonal gravitional harmonics of Saturn. The authors thank S. D. Gillam and V. Alwar (JPL) for their efforts in reducing the Cassini optical navigation data.
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Geodynamics concerns the internal structure, differentiation and convection, and tectonics of worlds. With respect to icy satellites there exists an excellent literature (e.g., Burns, 1986), and for the Earth a formidable body of new research results. In this review, I update some of the perspectives on how the icy satellites operate geodynamically, addressing the interplay between rheology, petrology, convection, and tectonics, and focusing on convection as a predominant endogenic process. Icy satellites, if they do undergo internal convection, are generally in the stagnant lid regime as defined by Solomatov, because the viscosity of water ice is strongly temperature-dependent. The Rayleigh number, a measure of the vigor of convection, for the actively convecting interior of an icy satellite is a very strong function of satellite radius (going at least as the sixth power). Convection was probable (if not vigorous) in all but the smallest middle-sized icy satellites early in solar system history. Today, vigorous convection only occurs in Ganymede, Callisto, and Titan, with weak convection occurring in Triton and Pluto. The pronounced polymorphism of the predominant ice, water ice, is expected to strongly modulate convective flow. The ice I-to-II transition should augment convective vigor, while both the ice I-to-III and II-to-V transitions should, by themselves, inhibit convective penetration. Convection within the larger icy satellites should be or have been layered. The negative activation volume for ice I ensures that convective flow in ice I is strongly coupled to the overlying icy lithosphere, which may in some circumstances generate sufficient stress in the lithosphere to induce brittle failure and surface tectonics.
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We have analyzed Hubble Space Telescope Wide Field Camera observations of Pluto, Charon, and a reference star, acquired in 1991 and 1993, to observe Pluto's barycentric motion and determine the Charon/Pluto mass ratio, 0.124±0.008. Solution values for Charon orbital elements include the semimajor axis, 19662±81 km inclination, 96.57±0.24 deg; eccentricity, 0.0072±0.0067; longitude of periapsis, 2±35 deg; and mean longitude, 123.58±0.43 deg. These elements are referred to the J2000 Earth equator and equinox at epoch JED 2446600.5.
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Kuiper Belt objects (KBOs) have been discovered with radii up to ~1000 km. Here I examine the early (≤500 Myr) thermal evolution of the larger of these bodies (≥100 km), up to the scale of Varuna. Only those KBOs that have resided in the Kuiper Belt since their formation are considered, so their thermal evolution is dominated by radiogenic heating, although impact heating of accreting surfaces by 10s of K is possible for the largest KBOs. I argue that the likely time scales of KBO accretion justify the neglect of 26Al in the models. The role of initial porosity (≍20-40%) is stressed in lowering conductivities of rock, amorphous ices, and carbonaceous matter, allowing for more rapid internal heating, although internal pressures strongly mitigate against the ultralow conductivities of vapor-deposited water ice. Release of occluded CO and other species as the water-ice crystallization temperature is reached (~85-90K for pure ice), increases the effective conductivity by means of vapor transport, and lowers the exothermic temperature excursion, if any. Transformation plasticity accompanying the amorphous-to-crystalline phase change could result in porosity reduction. Part of the color systematics of KBOs may be attributable to early geologic activity on the largest bodies vs. the likely cosmochemically primordial character of KBOs under ~75-to-225 km in radius.
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An assembly of present knowledge on the creep properties of ice including information derived from laboratory experiments and future measurements on glaciers, ice sheets and ice shelves. Principal papers since earlier reviews published between 1973 and 1981 are covered.-K.Clayton
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The tidal response of Europa to a time-varying potential is determined by solving the quasi-static equilibrium equations for a body composed of several uniform, Maxwell viscoelastic layers. Models in which Europa's surface ice extends all the way to the silicate mantle have weak tidal responses (~1 m vertical deflection) while models in which a liquid "ocean" underlies the ice have about 30 m peak vertical deflection. The phase-lag of the tidal response is very small (<2°) in the case of an ocean, but may be large if there is no ocean and the ice has a viscosity around 1013 Pa s. For models with an ocean, the product of the thickness and strength of the ice determines the tidal response, making it difficult to determine the thickness of the ice by observing the tidal deflection of Europa.
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We performed high-pressure experiments on the crystallization of water ice I and III in the ammonia–water (NH3)x(H2O)(1−x) system, and apply the results to the interiors of icy bodies in the Solar System. Phase equilibrium lines between an entirely liquid solution and a liquid solution in which water ice forms (liquidus lines) were determined for ammonia concentration by mass X equal to 0.034, 0.0472, 0.111, 0.176, and 0.229. Growth–melting of ice I as well as ice III crystals were observed. Application of the results to icy satellites that are potential bearers of ammonia shows that ammonia admixture decreases the depth of the liquidus surface. A shift of the liquidus temperature within a satellite depends on three parameters: the ammonia concentration, X; the temperature gradient, α; and the product of density and gravity, ρg.
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Key Words high-pressure phases, grain-size-sensitive creep, deformation mechanisms, brittle-to-ductile transition, Europa s Abstract The icy moons of the outer solar system have not been quiescent bodies, in part because many have a substantial water component and have experi-enced significant internal heating. We can begin to understand the thermal evolution of the moons and the rate of viscous relaxation of surface topography because we now have good constraints on how ice (in several of its polymorphic forms) flows under deviatoric stress at planetary conditions. Details of laboratory-derived flow laws for pure, polycrystalline ice are reviewed in detail. One of the more important questions at hand is the role of ice grain size. Grain size may be a dynamic quantity within the icy moons, and it may (or may not) significantly affect rheology. One recent benefi-ciary of revelations about grain-size-sensitive flow is the calculation of the rheological structure of Europa's outer ice shell, which may be no thicker than 20 km.
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1] A cooling viscoelastic ice shell overlying an ocean develops stresses due to two effects: thermal contraction of the ice due to cooling and the expansion of the shell due to the ice-water volume change. The former effect generates near-surface compression and deeper extension; the second effect generates extension only. In both cases, stresses are smaller at depth due to viscous creep. The resulting combined stresses are extensional except at shallow (<1 km) depths in thin ice shells. For ice shells thicker than 45 km, stresses are extensional throughout. The extensional stresses exceed 10 MPa for shells thicker than 20 km and thus dominate all other likely sources of stress as long as shell cooling occurs. The dominantly extensional nature of the stresses may help to explain the puzzling lack of compression observed on Europa and other large icy satellites. However, after 100 Myr of conductive cooling the maximum theoretical elastic strains for Europa are \$0.35%, which are probably insufficient to explain the total amount of observed extension.
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Thermal history models for the mid-sized saturnian satellites Mimas, Tethys, Dione, Iapetus, and Rhea have been calculated assuming stagnant lid convection in undifferentiated satellites and varying parameter values over broad ranges. Of all five satellites under consideration, only Dione, Rhea and Iapetus do show significant internal activities related to convective overturn for extended periods of time. The interiors of Mimas and Tethys do not convect or do so only for brief periods of time early in their thermal histories. Although we use lower densities than previous models, our calculations suggest higher interior temperatures but also thicker rigid shells above the convecting regions. Temperatures in the stagnant lid will allow melting of ammonia-dihydrate. Dione, Rhea and Iapetus may differentiate early and form early oceans, Iapetus only if ammonia is present. Mimas and Tethys with ammonia may differentiate if they accreted in an optically thick nebula with ambient temperatures around 250 K. Our models suggest that the outer shells of the satellites are largely primordial in composition even if the satellites differentiated. In these cases the deep interior may be layered with a pure ice shell underlain by an ammonia dihydrate layer and a rock core.
Article
Every three years the IAU/IAG Working Group on cartographic coordinates and rotational elements of the planets and satellites revises tables giving the directions of the north poles of rotation and the prime meridians of the planets, satellites, and asteroids. Also presented are revised tables giving their sizes and shapes. Changes since the previous report are summarized in the Appendix.
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The Cassini/Huygens mission will provide an accurate description of Titan's surface features. One important outcome of these data is that it will help for understanding the processes of methane exchange between Titan's interior and its atmosphere. Such a correlation between surface features and internal processes involving methane will be highly simplified if the nature of methane reservoirs is understood. In this paper, the behavior of methane within Titan is investigated using both data on methane clathrate stability and data on the ammonia–water system.A mathematical description of the different liquidus of the ammonia–water system is proposed. It is shown that the low pressure and water rich domain of the system is very well constrained. On the contrary, both high pressure ices and ammonia hydrates domains are still very badly understood because of the lack of experimental data. Nonetheless, several important characteristics of both ices and hydrates stability are described. These data are used for proposing a new model which computes the thermodynamical characteristics of the liquid layer within Titan. This provides new constraints on the temperature and composition fields within the liquid layer of Titan which indicates that the dissociation of methane clathrates in the deep interior is almost impossible.In the last part, the methane clathrate behavior within the different layer of Titan's interior is investigated. Due to the density contrasts between methane clathrates and ices, it will be shown that methane is certainly trapped within large clathrate reservoirs below the upper conductive lid of Titan. Further ascent and dissociation of clathrate into gaseous methane + ice must then be associated with tectonic and/or volcanic processes which allow rapid ascent without cooling of clathrates. Indeed, the dissociation is only possible at very shallow depth only if hot material from the ice layer can reach the surface rapidly.
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The origin of methane at the present surface of Titan is modeled in light of new high-pressure phase diagrams of ammonia-water compounds and clathrate hydrate. Using recently published experimental data on the ammonia-water system at kilobar pressures, temperature-composition slices of the phase diagram are constructed at a series of pressures up to 12 kbar. A new phase of ammonia dihydrate is proposed and incorporated in the diagrams, to allow consistency with low-pressure data. These results, along with the high-pressure phase diagram of methane clathrate hydrate recently caculated by J. I. Lunine and D. J. Stevenson (1985a, Astrophys. J. Suppl.58, 493–531) are applied to a model for the origin of the methane presently on the surface of Titan. Using simple bounds on the accretional temperatures and postaccretional state of an ammonia-rich Titan, we show that an unstable interior configuration is likely immediately after accretion, in which a rock layer is positioned above a lower-density rock-ice core. When core overturns begins the methane in the core, which is released from the clathrate structure by virtue of the high pressures, migrates upward. A model for the cooling and freezing of an ammonia-water ocean in the upper mantle of Titan, based on the phase diagram, is applied and it is concluded that insufficient liquid water exists to retrap all of the upwelling methane as clathrate. However, alternative interpretations of the phase diagram permit an ocean thick enough to entrap the methane. For the bulk of the range of plausible accretion models, enough methane is available from the interior to account for the present-day surface hydrocarbon abundance; however, the amount of nitrogen extruded in this model may be much smaller.
Article
Limb coordinates are used to find the radii, shapes, and local topography of the five large satellites of Uranus. The technique provides a direct measure of ellipsoidal shapes of satellites and fixes radii to subpixel accuracy. Umbriel, Titania, and Oberon are best fit by spheres. Miranda and Ariel are ellipsoids whose equatorial bulges are consistent with published mean densities. Limb topography on Miranda shows substantial deformation of both old cratered terrain and the younger coronae and complex faulting and uplift at the margins of the coronae. The maximum deformation is about 10 km. Umbriel's limb shows a basin of undetermined origin about 500 km across and 6 km deep.
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We have elaborated an evolutionary turbulent model of the subnebula of Saturn derived from that of Dubrulle (1993, Icarus106, 59–76) for the solar nebula, which is valid for a geometrically thin disk. We demonstrate that if carbon and nitrogen were in the form of CO and N2, respectively, in the early subnebula, these molecules were not subsequently converted into CH4 and NH3 during the evolution of the disk, contrary to the current scenario initially proposed by Prinn and Fegley (1981, Astrophys. J., 249, 308–317). However, if the early subnebula contained some CH4 and NH3, these gases were not subsequently converted into CO and N2. We argue that Titan must have been formed from planetesimals migrating from the outer part of the subnebula to the present orbit of the satellite. These planetesimals were relics of those embedded in the feeding zone of Saturn prior to the completion of the planet and contained hydrates of NH3 and clathrate hydrates of CH4. It is shown that, for plausible abundances of CH4 and NH3 in the solar nebula at 10 AU, the masses of methane and nitrogen trapped in Titan were higher than the estimate of masses of these components in the primitive atmosphere of the satellite. If our scenario is valid and if our turbulent model properly describes the structure and the evolution of the actual subnebula of Saturn, the Xe/C ratio should be six times higher in Titan's atmosphere today than in the Sun, while the current scenario would probably result in a quasi solar Xe/C ratio. The mass spectrometer and gas chromatograph instrument aboard the Huygens Titan probe of the Cassini mission has the capability of measuring this ratio in 2004, thus permitting us to discriminate between the current scenario and the one proposed in this report.