Planetary and Space Science 01/2013; in preparation. · 2.22 Impact Factor
ABSTRACT: Saturn’s satellite Titan is a particularly interesting body in our solar system. It is the only satellite with a dense atmosphere,
which is primarily made of nitrogen and methane. It harbours an intricate photochemistry, that populates the atmosphere with
aerosols, but that should deplete irreversibly the methane. The observation that methane is not depleted led to the study
of Titan’s methane cycle, starting with its atmospheric part. The features that inhabit Titan’s atmosphere can last for timescales
varying from year to day. For instance, the reversal of the north–south asymmetry is linked to the 16-year seasonal cycle.
Diurnal phenomena have also been observed, like a stratospheric haze enhancement or a possible tropospheric drizzle. Furthermore,
clouds have been reported on Titan since 1993. From these first detections and up to now, with the recent inputs from the
Cassini–Huygens mission, clouds have displayed a large range of shapes, altitudes, and natures, from the flocky tropospheric
clouds at the south pole to the stratiform ones in the northern stratosphere. It is still difficult to compose a clear picture
of the physical processes governing these phenomena, even though of lot of different means of observation (spectroscopy, imaging)
are available now. We propose here an overview of the phenomena reported between 1993 and 2008 in the low atmosphere of Titan,
with indications on the location, altitude, and their characteristics in order to give a perspective of our up-to-date understanding
of Titan’s meteorological manifestations. We shall focus mainly on direct imaging observations, from both space- and ground-based
facilities. All of these observations, published in more than 30 different refereed papers to date, allow us to build a precise
chronology of Titan’s atmospheric changes (including the north–south asymmetry, diurnal and seasonal effects, etc). Since
the interpretation is at an early stage, we only briefly mention some of the current theories regarding the features’ nature.
Astronomy and Astrophysics Review 04/2012; 17(2):105-147. · 11.53 Impact Factor
ABSTRACT: The Titan Saturn System Mission (TSSM) concept is composed of a TSSM orbiter provided by NASA that would carry two Titan in
situ elements provided by ESA: the montgolfière and the probe/lake lander. One overarching goal of TSSM is to explore in situ
the atmosphere and surface of Titan. The mission has been prioritized as the second Outer Planets Flagship Mission, the first
one being the Europa Jupiter System Mission (EJSM). TSSM would launch around 2023–2025 arriving at Saturn 9years later followed
by a 4-year science mission in the Saturn system. Following delivery of the in situ elements to Titan, the TSSM orbiter would
explore the Saturn system via a 2-year tour that includes Enceladus and Titan flybys before entering into a dedicated orbit
around Titan. The Titan montgolfière aerial vehicle under consideration will circumnavigate Titan at a latitude of ~20° and
at altitudes of ~10km for a minimum of 6months. The probe/lake lander will descend through Titan’s atmosphere and land on
the liquid surface of Kraken Mare (~75° north latitude). As for any planetary space science mission, and based on the Cassini–Huygens
experience, Earth-based observations will be synergistic and enable scientific optimization of the return of such a mission.
Some specific examples of how this can be achieved (through VLBI and Doppler tracking, continuous monitoring of atmospheric
and surface features, and Direct-to-Earth transmission) are described in this paper.
Earth Moon and Planets 04/2012; 105(2):135-142. · 0.67 Impact Factor
ABSTRACT: Internal processes in icy satellites, e.g. the exchange of material from the subsurface to the surface or processes leading
to volcanism and resurfacing events, are a consequence of the amount of energy available in the satellites’ interiors. The
latter is mainly determined shortly after accretion by the amount of radioactive isotopes incorporated in the silicates during
the accretion process. However, for satellites—as opposed to single objects—important contributions to the energy budget on
long time-scales can come from the interaction with other satellites (forcing of eccentricities of satellites in resonance)
and consequently from the tidal interaction with the primary planet. Tidal evolution involves both changes of the rotation
state—usually leading to the 1:1 spin orbit coupling—and long-term variations of the satellite orbits. Both processes are
dissipative and thus connected with heat production in the interior. The way heat is transported from the interior to the
surface (convection, conduction, (cryo-) volcanism) is a second main aspect that determines how internal processes in satellites
work. In this chapter we will discuss the physics of heat production and heat transport as well as the rotational and orbital
states of satellites. The relevance of the different heat sources for the moons in the outer solar system are compared and
KeywordsSatellites-Energy sources-Rotation-Tides-Orbital dynamics-Heat transfer
Space Science Reviews 04/2012; 153(1):317-348. · 3.61 Impact Factor
ABSTRACT: The exploration of the Jovian System and its fascinating satellite Europa is one of the priorities presented in ESA’s “Cosmic
Vision” strategic document. The Jovian System indeed displays many facets. It is a small planetary system in its own right,
built-up out of the mixture of gas and icy material that was present in the external region of the solar nebula. Through a
complex history of accretion, internal differentiation and dynamic interaction, a very unique satellite system formed, in
which three of the four Galilean satellites are locked in the so-called Laplace resonance. The energy and angular momentum
they exchange among themselves and with Jupiter contribute to various degrees to the internal heating sources of the satellites.
Unique among these satellites, Europa is believed to shelter an ocean between its geodynamically active icy crust and its
silicate mantle, one where the main conditions for habitability may be fulfilled. For this very reason, Europa is one of the
best candidates for the search for life in our Solar System. So, is Europa really habitable, representing a “habitable zone”
in the Jupiter system? To answer this specific question, we need a dedicated mission to Europa. But to understand in a more
generic way the habitability conditions around giant planets, we need to go beyond Europa itself and address two more general
questions at the scale of the Jupiter system: to what extent is its possible habitability related to the initial conditions
and formation scenario of the Jovian satellites? To what extent is it due to the way the Jupiter system works? ESA’s Cosmic
Vision programme offers an ideal and timely framework to address these three key questions. Building on the in-depth reconnaissance
of the Jupiter System by Galileo (and the Voyager, Ulysses, Cassini and New Horizons fly-by’s) and on the anticipated accomplishments
of NASA’s JUNO mission, it is now time to design and fly a new mission which will focus on these three major questions. LAPLACE,
as we propose to call it, will deploy in the Jovian system a triad of orbiting platforms to perform coordinated observations
of its main components: Europa, our priority target, the Jovian satellites, Jupiter’s magnetosphere and its atmosphere and
interior. LAPLACE will consolidate Europe’s role and visibility in the exploration of the Solar System and will foster the
development of technologies for the exploration of deep space in Europe. Its multi-platform and multi-target architecture,
combined with its broadly multidisciplinary scientific dimension, will provide an outstanding opportunity to build a broad
international collaboration with all interested nations and space agencies.
Experimental Astronomy 04/2012; 23(3):849-892. · 1.82 Impact Factor
ABSTRACT: a b s t r a c t We present models of the near-infrared (1–5 lm) spectra of Saturn's F ring obtained by Cassini's Visual and Infrared Mapping Spectrometer (VIMS) at ultra-high phase angles (177.4–178.5°). Modeling this spectrum constrains the size distribution, composition, and structure of F ring particles in the 0.1– 100 lm size range. These spectra are very different from those obtained at lower phase angles; they lack the familiar 1.5 and 2 lm absorption bands, and the expected 3 lm water ice primary absorption appears as an unusually narrow dip at 2.87 lm. We have modeled these data using multiple approaches. First, we use a simple Mie scattering model to constrain the size distribution and composition of the particles. The Mie model allows us to understand the overall shapes of the spectra in terms of dominance by diffraction at these ultra-high phase angles, and also to demonstrate that the 2.87 lm dip is associated with the Christiansen frequency of water ice (where the real refractive index passes unity). Second, we use a com-bination of Mie scattering with Effective Medium Theory to probe the effect of porous (but structureless) particles on the overall shape of the spectrum and depth of the 2.87 lm band. Such simple models are not able to capture the shape of this absorption feature well. Finally, we model each particle as an aggregate of discrete monomers, using the Discrete Dipole Approximation (DDA) model, and find a better fit for the depth of the 2.87 lm feature. The DDA models imply a slightly different overall size distribution. We present a simple heuristic model which explains the differences between the Mie and DDA model results. We conclude that the F ring contains aggregate particles with a size distribution that is distinctly nar-rower than a typical power law, and that the particles are predominantly crystalline water ice.
ABSTRACT: We present our study on Titan's geology in order to develop our current
understanding of the satellite's active zones ,. The key aim is to
study Titan's geology holistically, by means of internal activity and
surface properties, in addition to terrestrial comparisons. We have
applied the Principal Components Analysis (PCA) method in order to
collect combined information of the seven infrared spectral windows,
using the Cassini Mission Visual and Infrared Mapping Spectrometer
(VIMS) data. The study areas for the moment are Tui Regio (located at
20°S, 130°W) and Hotei Regio (located at 26°S, 78°W).
The main goal is to identify the composition as well as the alterations
of the components that compose the possible calderas and lava flows ,
by using the principal components of the PCA method. Principal component
analysis (PCA) is recommended, as our primary concern is to determine
the minimum number of factors that will account for the maximum variance
in the data in use in this particular multivariate analysis. Moreover,
Cassini/Radar images have been processed  in order to study
morphologically the active zones within the areas of Tui and Hotei Regio
and to identify any analogues with terrestrial features. Both VIMS and
Radar data  have provided significant information regarding the
geology of the two areas, which should enable us to determine a possible
internal activity as well as to identify superficial geologic
structures. References  Nelson, R. M. (2009) Icarus 199, 429-441.
 Solomonidou, A. (2009) European Planetary Science Congress Vol. 4,
EPSC2009-710.  Sotin, C. (2005) Nature, Vol 435.  Bratsolis, E.
& Sigelle, M. (2003) IEEE Transactions on Geoscience and Remote
Sensing, 41, pp. 2890-2899.  Le Mouélic, S. (2008) Journal of
Geophysical Research, Volume 113, Issue E4.
Scientific American 03/2010; 302(3):36-43. · 2.37 Impact Factor
ABSTRACT: We analyze observations taken with Cassini’s Visual and Infrared Mapping Spectrometer (VIMS), to determine the current methane and haze latitudinal distribution between 60°S and 40°N. The methane variation was measured primarily from its absorption band at 0.61 μm, which is optically thin enough to be sensitive to the methane abundance at 20–50 km altitude. Haze characteristics were determined from Titan’s 0.4–1.6 μm spectra, which sample Titan’s atmosphere from the surface to 200 km altitude. Radiative transfer models based on the haze properties and methane absorption profiles at the Huygens site reproduced the observed VIMS spectra and allowed us to retrieve latitude variations in the methane abundance and haze. We find the haze variations can be reproduced by varying only the density and single scattering albedo above 80 km altitude. There is an ambiguity between methane abundance and haze optical depth, because higher haze optical depth causes shallower methane bands; thus a family of solutions is allowed by the data. We find that haze variations alone, with a constant methane abundance, can reproduce the spatial variation in the methane bands if the haze density increases by 60% between 20°S and 10°S (roughly the sub-solar latitude) and single scattering absorption increases by 20% between 60°S and 40°N. On the other hand, a higher abundance of methane between 20 and 50 km in the summer hemisphere, as much as two times that of the winter hemisphere, is also possible, if the haze variations are minimized. The range of possible methane variations between 27°S and 19°N is consistent with condensation as a result of temperature variations of 0–1.5 K at 20–30 km. Our analysis indicates that the latitudinal variations in Titan’s visible to near-IR albedo, the north/south asymmetry (NSA), result primarily from variations in the thickness of the darker haze layer, detected by Huygens DISR, above 80 km altitude. If we assume little to no latitudinal methane variations we can reproduce the NSA wavelength signatures with the derived haze characteristics. We calculate the solar heating rate as a function of latitude and derive variations of ∼10–15% near the sub-solar latitude resulting from the NSA. Most of the latitudinal variations in the heating rate stem from changes in solar zenith angle rather than compositional variations.
ABSTRACT: Observations of Titan obtained by the Cassini Visual and Infrared Mapping Spectrometer (VIMS) have revealed Selk crater, a geologically young, bright-rimmed, impact crater located ∼800 km north-northwest of the Huygens landing site. The crater rim-crest diameter is ∼90 km; its floor diameter is ∼60 km. A central pit/peak, 20–30 km in diameter, is seen; the ratio of the size of this feature to the crater diameter is consistent with similarly sized craters on Ganymede and Callisto, all of which are dome craters. The VIMS data, unfortunately, are not of sufficient resolution to detect such a dome. The inner rim of Selk crater is fluted, probably by eolian erosion, while the outer flank and presumed ejecta blanket appear dissected by drainages (particularly to the east), likely the result of fluvial erosion. Terracing is observed on the northern and western walls of Selk crater within a 10–15 km wide terrace zone identified in VIMS data; the terrace zone is bright in SAR data, consistent with it being a rough surface. The terrace zone is slightly wider than those observed on Ganymede and Callisto and may reflect differences in thermal structure and/or composition of the lithosphere. The polygonal appearance of the crater likely results from two preexisting planes of weakness (oriented at azimuths of 21° and 122° east of north). A unit of generally bright terrain that exhibits similar infrared-color variation and contrast to Selk crater extends east-southeast from the crater several hundred kilometers. We informally refer to this terrain as the Selk “bench.” Both Selk and the bench are surrounded by the infrared-dark Belet dune field. Hypotheses for the genesis of the optically bright terrain of the bench include: wind shadowing in the lee of Selk crater preventing the encroachment of dunes, impact-induced cryovolcanism, flow of a fluidized-ejecta blanket (similar to the bright crater outflows observed on Venus), and erosion of a streamlined upland formed in the lee of Selk crater by fluid flow. Vestigial circular outlines in this feature just east of Selk’s ejecta blanket suggest that this might be a remnant of an ancient, cratered crust. Evidently the southern margin of the feature has sufficient relief to prevent the encroachment of dunes from the Belet dune field. We conclude that this feature either represents a relatively high-viscosity, fluidized-ejecta flow (a class intermediate to ejecta blankets and long venusian-style ejecta flows) or a streamlined upland remnant that formed downstream from the crater by erosive fluid flow from the west-northwest.
ABSTRACT: Clouds on Titan result from the condensation of methane and ethane and, as on other planets, are primarily structured by circulation of the atmosphere. At present, cloud activity mainly occurs in the southern (summer) hemisphere, arising near the pole and at mid-latitudes from cumulus updrafts triggered by surface heating and/or local methane sources, and at the north (winter) pole, resulting from the subsidence and condensation of ethane-rich air into the colder troposphere. General circulation models predict that this distribution should change with the seasons on a 15-year timescale, and that clouds should develop under certain circumstances at temperate latitudes ( approximately 40 degrees ) in the winter hemisphere. The models, however, have hitherto been poorly constrained and their long-term predictions have not yet been observationally verified. Here we report that the global spatial cloud coverage on Titan is in general agreement with the models, confirming that cloud activity is mainly controlled by the global circulation. The non-detection of clouds at latitude approximately 40 degrees N and the persistence of the southern clouds while the southern summer is ending are, however, both contrary to predictions. This suggests that Titan's equator-to-pole thermal contrast is overestimated in the models and that its atmosphere responds to the seasonal forcing with a greater inertia than expected.
Nature 07/2009; 459(7247):678-82. · 36.28 Impact Factor
ABSTRACT: The majority of planetary aurorae are produced by electrical currents flowing between the ionosphere and the magnetosphere which accelerate energetic charged particles that hit the upper atmosphere. At Saturn, these processes collisionally excite hydrogen, causing ultraviolet emission, and ionize the hydrogen, leading to H(3)(+) infrared emission. Although the morphology of these aurorae is affected by changes in the solar wind, the source of the currents which produce them is a matter of debate. Recent models predict only weak emission away from the main auroral oval. Here we report images that show emission both poleward and equatorward of the main oval (separated by a region of low emission). The extensive polar emission is highly variable with time, and disappears when the main oval has a spiral morphology; this suggests that although the polar emission may be associated with minor increases in the dynamic pressure from the solar wind, it is not directly linked to strong magnetospheric compressions. This aurora appears to be unique to Saturn and cannot be explained using our current understanding of Saturn's magnetosphere. The equatorward arc of emission exists only on the nightside of the planet, and arises from internal magnetospheric processes that are currently unknown.
Nature 12/2008; 456(7219):214-7. · 36.28 Impact Factor
Science 04/2008; 319(5870):1629-30. · 31.20 Impact Factor
ABSTRACT: Data from the Cassini-Huygens mission indicate that an ocean may exist beneath the solid surface of Saturn's moon Titan.
Science 03/2008; 319(5870):1629-1630. · 31.20 Impact Factor
02/2008: pages 253 - 271; , ISBN: 9783527618996
ABSTRACT: The surface of Titan has been revealed by Cassini observations in the infrared and radar wavelength ranges as well as locally by the Huygens lander instruments. Sand seas, recently discovered lakes, distinct landscapes and dendritic erosion patterns indicate dynamic surface processes. This study focus on erosional and depositional features that can be used to constrain the amount of liquids involved in the erosional process as well as on the compositional characteristics of depositional areas. Fluvial erosion channels on Titan as identified at the Huygens landing site and in RADAR and Visible and Infrared Mapping Spectrometer (VIMS) observations have been compared to analogous channel widths on Earth yielding average discharges of up to 1600 m 3 /s for short recurrence intervals that are sufficient to move centimeter-sized sediment and significantly higher discharges for long intervals. With respect to the associated drainage areas, this roughly translates to 1–150 cm/day runoff production rates with 10 years recurrence intervals and by assuming precipitation this implies 0.6–60 mm/h rainfall rates. Thus the observed surface erosion fits with the methane convective storm models as well as with the rates needed to transport sediment. During Cassini's T20 fly-by, the VIMS observed an extremely eroded area at 30 • W, 7 • S with resolutions of up to 500 m/pixel that extends over thousands of square kilometers. The spectral characteristics of this area change systematically, reflecting continuous compositional and/or particle size variations indicative of transported sediment settling out while flow capacities cease. To account for the estimated runoff production and widespread alluvial deposits of fine-grained material, release of area-dependent large fluid volumes are required. Only frequent storms with heavy rainfall or cryovolcanic induced melting can explain these erosional features.
Icarus 01/2008; 197:526-538. · 3.38 Impact Factor
ABSTRACT: Grüneisen's and third-order finite-strain theories are used to compute the density and seismic-wave velocities of minerals. Assuming the minealogical model of Ito & Takahashi (1987), seismic velocities of the upper mantle are calculated using the Hashin–Shtrikman averaging procedure. 1-D profiles are first obtained along adiabats, and compared to the IASP91 model. Different adiabats are considered in order to take into account the thermal effect of phase transitions. The best results are found with adiabats initiated at 1473, 1573 and 1613 K for α-olivine, β- and γ-spinel, respectively. The incorporation of thermal effects resulting from phase transitions gives velocity jumps at discontinuities close to those of IASP91.Next, a model of convection constructed by Dupeyrat, Sotin & Parmentier (1995), incorporating plate tectonics, is used to compute 1-D profiles and 2-D fields of seismic anomalies in the upper mantle. Averaged profiles show seismic-velocity gradients very close to those of IASP91, but individual values are much too high, suggesting that the mean temperature profile of the convection model is too cold by 400 K. When low-pass filtered to the resolution scale of presently available tomogrphic models, both the amplitude and shape of the computed seismic anomalies are consistent with the results of tomographic studies. The amplitude of the anomalies ranges between – 2.7 and 3.8% for P-wave slownesses, and from – 3.3 to 4.5% for S-wave slownesses. These anomalies correspond to lateral temperature variations of –465 to 520 K. These calculations are used (1) as an aid to the interpretation of global tomographic models, for instance by computing spectra of lateral heterogeneities, and (2) to test the adequacy of the basic assumptions used in the computation of numerical models of mantle convection, and to build a theoretical temperature profile that would give the best fit to IASP91. In the uppermost mantle this theoretical model has a shape close to both the convection model and the 1473 K adiabat, but in the transition zone the profile is highly subadiabatic. The spectra obtained for the synthetic seismic anomalies resemble that of tomographic studies, with most of the energy contained within gravest angular orders l, and a fast decrease of energy with increasing l. The spatial filtering has clearly different effects on heterogeneities, depending on their respective wavelengths. It is suggested that the change of decreasing rate observed in tomographic models at l= 7 is closely related to the filter wavelength and may correspond at a lesser extent to a characteristic wavelength to mantle heterogeneities.
Geophysical Journal International 04/2007; 124(1):45 - 56. · 2.42 Impact Factor
ABSTRACT: Global mineralogical mapping of Mars by the Observatoire pour la Mineralogie, l'Eau, les Glaces et l'Activité (OMEGA) instrument on the European Space Agency's Mars Express spacecraft provides new information on Mars' geological and climatic history. Phyllosilicates formed by aqueous alteration very early in the planet's history (the "phyllocian" era) are found in the oldest terrains; sulfates were formed in a second era (the "theiikian" era) in an acidic environment. Beginning about 3.5 billion years ago, the last era (the "siderikian") is dominated by the formation of anhydrous ferric oxides in a slow superficial weathering, without liquid water playing a major role across the planet.
Science 05/2006; 312(5772):400-4. · 31.20 Impact Factor
ABSTRACT: Saturn's largest satellite, Titan, has a massive nitrogen atmosphere containing up to 5 per cent methane near its surface. Photochemistry in the stratosphere would remove the present-day atmospheric methane in a few tens of millions of years. Before the Cassini-Huygens mission arrived at Saturn, widespread liquid methane or mixed hydrocarbon seas hundreds of metres in thickness were proposed as reservoirs from which methane could be resupplied to the atmosphere over geologic time. Titan fly-by observations and ground-based observations rule out the presence of extensive bodies of liquid hydrocarbons at present, which means that methane must be derived from another source over Titan's history. Here we show that episodic outgassing of methane stored as clathrate hydrates within an icy shell above an ammonia-enriched water ocean is the most likely explanation for Titan's atmospheric methane. The other possible explanations all fail because they cannot explain the absence of surface liquid reservoirs and/or the low dissipative state of the interior. On the basis of our models, we predict that future fly-bys should reveal the existence of both a subsurface water ocean and a rocky core, and should detect more cryovolcanic edifices.
Nature 04/2006; 440(7080):61-4. · 36.28 Impact Factor
ABSTRACT: Observations from the Cassini Visual and Infrared Mapping Spectrometer show an anomalously bright spot on Titan located at 80 degrees W and 20 degrees S. This area is bright in reflected light at all observed wavelengths, but is most noticeable at 5 microns. The spot is associated with a surface albedo feature identified in images taken by the Cassini Imaging Science Subsystem. We discuss various hypotheses about the source of the spot, reaching the conclusion that the spot is probably due to variation in surface composition, perhaps associated with recent geophysical phenomena.
Science 11/2005; 310(5745):92-5. · 31.20 Impact Factor