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Regional variations of Mercury's crustal density and porosity from MESSENGER gravity data
Abstract
A new solution of Mercury's gravity field to degree and order 160, named HgM009, is retrieved through a reprocessing of MESSENGER radio science measurements. By combining our latest gravity field with topography data, localized spectral admittance analyses are carried out to investigate Mercury's crustal and lithospheric properties across the northern hemisphere. The measured spectra are compared with admittances predicted by lithospheric flexure models. The localized gravity/topography admittance analyses yield key information on the lateral variations of the bulk density of the upper crust. Elastic and crustal thicknesses are also adjusted in our study, but the local admittance spectra allow us to constrain these parameters only over a few regions. The average bulk density across the observed areas in the northern hemisphere is 2540 ±60 kg m⁻³. The crustal porosity is then constrained by using an estimate of the pore-free grain density of surface materials with our measured bulk density. Our estimate of the mean porosity is 14.7 ±1.6 %, which is comparable to, but slightly higher than, the average value measured on the Moon. Larger crustal porosities are observed over heavily cratered regions, suggesting that impact bombardment is the main cause of the crustal porosity.
... Classical inversions for crustal thickness rely on appropriate knowledge of the planetary gravity field and topography (e.g., Wieczorek et al., 2022) and, if available, additional constraints on the density and thickness of the crust from orbital analyses and seismic measurements (Knapmeyer-Endrun et al., 2021;Wieczorek et al., 2013). Given the lack of high-resolution gravity data for Mercury due to the high-altitude and elliptical orbit of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft , the thickness and in particular the porosity of the planet's crust have remained poorly known (Beuthe et al., 2020;Genova et al., 2023). ...
... Compared to the Moon with degree-1,200 gravity field models (Goossens et al., 2020), Mercury's gravity field is poorly known with resolution ranging from degree 10 (wavelength of ∼1,500 km) in the southern hemisphere to 90 and up to 160 in the north (∼95-170 km, e.g., Konopliv et al., 2020). At such resolutions, crustal porosity is difficult to decouple from crustal thickness and elastic thickness variations based on gravity and topography inversions alone (Genova et al., 2023;Goossens et al., 2022). Given the similarity in size between the Moon and Mercury and the lack of major erosion processes on both planetary bodies, a lunar-like relationship between Mercury's crater population and crustal porosity can be expected. ...
... Regions affected by smooth plains volcanism are treated separately and porosity differences with the cratered terrains are discussed. Our inferred porosity maps are compared to local estimates from gravity and topography, where gravity data are well resolved (Genova et al., 2023), to infer the best fit parameters relating the crater population to the crustal porosity. Crustal thickness models that consider porosity and grain density variations are then constructed, thereby providing valuable insights into the structure of Mercury's crust in the framework of the upcoming BepiColombo mission (Benkhoff et al., 2021). ...
Plain Language Summary
The crust of a planet is a thermal barrier, which controls how fast heat escapes to space. Depending on its thickness, the crust can strongly insulate the planet's interior preventing efficient cooling. Therefore, knowing the structure of the crust is critical to unraveling the geologic history of planetary bodies. Crustal thickness is typically inverted from gravity and topography data. One critical parameter for these inversions is the bulk density of the crust, which is primarily driven by porosity variations. While high‐resolution gravity field mapping allowed constraining the bulk density and porosity of the lunar crust, crustal porosity on other planetary bodies has remained unknown. In this work, we use a model that was calibrated to the Moon to relate Mercury's impact crater population and long‐wavelength crustal porosity in the cratered terrains. We show that crustal porosity in the cratered terrains ranges from 9% to 18% with an average and standard deviation of 13% ± 2%, indicating lunar‐like low bulk densities of 2,565 ± 70 kg m−3 .
... During the final year of mission operations, the spacecraft reached altitudes as low as 15-25 km (i.e., low-altitude campaign), carrying out surface observations at unprecedented spatial resolutions. The radiometric data collected during this mission phase were most sensitive to the shortwavelength components of the planet's gravity field, providing crucial information to investigate the physical properties of the outer layers through local admittance and gravity analyses [6]. At such low altitudes, the spacecraft orbit was also strongly perturbed by the planet's radiation. ...
... A reprocessing of the entire MESSENGER data set, including Doppler and range radiometric measurements, was conducted to retrieve a new estimate of Mercury's gravity field, HgM009, in spherical harmonics to degree and order 160 [6]. In this section, we describe the methods adopted to refine our estimates of the gravitational forces, leading to an enhanced reconstruction of the spacecraft orbit. ...
... This global inversion of the entire MESSENGER data set led to the estimation of the spacecraft's orbit and thermo-optical properties and the gravitational field of Mercury, including orientation parameters and tides [10]. In this work, we present the reconstructed trajectory of the spacecraft that enabled the estimation of the latest Mercury's gravity field to degree and order 160 [6]. The gravitational forces account for the rotational model and Love number k 2 estimated in the previous study by Genova et al. [10]. ...
The NASA MESSENGER mission explored Mercury for more than four years to investigate the properties of the planet. To safely operate in the harsh conditions around Mercury, the spacecraft was in a highly eccentric orbit with a low periapsis altitude. The radiation environment had a strong impact on the spacecraft orbit evolution because of the proximity of Mercury to the Sun. A detailed modeling of the nonconservative forces is then a key factor to enhance the precise orbit determination of the spacecraft. We present here refined models of the nonconservative forces, including thermal reradiation effects, that enabled significant improvements in the trajectory reconstruction. A crossover analysis based on the Mercury Laser Altimeter (MLA) data was carried out to cross-check the accuracy of the orbit determination results. The trajectories retrieved by using the refined spacecraft dynamical model provide reduced height misfit at crossover points, indicating a high-quality reconstruction. Our new solutions of the spacecraft orbits are then archived to be used as auxiliary information for the data analysis of other MESSENGER instruments.
... The Bouguer correction is applied, effectively removing the gravitational effects of surface topography between the observation point and the reference level, taking the elevation difference and the average density of the rocks above the reference level into account (Wieczorek, 2015). Our calculations assume a global crustal density of 2,800 kg m 3 (Genova et al., 2019(Genova et al., , 2023Goossens et al., 2022;Konopliv et al., 2020) and consider finite-amplitude corrections (see Section S1 in Supporting Information S1). Impact basins are typically divided into three regions. ...
... This offset can potentially be attributed to a larger density difference between the crust and mantle. Mercury's crust-mantle density contrast is approximately 400 kg m 3 (Genova et al., 2023), while the lunar density contrast is 700 kg m 3 . For larger diameters, Mercury shows drastically lower Bouguer contrasts compared to the Moon. ...
The crustal structure of Mercury's large impact basins provides valuable insights into the planet's geological history. For a warm crust, a post‐impact basin structure will viscously relax with inward flow of crustal materials toward the basin center. This effect drastically diminishes the crustal thickness contrasts and associated Bouguer gravity contrasts between the basin center and its surroundings. Here, we analyze Bouguer contrasts of 36 basins (diameter > 300 km) located in the northern hemisphere as a proxy for viscoelastic relaxation. Thermal evolution models, assuming the present 3:2 spin‐orbit configuration, are used to predict crustal temperatures. Our analysis reveals that the expected correlation between zones of warm crust and low Bouguer contrast from relaxation is not observed in the available data. This suggests that crustal temperatures have changed in the past, potentially due to a change in Mercury's orbit or to a major volcanic event associated with smooth plain formation.
... In addition to the above-mentioned research questions, the gravity anomalies have been widely used in estimation of the elastic thickness and have been extended from Earth to other planets where gravity data is available (Broquet and Wieczorek, 2019;Genova et al., 2023). Since the routine Pratt and Airy compensation modes require a lithosphere with an unrealistic and highly anisotropic mechanical behavior, the flexure model has been extensively used to interpret short-wavelength gravity anomalies due to variations in crustal thickness. ...
... Based on this, we then computed the sensitivity error of the depth to crust-mantle interface (Eq. (11) in "Methods"), yielding a 5% error. ...
We modeled gravity data to explore Mercury’s internal structure and show the presence of crustal heterogeneities in density. We first evaluated the lithospheric flexure occurring in the spherical harmonic degree range 5–80, according to the flexural isostatic response curve. We thus estimated a mean elastic lithosphere thickness of about 30 ± 10 km and modeled the crust-mantle interface, which varies from 19 to 42 km depth, according to a flexural compensation model. The isostatic gravity anomalies were then obtained as the residual field with respect to the contributions from topography and lithospheric flexure. Isostatic anomalies are mainly related to density variations in the crust: gravity highs mostly correspond to large-impact basins suggesting intra-crustal magmatic intrusions as the main origin of these anomalies. Isostatic gravity lows prevail, instead, above intercrater plains and may represent the signature of a heavily fractured crust.
... Local minima of our predicted crustal thickness profile (Figure 1b) coincide with local maxima of ε plastic which is consistent with models of Orientale's formation . However, the degree strength of Mercury's gravity field-from which crustal thickness maps are calculated-is latitudinally variable, ranging from ∼160 at the north pole to ∼30 at Caloris' southern margin (e.g., Genova et al., 2019Genova et al., , 2023. Thus, short wavelength variations in Caloris' crustal profile remain unresolved. ...
In contrast to multiring basins, megabasins lack clear ring structures and display crustal profiles characterized by a thin layer of crust at their center that gradually increases in thickness to the surrounding region. The 1,550‐km‐diameter Caloris basin on Mercury is often speculated to be a multiring basin; however, its crustal structure is indicative of a megabasin, perhaps explaining the lack of discernible rings. Here, we model the formation of Caloris basin using iSALE‐2D under a range of preimpact thermal conditions to determine whether the processes responsible for its megabasin crustal structure could lead to the formation of basin rings. We find that thermal gradients between 22 and 30 K/km—hotter than previously inferred—facilitate the reproduction of Caloris basin's crustal structure through crustal flowback, though such preimpact thermal structures preclude its formation as a multiring basin. Instead, repeated thrusting and necking events during transient crater collapse induce instances of fault reactivation, which diminish initial fault offsets and produce a series of discrete crustal blocks. Models assuming cooler thermal gradients lead to overly thin or non‐existent crust at the basin center, in contrast to observations. Even if crustal deficits were made up via a differentiating melt pool, the crust elsewhere would be overly thick, supporting the idea that the crustal structure of megabasins is primarily associated with transient crater collapse. We conclude that the transition to a megabasin morphology on terrestrial planetary surfaces, like the multiring transition, is dependent upon the size of the basin compared to the lithospheric thickness.
Knowing the structure of the crust is critical to understanding a planet’s geologic evolution. Crustal thickness inversions rely on bulk density estimates, which are primarily affected by porosity. Due to the absence of high-resolution gravity data, Mercury’s crustal porosity has remained unknown. Here, we use a model that was calibrated to the Moon to relate Mercury’s impact crater population and crustal porosity. Therein, porosity is created by large impacts and then decreased as the surface ages due to pore compaction by smaller impacts and overburden pressure. Our preferred model fits independent porosity estimates in the northern regions, where gravity data is well resolved. Porosity is found to be 9 to 17\% with an average and standard deviation of 142\%, indicating lunar-like crustal bulk densities of 255575 kg m from which updated crustal thickness maps are constructed. BepiColombo’s improved gravity models will allow comparing porosity generation on Mercury and the Moon.
The ESA/JAXA BepiColombo mission is currently on its way to Mercury, aiming to unveil the secrets held by the innermost planet of the Solar System. The radio tracking measurements, in combination with the data acquired by the onboard accelerometer, will enable a precise navigation of the spacecraft addressing the scientific objectives of the mission. A series of planetary flybys are required during the interplanetary transfer orbit to save propellant for the orbital insertion around Mercury. We present here a joint analysis of the radio tracking and accelerometer measurements collected during the second Venus flyby and the first two Mercury flybys. A precise orbit determination process is carried out to yield a combined adjustment of the spacecraft trajectory and parameters associated with dynamical and observational modeling. The accelerometer data enabled the detection of unmodeled dynamical perturbations including outgassing and thermal recoil. Our analysis of Mercury flybys allowed us for the first time to combine BepiColombo and MESSENGER radiometric measurements, leading to significant improvements in the modeling of the gravitational forces.
We have analyzed the entire set of radiometric tracking data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. This analysis employed a method where standard Doppler tracking data were transformed into line-of-sight accelerations. These accelerations have greater sensitivity to small-scale features than standard Doppler. We estimated a gravity model expressed in spherical harmonics to degree and order 180 and showed that this model is improved, as it has increased correlations with topography in areas where tracking data were collected when the spacecraft altitude was low. The new model was used in an analysis of the localized admittance between gravity and topography to determine properties of Mercury’s lithosphere. Four areas with high correlations between gravity and topography were selected. These areas represent different terrain types: the high-Mg region, the Strindberg crater plus some lobate scarps, heavily cratered terrain, and smooth plains. We employed a Markov Chain Monte Carlo method to estimate crustal density, load density, crustal thickness, elastic thickness, load depth, and a load parameter that describes the ratio between surface and depth loading. We find densities around 2600 kg m ⁻³ for three of the areas, with the density for the fourth area, the northern rise, being higher. The elastic thickness is generally low, between 11 and 30 km.
In preparation for the ESA/JAXA BepiColombo mission to Mercury, thematic working groups had been established for coordinating the activities within the BepiColombo Science Working Team in specific fields. Here we describe the scientific goals of the Geodesy and Geophysics Working Group (GGWG) that aims at addressing fundamental questions regarding Mercury’s internal structure and evolution. This multidisciplinary investigation will also test the gravity laws by using the planet Mercury as a proof mass. The instruments on the Mercury Planetary Orbiter (MPO), which are devoted to accomplishing the GGWG science objectives, include the BepiColombo Laser Altimeter (BELA), the Mercury orbiter radio science experiment (MORE), and the MPO magnetometer (MPO-MAG). The onboard Italian spring accelerometer (ISA) will greatly aid the orbit reconstruction needed by the gravity investigation and laser altimetry. We report the current knowledge on the geophysics, geodesy, and evolution of Mercury after the successful NASA mission MESSENGER and set the prospects for the BepiColombo science investigations based on the latest findings on Mercury’s interior. The MPO spacecraft of the BepiColombo mission will provide extremely accurate measurements of Mercury’s topography, gravity, and magnetic field, extending and improving MESSENGER data coverage, in particular in the southern hemisphere. Furthermore, the dual-spacecraft configuration of the BepiColombo mission with the Mio spacecraft at higher altitudes than the MPO spacecraft will be fundamental for decoupling the internal and external contributions of Mercury’s magnetic field. Thanks to the synergy between the geophysical instrument suite and to the complementary instruments dedicated to the investigations on Mercury’s surface, composition, and environment, the BepiColombo mission is poised to advance our understanding of the interior and evolution of the innermost planet of the solar system.
Plain Language Summary
Mercury's crust has a complex structure resulting from a billion years of volcanism. The surface variations in chemical composition have been identified from orbit by the spacecraft MESSENGER. Combining these measurements with laboratory experiments on partial melting, we estimate which variations in surface density and degree of mantle melting are required to produce surface rocks. If the surface density is representative of the deep crustal density, more than one half of crustal thickness variations in the Northern Hemisphere are explained by lateral variations in mantle melting. The crust is thin below the magnesium‐poor northern volcanic plains, whereas the thickest crust is found in the magnesium‐rich region located at middle northern latitudes in the Western Hemisphere. The magnesium‐rich region is thus not due to an early impact but rather to extensive mantle melting. The thickness‐melting relation has also been observed for the oceanic crust on Earth and might be a common feature of terrestrial planets.
By modeling the elastic flexure of the Martian lithosphere under imposed loads, we provide a systematic study of the old and low‐relief volcanoes (>3.2 Ga, 0.5 to 7.4 km) and the younger and larger prominent constructs within the Tharsis and Elysium provinces (<3 Ga, 5.8 to 21.9 km). We fit the theoretical gravitational signal to observations in order to place constraints on 18 volcanic structures. Inverted parameters include the bulk density of the load, the elastic thickness required to support the volcanic edifice at the time it was emplaced (Te), the heat flow, the volume of extruded lava, and the ratio of volcanic products that form within (Vi) and above the preexisting surface (Ve). The bulk density of the volcanic structures is found to have a mean value of 3,206±190 kg/m³, which is representative of iron‐rich basalts as sampled by the Martian basaltic meteorites. Te beneath the small volcanoes is found to be small, less than 15 km, which implies that the lithosphere was weak and hot when these volcanoes formed. Conversely, most large volcanoes display higher values of Te, which is consistent with the bulk of their emplacement occurring later in geologic history, when the elastic lithosphere was colder and thicker. Our estimates for the volumes of volcanic edifices are about 10 times larger than those that neglect the flexure of the lithosphere. Constraints on the magnitude of subsurface loads imply that the ratio Vi/Ve is generally 3:5, which is smaller than for the Hawaiian volcanoes on Earth.
Geodetic analysis of radio tracking measurements of the MESSENGER spacecraft while in orbit about Mercury has yielded new estimates for the planet's gravity field, tidal Love number, and pole coordinates. The derived right ascension (α = 281.0082° ± 0.0009°; all uncertainties are 3 standard deviations) and declination (δ =61.4164° ± 0.0003°) of the spin pole place Mercury in the Cassini state. Confirmation of the equilibrium state with an estimated mean (whole-planet) obliquity ϵ of 1.968 ± 0.027 arcmin enables the confident determination of the planet's normalized polar moment of inertia (0.333 ± 0.005), which indicates a high degree of internal differentiation. Internal structure models generated by a Markov-Chain Monte Carlo process and consistent with the geodetic constraints possess a solid inner core with a radius (r
ic
) between 0.3 and 0.7 that of the outer core (r
oc
).
Geophysical analyses are often performed in spherical geometry and require the use of spherical harmonic functions to express observables or physical quantities. When expanded to high degree, the accuracy and speed of the spherical harmonic transforms and reconstructions are of paramount importance. SHTools is a time and user-tested open-source archive of both Fortran 95 and Python routines for performing spherical harmonic analyses. The routines support all spherical-harmonic normalization conventions used in the geosciences, including 4π-normalized, Schmidt seminormalized, orthonormalized, and unnormalized harmonics, along with the option of employing the Condon-Shortley phase factor of (Formula presented.). Data on the sphere can be sampled on a variety of grid formats, including equally spaced cylindrical grids and grids appropriate for integration by Gauss-Legendre quadrature. The spherical-harmonic transforms are proven to be fast and accurate for spherical harmonic degrees up to 2800. Several tools are provided for the geoscientist, including routines for performing localized spectral analyses and basic operations related to global gravity and magnetic fields. In the Python environment, operations are very simple to perform as a result of three class structures that encompass all operations on grids, spherical harmonic coefficients, and spatiospectral localization windows. SHTools is released under the unrestrictive BSD 3-clause license.
The NASA MESSENGER mission explored the innermost planet of the solar system and obtained a rich data set of range measurements for the determination of Mercury's ephemeris. Here we use these precise data collected over 7 years to estimate parameters related to general relativity and the evolution of the Sun. These results confirm the validity of the strong equivalence principle with a significantly refined uncertainty of the Nordtvedt parameter η = (-6.6 ± 7.2) × 10-5. By assuming a metric theory of gravitation, we retrieved the post-Newtonian parameter β = 1 + (-1.6 ± 1.8) × 10-5 and the Sun's gravitational oblateness, [Formula: see text] = (2.246 ± 0.022) × 10-7. Finally, we obtain an estimate of the time variation of the Sun gravitational parameter, [Formula: see text] = (-6.13 ± 1.47) × 10-14, which is consistent with the expected solar mass loss due to the solar wind and interior processes. This measurement allows us to constrain [Formula: see text] to be <4 × 10-14 per year.
We develop a method to estimate the power spectrum of a stochastic process on the sphere from data of limited geographical coverage. Our approach can be interpreted either as estimating the global power spectrum of a stationary process when only a portion of the data
are available for analysis, or estimating the power spectrum from local data under the assumption that the data are locally stationary in a specified region. Restricting a global function to a spatial subdomain -- whether by necessity or by design -- is a windowing operation, and an equation like a convolution in the spectral domain relates the expected value of the windowed power spectrum to the underlying global power spectrum and the known power spectrum of the localization window.
The best windows for the purpose of localized spectral analysis have their energy concentrated in the region of interest while possessing the smallest effective bandwidth as possible. Solving an optimization problem in the sense of Slepian (1960) yields a family of orthogonal windows of diminishing spatiospectral localization, the best concentrated of which we propose to use to form a weighted multitaper spectrum estimate in the sense of Thomson (1982). Such an estimate is both more representative of the target region and reduces the estimation variance when compared to estimates formed by any single bandlimited window. We describe how the weights applied to the individual spectral estimates in forming the multitaper estimate can be chosen such that the variance of the estimate is minimized.
We have analyzed three years of radio tracking data from the MESSENGER spacecraft in orbit around Mercury and determined the gravity field, planetary orientation, and ephemeris of the innermost planet. With improvements in spatial coverage, force modeling, and data weighting, we refined an earlier global gravity field both in quality and resolution, and we present here a spherical harmonic solution to degree and order 50. In this field, termed HgM005, uncertainties in low-degree coefficients are reduced by an order of magnitude relative to the earlier global field, and we obtained a preliminary value of the tidal Love number k2 of 0.451 ± 0.014. We also estimated Mercury's pole position, and we obtained an obliquity value of 2.06 ± 0.16 arcmin, in good agreement with analysis of Earth-based radar observations. From our updated rotation period (58.646146 ± 0.000011 days) and Mercury ephemeris, we verified experimentally the planet's 32 spin-orbit resonance to greater accuracy than previously possible. We present a detailed analysis of the HgM005 covariance matrix, and we describe some near-circular frozen orbits around Mercury that could be advantageous for future exploration.
Earth-based radar observations of the spin state of Mercury at 35 epochs
between 2002 and 2012 reveal that its spin axis is tilted by (2.04
± 0.08) arc min with respect to the orbit normal. The direction
of the tilt suggests that Mercury is in or near a Cassini state.
Observed rotation rate variations clearly exhibit an 88-day libration
pattern which is due to solar gravitational torques acting on the
asymmetrically shaped planet. The amplitude of the forced libration,
(38.5 ± 1.6) arc sec, corresponds to a longitudinal
displacement of ˜450 m at the equator. Combining these measurements
of the spin properties with second-degree gravitational harmonics (Smith
et al., 2012) provides an estimate of the polar moment of inertia of
Mercury C/MR2 = 0.346 ± 0.014,
where M and R are Mercury's mass and radius. The fraction of the moment
that corresponds to the outer librating shell, which can be used to
estimate the size of the core, is
Cm/C = 0.431 ± 0.025.
We have compiled a near-global catalog of impact craters on Mercury for diameters >=20 km from images obtained during the flybys of the Mariner 10 and MESSENGER spacecraft. The observed variations in crater density suggest that the smooth plains within and around the Caloris basin are the most prominent contiguous, comparatively young regions on Mercury; no other comparably large area appears as young. In more heavily cratered terrain, even the most densely cratered regions on Mercury are deficient in craters between 20 km and ˜100 km in diameter compared with the Moon, a result that extends an observation made from Mariner 10 images of a smaller fraction of Mercury's surface. This deficit is interpreted to reflect crustal resurfacing of Mercury early in its history, most likely by volcanic processes. For craters larger than ˜100 km in diameter, the density of craters on Mercury in a given size range is similar to that of the Moon. Because such larger craters are less easily removed by volcanic resurfacing, we interpret this similarity to be the result of impact saturation effects on both bodies.
Radio tracking of the MESSENGER spacecraft has provided a model of Mercury’s gravity field. In the northern hemisphere, several
large gravity anomalies, including candidate mass concentrations (mascons), exceed 100 milli-Galileos (mgal). Mercury’s northern
hemisphere crust is thicker at low latitudes and thinner in the polar region and shows evidence for thinning beneath some
impact basins. The low-degree gravity field, combined with planetary spin parameters, yields the moment of inertia C/MR2 = 0.353 ± 0.017, where M and R are Mercury’s mass and radius, and a ratio of the moment of inertia of Mercury’s solid outer shell to that of the planet
of Cm/C = 0.452 ± 0.035. A model for Mercury’s radial density distribution consistent with these results includes a solid silicate
crust and mantle overlying a solid iron-sulfide layer and an iron-rich liquid outer core and perhaps a solid inner core.
The Holy GRAIL?
The gravity field of a planet provides a view of its interior and thermal history by revealing areas of different density. GRAIL, a pair of satellites that act as a highly sensitive gravimeter, began mapping the Moon's gravity in early 2012. Three papers highlight some of the results from the primary mission. Zuber et al. (p. 668 , published online 6 December) discuss the overall gravity field, which reveals several new tectonic and geologic features of the Moon. Impacts have worked to homogenize the density structure of the Moon's upper crust while fracturing it extensively. Wieczorek et al. (p. 671 , published online 6 December) show that the upper crust is 35 to 40 kilometers thick and less dense—and thus more porous—than previously thought. Finally, Andrews-Hanna et al. (p. 675 , published online 6 December) show that the crust is cut by widespread magmatic dikes that may reflect a period of expansion early in the Moon's history.
The objective of the Dawn gravity investigation is to use high precision X-band Doppler tracking and landmark tracking from
optical images to measure the gravity fields of Vesta and Ceres to a half-wavelength surface resolution better than 90-km
and 300-km, respectively. Depending on the Doppler tracking assumptions, the gravity field will be determined to somewhere
between harmonic degrees 15 and 25 for Vesta and about degree 10 for Ceres. The gravity fields together with shape models
determined from Dawn’s framing camera constrain models of the interior from the core to the crust. The gravity field is determined
jointly with the spin pole location. The second degree harmonics together with assumptions on obliquity or hydrostatic equilibrium
may determine the moments of inertia.
KeywordsDawn–Vesta–Ceres–Gravity–Geophysics
Mercury holds answers to several critical questions regarding the formation and evolution of the terrestrial planets. These questions include the origin of Mercury's anomalously high ratio of metal to silicate and its implications for planetary accretion processes, the nature of Mercury's geological evolution and interior cooling history, the mechanism of global magnetic field generation, the state of Mercury's core, and the processes controlling volatile species in Mercury's polar deposits, exosphere, and magnetosphere. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission has been designed to fly by and orbit Mercury to address all of these key questions. After launch by a Delta 2925H-9.5, two flybys of Venus, and two flybys of Mercury, orbit insertion is accomplished at the third Mercury encounter. The instrument payload includes a dual imaging system for wide and narrow fields-of-view, monochrome and color imaging, and stereo; X-ray and combined gamma-ray and neutron spectrometers for surface chemical mapping; a magnetometer; a laser altimeter; a combined ultraviolet–visible and visible-near-infrared spectrometer to survey both exospheric species and surface mineralogy; and an energetic particle and plasma spectrometer to sample charged species in the magnetosphere. During the flybys of Mercury, regions unexplored by Mariner 10 will be seen for the first time, and new data will be gathered on Mercury's exosphere, magnetosphere, and surface composition. During the orbital phase of the mission, one Earth year in duration, MESSENGER will complete global mapping and the detailed characterization of the exosphere, magnetosphere, surface, and interior.
The Mercury gravity field, spin-pole axis, rotation period, Love number, and ephemeris have been determined using the complete four years of MESSENGER tracking data from March 2011 to April 2015. The pole location and obliquity (1.99 ± 0.12 arcmin) is consistent with previous determinations. Since MESSENGER was in a highly elliptical orbit with periapsis in the far northern hemisphere, the gravity resolution over the surface of Mercury varies greatly from harmonic degree n = 12 at the south pole to n = 154 in a small region near the north pole which was covered with exceptionally low periapsis data near the end of mission. The gravity field MESS160A is determined to n = 160 and shows notable improvement in the correlation with topography. Three different constraint methods are used to generate the gravity field. The nominal method is a Kaula power law of 5 × 10⁻⁵/n² to constraint the coefficients. One alternate constraint uses surface acceleration measurements, which only constrains the unobserved portion of the gravity field. The other constraint method sets the gravity uncertainties using the harmonic spectrum of the gravity derived from topography. The Mercury tidal Love number solution k2 = 0.53 ± 0.03 is larger than previous results but within suggested error bounds. In addition, the technique for estimating the Mercury ephemeris is discussed.
Unique physical and chemical characteristics of Mercury have been revealed by measurements from NASA’s MESSENGER spacecraft. The closest planet to our Sun is made up of a large metallic core that is partially liquid, a thin mantle thought to be formed by solidification of a silicate magma ocean, and a relatively thick secondary crust produced by partial melting of the mantle followed by volcanic eruptions. However, the origin of the large metal/silicate ratio of the bulk planet and the conditions of accretion remain elusive. Metal enrichment may originate from primordial processes in the solar nebula or from a giant impact that stripped most of the silicate portion of a larger planet leaving Mercury as we know it today.
Previous work suggested that the lithosphere of Mercury could undergo folding in response to global contraction, and indeed observations from the MESSENGER mission revealed several regions where long-wavelength topography is present. Here, we test, via finite-element simulations that use a more realistic rheological model than that earlier work, lithospheric folding as a formation mechanism for long-wavelength topography on Mercury from interior secular cooling over the last 3.8 Gyr. This radial contraction has been estimated from geological observations to be less than 10 km, which translates into small amounts of horizontal shortening of < 0.3%. Under expected surface temperatures of ∼440 K, the development of even modest fold amplification in such low strain environments is untenable. The scenarios under which there is this positive fold amplification begin with a fully compensated crust, but amplifications are small (factors < 1.1). Under other, non-compensated scenarios (e.g., a constant thickness crust), the collapse to isostasy overwhelms any folding instability. In order to produce lithospheric fold amplitudes that match those observed on Mercury, unrealistically large amounts of horizontal shortening (in excess of 10%, corresponding to hundreds of kilometers of radius change) are required. Therefore, we find that lithospheric folding cannot produce the observed long-wavelength topography on Mercury, and conclude that this topography must be buoyantly supported.
When a spacecraft visits a new planetary body, it is useful to know the properties of its shape and gravity field. This knowledge helps predict the magnitude of the perturbations in the motion of the spacecraft due to nonsphericity of a body's gravity field as well as planning for an observational campaign. It has been known for the terrestrial planets that the power spectrum of the gravity field follows a power law, also known as the Kaula rule (Kaula, 1963, https://doi.org/10.1029/RG001i004p00507; Rapp, 1989, https://doi.org/10.1111/j.1365-246X.1989.tb02031.x). A similar rule was derived for topography (Vening Meinesz, 1951). The goal of this study is to generalize the power law dependence of the gravity and topography spectra for solid surface solar system bodies across a wide range of body sizes. Traditionally, it is assumed that the gravity and topography power spectra of planets scale as g⁻², where g is the surface gravity. This gravity scaling also works for the minor bodies to first order. However, we find that a better fit can be achieved using a more general scaling based on the body's radius and mean density. We outline a procedure on how to use this general scaling for topography to provide an a priori estimate for the gravity power spectrum. We show that for irregularly shaped bodies the gravity power spectrum is no longer a power law even if their topography spectrum is a power law. Such a generalization would be useful for observation planning in the future space missions to the minor bodies for which little is known.
We examine the global distribution and spectral properties of low-reflectance material (LRM) across Mercury to estimate the relative carbon abundance of prominent exposures and to test hypotheses for the origin of carbon in LRM. Our mapping demonstrates that LRM is consistently excavated from depth and that the spectral curvature attributed to carbon becomes more subdued as these surface deposits age. We find that the 600-nm band depth in LRM deposits is related to carbon content and can be used to estimate carbon enrichment. LRM deposits excavated by basins and large craters may be enriched with as much as 4 wt% carbon over the local mean. Regional deposits, associated with the most heavily cratered terrains, are enriched by an average of ~2.5 wt% carbon. The association of LRM with excavation from depth shows that the carbon in LRM is native to the planet, rather than deposited over time by impacts.
This chapter reviews our current knowledge of the gravity and topography of the terrestrial planets and describes the methods that are used to analyze these data. A general review of the mathematical formalism that is used in describing gravity and topography is first given. Next, the basic properties of Earth, Venus, Mars, Mercury, and the Moon are characterized. Following this, the relationship between gravity and topography is quantified, and techniques by which geophysical parameters can be constrained are detailed. Analysis methods include crustal thickness modeling, geoid/topography ratios, spectral admittance and correlation functions, and localized spectral analysis and wavelet techniques. Finally, the major results that have been obtained by modeling the gravity and topography of Earth, Venus, Mars, Mercury, and the Moon are summarized.
Crustal thickness is a crucial geophysical parameter in understanding the geology and geochemistry of terrestrial planets. Recent development of mathematical techniques suggests that previous studies based on assumptions of isostasy overestimated crustal thickness on some of the solid bodies of the solar system, leading to a need to revisit those analyses. Here, I apply these techniques to Mercury. Using MESSENGER-derived elemental abundances, I calculate a map of grain density (average 2974 ± 89 kg/m³) which shows that Pratt isostasy is unlikely to be a major compensation mechanism of Mercury's topography. Assuming Airy isostasy, I find the best fit value for Mercury's mean crustal thickness is 26 ± 11 km, 25% lower than the most recently reported and previously thinnest number. Several geological implications follow from this relatively low value for crustal thickness, including showing that the largest impacts very likely excavated mantle material onto Mercury's surface. The new results also show that Mercury and the Moon have a similar proportion of their rocky silicates composing their crusts, and thus Mercury is not uniquely efficient at crustal production amongst terrestrial bodies. Higher resolution topography and gravity data, especially for the southern hemisphere, will be necessary to refine Mercury's crustal parameters further.
Knowledge of the average density of the crust of a planet is important in determining its interior structure. The combination of high-resolution gravity and topography data has yielded a low density for the Moon's crust, yet for other terrestrial planets the resolution of the gravity field models has hampered reasonable estimates. By using well-chosen constraints derived from topography during gravity field model determination using satellite tracking data, we show that we can robustly and independently determine the average bulk crustal density directly from the tracking data, using the admittance between topography and imperfect gravity. We find a low average bulk crustal density for Mars, 2582 ± 209kgm⁻³. This bulk crustal density is lower than that assumed until now. Densities for volcanic complexes are higher, consistent with earlier estimates, implying large lateral variations in crustal density. In addition, we find indications that the crustal density increases with depth.
During its four years in orbit around Mercury, the MErcury Surface, Space Environment, GEochemistry, and Ranging (MESSENGER) spacecraft's X-Ray Spectrometer revealed a large geochemical terrane in the northern hemisphere that hosts the highest Mg/Si, S/Si, Ca/Si, and Fe/Si and lowest Al/Si ratios on the planet. Correlations with low topography, thin crust, and a sharp northern topographic boundary led to the proposal that this high-Mg region (HMR) is the remnant of an ancient, highly degraded impact basin. Here we use a numerical modeling approach to explore the feasibility of this hypothesis and evaluate the results against multiple mission-wide datasets and resulting maps from MESSENGER. We find that a ~3000-km diameter impact basin easily exhumes Mg-rich mantle material but that the amount of subsequent modification required to hide basin structure is incompatible with the strength of the geochemical anomaly, which is also present in maps of Gamma Ray and Neutron Spectrometer data. Consequently, the high-Mg region is more likely to be the product of high-temperature volcanism sourced from a chemically heterogeneous mantle than the remains of a large impact event.
We analyze radio tracking data obtained during 1311 orbits of the MESSENGER spacecraft in the period March 2011 to April 2014. A least-squares minimization of the residuals between observed and computed values of two-way range and Doppler allows us to solve for a model describing Mercury's gravity, tidal response, and spin state. We use a spherical harmonic representation of the gravity field to degree and order 40 and report error bars corresponding to 10 times the formal uncertainties of the fit. Our estimate of the product of Mercury's mass and the gravitational constant, kms, is in excellent agreement with published results. Our solution for the geophysically important second-degree coefficients (, ) confirms previous estimates to better than 0.4\% and, therefore, inferences about Mercury's moment of inertia and interior structure. Our estimate of the tidal Love number indicates that Mercury's mantle may be hotter and weaker than previously thought. Our spin state solution suggests that gravity-based estimates of Mercury's spin axis orientation are marginally consistent with previous measurements of the orientation of the crust.
Crater size–frequency analyses have shown that the largest volcanic plains deposits on Mercury were emplaced around 3.7 Ga, as determined with recent model production function chronologies for impact crater formation on that planet. To test the hypothesis that all major smooth plains on Mercury were emplaced by about that time, we determined crater size–frequency distributions for the nine next-largest deposits, which we interpret also as volcanic. Our crater density measurements are consistent with those of the largest areas of smooth plains on the planet. Model ages based on recent crater production rate estimates for Mercury imply that the main phase of plains volcanism on Mercury had ended by ~3.5 Ga, with only small-scale volcanism enduring beyond that time. Cessation of widespread effusive volcanism is attributable to interior cooling and contraction of the innermost planet.
This book presents fundmentals of orbit determination--from weighted least squares approaches (Gauss) to todays high-speed computer algorithms that provide accuracy within a few centimeters. Numerous examples and problems are provided to enhance readers understanding of the material. * Covers such topics as coordinate and time systems, square root filters, process noise techniques, and the use of fictitious parameters for absorbing un-modeled and incorrectly modeled forces acting on a satellite. * Examples and exercises serve to illustrate the principles throughout each chapter. * Detailed solutions to end-of-chapter exercises available to instructors.
We model the formation of lunar complex craters and investigate the effect of preimpact
porosity on their gravity signatures. We find that while preimpact target porosities less than ~7% produce
negative residual Bouguer anomalies (BAs), porosities greater than ~7% produce positive anomalies whose
magnitude is greater for impacted surfaces with higher initial porosity. Negative anomalies result from
pore space creation due to fracturing and dilatant bulking, and positive anomalies result from destruction
of pore space due to shock wave compression. The central BA of craters larger than ~215 km in diameter,
however, are invariably positive because of an underlying central mantle uplift. We conclude that the striking
differences between the gravity signatures of craters on the Earth and Moon are the result of the higher
average porosity and variable porosity of the lunar crust.
The degree and depth of fracturing of the lithospheres of Mars, Mercury, and the Moon remain poorly known. It is these two properties, however, that govern the mechanical behavior of a planetary lithosphere. Considering the lithosphere as a cohesive rock mass that consists of small and large blocky, interlocked rock fragments, as opposed to an intact body or a body entirely lacking cohesion, provides insight into the effect of lithospheric fracturing on tectonic processes on these bodies. Characterization of the near-surface lithospheric brittle strength that incorporates the degree of fracturing is necessary for a realistic assessment of the lithospheric response to global contraction resulting from interior secular cooling. Such an assessment shows that all of these bodies could have accommodated substantial amounts of global contraction prior to the formation of thrust fault-related landforms. In fact, their lithospheres were sufficiently strong so as to experience changes in radius of as much as 2.2±0.4km (Mars), 2.1±0.4km (Mercury), and 1.4±0.3km (the Moon) prior to the onset of widespread thrust faulting. These values imply that the process of global contraction begins before any evidence of it is established in the geologic record, requiring an earlier onset, and for Mars and Mercury a faster initial strain rate, of global contraction than previously thought. Such results add a heretofore unrecognized component of planetary radial decrease with implications for timing and strain rate to studies of global contraction on Mars, Mercury, and the Moon, in particular, and to planetary bodies, in general.
We have mapped the major-element composition of Mercury's surface from orbital MESSENGER X-Ray Spectrometer measurements. These maps constitute the first global-scale survey of the surface composition of a Solar System body conducted with the technique of planetary X-ray fluorescence. Full maps of Mg and Al, together with partial maps of S, Ca, and Fe, each relative to Si, reveal highly variable compositions (e.g., Mg/Si and Al/Si range over 0.1–0.8 and 0.1–0.4, respectively). The geochemical variations that we observe are consistent with those inferred from other MESSENGER geochemical remote sensing datasets, but they do not correlate well with units mapped previously from spectral reflectance or morphology. Location-dependent, rather than temporally evolving, partial melt sources were likely the major influence on the compositions of the magmas that produced different geochemical terranes. A large ( ) region with the highest Mg/Si, Ca/Si, and S/Si ratios, as well as relatively thin crust, may be the site of an ancient and heavily degraded impact basin. The distinctive geochemical signature of this region could be the consequence of high-degree partial melting of a reservoir in a vertically heterogeneous mantle that was sampled primarily as a result of the impact event.
Orbital multispectral mapping of Mercury reveals stratigraphic relations of plains units and evidence for origin of low-reflectance material.
The range in density and compressibility of Mercurian melt compositions was determined to better understand the products of a possible Mercurian magma ocean and subsequent volcanism. Our experiments indicate that the only mineral to remain buoyant with respect to melts of the Mercurian mantle is graphite; consequently, it is the only candidate mineral to have composed a primary floatation crust during a global magma ocean. This exotic result is further supported by Mercury's volatile-rich nature and inexplicably darkened surface. Additionally, our experiments illustrate that partial melts of the Mercurian mantle that compose the secondary crust were buoyant over the entire mantle depth and could have come from as deep as the core-mantle boundary. Furthermore, Mercury could have erupted higher percentages of its partial melts compared to other terrestrial planets because magmas would not have stalled during ascent due to gravitational forces. These findings stem from the FeO-poor composition and shallow depth of Mercury's mantle, which has resulted in both low-melt density and a very limited range in melt density responsible for Mercury's primary and secondary crusts. The enigmatically darkened, yet low-FeO surface, which is observed today, can be explained by secondary volcanism and impact processes that have since mixed the primary and secondary crustal materials.
To explore the mechanisms of support of surface topography on Mercury, we have determined the admittances and correlations of topography and gravity in Mercury's northern hemisphere from measurements obtained by NASA's MESSENGER spacecraft. These admittances and correlations can be interpreted in the context of a number of theoretical scenarios, including flexural loading and dynamic flow. We find that long-wavelength (spherical harmonic degree l < 15) surface topography on Mercury is primarily supported through a combination of crustal thickness variations and deep mass anomalies. The deep mass anomalies may be interpreted either as lateral variations in mantle density or as relief on compositional interfaces. Domical topographic swells are associated with high admittances and are compensated at 300–400 km depth, in the lower reaches of Mercury's mantle. Quasi-linear topographic rises are primarily associated with shallow crustal compensation and are weakly correlated with positive mass anomalies in the mantle. The center of the Caloris basin features some of the thinnest crust on the planet, and the basin is underlain by a large negative mass anomaly. We also explore models of dynamic flow in the presence of compositional stratification above the liquid core. If there is substantial compositional stratification in Mercury's solid outer shell, relaxation of perturbed compositional interfaces may be capable of creating and sustaining long-wavelength topography.
To gain insight into the thickness of the crust of Mercury we use gravity and topography data acquired by the MESSENGER spacecraft to calculate geoid-to-topography ratios over the northern hemisphere of the planet. For an Airy model for isostatic compensation of variations in topography, we infer an average crustal thickness of 35±18km Combined with the value of the radius of the core of Mercury, this crustal thickness implies that Mercury had the highest efficiency of crustal production among the terrestrial planets. From the measured abundance of heat-producing elements on the surface we calculate that the heat production in the mantle from long-lived radioactive elements 4.45 Ga ago was greater than 5.4×10−12 W/kg. By analogy with the Moon, the relatively thin crust of Mercury allows for the possibility that major impact events, such as the one that formed the Caloris basin, excavated material from Mercury's mantle.
Orbital observations by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft are used to re-evaluate the nature and origin of the oldest mapped plains deposits on Mercury, the intercrater and intermediate plains units defined by Mariner 10 investigators. Despite the large areal extent of these plains, which comprise approximately one-third of the planetary surface area viewed by Mariner 10, their formation mechanism was not well constrained by Mariner 10 imaging. One hypothesis attributed plains formation to ponding of fluidized impact ejecta to create relatively smooth surfaces. Another hypothesis was that these plains are of volcanic origin. To assess the origin of these older plains and the contribution of early volcanism to resurfacing on Mercury, we have used MESSENGER data to analyze the morphology, spectral properties, impact crater statistics, and topography of Mariner 10 type-areas of intercrater and intermediate plains. On the basis of new criteria for the identification of intercrater and intermediate plains derived from these observations, we have remapped 18% of the surface of Mercury. We find that the intercrater plains are a highly textured unit with an abundance of secondary craters, whereas the intermediate plains are composed of both intercrater and smooth plains. We suggest that the term “intermediate plains” not be used to map the surface of Mercury henceforth, but rather this unit should be subdivided into its constituent intercrater and smooth plains units. We argue that a substantial percentage of the intercrater plains are composed of volcanic materials on the basis of (1) examples of areas where ejecta from a small number of superposed craters have transformed smooth plains deposits of volcanic origin into a unit indistinguishable from intercrater plains; (2) the range in ages of intercrater plains deposits as interpreted from crater size–frequency distributions; and (3) the near-global distribution of intercrater plains compared with the uneven distribution of impact basins and their associated ejecta deposits.
A key objective of the MESSENGER mission is to collect and characterize global topographic measurements of Mercury. Global DEMs will be created from this work.
We present the analysis of 205 spatially resolved measurements of the
surface composition of Mercury from MESSENGER's X-Ray Spectrometer. The
surface footprints of these measurements are categorized according to
geological terrain. Northern smooth plains deposits and the plains
interior to the Caloris basin differ compositionally from older terrain
on Mercury. The older terrain generally has higher Mg/Si, S/Si, and
Ca/Si ratios, and a lower Al/Si ratio than the smooth plains. Mercury's
surface mineralogy is likely dominated by high-Mg mafic minerals (e.g.,
enstatite), plagioclase feldspar, and lesser amounts of Ca, Mg, and/or
Fe sulfides (e.g., oldhamite). The compositional difference between the
volcanic smooth plains and the older terrain reflects different
abundances of these minerals and points to the crystallization of the
smooth plains from a more chemically evolved magma source. High-degree
partial melts of enstatite chondrite material provide a generally good
compositional and mineralogical match for much of the surface of
Mercury. An exception is Fe, for which the low surface abundance on
Mercury is still higher than that of melts from enstatite chondrites and
may indicate an exogenous contribution from meteoroid impacts.
The Mercury Surface, Space Environment, GEochemestry, and Ranging (MESSENGER) spacecraft, launched on August 3, 2004, was inserted in a highly elliptical polar orbit around the planet on March 18, 2011. One of the main mission goals is the determination of the interior structure of the planet, enabled by a suite of instruments that includes the radio system and a laser altimeter. Thanks to altimetric and radio observables, the topography and the gravity field of the planet have been retrieved with good accuracy, especially in the north polar region, where the spacecraft altitude is lower. In September, 2011, the radio tracking data of the first 6 months of operations were published with the ancillary information necessary for the MESSENGER orbit determination. This data set offers an excellent opportunity to test the orbit determination procedures developed in view of a similar, but more accurate, experiment hosted onboard BepiColombo, the ESA mission to Mercury. We present here the results of our analysis, which provide the spacecraft orbit, a 20×20 gravity field and a linear update of Mercury's ephemeris. The estimated gravity field is fully compatible with the one published by Smith et al. (2012).
The Tyrrhena Patera highland volcano, Mars, is associated with a relatively well localized gravity anomaly and we have carried out a localized admittance analysis in the region to constrain the density of the volcanic load, the load thickness, and the elastic thickness at the time of load emplacement. The employed admittance model considers loading of an initially spherical surface, and surface as well as subsurface loading is taken into account. Our results indicate that the gravity and topography data available at Tyrrhena Patera is consistent with the absence of subsurface loading, but the presence of a small subsurface load cannot be ruled out. We obtain minimum load densities of 2960 kg m−3, minimum load thicknesses of 5 km, and minimum load volumes of 0.6 × 106 km3. Photogeological evidence suggests that pyroclastic deposits make up at most 30% of this volume, such that the bulk of Tyrrhena Patera is likely composed of competent basalt. Best fitting model parameters are a load density of 3343 kg m−3, a load thickness of 10.8 km, and a load volume of 1.7 × 106 km3. These relatively large load densities indicate that lava compositions are comparable to those at other martian volcanoes, and densities are comparable to those of the martian meteorites. The elastic thickness in the region is constrained to be smaller than 27.5 km at the time of loading, indicating surface heat flows in excess of 24 mW m−2.
A new technique for calculating potential anomalies on a sphere due to finite amplitude relief has been developed. We show that by raising the topography to the nth power and expanding this field into spherical harmonics, potential anomalies due to topography on spherical density interfaces can be computed to arbitrary precision. Using a filter for downward continuing the Bouguer anomaly, we have computed a variety of crustal thickness maps for the moon, assuming both a homogeneous as well as a dual-layered crust. The crustal thickness maps for the homogeneous model give plausible results, but this model is not consistent with the seismic data, petrologic evidence, and geoid to topography ratios, all of which suggest some form of crustal stratification. Several dual-layered models were investigated, and it was found that only models with both upper and lower crustal thickness variations could satisfy the gravity and topography data. These models predict that the entire upper crust has been excavated beneath the major near-side multiring basins. Significant amounts of lower crustal material was excavated from these basins, especially beneath Crisium. This model also predicts that mantle material should not have been excavated during the South-Pole Aitken basin-forming event, and that lower crustal material should be exposed at the surface in this basin.
The most heavily cratered terrains on Mercury have been estimated to be about 4 billion years (Gyr) old, but this was based on images of only about 45 per cent of the surface; even older regions could have existed in the unobserved portion. These terrains have a lower density of craters less than 100 km in diameter than does the Moon, an observation attributed to preferential resurfacing on Mercury. Here we report global crater statistics of Mercury's most heavily cratered terrains on the entire surface. Applying a recent model for early lunar crater chronology and an updated dynamical extrapolation to Mercury, we find that the oldest surfaces were emplaced just after the start of the Late Heavy Bombardment (LHB) about 4.0-4.1 Gyr ago. Mercury's global record of large impact basins, which has hitherto not been dated, yields a similar surface age. This agreement implies that resurfacing was global and was due to volcanism, as previously suggested. This activity ended during the tail of the LHB, within about 300-400 million years after the emplacement of the oldest terrains on Mercury. These findings suggest that persistent volcanism could have been aided by the surge of basin-scale impacts during this bombardment.
It is thought that Mercury, the moon, and many large satellites of the
major planets have been tidally despun from an initially faster
rotation, and that these bodies probably possessed equatorial bulges
which relaxed as they lost their spin. An analysis of the stresses
induced in an elastic shell by the relaxation of an equatorial bulge
indicates that differential stresses may reach a few kilobars and that
the tectonic pattern developed depends mainly upon the shell thickness.
In every model studied the azimuthal stress is larger (more compressive)
than the meridional stress. Tectonic patterns expected for the cases of
a thin elastic shell and a thicker elastic shell are discussed. It is
suggested that observations of the polar regions of a despun planet will
help determine whether a given lineament system is due to stresses
induced by the relaxation of the planet's equatorial bulge.
The primary crater population on Mercury has been modified by volcanism and secondary craters. Two phases of volcanism are recognized. One volcanic episode that produced widespread intercrater plains occurred during the period of the Late Heavy Bombardment and markedly altered the surface in many areas. The second episode is typified by the smooth plains interior and exterior to the Caloris basin, both of which have a different crater size-frequency distribution than the intercrater plains, consistent with a cratering record dominated by a younger population of impactors. These two phases may have overlapped as parts of a continuous period of volcanism during which the volcanic flux tended to decrease with time. The youngest age of smooth plains volcanism cannot yet be determined, but at least small expanses of plains are substantially younger than the plains associated with the Caloris basin. The spatial and temporal variations of volcanic resurfacing events can be used to reconstruct Mercury's geologic history from images and compositional and topographic data to be acquired during the orbital phase of the MESSENGER mission.Highlights► Mercury's crater population has been modified by volcanism and secondary cratering. ► Two possibly overlapping phases of volcanism are recognized on Mercury. ► One volcanic episode produced intercrater plains during the Late Heavy Bombardment. ► Later volcanism is typified by smooth plains exterior and interior to Caloris basin.
NASA’s MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission will further the understanding of
the formation of the planets by examining the least studied of the terrestrial planets, Mercury. During the one-year orbital
phase (beginning in 2011) and three earlier flybys (2008 and 2009), the X-Ray Spectrometer (XRS) onboard the MESSENGER spacecraft
will measure the surface elemental composition. XRS will measure the characteristic X-ray emissions induced on the surface
of Mercury by the incident solar flux. The Kα lines for the elements Mg, Al, Si, S, Ca, Ti, and Fe will be detected. The 12°
field-of-view of the instrument will allow a spatial resolution that ranges from 42km at periapsis to 3200 km at apoapsis
due to the spacecraft’s highly elliptical orbit. XRS will provide elemental composition measurements covering the majority
of Mercury’s surface, as well as potential high-spatial-resolution measurements of features of interest. This paper summarizes
XRS’s science objectives, technical design, calibration, and mission observation strategy.
The Magellan Doppler radiometric tracking data provides unprecedented precision for spacecraft-based gravity measurements with the maximum resolution approaching spherical harmonic degree and order 180 in selected equatorial regions. Determining a gravity field to degree 180 with a complete covariance containing the correlations between all the spherical harmonic coefficients (a 4.5-GB binary file for the triangular matrix) would be an extensive computational task even on the JPL/Caltech supercomputer that we used. Instead we determined a gravity field complete to degree and order 180 but in three separate steps. This gravity solution (MGNP180U) was determined first to degree and order 120 with a complete covariance for all the coefficients to degree 120. The second step solved for the coefficients from degree 116 to 155 only and the third step from degree 154 to 180. MGNP180U shows substantial improvement over previous solutions (up to and including MGNP120PSAAP, A. S. Konoplivet al.1996a, presented at1996 AGU Fall Meeting,San Francisco, CA) especially in the medium to shorter wavelengths (harmonic degree 80 and greater). The RMS magnitude power in the spectrum has increased as well as the correlations with topography. The amplitudes of various features have increased substantially (up to 33%, e.g., Bell Regio and Maat Mons). This will allow for better investigation of lithospheric modeling for shorter wavelength features such as coronae, volcanoes, and impact basins.
We estimate the impact flux and cratering rate as a function of latitude on the terrestrial planets using a model distribution of planet crossing asteroids and comets [Bottke, W.F., Morbidelli, A., Jedicke, R., Petit, J.-M., Levison, H.F., Michel, P., Metcalfe, T.S., 2002. Icarus 156, 399–433]. After determining the planetary impact probabilities as a function of the relative encounter velocity and encounter inclination, the impact positions are calculated analytically, assuming the projectiles follow hyperbolic paths during the encounter phase. As the source of projectiles is not isotropic, latitudinal variations of the impact flux are predicted: the calculated ratio between the pole and equator is 1.05 for Mercury, 1.00 for Venus, 0.96 for the Earth, 0.90 for the Moon, and 1.14 for Mars over its long-term obliquity variation history. By taking into account the latitudinal dependence of the impact velocity and impact angle, and by using a crater scaling law that depends on the vertical component of the impact velocity, the latitudinal variations of the cratering rate (the number of craters with a given size formed per unit time and unit area) is in general enhanced. With respect to the equator, the polar cratering rate is about 30% larger on Mars and 10% on Mercury, whereas it is 10% less on the Earth and 20% less on the Moon. The cratering rate is found to be uniform on Venus. The relative global impact fluxes on Mercury, Venus, the Earth and Mars are calculated with respect to the Moon, and we find values of 1.9, 1.8, 1.6, and 2.8, respectively. Our results show that the relative shape of the crater size-frequency distribution does not noticeably depend upon latitude for any of the terrestrial bodies in this study. Nevertheless, by neglecting the expected latitudinal variations of the cratering rate, systematic errors of 20–30% in the age of planetary surfaces could exist between equatorial and polar regions when using the crater chronology method.