High-resolution gravity data obtained from the dual Gravity Recovery and Interior Laboratory (GRAIL) spacecraft show that the bulk density of the Moon's highlands crust is 2550 kilograms per cubic meter, substantially lower than generally assumed. When combined with remote sensing and sample data, this density implies an average crustal porosity of 12% to depths of at least a few kilometers. Lateral variations in crustal porosity correlate with the largest impact basins, whereas lateral variations in crustal density correlate with crustal composition. The low-bulk crustal density allows construction of a global crustal thickness model that satisfies the Apollo seismic constraints, and with an average crustal thickness between 34 and 43 kilometers, the bulk refractory element composition of the Moon is not required to be enriched with respect to that of Earth.

New gravity measurements greatly improve the Moon’s preserved impact basin inventory.

[1] The Meridiani Planum region of Mars has been identified as a region of past aqueous activity by a combination of orbital and in situ observations that revealed evidence for sulfate-rich dirty evaporites formed in a playa setting. We investigate the hydrology and sedimentary record of this area using global and regional hydrological models in which groundwater flow is driven by a combination of precipitation, evaporation, and the surface topography. Groundwater evaporation results in evaporite precipitation and cementation of aeolian sediments, allowing the accumulation of deposits of substantial thickness, which then affect the subsequent patterns of groundwater flow. Hydrological activity is initially predicted to be isolated to the deepest craters and depressions, primarily within the Arabia Terra region surrounding Meridiani. As these depressions fill with sediments, the groundwater upwelling spreads laterally across broad regions of Arabia Terra, including Meridiani Planum, as well as regional topographic lows such as the northern lowlands and large impact basins. The model predictions are borne out by observations of large intracrater deposits, inverted valley networks, finely layered deposits, spectral evidence for hydrated sulfates, and pedestal craters that preserve the remnants of a much larger deposit that once covered much of Arabia Terra. The results suggest that the inferred playa at Meridiani was part of a regionally extensive zone of groundwater upwelling. This hydrological cycle requires that conditions in the late Noachian to early Hesperian must have been conducive to the existence of liquid water at the surface throughout much of the low latitudes of Mars.

The Gravity Recovery and Interior Laboratory (GRAIL) mission has sampled lunar gravity with unprecedented accuracy and resolution. The lunar GM, the product of the gravitational constant G and the mass M, is very well determined. However, uncertainties in the mass and mean density, 3345.56 ± 0.40 kg/m 3 , are limited by the accuracy of G. Values of the spherical harmonic degree-2 gravity coefficients J 2 and C 22 , as well as the Love number k 2 describing lunar degree-2 elastic response to tidal forces, come from two independent analyses of the 3 month GRAIL Primary Mission data at the Jet Propulsion Laboratory and the Goddard Space Flight Center. The two k 2 determinations, with uncertainties of~1%, differ by 1%; the average value is 0.02416 ± 0.00022 at a 1 month period with reference radius R = 1738 km. Lunar laser ranging (LLR) data analysis determines (C À A)/B and (B À A)/C, where A < B < C are the principal moments of inertia; the flattening of the fluid outer core; the dissipation at its solid boundaries; and the monthly tidal dissipation Q = 37.5 ± 4. The moment of inertia computation combines the GRAIL-determined J 2 and C 22 with LLR-derived (C À A)/B and (B À A)/C. The normalized mean moment of inertia of the solid Moon is I s /MR 2 = 0.392728 ± 0.000012. Matching the density, moment, and Love number, calculated models have a fluid outer core with radius of 200-380 km, a solid inner core with radius of 0-280 km and mass fraction of 0-1%, and a deep mantle zone of low seismic shear velocity. The mass fraction of the combined inner and outer core is ≤1.5%.

We analyzed data from the Gravity Recovery and Interior Laboratory (GRAIL) mission using a localized admittance approach to map out spatial variations in the vertical density structure of the lunar crust. Mare regions are characterized by a distinct decrease in density with depth, while the farside is characterized by an increase in density with depth at an average gradient of ∼35 kg m −3 km −1 and typical surface porosities of at least 20%. The Apollo 12 and 14 landing site region has a similar density structure to the farside, permitting a comparison with seismic velocity profiles. The interior of the South Pole-Aitken (SP-A) impact basin appears distinct with a near-surface low-density (porous) layer 2-3 times thinner than the rest of the farside. This result suggests that redistribution of material during the large SP-A impact likely played a major role in sculpting the lunar crust.

The most prominent feature on the surface of Mars is the near-hemispheric dichotomy between the southern highlands and northern lowlands. The root of this dichotomy is a change in crustal thickness along an apparently irregular boundary, which can be traced around the planet, except where it is presumably buried beneath the Tharsis volcanic rise. The isostatic compensation of these distinct provinces and the ancient population of impact craters buried beneath the young lowlands surface suggest that the dichotomy is one of the most ancient features on the planet. However, the origin of this dichotomy has remained uncertain, with little evidence to distinguish between the suggested causes: a giant impact or mantle convection/overturn. Here we use the gravity and topography of Mars to constrain the location of the dichotomy boundary beneath Tharsis, taking advantage of the different modes of compensation for Tharsis and the dichotomy to separate their effects. We find that the dichotomy boundary along its entire path around the planet is accurately fitted by an ellipse measuring approximately 10,600 by 8,500 km, centred at 67 degrees N, 208 degrees E. We suggest that the elliptical nature of the crustal dichotomy is most simply explained by a giant impact, representing the largest such structure thus far identified in the Solar System.

The Opportunity Mars Exploration Rover found evidence for groundwater activity in the Meridiani Planum region of Mars in the form of aeolian and fluvial sediments composed of sulphate-rich grains. These sediments appear to have experienced diagenetic modification in the presence of a fluctuating water table. In addition to the extensive secondary aqueous alteration, the primary grains themselves probably derive from earlier playa evaporites. Little is known, however, about the hydrologic processes responsible for this environmental history-particularly how such extensive evaporite deposits formed in the absence of a topographic basin. Here we investigate the origin of these deposits, in the context of the global hydrology of early Mars, using numerical simulations, and demonstrate that Meridiani is one of the few regions of currently exposed ancient crust predicted to have experienced significant groundwater upwelling and evaporation. The global groundwater flow would have been driven primarily by precipitation-induced recharge and evaporative loss, with the formation of the Tharsis volcanic rise possibly playing a role through the burial of aquifers and induced global deformation. These results suggest that the deposits formed as a result of sustained groundwater upwelling and evaporation, rather than ponding within an enclosed basin. The evaporite formation coincided with a transition to more arid conditions that increased the relative impact of a deep-seated, global-scale hydrology on the surface evolution.

[1] Columbus crater in the Terra Sirenum region of the Martian southern highlands contains light-toned layered deposits with interbedded sulfate and phyllosilicate minerals, a rare occurrence on Mars. Here we investigate in detail the morphology, thermophysical properties, mineralogy, and stratigraphy of these deposits; explore their regional context; and interpret the crater's aqueous history. Hydrated mineral-bearing deposits occupy a discrete ring around the walls of Columbus crater and are also exposed beneath younger materials, possibly lava flows, on its floor. Widespread minerals identified in the crater include gypsum, polyhydrated and monohydrated Mg/Fe-sulfates, and kaolinite; localized deposits consistent with montmorillonite, Fe/Mg-phyllosilicates, jarosite, alunite, and crystalline ferric oxide or hydroxide are also detected. Thermal emission spectra suggest abundances of these minerals in the tens of percent range. Other craters in northwest Terra Sirenum also contain layered deposits and Al/Fe/Mg-phyllosilicates, but sulfates have so far been found only in Columbus and Cross craters. The region's intercrater plains contain scattered exposures of Al-phyllosilicates and one isolated mound with opaline silica, in addition to more common Fe/Mg-phyllosilicates with chlorides. A Late Noachian age is estimated for the aqueous deposits in Columbus, coinciding with a period of inferred groundwater upwelling and evaporation, which (according to model results reported here) could have formed evaporites in Columbus and other craters in Terra Sirenum. Hypotheses for the origin of these deposits include groundwater cementation of crater-filling sediments and/or direct precipitation from subaerial springs or in a deep (∼900 m) paleolake. Especially under the deep lake scenario, which we prefer, chemical gradients in Columbus crater may have created a habitable environment at this location on early Mars.