Michael M. Sori

Lunar impact basins revealed by Gravity Recovery and Interior Laboratory measurements

New gravity measurements greatly improve the Moon’s preserved impact basin inventory. Observations from the Gravity Recovery and Interior Laboratory (GRAIL) mission indicate a marked change in the gravitational signature of lunar impact structures at the morphological transition, with increasing diameter, from complex craters to peak-ring basins. At crater diameters larger than ~200 km, a central positive Bouguer anomaly is seen within the innermost peak ring, and an annular negative Bouguer anomaly extends outward from this ring to the outer topographic rim crest. These observations demonstrate that basin-forming impacts remove crustal materials from within the peak ring and thicken the crust between the peak ring and the outer rim crest. A correlation between the diameter of the central Bouguer gravity high and the outer topographic ring diameter for well-preserved basins enables the identification and characterization of basins for which topographic signatures have been obscured by superposed cratering and volcanism. The GRAIL inventory of lunar basins improves upon earlier lists that differed in their totals by more than a factor of 2. The size-frequency distributions of basins on the nearside and farside hemispheres of the Moon differ substantially; the nearside hosts more basins larger than 350 km in diameter, whereas the farside has more smaller basins. Hemispherical differences in target properties, including temperature and porosity, are likely to have contributed to these different distributions. Better understanding of the factors that control basin size will help to constrain models of the original impactor population.

The fractured Moon: Production and saturation of porosity in the lunar highlands from impact cratering

We have analyzed the Bouguer anomaly (BA) of ~1200 complex craters in the lunar highlands from Gravity Recovery and Interior Laboratory observations. The BA of these craters is generally negative, though positive BA values are observed, particularly for smaller craters. Crater BA values scale inversely with crater diameter, quantifying how larger impacts produce more extensive fracturing and dilatant bulking. The Bouguer anomaly of craters larger than 93 þ47 � 19 km in diameter is independent of crater size, indicating that there is a limiting depth to impact-generated porosity, presumably from pore collapse associated with either overburden pressure or viscous flow. Impact-generated porosity of the bulk lunar crust is likely in a state of equilibrium for craters smaller than ~30km in diameter, consistent with an ~8km thick lunar megaregolith, whereas the gravity signature of larger craters is still preserved and provides new insight into the cratering record of even the oldest lunar surfaces.

The vanishing cryovolcanoes of Ceres

Ahuna Mons is a 4-km-tall mountain on Ceres interpreted as a geologically young cryovolcanic dome. Other possible cryovolcanic features are more ambiguous, implying that cryovolcanism is only a recent phenomenon or that other cryovolcanic structures have been modified beyond easy identification. We test the hypothesis that Cerean cryovolcanic domes viscously relax, precluding ancient domes from recognition. We use numerical models to predict flow velocities of Ahuna Mons to be 10–500 m/Myr, depending upon assumptions about ice content, rheology, grain size, and thermal parameters. Slower flow rates in this range are sufficiently fast to induce extensive relaxation of cryovolcanic structures over 108–109 years, but gradual enough for Ahuna Mons to remain identifiable today. Positive topographic features, including a tholus underlying Ahuna Mons, may represent relaxed cryovolcanic structures. A composition for Ahuna Mons of >40% ice explains the observed distribution of cryovolcanic structures because viscous relaxation renders old cryovolcanoes unrecognizable.

Stratigraphy of the north polar layered deposits of Mars from high-resolution topography

NASA Earth and Space Science Fellowship [NNX13AO55H]; NASAs Mars Reconnaissance Orbiter project (HiRISE)

Viscous flow rates of icy topography on the north polar layered deposits of Mars

We investigate the importance of viscous flow in shaping topography at the north polar layered deposits (NPLD) of Mars by using finite element modeling to calculate the distribution of stresses and flow velocities. Present-day impact craters on theNPLDare too small and cold for viscous relaxation tohavebeen an important mechanism in controlling their current dimensions; this effect may be ignored when analyzing crater size-frequency distributions. Scarps at the NPLDmargins, where avalanches of dust and carbon dioxide frost occur, are sufficiently steep, high, andwarm to experience significant viscous flow.We find flow velocities at the base of these steep scarps on the order of tens to hundreds of cm/yr, which are fast enough to significantly affect their slopeover kiloyear timescales. Alternatively, the scarps could be close to steady state in which observed block falls provide a competing effect to viscous flow.

Signals of astronomical climate forcing in the exposure topography of the North Polar Layered Deposits of Mars

NASA Earth and Space Science Fellowship [NNX13AO55H]; NASAs Mars Reconnaissance Orbiter project

Isostatic Compensation of the Lunar Highlands: Isostatic Compensation of the Lunar Highlands

The lunar highlands are isostatically compensated at large horizontal scales, but the specific compensation mechanism has been difficult to identify. With topographic data from the Lunar Orbiter Laser Altimeter and gravity data from the Gravity Recovery and Interior Laboratory, we investigate support of highland topography. Poor correlation between crustal density and elevation shows that Pratt compensation is not important in the highlands. Using spectrally weighted admittance, we compared observed values of geoid-to-topography ratio (GTR) with those predicted by isostatic models. Observed GTRs are 25.8 +7.5-5.7 m/km for the nearside highlands and 39.3 +5.7-6.2 m/km for the farside highlands. These values are not consistent with flexural compensation of long-wavelength topography or Airy isostasy defined under an assumption of equal mass in crustal columns. Instead, the observed GTR values are consistent with models of Airy compensation in which isostasy is defined under a requirement of equal pressures at equipotential surfaces at depth. The gravity and topography data thus reveal that long-wavelength topography on the Moon is most likely compensated by variations in crustal thickness, implying that highland topography formed early in lunar history before the development of a thick elastic lithosphere.

Episodes of Aqueous Flooding and Effusive Volcanism Associated With Hrad Vallis, Mars

NASA Planetary Geology and Geophysics Program NASA Planetary Science Division [NNX13AR14G, 80NSSC17K0307]; NASA Earth and Space Sciences Fellowship (NESSF) Program [NNX16AP09H]