Peter B. James’s research while affiliated with Baylor University and other places

Impact craters that form on every planetary body provide a record of planetary surface evolution. On heavily cratered surfaces, new craters that form often overlap antecedent craters, but it is unknown how the presence of antecedent craters alters impact crater formation. We use overlapping complex crater pairs on the lunar surface to constrain this process and find that crater rims are systematically lower where they intersect antecedent crater basins. The rim morphology of the new crater depends on the depth of the antecedent crater and the degree of overlap between the craters. Our observations suggest that new craters do not always obliterate underlying topography and that transient rim collapse is altered by antecedent topography. This study represents the first formalization of the influence of antecedent topography on rim morphology and provides process insight into a common impact scenario relevant to the geology of potential Artemis landing sites.

Impact craters that form on every planetary body provide a record of planetary surface evolution. On heavily-cratered surfaces, new craters that form often overlap older craters, but it is unknown how the presence of older craters alters impact crater formation. We use overlapping complex crater pairs on the lunar surface to constrain this process and find that crater rims are systematically lower where they intersect antecedent crater basins. However, the rim morphology of the new crater depends on both the depth of the antecedent crater and the degree of overlap between the two craters. Our observations suggest that transient rim collapse is altered by antecedent topography, leading to circumferential distribution of rim materials in the younger crater. This study represents the first formalization of the influence of antecedent topography on rim morphology and provides process insight into a common impact scenario relevant to the geology of potential Artemis landing sites.

The mass transport of volatiles on Mars represents a seasonally changing load onto the lithosphere of the planet. Much like on Earth, as mass is redistributed across the planet, the surface responds in a complex manner becoming displaced downwards or upwards. The magnitude and extent of displacement depend on the properties of the load and mechanical properties of the planetary interior. Based on new estimates of the height variation of the seasonal polar cap (SPC) we predict local surface displacements of up to tens of millimeters with a strong degree 1 signal throughout the Martian year. The long-wavelength portion of the displacement is potentially observable, with a magnitude of a few millimeters, located away from the seasonal polar cap where we could realistically measure it with a landed or orbital mission. We also model the direct contribution of this process to observable time variable gravity where we find the odd zonal coefficients to be in line with previous measurements, although with a smaller magnitude. Future measurements of this displacement could be used to help elucidate the composition of the mantle and crust of Mars, using this process as a probe into the Martian interior. Furthermore, more refined measurements of time-variable gravity would be a powerful tool in constraining the pole-to-pole volatile cycle present on Mars.

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.

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.

The crust of Mercury has experienced contraction on a global scale. Contractional deformation is expressed by a broadly distributed network of lobate thrust fault scarps. The most likely principal source of stress is global contraction from cooling of Mercury's interior. Global contraction alone would be expected to result in a uniformly distributed population of thrust faults. Mercury's fault scarps, however, often occur in long, linear clusters or bands. An analysis of the contractional strain as a function of crustal thickness, estimated in two crustal thickness models (CT1 and CT2) derived from gravity and topography data obtained during the MESSENGER mission, indicates the greatest contractional strain occurs in crust 50–60 km thick. On Earth, mantle downwelling can thicken and compress overlying crust, regionally concentrating thrust faults. Clusters of lobate scarps collocated with regions of thick crust suggest downward mantle flow contributed to the localization of lithosphere‐penetrating thrust faults.

Significance We have identified a pattern of tectonic deformation on Venus that suggests that many of the planet’s lowlands have fragmented into discrete crustal blocks, and that these blocks have moved relative to each other in the geologically recent past. These motions may be the result of mantle convection and, if so, constitute a style of interior–surface coupling not seen elsewhere in the inner Solar System except for continental interiors on Earth. Venus’ fragmented, mobile lithosphere may offer a framework for understanding how tectonics on Earth operated in the Archean.