Mikhail A. Kreslavsky

In the central area of the South Pole‐Aitken (SPA) basin, an intermediate albedo, mafic compositional anomaly (SPA Compositional Anomaly, SPACA) has been documented by previous studies, but its origin remains uncertain. We conducted an investigation of stratigraphic units defined based on morphology and composition and their relative ages, and placed these in the context of basin topography and the observed sequence of geological events, all helping to distinguish between SPACA origins from: (a) SPA impact melt, (b) volcanism induced by the SPA event and (c) lunar cryptomaria. We conclude that SPACA represents extensive traditional cryptomare deposits overlying the SPA impact melt. We interpret the basin center to be filled with cryptomare deposits at least one km thick (>1 × 10⁵ km³ in volume) with ages not younger than Early Imbrian. We attribute the relatively high albedo of SPACA to lateral mixing of ejecta from nearby highlands craters and basins, and conclude that the cryptomaria basalts are likely to be very similar to basalts on the nearside. Our findings imply a 0.5%–1.8% increase in the total volume of global lunar mare and cryptomare deposits. These results show that mare volcanism was common only in areas of thinnest crust on the lunar farside, a factor important in understanding lunar nearside‐farside asymmetries. Despite this significant increase in total cryptomare volume in the SPA basin center, SPA remains underfilled relative to nearside mascon basins. Return of mare basalts from the SPA region by Chang’E‐6 will help determine potential mantle source region differences and petrogenetic pathways.

On the Moon, the surface morphology at the scale of meters and tens of meters is typically smooth and subdued due to regolith gardening. Sharp, “crisp,” meter-scale morphologic features are observed only where the regolith is either thin or recently disturbed. Such crisp morphologies are typically created by geologically recent meteoritic impacts of different scales. The prominent exception is so-called irregular mare patches (IMPs), rare small features of debated origin. We report here on the discovery of previously unknown crisp immature morphological features (named “spiders” due to their central circular region and radiating “legs”) not related to impacts and even more rare. The spiders are meters-deep depressions with near-radial chutes open toward the center which make an incipient dendritic pattern 50–80 m in diameter. All spiders found thus far occur in clusters in the same region in Mare Tranquillitatis in the immediate proximity to small IMPs. We interpret spiders as the result of an energetic granular flow of the regolith draining into shallow subsurface voids following the sudden collapse of the roofs of the voids. Regolith gardening destroys the spiders’ legs rapidly, on a timescale of a million years. If the entrance into the subsurface void remains unclogged, a spider appears to evolve into a pit; otherwise it evolves into a gentle depression and finally disappears. Our interpretation of spiders provides a consistent explanation of all of their features, occurrence settings, and associations.

It has been predicted that high equatorial temperatures on Mercury could promote thermal annealing by Ostwald ripening, where nanophase metal particles (a product of space weathering) coalesce and grow into larger, microphase particles, resulting in lower albedo. Here, we test this prediction by studying the correlation between albedo and temperature in 1° spatial bins using newly recalibrated 1064 nm reflectance data acquired by the Mercury Laser Altimeter (MLA), low incidence angle data from the Mercury Dual Imaging System (MDIS), and newly modeled maximum surface temperatures (MSTs). Accounting for local geology and latitude, we compare the reflectance values of surfaces with MSTs >675 K (where Ostwald ripening is predicted to be most effective) to surfaces with MSTs 675 K are 10% and 12% darker than surfaces 675 K are 8% and 7% darker than surfaces <473 K. However, open questions remain regarding the causation of this darkening; statistical tests cannot distinguish whether the reflectance differences are systematic or the result of compositional variations that happen to correlate with MST. Along Mercury’s thermal longitudes, we find that reflectance is typically lower along hot poles than along the 90°E cold pole in the low-to-midlatitudes, especially in the smooth plains, consistent with previous work identifying a decrease in optical maturity along the 90°E cold pole. Longitudinal reflectance variations correlate with temperature variations, rather than variations in micrometeoroid or solar wind fluxes.

After its formation, Jupiter was substantially more luminous than it is at the present day (10⁴ times). Right after formation, the inner satellites Io and Europa would have received an order-of-magnitude larger incoming flux from Jupiter, than solar insolation. However, little work has been done to examine the impact this would have had on the Galilean satellites. In this work we explore the possibility that Jupiter’s early luminosity could have caused the loss of any initial water inventory on Io and Europa. We find that, depending on Jupiter’s luminosity after the proto-Jovian disk dissipates, heating from Jupiter could drive rapid water loss via hydrodynamic escape. If early tidal heating dehydrated Io’s accreted silicates, this may provide a mechanism to lose Io’s initial water inventory.

Gullies on Mars resemble water-carved channels on Earth, but they are mostly at elevations where liquid water is not expected under current climate conditions. It has been suggested that sublimation of carbon dioxide ice alone could have formed Martian gullies. We used a general circulation model to show that the highest-elevation Martian gullies coincide with the boundary of terrain that experienced pressures above the triple point of water when Mars’ rotational axis tilt reached 35°. Those conditions have occurred repeatedly over the past several million years, most recently ~630,000 years ago. Surface water ice, if present at these locations, could have melted when temperatures rose >273 kelvin. We propose a dual gully formation scenario that is driven by melting of water ice followed by carbon dioxide ice sublimation.

Impact craters are the dominant features on the Moon, and their degradation with time is the most common geological process on this body. This work is aimed at detailed quantitative characterization of this process. We quantitatively characterize crater materials by (1) the topographic roughness calculated from Lunar Orbiter Laser Altimeter and (2) rock abundances and nighttime soil temperatures derived from Diviner measurements. In contrast to preceding works, we separately consider crater material subunits: central peaks, floors, walls, and continuous ejecta to study their degradation in detail. We mapped totally 4,770 individual crater material subunits. All subunits of the youngest craters are characterized by increased roughness, rock abundance, and soil temperature. These parameters decrease with age and tend toward equilibrium, stable state; the continuous ejecta reach equilibrium more rapidly than other subunits. Kilometer‐baseline roughness of crater walls in the lunar maria is higher than in the highlands. Rock abundances and soil temperatures of the walls and floors of simple craters in the maria are also higher than in the highlands. We attribute these trends to differences in the mechanical properties of the target materials. Although the properties studied are not exact proxies for age, they can be used to assess individual age predictions; for example, several craters that were erroneously classified as Copernican have been detected. We also found that the unusual thermophysical signature of the walls of the Late Imbrian crater Bonpland D is due to recent regolith flows that could have been caused by a strong, shallow seismic event.

The roughness properties of impact craters are valuable indicators of crater degradation and can provide insight into crater ages. We evaluate the roughness of lunar craters from different geologic eras, confirming that young, Copernican craters are distinctly rougher than older craters. We evaluate the potential age of small (less than ~15 km) craters that are thought to host surface ice by quantifying the roughness inside these craters, as well as outside. Interior roughness may be subdued by slope processes or the presence of volatiles. The distribution of ice‐bearing craters is skewed toward roughness values higher than those of pre‐Imbrian craters, although no ice‐bearing craters are within the Copernican‐only domain in roughness space. All of the 15 rough, permanently shadowed craters that are found within the Copernican‐only domain lack water‐ice detections, suggesting that either ice has not been delivered to these young craters or that it has since been destroyed.