The Rise and Fall of Lunar Topography

Abstract

Landscape evolution on the Moon is driven by a relatively small number of physical mechanisms, making it an ideal laboratory for studying how the surfaces of airless planetary bodies change over time. By investigating the processes that shape topography, predominantly impact cratering, we can better interpret the present-day lunar surface. Decades of remote sensing observations have vastly improved our understanding of lunar topography and constrained the present-day crater formation rate. Coupled with high-resolution remote sensing datasets, numerical models offer a powerful tool for investigating the drivers of lunar geomorphology. In this thesis, I study the evolution of small-scale topography on the Moon across three main areas of focus. I first develop a general-purpose landscape evolution model for airless bodies that is calibrated by tuning the rate of diffusive degradation to match topographic roughness statistics on the maria. Using this model, I simulate the horizontal and vertical mixing of lunar regolith to constrain the timescales of surface exposure during which soil grains are exposed to space weathering effects. Over 3.5 Gyr of surface evolution, grains spend relatively little time at the uppermost surface due to rapid gardening by small impacts, with 98% of regolith tracer particles spending less than 20 Myr within a millimeter of the surface. By mapping the distribution of regolith exposure ages onto existing soil maturity measurements, I find that regolith grains reach a state of chemical maturity after 7-19 Myr of cumulative surface exposure. The same processes that excavate, transport, and bury regolith are also responsible for the erosion of lunar surface features and are therefore crucial for interpreting their ages. I next model the degradation of kilometer-scale craters and quantify the rate of topographic diffusion from small impacts to assess whether micrometeorite gardening has been the dominant erosional mechanism on the lunar surface over the last few billion years. Under commonly used lunar crater production functions, the erosion rate from small impacts is approximately 200 times lower than the value inferred from elevation profiles of degraded kilometer-scale craters on the maria. However, the abundance of fresh craters detected over the last decade is consistent with small impacts dominating the erosion of these features, but only if that abundance continues down to the sub-millimeter scale. My results also demonstrate that, regardless of the magnitude of diffusivity, mass transport from small impacts is fundamentally a nonlinear diffusion process and so are a revision to canonical lunar erosion models. Finally, the landforms resulting from impact processes, i.e., craters, can serve as reservoirs for thermally unstable species that record the delivery of volatiles to the inner solar system. Because of the Moon's low obliquity, small topographic depressions on the floors of large circumpolar craters can be doubly shadowed, shielded from both direct solar illumination and scattered light from nearby sunlit terrain. These locations are among the coldest in the solar system and could hold clues to the origins of the Moon's most volatile deposits. With illumination models applied to high-resolution digital terrain models, I derive the first map of double shadows at the lunar poles. At 30 m/pxl resolution, the total doubly shadowed surface area is 1.47 km$^2$ in the north and 5.37 km$^2$ in the south (~0.04% of singly shadowed area poleward of 85$^{\circ}$ latitude). The largest double shadows, nearly 600 meters across, could potentially be resolved with orbital temperature and reflectance measurements and are high-priority targets for future in situ exploration.