South Pole-Aitken Basin (SPA) Units Delineated by Measures of Surface Roughness: Implications for the History and Evolution of the Basin as Seen by Data from the Lunar Reconnaissance Orbiter (LRO)

Abstract:

The interior of SPA contains a range of morphologic units, from smooth plains and mare basalts to rough, ancient, terrains. Recent data, particularly from LRO provide unique measures of SPA surface properties. With each new dataset, the differences between the interior of SPA and its surroundings become more, or in some cases less, clearly defined. Here we explore recent datasets that offer insight into surface roughness at a variety of scales and assess implications for the origins of units across SPA. Identifying the origin of units in SPA is critical for identifying future sampling sites that address the science goal of determining the age of SPA.

The unique interior of SPA relative to the rest of the Moon is demonstrated by Mini-RF and LOLA derived products. Mini-RF data shows that the interior of SPA has a slightly higher average Circular Polarization Ratio than nearly any other terrain on the Moon, with the exception of the interior of the Orientale Basin. Cahill et al. [2014,Icarus] note that the average interior CPR value of SPA is similar but slightly higher than the mid-latitude farside highlands, suggesting that both are enhanced in blocks at the surface and near subsurface (to depths <2m). However, SPA is distinct relative to the interior of Orientale in the m-chi deconvolution, suggesting that the interior has a regolith with scatterers smaller than the wavelength of Mini-RF (12.6cm).

LOLA-derived roughness shows that the interior of SPA is smoother than the surrounding highlands [Rosenburg et al.,2011,JGR; Kreslavsky et al., 2013,Icarus]. LOLA roughness measures show roughness variations within the basin tied to volcanism. However the expanse of smooth areas is broader than the previously mapped volcanic units, suggesting that ancient volcanism may be contributing to interior smoothness.

The distinction between datasets is likely due to differences in spatial resolution and that Mini-RF data probes the near subsurface. These measurements of roughness offer insight into the origins of units across SPA, including the possible presence of extensive ancient volcanic deposits. Regional variations in LOLA-derived roughness, Mini-RF CPR, and other high resolution measures of surface roughness within SPA will be used to infer, delineate morphologic terrains, and distinguish volcanic and impact-generated units.