Analyzing the Evolution of Surface Roughness of Lunar

Introduction: About 17% of the lunar surface is covered by mare basalts [1], mainly concentrated on the lunar nearside. The main period of mare volcanism occurred during the late Imbrian Period, ~3.3-3.8 Ga [2], although volcanism continued to a lesser extent until ~1.2 Ga [3]. These dates were assigned by Hiesinger et al. using crater size-frequency distribution measurements applied to pre-defined mare units (Figure 1) [2, 3, 4]. These mare units were defined to be spectrally homogeneous according to Clementine multispectral data and represent a single eruptive phase [4]. After eruption, the constant bombardment by meteorites and radiation on the lunar surface leads to the buildup of regolith over time [5]. The regolith is understood to be between 2 and ~10 m thick on the mare [6], with a heterogeneity of particle sizes ranging from meter-sized rocks to submicron particles (most regolith material is under 1 mm in size) [5]. Regolith thickness increases with age as a surface is exposed to the space environment. The rate of regolith accumulation is thought to be ~2 m/Ga [5]. We sought to determine the relationship between surface roughness and age of lunar mare, in relation to regolith development. To do this, we compared surface roughness measurements using circular polarization ratio (CPR) values at both Sand P-band wavelengths (12.6 and 70 cm wavelengths, respectively), and various surface characteristics from Lunar Orbiter Laser Altimeter (LOLA) measurements. The comparison of these data sets allows us to evaluate roughness on a range of length scales. Methods: Based on the mare age calculations of Hiesinger et al. [2, 3, 4], ~6 units were chosen in each of three age categories: Young (~1.2-1.7 Ga), Middle (2.4-2.6 Ga), and Old (3.7-3.9 Ga) (Figure 1). Each unit was analyzed using a 6 x 6 km analysis box. A full description of the methods used in this project to determine roughness is provided in a companion abstract [7] and only a brief summary will be given here. LOLA Data. LOLA’s five spot configuration [8] allows for multiple measurements of the lunar surface to be grouped together and analyzed cohesively.