A Thicker Regolith on Mercury

The scale-dependence of roughness contrasts, and the morphology of small craters in smooth plains suggest that regolith on Mercury is thicker than on the Moon. Introduction: The surface of the Moon is covered with a layer of regolith, fragmental, highly heterogeneous material. Formation, modification and transport of the regolith occur due to meteoritic and micrometeoritic impacts and a number of other processes [1]. There is no doubt that a similar regolith layer exists on Mercury; however, observations about its nature and thickness have been limited by the absence of highresolution information. Here we analyze data obtained by the MESSENGER mission to Mercury and outline two lines of evidence suggesting that the regolith on Mercury is significantly thicker than on the Moon. Scale dependence of roughness contrasts: Highprecision data from orbital laser altimeters have been used to map kilometerand sub-kilometer-scale topographic roughness of the entire Moon [2] and for the northern circumpolar area of Mercury [3] where MLA data are dense and of highest quality. The primary feature of kilometer-scale roughness maps for both bodies is the dichotomy between topographically smooth volcanic plains (maria on the Moon and smooth plains [4,5] on Mercury), and topographically rough terrains (highlands on the Moon and cratered and intercrater plains [6] on Mercury). The roughness contrast between these smooth and rough terrains is higher at larger scales (longer baselines) and lower at smaller scales (shorter baselines) (Fig. 1). The observed decrease of the contrast at sub-kilometer baselines has been explained [2,3] by the fact that topography at these scales is primarily controlled by regolith formation and transport and is thus less sensitive to bedrock geology; the topographic roughness is defined by equilibration of roughening due to formation of subkilometer-size craters and smoothing due to regolith gardening. Fig. 1 shows that this roughness contrast decrease on Mercury occurs at longer baselines than on the Moon. This suggests that on Mercury the equilibrium between cratering and regolith gardening is reached at scales a factor of 3 longer. This in turn suggests more intensive regolith gardening and/or a thicker regolith on Mercury; we test this hypothesis by examining small fresh regolith impact craters. Morphology of fresh small craters: Many fresh small (10 to 100 m) craters on the Moon, mostly on maria, have specific morphologies characteristic of impacts into a layered target with a weaker (regolith) layer on top of stronger (bedrock) material [7]. These morphologies include central-mound, flatand hummocky-bottom, and concentric double-ring craters (Fig. 2); they can be used to estimate thickness of the regolith [8]. Thickness of lunar regolith measured with this method [9] varies from 2 5 m for maria to 5 8 m for highlands.