The surface roughness of Mercury from the Mercury Laser Altimeter: Investigating the effects of volcanism, tectonism, and impact cratering

Surface roughness is a statistical measure of change in surface height over a given spatial horizontal scale after the effect of broad-scale slope has been removed and can be used to understand how geologic processes produce and modify a planet's topographic character at different scales. The statistical measure of surface roughness employed in this study of Mercury was the root-mean-square deviation and was calculated from 45 to 90°N at horizontal baselines of 0.5–250 km with detrended topographic data from individual Mercury Laser Altimeter tracks. As seen in previous studies, the surface roughness of Mercury has a bimodal spatial distribution, with the cratered terrain (dominated by the intercrater plains) possessing higher-surface roughness than the smooth plains. The measured surface roughness for both geologic units is controlled by a trade-off between impact craters generating higher-surface roughness values and flood-mode volcanism decreasing surface roughness. The topography of the two terrain types has self-affine-like behavior at baselines from 0.5 to 1.5 km; the smooth plains collectively have a Hurst exponent of 0.88 ± 0.01, whereas the cratered terrains have a Hurst exponent of 0.95 ± 0.01. Subtle variations in the surface roughness of the smooth plains can be attributed to differences in regional differences in the spatial density of tectonic landforms. The northern rise, a 1000 km wide region of elevated topography centered at 65°N, 40°E, is not distinguishable in surface roughness measurements over baselines of 0.5–250 km.