Distribution of Surface Slopes and Roughness in Mercury's Northern Hemisphere

Since the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft (MESSENGER) was inserted into orbit about Mercury in March 2011, the Mercury Laser Altimeter (MLA) has been measuring the topography of the planet's northern hemisphere with high accuracy. MLA elevation data can be used to characterize slopes and roughness of the surface over a range of baselines. In our study the median absolute value of differential slope at a given baseline is adopted as a measure of surface roughness. Baseline lengths from 0.7 to 41.6 km are used, and the roughness is determined at scales of 0.7, 2.8, and 11.2 km to produce roughness maps of Mercury's northern hemisphere. From the resulting maps it is possible to distinguish clearly at least two broad geologic units on the basis of roughness characteristics: the northern smooth plains and heavily cratered terrain. The roughness values of the two terrains are compared at all baselines as an aid in the interpretation of the geological processes that formed and modified them. Over the range of baseline lengths considered, the northern smooth plains have significantly lower roughness( at least 0.2 degree less than for heavily cratered terrain). The observed lower roughnessat the shortest baseline is consistent with a volcanic origin for the northern plains, which have been interpreted on the basis of geological and elemental remote sensing observations as products of flood lavas of high temperature and low viscosity. The lower roughness values at intermediate baselines reflect the relatively younger age and lower overall density of impact craters on the plains unit compared with heavily cratered terrain.