THE DISTRIBUTION OF SURFACE ROUGHNESS AROUND COMPLEX IMPACT CRATERS ON MERCURY

Surface roughness is a quantitative measure of the change in topography over a given distance and can provide clues as to what geological process dominates a given scale of topography. Previous studies of Mercury have found that impact cratering dominates surface roughness from a few hundred meters to kilometer-scale baselines. This study compares surface roughness over a broad range of baselines within individual fresh complex craters and their deposits to understand how the impact cratering process, with its variations on Mercury, is reflected in surface roughness measurements. Such an assessment can provide evidence for the processes (e.g., target properties, impact velocity and angle) that cause craters on Mercury to differ from one another.

The surface roughness of Mercury is measured from topographic data obtained by the Mercury Laser Altimeter (MLA) on the MErcury, Surface Space, ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. Roughness from a few hundred meters to kilometer-scales is determined from separate MLA footprints within MLA tracks, while the spread in the MLA pulse-width at each return reflects both meter-scale roughness and local slope. These large- and small-scale roughness measurements cannot be compared quantitatively because they were derived by different means; however, qualitative comparisons are useful in interpreting the morphology of these craters.

Typical complex impact craters and their deposits have increased long-scale surface roughness than average roughness on the crater rim, near-field ejecta, and secondary fields. Small-scale surface roughness is highest on the crater rim and ejecta, but declines to background roughness values closer to the crater rim compared to long-scale roughness.

Variations in the long- and meter-scale surface roughness exist between craters. For example, ejecta of the crater Hokusai (95 km in diameter) have lower roughness values than the similarly sized adjacent crater Abedin (110 km in diameter). The higher-roughness values associated with the ejecta ends closer to the crater than a more typical crater such as Abedin. This may be due to extensive impact melt around Hokusai. By comparing the surface roughness with imaging and other datasets, permits the evaluation of hypothesis into the origin of unusual craters on Mercury.