VISIBLE ROCKS VS. HIDDEN 'ROCKINESS' - COMPARING THE LROC NAC AND DIVINER OBSERVATIONS AT FRACTURED MELT VENEERS OF COPERNICAN CRATERS

Continuous impact melt sheets are not particularly ‘rocky’ in thermal inertia maps, and lack of elevated night-time temperatures over impact melts was known well before the LRO mission from Earth based observations. Overlapping observations between the NAC and Diviner instruments show the correlation between the areal rock fraction derived using Diviner observations and density of blocks (>1 meter diameter). Typically, impact melt was deposited over the proximal continuous ejecta, and the areal rock fraction is low. Excavated blocks are generally visible within impact melt deposits where the areal rock fraction is high. However, we find that this qualitative correlation is not consistent at the impact melt veneers outside the crater rims. It is expected that impact melts are associated with low areal rock fraction but adjacent or nearly adjacent sample areas with impact melt can be shown to have both low and high areal rock fractions. In this work, we explore and discuss possible causes and implications of such observations in the context of large (D > 10 km) Copernican craters.

Impact melt veneers can be found on the floor, wall, rim, and proximal ejecta of Copernican craters. After formation, impact melt flows outside the crater rim forming veneers near the rim and often, over the already emplaced proximal ejecta. As the melt cools and solidifies, change in melt volume and shape of the crater during formation causes fractures in the melt to develop. Many of these fractures are visible in NAC images and comparing with Diviner derived rock fractions we find that high areal rock fractions are often spatially associated with these fractured impact melts, while the density of visible rocks may be quite low. Following crater formation, thermal stresses, isostatic rebound and subsequent impacts could have caused these fractures to become more extensive, leading to the formation of tightly clustered blocks. Meteoritic bombardment and seismic activity can further dislodge blocks, widening the intra-cluster gaps. The Diviner areal rock fraction appears to distinguish between fractured and relatively smooth impact melt, from our initial analysis. Fractured sections of impact melt veneers show similar response as other areas on the continuous ejecta with higher block populations. Further, pervasive presence of rocks close to the crater rim is considered a signature of recent impact, but a similar inference based on areal rock fraction is complicated since melt veneer fracturing due to seismic or meteoritic impacts occurred subsequent to the impact.