Bright and Dark Ejecta

A relatively recent impact event distributed ejecta across the lunar surface. Smaller craters punch through the ejecta to reveal a darker substrate. LROC NAC M139782204LE, image width is 500 m [NASA/GSFC/Arizona State University].

Fresh (young) impacts on the Moon often display magnificent ejecta blankets (so called because they "blanket" the surrounding terrain). Ejecta is unevenly distributed, which gives rise to its interfingered appearance. Since space weathering tends to lower the albedo of material on an airless planet, the relative brightness of this ejecta blanket speaks to the young age of the parent crater.

Context image of today's Featured Image, located at 4.408 N, 19.230 E. A nearby linear depression may have provided the darker substrate. Image width is 100 km [NASA/GSFC/Arizona State University].

In this case, the parent crater is just to the south of the opening image, and can be seen in the context image. But what is providing the small circular patches of dark material? Were the patches formed as part of the impact that formed the ejecta blanket, or later? Was the material excavated from below the bright ejecta? Most likely secondary craters (late stage ejecta) from the initial impact, hit and dug up dark mare material (original surface) from below the thin ejecta blanket. Can we test this idea? How dark is dark? In a more precise sense - do the albedos of the small low reflectance spots match that of the surrounding mare? Your eye could be fooled by all the changes in reflectance. The small dark patches have a higher albedo than the mare (0.07 vs 0.06), which would be consistent with mare material mixing with the brighter (0.09 to 0.11) ejecta blanket. This observation is consistent with the secondary crater interpretation (the underlying mare is mixed with a small amount of the bright immature ejecta). If the reflectance of the dark patches was lower than that of the mare, then something else would have to be at work.

Can you think of other explanations while browsing the full LROC NAC frame?

Related Posts:

Beautiful Ejecta Patterns

DMD Excavations

Symmetric Ejecta

Published by Drew Enns on 10 September 2013