Tuesday, June 7, 2011

Dark-haloed crater in Mare Humorum

Dark- and light-toned lunar regolith contrasts sharply in this striking image of a dark-haloed crater in Mare Humorum (24.8°S, 315.0°E).LROC Narrow Angle Camera (NAC) observation M140203430R; solar incidence angle 24° LRO orbit 5795, September 27, 2010; field of view is around 180 meters across. See the full-size LROC Featured Image HERE [NASA/GSFC/Arizona State University].

James Ashley
LROC News System

Features such as this "dark-haloed" crater are not common on the Moon, but where found there tend to be occurrences of both mature volcanic deposits, and fresher (more recent) impact ejecta deposits. This circumstance provides clues for solving the light/dark mystery in a straightforward manner. For example, this crater is located to the north of Liebig J, a relatively young, bright-rayed crater in Mare Humorum.

The Featured Image impact clearly occurred within the Liebig J ejecta blanket, which is less mature and therefore of higher albedo than the surrounding dark mare rock. When the small impact took place, it penetrated the Liebig J ejecta and excavated the darker material from beneath.

Note that some of the material within the crater wall is actually brighter than surrounding material. This is not too unusual with fresh craters. But why was the dark ejecta not distributed in a perfectly even and symmetrical way? It would probably require the collection of samples and field mapping on the ground to answer this completely. But part of the story may be due to textural heterogeneity (clumpiness) of the Liebig J ejecta deposit in this area. When the dark-halo impact occurred, such clumpiness may have caused the impact energy to disperse in an uneven way. Note also that more recent, smaller craters have punctured through the dark ray pattern to re-expose the brighter deposits beneath. The end result is a complex local stratigraphy, but one which can be unraveled through a careful study of the effects of impacts, their energies, and locations. The context image below shows the albedo difference between the Liebig J ejecta deposits and the surrounding mare deposits.

A portion of the global WAC mosaic showing the Liebig J crater region of western Mare Humorum and its bright ejecta [NASA/GSFC/Arizona State University].

Explore the full NAC image HERE; notice the ejecta of the prominent Liebig J crater. How many similar dark-haloed features can you find?

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