Wednesday, October 21, 2009

Illumination Comparison of a Mare Crater

These two images, subsets of M104670019L (incidence angle 50.2°) and M107035386L (incidence angle 24.6°) illustrate the effects of illumination angle on recognition of different aspects of the lunar surface. Each scene is about 550 meters wide and 1.1 km tall.

On the left, the morphology of this small mare crater (200 m diameter) is clear, whereas in the image on the right differences in brightness (reflectance or albedo) are highlighted defining ejecta patterns. Explorers might prefer the low-Sun image for landing or planning traverses, while the high-Sun image guides the eye to ejecta. One of the strengths of the LRO mission is that it passes over the same region numerous times, and each time the Sun angle is different allowing LROC to build up an image set over a wide range of angles.

Same crater from two LROC NAC frames under very different lighting. On the left the Sun was low to the horizon and on the right the Sun was high, approaching noon. Under low sun surface roughness is quite evident; under high sun (right) variations in brightness (reflectance) dominate the scene [NASA/GSFC/Arizona State University].

This crater also illustrates the influence of pre-existing regolith (soil) structure on crater morphology. When the regolith is relatively thin, over more-or-less intact bedrock, small craters often do not have a simple bowl shape. This crater is filled with rubble and there are numerous blocks up to 10 m in diameter scattered around the rim and the interior, indicating that bedrock was excavated during the impact. In cases where impacts occur in a thick regolith, the crater does not excavate blocky rocks but rather fine-grained soil, resulting a nicely defined bowl shape.

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