Thursday, July 28, 2011

Dark-haloed crater on ejecta blanket

A small dark halo crater on the ejecta of Censorinus A. Image scale is 50 centimeters per pixel, image field of view is 200 meters, solar illumination incidence 45°, from the east. LROC Narrow Angle Camera (NAC) observation M144409490L, LRO orbit 6415, November 14, 2010. See the full-size LROC Featured Image release HERE [NASA/GSFC/Arizona State University].

Hiroyuki Sato
LROC News System

Fresh impact crater ejecta is generally brighter than the surroundings because it is "immature". As material rests on the surface of the Moon it is slowly altered by solar wind sputtering and micrometeorite bombardment that results in a general darkening and shift in color.

This change in reflectance properties is known as space weathering, or maturation. Impacts excavate materials from beneath the mature surface resulting high reflectance rays. If you are very patient, and waited around for 500 million years you could observe rays slowing fade as they mature. But the small crater (25 meter diameter) in today's Featured Image displays dark ejecta deposits, so what is happening here?

Ejecta blanket at north of the rim (lower left) of Censorinus A. field of view 2.3 km, nearly the full width of LROC NAC M1144409490L. The white rectangle indicates the area of within the LROC Featured Image, July 28, 2011. [NASA/GSFC/Arizona State University].

This tiny crater is located near near Mare Tranquillitatis, 3 km north of the rim of Censorinus A crater. The surrounding area is covered by ejecta coming from Censorinus, which is about 12 km distant (the surface streaks point back to the crater). The dark and bright areas are intermixed (see middle image), and the tiny dark ejecta crater is located on the brighter portion of the ejecta. The brighter ray materials are likely on the top of darker materials, which are perhaps simply mature soils or maybe impact melt from the earlier Censorinus impact event. So when the small crater was formed, it excavated dark material from beneath a bright ray.

Censorinus (lower left) and Censorinus A (lower right) in a simulated oblique image stitched from two LROC Wide Angle Camera (WAC) monochrome (643nm) observations swept up in consecutive orbital passes (orbits 2763 & 64) on February 1, 2010 [NASA/GSFC/Arizona State University].

Explore dark ejecta deposits and surrounding areas in the full NAC frame!

Related posts:
Dark-haloed crater in Mare Humorum
Action Shot
Splendors of Mare Smythii
Rima Bode: Constellation ROI
Dark Craters on a Bright Ejecta Blanket

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