Dark halo crater

A small dark halo crater on the ejecta of Censorinus A crater. Image scale is 0.5 m/pixel, image width is 500m, incidence angle 45°, sunlight is from right side [NASA/GSFC/Arizona State University].

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 Censorinus A crater. Image width is about 1.2 km. While box indicates the area of today's Featured Image [NASA/GSFC/Arizona State University].

This tiny crater is located near near Mare Tranquillitatis, 200 m north 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.

LROC WAC 100m/pixel mosaic near Censorinus crater. Image center is latitude 0.41°S, longitude 32.97°E. Blue box corresponds to NAC full frame footprint [NASA/GSFC/Arizona State University].

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

Related posts:
Dark-haloed crater in Mare Humorum, Action Shot, Splendors of Mare SmythiiRima Bode: Constellation region of interestDark Craters on a Bright Ejecta Blanket


Posted by Hiroyuki Sato on July 28, 2011 12:21.