High-reflectance ejecta and low-reflectance impact melt streamers surround this fresh impact crater. NAC M189994606LR, image width 2.25 km [NASA/GSFC/Arizona State University].

High-reflectance ejecta blankets the terrain surrounding a ~650 m diameter crater (26.525°N, 93.518°E). From samples collected during the Apollo missions we know that high-reflectance ejecta represents recently exposed material that has not yet been affected by space weathering processes (maturity rays) or material exposed that is a different composition than the surrounding area (compositional rays). The impact crater in the opening image formed near the base of the central peak of Joliot crater (~173 km diameter, 25.79°N, 93.39°E), the floor of which was partially flooded with volcanic material. What type of ejecta rays are observed in today's Featured Image - compositional or maturity?

LROC WAC monochrome mosaic of Joliot crater, with a fresh impact (arrow) at the base of the central peak [NASA/GSFC/Arizona State University].

The crater formed on the base of the central peak which is likely highlands material. The rays extend outward more than two crater diameters onto the mare material. Thus we have an example that is both a maturity ray and compositional ray. Over time as the ejecta matures, the portion on the highlands material will be indistinguishable, while the portion on the mare will still be visible. The much larger crater Tycho (93 km diameter) shows the same combination maturity-compositional rays.

Explore the full LROC NAC image for yourself; do you see evidence for impact melt and if so, what do you see (ponds, streamers, flows)?

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Ejecta Starburst
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Ejecta Sweeps The Surface
Minty Fresh

Published by Lillian Ostrach on 18 June 2013