Ejecta Starburst

High-reflectance ejecta created a starburst pattern originating from an unnamed ~270 m diameter crater. NAC M159059694R, image width 855 m [NASA/GSFC/Arizona State University].

Small, Copernican-aged craters abound on the Moon and their ejecta blankets often look like miniature starbursts. For young craters like this one, located on the farside at 25.876°S, 136.081°E, the ejecta is high-reflectance because it was recently exposed by the impact process and is thus really fresh material

Taking a small step back, the resampled NAC image provides context for the remainder of the ejecta blanket surrounding this beautiful fresh crater. In fact, this step back allows us to see that the ejecta blanket is more expansive on the eastern side of the crater, perhaps because the impact angle was slightly oblique. NAC M159059694R resampled to 2m/pixel, image width is ~2.6 km [NASA/GSFC/Arizona State University].

When we observe the ejecta blanket in detail, there are variations in reflectance within the ejecta and it looks as though the ejecta swept out from the crater in sheets. During the impact event, material is ejected from the growing crater and is emplaced over a short period of time. However, the emplacement is not instantaneous and the ejecta is expelled from the growing crater at different speeds and angles depending on where within the impact cavity it originates. Some of the target rock is melted and is also sprayed out of the crater with the pulverized target material. The bright and dark fingers of ejecta seen in the opening image may represent granular and melt materials, respectively. Furthermore, the ejecta farthest away from the crater is thinner and less continuous than the ejecta closest to the crater. At the distal margins of the ejecta blanket, contrasts may simply be due to original mature material showing through between fingers of ejected fresh material. Over geologic time, the starburst pattern of ejecta will gradually disappear as the material matures, and eventually no ejecta blanket will be visible in the NAC images at all.

Because of its small size, the fresh crater in the images above is barely resolvable in the LROC WAC monochrome mosaic. The fresh crater is < 300 m in diameter and thus is not even 3 pixels across in the 100 m/pixel mosaic. Good thing we have the high resolution NAC image to observe the spectacular detail! Asterisk notes location of crater [NASA/GSFC/Arizona State University].

Within the full LROC NAC image, how far can you trace the streamers of high-reflectance ejecta?

Related Posts: Action Shot

Slice of Mare

Small crater at the southern rim of Menelaus

Ejecta Blanket

Farside impact!

Intricate young ejecta blanket in ancient Murchison Crater

Published by Lillian Ostrach on 28 March 2012