Dark Ejecta

Southwestern rim of crater Copernicus H. NAC M186005514L, image width is 930 m, incidence angle is 10° [NASA/GSFC/Arizona State University].

Copernicus H (about 4.4 km in diameter, latitude 6.88°N, longitude 341.71°E) , a satellite crater of Copernicus crater, is located 56 km southeast of Copernicus (93 km diameter crater located in eastern Oceanus Procellarum). The opening image highlights a part of the Copernicus H crater rim (see blue square in the next image). The upper right portion of the image with bright/dark streaks corresponds to the crater wall, and the bottom left low reflectance part is the surrounding flat area. The low reflectance materials extend from the rim down the slope into the crater. What is the origin of these dark materials? Is the material mature soil? Impact melts? Pyroclastic deposits?

Zoomed out view of NAC M186005514L. Image width is about 5.7 km. Blue box indicates the location of today's Featured Image [NASA/GSFC/Arizona Steta University].

As seen in the subsampled NAC image (above) and WAC context image (below), the ejecta blanket of Copernicus H shows relatively lower reflectance than the surrounding area. Normally crater ejecta has a higher reflectance because the newly-exposed material is "immature" in a space-weathered sense. Notice that the crater floor is largely filled by lower reflectance impact melt, and the high reflectance materials extend radially from the floor to the crater rim. These brighter materials are the debris on the slope of the crater wall that are mass-wasting gravitationally toward the crater center.

Looking at the WAC context image below, it is likely that Copernicus H excavated some low reflectance materials underlying the ejecta of Copernicus. Several craters located at least 45 km away from the Copernicus rim also show similar dark ejecta. Therefore, the underlying dark material must be widely distributed. What might these materials be? Pyroclastic deposits at Southern Sinus Aestuum are located at about 90 km southeast of Copernicus H, and are underlying the Copernicus ejecta. If this layer is extensively distributed under the Copernicus ejecta, subsequent impacts with sufficient energy could have excavated the dark materials. 

To unravel these complicated stratigraphic relations from orbit, accurate spectral data and an understanding of the effects of space weathering are necessary. New measurements from LRO, Selene, and Chandrayaan are providing lunar scientists with the needed information!

Copernicus H and surrounding area in WAC monochrome mosaic (100 m/pix). Image center is 6.88°N, 341.73°E. The blue box indicates the location of full NAC frame [NASA/GSFC/Arizona State University].

Explore this dark ejecta in full LROC NAC image yourself! 

Related Posts:

Dark Craters on a Bright Ejecta Blanket

Dark halo crater

Just Another Crater?

Dark Impact Melt Sheet

A Beautiful Impact

Pyroclastic Excavation


Posted by Hiroyuki Sato on September 04, 2012 10:55.