Thursday, October 31, 2013

Dark ejecta, clue to cryptomare

A 78 meter diameter crater with distinct dark ejecta surrounding its rim, in the farside southern  highlands (31.131°S, 147.536°E) north of Jules Verne. LROC Narrow Angle Camera (NAC) frame M1115555142L, LRO orbit 16629, February 16, 2013; 40.36° angle of incidence, 74 cm per pixel resolution from 71.92 km [NASA/GSFC/Arizona State University].
Jeffrey Plescia
LROC News System

Impact crater ejecta is typically brighter than the surrounding material because it is fine-grained and immature (unweathered); even on the dark mare, fresh craters usually have bright ejecta. Craters with distinctly dark ejecta do occur, but they are rare (e.g., Shorty Crater at the Apollo 17 landing site). When craters have dark ejecta, the interpretation is that a layer of low reflectance rock or soil at depth was excavated and distributed around the margin of the crater. In the case of Shorty Crater, the conclusion is that a layer of dark pyroclastics was hidden just beneath the surface; in other cases, dark halo craters are interpreted to indicate mare material at depth (cryptomare). Several of the craters formed by the impact of spacecraft hardware, such as the Apollo 13 S-IVB, into the surface also have dark ejecta rays, and this observation is not yet well understood.

Details of the crater morphology can be seen in this expanded view from LROC NAC M1115555142L [NASA/GSFC/Arizona State University].
Our dark halo crater (31.131°S, 147.536°E) has a diameter of about 78 m, although it is slightly elongate in the north-south direction. Rays of dark ejecta extend for almost 200 m from the crater rim. Morphologically, the crater is not the normal simple bowl shape for a crater of this size. Rather, a depression on the crater floor in the center is surrounded by a low ridge about 33 m in diameter; beyond that annular ridge to the crater wall the floor appears to be flat. Boulders are scattered on the crater floor and on the ejecta to the east; the largest boulders on the east side are as large as 6 m across.
Regional view of the dark ejecta crater. Note the crater formed on the western flank of an older, larger crater. Relatively low albedo, smooth plains spread out immediately to the south. These plains (see next image) may be mare material and may underlie the area of the small impact. LRO NAC frame M167241339R (spacecraft orbit 9780, August 6, 2011; 55.96° angle of incidence, 65 cm per pixel resolution from 62.95 km)  [NASA/GSFC/Arizona State University].
LROC Wide Angle Camera (GLD100) context, at 64 meters per pixel resolution, shows the location of the crater of interest in relation to the arc of exposed mare material to the south and west [NASA/GSFC/Arizona State University].
This crater formed on the outer flank of a larger older, degraded 635 m crater. Highlands ejecta (higher reflectance than mare) from this larger crater buried the mare. Later the impact that formed the younger dark halo crater punched through the bright highland ejecta and brought up mare from 10 meters or more depth.

Explore this region of the Moon in the full NAC image, HERE.

Related Posts:
Bright and Dark Ejecta (September 11, 2013)
Excavating Dark Deposits (September 3, 2013)
Dark haloed crater in Mare Humorum (June 7, 2011)
Dark Halo Crater in Orientale (March 4, 2011)
Dark craters on a bright ejecta blanket (November 17, 2009)

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