Saturday, April 16, 2011

Bouldery crater near Mare Australe

Drew Enns of the Lunar Reconnaissance Orbiter Camera (LROC) team at Arizona State University presented twin postings, this past week, discussing features in the vicinity of Mare Australe, a less well-known ancient impact basin on the nearside's eastern limb and almost beyond line of sight view from Earth. LRO, closing in on completing its second year in polar orbit (8,370 by April 15, 2011), has, of course, not been confined to any one plane in relation to the Moon and continues filling in the gaps and furthering our understanding of the entire lunar surface. Here, utilizing an ever-more resource-rich web-based map index of LROC's already massive library of available images, is a "lower" resolution context view showing the locations of both of this week's LROC Featured Images. (The mare-filled crater Brisbane Z is approximately 70 kilometers wide) [NASA/GSFC/Arizona State University].

Low Sun image of a fresh crater in the lunar highlands near Mare Australe. Boulders are scattered in and around the 550 meter-wide impact crater, image field of view is 950 meter; LROC Narrow Angle Camera (NAC) observation M150062296R, LRO orbit 7248, January 19, 2011. See the full-sized Featured Image HERE [NASA/GSFC/Arizona State University].

Drew Enns
LROC News System

Small craters on the Moon come in a variety of morphologies, and knowing how they differ is key to understanding their formation. This crater (51.42°S, 68.73°E) is surrounded with a high density of boulders. Was this crater formed by a primary or secondary impact? When a bolide (anything that impacts a planet, usually an asteroid or comet) impacts a more coherent material (solid rock for example), the ejecta contains a high percentage of boulders. So the boulders seen above could indicate a stronger subsurface.

While primary bolides typically impact the Moon at speeds of 15-20 km/s, secondary impacts occur at much slower speeds. Any ejecta traveling faster than the escape velocity (2.38 km/s) will not impact the Moon. Because of the lower speed, ejecta impacts the Moon with less energy, which could lead to a smaller and more bouldery crater if the ejecta itself was a large piece of solid rock that shattered upon impact. In either case, these boulders would make excellent targets for sample return so that scientists can better understand how to definitively tell the difference between primary and secondary craters.

Backing away nearly to the full ~ 4.8 kilometer width of LROC NAC observation M150062296, January 19, 2011), the extent of the brighter and less weathered crater's ray material can be better appreciated [NASA/GSFC/Arizona State University].

LROC Wide Angle Camera (WAC) monochrome mosaic of the region surrounding LROC's Featured Image, an illustration of early lunar crater morphology, a fresh and rough, less space-weathered (and thus brighter) crater standing out from a nominal lunar scene characterized by "crater saturation." The image is cropped from the original. The arrow points to the location of the high-resolution Featured Image [NASA/GSFC/Arizona State University].

Search for more bouldery craters in the full NAC image!

Related Posts:
Scouring secondary ejecta
Small crater in Oceanus Procellarum
Chain of secondaries in Mare Orientale

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