Tuesday, October 8, 2013

Secondaries of all sizes near Mare Moscoviense

A cluster of secondary craters found in the highlands adjacent to Mare Moscoviense (28.803°S; 142.876°E). LROC Narrow Angle Camera (NAC) frame M1104980770R, LRO orbit 15145, October 15, 2012; 40.69° angle of incidence, resolution 1.08 meters per pixel, field of view about 1.4 km across [NASA/GSFC/Arizona State University].
James Ashley
LROC News System

Secondary craters are produced by debris lofted during excavation of primary impacts. They can be difficult to distinguish from primary impact craters if they are circular with well-developed ejecta patterns. That ambiguity is a concern to scientists when counting craters to determine the ages of planetary surfaces. This age dating approach assumes that impact flux has held steady throughout geologic time following the Late Heavy Bombardment period ending approximately 3.8 billion years ago. The model thus assumes that older surfaces present more craters than younger surfaces, and that one can determine relative ages by counting the number of impact craters per unit area.

M1109480770R at 50 percent
A wide field of view shows a wide variety of secondary crater cluster, from near and far, nearby. LROC NAC M1104980770R; about 2.14 km across [NASA/GSFC/Arizona State University].
Crater counts give reliable relative ages, but can also be translated to absolute ages using the radiometric age dates of rocks returned by the Apollo and Luna missions. Obviously counting statistics are skewed if secondary craters are included in the count; these "extra craters" create the false impression of an older surface.

Swarms of secondaries abound. LROC Wide Angle Camera context for the LROC Featured Image, highlighting the area designated with the arrow, on the rim of Tereshkova U, in this 53.5 km field of view, the full width of LROC WAC monochrome (604 nm) observation M167267012CE, LRO orbit 9784, August 8, 2011; spacecraft and camera slewed 8.68° off nadir, angle of incidence 56.66° at 67.5 meters per pixel resolution from 50.59 km over 31.09°N, 142.86°E [NASA/GSFC/Arizona State University].
Often, as in today's Featured Image, the distinction between primary and secondary craters is unambiguous. Here we see a large number of impact craters in a tight cluster. Particularly apparent when exploring the full NAC frame, we can see a clear association within this cluster, not merely a coincidental grouping of separate features spaced in time across many eons. Their ejecta exhibit higher reflectance than most of the craters in the surrounding areas, and they form a tighter grouping than other fresh-looking craters seen elsewhere in the frame.  

At 500 meters per pixel, there's no let up in the secondaries peppering the relatively young Moscoviense basin. The cross at center further designates the location, on the rim of Tereshkova U, shown at high-resolution in the LROC Featured Image. LROC WAC mosaic (GLD100) through the LROC Quick Map feature [NASA/GSFC/Arizona State University].
More secondary impact features can be found across the full NAC frame, HERE. Other secondary craters can be explored in earlier LROC Featured Image releases "Swarm of Secondaries," "Dark Secondary Crater Cluster," and "The Rays of Messier A."

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