Friday, October 21, 2011

LROC: Brush strokes of ejecta

Ejecta with a subtle chevron texture drapes the Anaximander crater group region in the highlands of the nearside north. The ejecta is pockmarked by few small craters, suggesting it is from a recent impact. LROC Narrow Angle Camera (NAC) M170937919R, LRO orbit 10325, September 17, 2011, image field of view is 500 meters. View the full size LROC Featured Image HERE [NASA/GSFC/Arizona State University].
Drew Enns
LROC News System

What created the chevrons in today's featured image? Chevron textures like these are secondary results of impacts. As ejecta is thrown out of its parent crater, the ejecta crashes back into the surface creating secondary craters and small linear streams of locally derived immature material.

Even better, the chevrons created during this process point back towards their parent crater! These features are not scale dependent, as we observe chevrons on both small and large craters.

So if these chevrons were formed from a recent impact, where is the parent crater?

LROC Wide Angle Camera (WAC) 643 nm mosaic from seven consecutive orbital observation opportunities beginning March 24, 2011. Though the Sun was near its highest, at this high latitude (near 69.2° North) the incidence angle averaged 71.5° (The overall resolution was 60.5 meters from 43.6 kilometers altitude). To the northeast (upper right) is 9 km-wide crater Carpenter T (70.17°N, 302.39°E). Note the differences between Carpenter T and the smaller crater, northwest of the slope captured in high resolution in the Featured Image. The smaller crater is younger, less smooth and "gardened" by relentless space weathering. It's inner rims are brighter, rougher and more reflective, so bright reflected scattered sunlight has illuminated an otherwise deeply shadowed south wall; unlike Carpenter T, where a patch of apparent impact melt is revealed only in direct sunlight [NASA/GSFC/Arizona State University].
A wider sample (114 km-wide at the latitude designated by the yellow arrow, location of the LROC Featured Image) of the 643 nm mosaic shows the floor of ancient composite crater Anaximander B has been superimposed with a variety of ejecta patterns, some deeper grooves point to the Imbrium basin-forming impact. A fresh crater has left it's sign at very bottom left. The narrowing of the overall mosaic is the result of converging orbital passes as LRO passes closer to the lunar North Pole [NASA/GSFC/Arizona State University].

The (original) context image (accompanying the LROC Featured Image) doesn't reveal any obvious choices, but they could be outside of this image. Try looking for potential candidates using the LROC QuickMap (don't forget to change the projection for the north pole).

Ed. Note:  Drew Enns is correct about the original WAC mosaic showing no obvious crater of origin for the ejecta field and pattern shown in detail in the Featured Image. It "could be outside" the area covered in that same image, and thus also in the WAC mosaic, shown at 60 meters and 240 meters per pixel resolution, respectively, immediately above. But it isn't. Comparing the narrow and wide angle fields of view above, the subtle ejecta patterns and especially differences in crater ages in the area appears to point toward the unnamed ~3 km crater about 12 kilometers up over the crest of the slope from the area highlighted in the Featured Image. But, if the welcome exercise draws interest to the dramatic improvements in the LROC QuickMap, it was well worth the investigation!

Are there more ejecta deposits within the full NAC frame?

Related Posts:
Ejecta Blanket
Ejecta from Van de Graaff Crater

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