Friday, February 10, 2012

LROC Shattering Consequences

Debris of variable reflectivity and size litters the floor of a small crater in the nearside lunar highlands (31.3°N, 34.45°E). Illumination is from the south, image field of view is approximately 410 meter across; from LROC Narrow Angle Camera (NAC) observation M170334512L, orbit 10236, September 10, 2011, native resolution 0.47 meters per pixel from 38.77 kilometers. View the full size (1000 x1000) LROC Featured Image HERE [NASA/GSFC/Arizona State University].
James Ashley
LROC News System

What causes such an astonishing range in textures and tones as we find in this image? Here we see a jumbled mixture of light and dark, blocky boulders (many the size of small buildings), finer deposits between and around the boulders, and dark (low albedo), flat zones that are almost completely free of boulders. A landscape formed in an instant - ground zero for one of the most violent processes found in planetary studies.

The tremendous energy released when an asteroid or comet slams into the Moon results in compression, shattering, accelerating, melting, and even vaporizing of target and impactor. Why so much energy? Hypervelocity is the answer - the bolide (comet or asteroid) is typically traveling greater than 15 km/second; scientists estimate the energy of impact in terms of megatons equivalent of TNT because the interaction is truly explosive. Hyperveolcity is different from a low-velocity impact, which might displace some rock and make a thud, but will not produce an explosion and larger crater. Hypervelocity craters are many 10s of times the diameter of the bolide that creates them.

A wider view across the width of the LROC NAC frame provides context for the featured area deeper within the impact crater (image field of view roughly 1800 meters across. View the larger (1000 x 1000) context image release HERE [NASA/GSFC/Arizona State University].

Following an impact, some material falls back into the cavity, either as mass wasting off the crater walls or as direct in-fall from the ejecta plume. Depending on the velocity of the impactor, the impact angle, the size of the impactor, and gravity, the amount of material that is ejected or stays within the crater varies widely. Impact melt, which tends to be a darker deposit than many types of bedrock because of its glass content, will flow and pool in low-lying areas and also coat some of the boulders in encounters on its way. This accounts for the range in reflectivity and the smooth, flat areas on the crater floor in this Featured Image. This crater also has a high-reflectance ejecta blanket that can be seen in the regional WAC mosaic, provided below. Exploration of craters like this provide subsurface samples to future explorers, which in turn provide insights into the local three dimensional geology and the impact process that formed the crater.

WAC monochrome (604 nm) mosaic from four sequential orbital observation opportunities (LRO orbits7281-7284) January 21, 2011 reduced from an original resolution averaging 60 meters per pixel from 43.5 kilometers. The image shows some of the complex region in the vicinity of the small crater. A light ejecta blanket surrounding the impact is easier to see in the original context image accompanying the LROC Featured Image HERE. (FOV above is approximately 110 kilometers across [NASA/GSFC/Arizona State University].

Examine the full NAC image for additional occurrences of impact melt and other features. More examples of small, young craters can be found in Featured Image postings Brayley G, Recent Impact in Oceanus Procellarum, and Farside Impact!

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