LROC: Bench Crater in Plato

A small impact feature in the lava-filled crater Plato (51.6°N, 350.7°E) exhibits an interesting morphology. LROC Narrow Angle Camera (NAC) observation M137610258L, LRO Orbit 5413, August 28, 2010; incidence angle 60.43° field of view 550 meters from 48.4 km attitude. View the full size LROC Featured Image HERE [NASA/GSFC/Arizona State University].

James Ashley
LROC News System

Impact craters of the size shown above are often bowl-shaped, but can also present flat bottoms and concentric or 'bench' features like those seen here.

This small (~140 m diameter) crater is characterized by a low-relief rim, shallow and hummocky floor containing a small central crater, and a large population of associated blocks or boulders. Blocky and irregular craters are often the result of low-velocity secondary impacts, but can result from high velocity as well. The circularity of this crater suggests that it is the result of a high-velocity, primary impact.

LROC Wide Angle Camera 100m monochrome mosaic affixed to to LOLA elevation data in NASA's ILIADS lunar mapper program  are used to create an oblique view centered on the small crater, from a point at 5000 meters elevation and 18 km south. The elevation of the crater (yellow circle) and a few selected points are used to illustrate the stark slope of the 2 kilometer high inner walls of Plato.

The view from directly overhead: LROC WAC observation M119931570M, LRO Orbit 2808, February 4, 2010 very near to its full 54.08 meter per pixel resolution from 38.6 km altitude; incidence angle 66.15° The yellow arrow marks the location of the small bench crater, barely visible at the limit of unprocessed visibility [NASA/GSFC/Arizona State University].

Plato in a field of view assembled from four sequential LROC WAC orbital observation opportunities, February 4, 2010. Even an image that seems to take all of Plato in fails to allow a full appreciation of the topographic impact, so to speak, of the crater on its surroundings, part of the outer rim of the Imbrium impact event. Three sinuous channels radiate from Plato, perhaps out from under its rim. One of these is a Constellation Region of Interest [NASA/GSFC/Arizona State University].

Plato from Zottengem, Belgium, February 17, 2008 (C9.25@F/20, DMK31AF@30fps) An example of the increasingly spectacular work by talented observers of the Moon spotlighted on Charles Wood's justifiably popular website Lunar Picture of the Day [Bart Declercq].

Experiments were conducted in the late 1960's using a high-speed gun to fire projectiles at targets in an attempt to understand the process of small crater formation. Loose sand and epoxy resin-bonded sand was used to simulate lunar soil (regolith) over a hard bedrock substrate. These experiments determined that different types of small crater morphologies result from different thicknesses of lunar soil. The bench crater morphology shown in today's Featured Image forms when the regolith is thin with respect to the crater's final diameter.

Our featured impact had enough energy to penetrate the lunar regolith layer to the hard basaltic bedrock beneath. But because the lunar soil is unconsolidated, this energy was more effective in displacing the soil than the bedrock. Hence, we see a wide impact feature with a shallow bottom instead of a bowl. The substrate does not have to be bedrock to produce a bench crater, but there must be a contrast in target strength. The clear presence of boulders in the case of today's featured crater does, however, indicate that bedrock fragmentation was involved in its production.

Are there any similar craters visible in the full NAC image?

See other examples of interesting small craters here, here, and here.