Thursday, October 6, 2011

An ill-defined portion of an otherwise circular rim

The bright craterlet rim of Nearside landmark Censorinus (upper right) is abruptly truncated by a pile of debris (lower left). LROC Narrow Angle Camera (NAC) observation M139694087R, LRO orbit 5720. September 21, 2010 (north is up), incidence angle is 9.32° and the field of view is 296 meter. See the full size LROC Featured Image HERE [NASA/GSFC/Arizona State University].
James Ashley
LROC News System

Censorinus is a fresh impact crater along the southeastern shore of Mare Tranquillitatis (0.47°S, 32.73°E). The amount of detail in the NAC frame is staggering, and worthy of a baker's dozen Featured Images. Noteworthy, however, is how well-formed and highly circular the rim is with the exception of this small portion along the southeast perimeter. Are there good geologic reasons for something like this, or is it simply the luck of the draw which portion of a crater rim will get a little sloppy? Such things are not much more than curiosities, but they are great for initiating geologic discussion!

The full-width of the featured NAC frame shows context of the Featured Image above (white rectangle) and the circularity that persists along most of the eastern Censorinus rim. LROC NAC M169398317R, field of view ~2.4 km (north is up). View the higher-resolution original LROC context image accompanying the Featured Image release HERE [NASA/GSFC/Arizona State University].
A neat, circular crater is usually a sign of sufficiently high energy from a hypervelocity projectile to create an explosive outcome. Censorinus is clearly the result of just such a high-energy, explosive event. In the case of the featured area of the rim, however, we see that energies were not high enough to excavate the target material radially all the way to meet the rest of the rim, but stopped short, as though making a smaller-diameter feature. This action resulted in littering the surface adjacent outside the wall with loose ejecta blocks, giving the impression that the rim stops suddenly in the Featured Image.

Censorinus (left) and Censorinus A (right) in a single band (566 nm) color LROC Wide Angle Camera (WAC) mosaic swept up during two orbital passes over this area, on the south frontier of Mare Tranquillitatis June 7, 2011, about 40 hours after local sunrise, when the Moon was barely more than 6 days old. At this scale the discontinuity of the south-southeastern rim of Censorinus is easy to see, perhaps the result of slumping of material from adjacent high ground in that direction [NASA/GSFC/Arizona State University]/
Why was material in this portion not accelerated to create a neat rim? Since the 'ground zero' target rock was destroyed in the impact, it is difficult to tell what kind of influence the local geology may have had on the energy distribution. The final answer therefore remains a bit of a mystery, but it is probably safe to speculate that target lithologies played a role here. The terrain along this portion of Mare Tranquillitatis shoreline is fairly rugged, and may contain a variety of rock types, some of which may be more resistant to excavation than others, thus creating a heterogeneous target for an incoming bolide!. A partially buried block of hard rock or even greater-than-average compaction in the regolith might be all that is needed to explain today's Featured Image.

This WAC mosaic context image shows the location of Censorinus and Censorinus A in relation to Mare Tranquillitatis, two relatively fresh craters. Though Censorinus A is larger than Censorinus, the latter is probably younger and thus more reflective and less "gardened" by impacts into darker "optical maturity." Censorinus was identified in early telescopes and Moon maps much sooner.Image field of view is roughly 360 km in height. See the higher-resolution LROC context image HERE [NASA/GSFC/Arizona State University].
A highly reduced "thumbnail" version of a much larger black and white mosaic assembled from several digital stills by Yuri Goriachko of Astronominsk in Belarus. Because the landmark Nearside equatorial crater stands out so well from the background, despite its small size, it almost seems unnecessary to mark bright Censorinus with a yellow arrow [Astronominsk].
The high-reflectance spot that marks the location of the Censorinus - Censorinus A crater pair may just be visible through a small telescope at the southwest edge of Mare Tranquillitatis beginning about the First Quarter phase of the lunar cycle, but albedo differences will become more accentuated as the Moon waxes to full and the shadows shorten.

What additional clues can you find in the full NAC frame

Other examples of asymmetry in impact features can be found in the distribution of rubble on the floor of this crater, and the distribution of impact melt around this crater.

No comments: