Oblong Roche V

The boundary between the flooded crater floor and the Roche V crater wall is subtle, except for locations where a distinct pinch represents the contact between units. LROC NAC M159100166R, image width is 590 m [NASA/GSFC/Arizona State University].

Roche V (38.86°S, 129.62°E) is one of five satellite craters associated with Roche crater. Compared to its siblings, Roche V is the ugly duckling of the bunch: it has an oblong, irregular shape, floor fractures, and mare fill. But, similar to the ugly duckling in the story, the geologic features that stand out and catch our attention make this 29 km diameter crater quite special indeed. The irregular shape tells us something about the crater formation, probably because the impactor hit the lunar surface at a highly oblique angle. But maybe more exciting is the flooded mare material in the crater, along with both linear and arcuate fractures. There are many floor fractured craters on the Moon, but most of them are much larger than Roche V (for example, Gassendi is 110 km in diameter). The cause of fractures in these larger craters remains poorly understood, but scientists do know that the floors of these craters are uplifted and many contain smooth deposits of basalt. Given the size difference between Roche V and these larger craters, can we explain the geology of the Roche V crater floor?

LROC WAC monochrome mosaic of satellite crater Roche V. Roche, the parent crater, is located to the southeast (toward the lower right) of this image. Asterisk marks location of opening image [NASA/GSFC/Arizona State University].

Taking a look at the WAC mosaic, there is a striking difference between the fractures in Roche V and those observed in other floor fractured craters, such as Atlas (87 km diameter). The fractures in Atlas and other floor fractured craters are large, somewhat resembling linear rilles in appearance. However, the fractures in Roche V are not as sharply defined and appear visually shallow compared to those observed in Atlas. The fractured and rougher mare fill (including the location of today's Featured Image) has a higher albedo than the smooth mare. Maybe the Roche V mare fill is partially covered by high albedo ejecta from a nearby recent impact; can you find any craters with high albedo ejecta blankets using the LROC WMS viewer?

The surface features of the floor material in Roche V are visually similar to the surface of a cornstarch and water mixture in a bowl as it dries. The material that lapped up on the sides of the crater stuck there as the lava cooled, but as the lava cooled it experienced a small volume change and shrank. The fractures observed may be the result of cooling within the lava pond. Much of the contact between crater wall and floor-fill is currently blurred and smoothed due to post-impact modification and regolith formation and at the NAC scale are very difficult to discern, but there are still a few distinct contacts visible in LROC NAC images.

What do you think - what formed these fractures? Take a look in the full LROC NAC image!

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Published by Lillian Ostrach on 6 December 2011