MoonWatch
May 1998
Cauchy, Rille and Scarp
2 January 1998 100mm Mks Peter Grego
Across the north-eastern plains of the Sea of Tranquiility, more than 500 km from the Apolio 11 landing site, lie two magnificent near-parallel cracks in the Moon's surface, both more than 200 km long. Rupes and Rima Cauchy (the Cauchy scarp and rille) are named after the bright 12 km diameter impact crater Cauchy that lies between them. Both features were formed after the massive lava flows that formed the sea had cooled and had begun to settle, and they were caused by a localised stretching apart of the lunar crust.
Like the crust of the Earth, the Moon's crust is capable of being deformed by stresses. In some lunar seas there are examples of wrinkle features that have arisen as a result of the Moon's crust buckling under pressure. But beyond a critical degree of compression or tension the rocks suddenly fault, giving rise to fault scarps, ridges and rilles. Some faults can be very deep-seated, like the so-called Straight Wall (Rupes Recta) in Mare Nubium which undoubtedly cuts through the upper lava strata into the very bedrock of the Nubian basin.
Rupes Cauchy is part rille, part escarpment, and appears a little more complicated than the clean scarp of Rupes Recta or the chisel-cut rille of the Rima Cauchy. Rupes Cauchy begins in the west at a pair of tiny oval craterlets (visible in a 150 mm reflector or larger) and several larger craters lie just along its length. Rupes Cauchy extends eastward to meet with the ragged mountain border of Mare Tranquillitatis, and there is evidence that it cuts through the mountains to meet with the small ruined crater Lawrence. What is to be made of the craters that lie along the Rupes Cauchy? Many of the Moon's rilles appear to be composed (at least in part) by elongated craterlets or chains of craters. In some cases the original faulting may have instigated fresh bouts of volcanic activity that produced small volcanic vents along the fissure. There certainly has been a good deal of volcanic activity in this region, forjust to the south of Rupes Cauchy lie two beautiful volcanic domes (squat volcanoes) designated Tau and Omega, the latter sporting a small summit craterlet.
The crater Cauchy is easily visible in a pair of binoculars as a bright spot at high illuminations. Rupes and Rima Cauchy, along with the two nearby domes, are all visible in a 60 mm refractor when they are positioned close to the terminator at low angles of illumination. At lunar sunrise Rupes Cauchy (like Rupes Recta) casts a dark shadow onto the plain to the east. At sunset the feature appears as a bright line, its western scarp face being brightly illuminated. In order to see the summit craterlet on Omega you will require at least a 150 mm reflector at high magnification under good seeing conditions. This fascinating region is suitably illuminated on the morning of May 15 (lunar sunset) and a fortnight later during the next lunation on the evening of May 30, at lunar sunrise.
