image by Clementine viaUSGS Map-A-Planet, Flagstaff, Arizona
I like Poincare. It was one of the basins that Bill Hartmann and I found in 1971 when we were studying basins on the farside using the then relatively new Lunar Orbiter images. We recognized Poincare as a two ring basin, with a possible inner mare ridge ring. There has been very little work done on the basin since our paper, but this Clementine mosaic tells more of its history. The main 335 km wide basin rim is visible on its south, west and north sides, passing by Poincare R to Hopmann and disappearing near Hess. The 180 km diameter inner ring is mostly marked by rough topography that borders the mare interior. Bill and I thought we could see on the oblique Lunar Orbiter images a darker ring of maria that we speculated might represent an inner ring of 70 km width; the Clementine image makes that seem less likely. What is much better seen here are many rilles that occur in the moat region between the inner and main rings, especially on the north and south sides of the basin. The crater Poincare Z contains a concentric rille on its south side and it looks like a lip of viscous lava is on the inside of the souther rim. Just southwest of Z is a mostly buried crater with rim and rilles preserved. This appears to be the uplifted floor of an otherwise mostly flooded floor-fractured crater. Another possible volcanic manifestation is the concentric crater to the east of Poincare R. From the Orbiter images it appeared that there was lighter and darker mare areas within Poincare, but this image shows that the lightening is due to rays from Poincare Y. Poincare apparently has never been the focus of a detailed geologic investigation, but its rilles and volcanic features - which are rare on the farside - suggest that it should be!
Yesterday's LPOD: Where's Clavius?
Tomorrow's LPOD: Making Rilles Visible