At the southeastern edge of Mare Imbrium, about 25 km west of Rima Hadley, there is a small shiny mound on a dark and flat mare basalt plain, which looks like a white sand island in the middle of a black ocean. The size of this mound is about 2.7 by 2.2 km across. Normally fresh slopes and fresh ejecta have high reflectance due to less space weathering. But this mound is brightest at its top, not down the slopes, and is brighter than nearby ejecta, implying that the mound is composed of higher-reflectance materials than the mare basalts. Then the question is, how was this shiny island was formed?
Most likely, it is a remnant of highlands sticking through the mare. A hummock of plagioclase-rich highlands materials was embayed by mare basalt lava, burying all except its summit. If so, mare basalt is overlapping the mound's skirt. Can you see the an overlap contact in today's Featured Image?
Unfortunately, the contact is not clear, or sharp. Over time such sharp contacts are blurred by micrometorite bombardment. If we are lucky, in the future, a small impact may occur right at the contact once again revealing the sharp contact. Or perhaps a future explorer might take a shovel to this spot and settle the question!
Explore this shiny mound in full NAC image!
Posted by Hiroyuki Sato on November 18, 2011 10:00 UTC.