October 10, 2015
Originally published October 9, 2004
East of Crisium, when the lighting is right and libration favorable, the limb is dark and smooth with the odd mare called Marginis. This is a very appropriate name, for this patch of dark mare material is at the visible margin of the Moon, but its also only marginally deserving the name "mare" and unlike most mare, there is almost no evidence that it is inside an impact basin. The mare itself covers a small area between Goddard and Neper, and those craters and a few others are filled with dark lava. Two roughly N-S parallel patches of mare west of Marginis are quite noticeable, but were unnamed, so in The Modern Moon I christened them Lacus Risus Felis - the Cat's Smile. According to Clementine altimetry, the floor of Mare Marginis is 3 km lower than the average lunar radius, and there is also a small mascon - a mass concentration - so perhaps there is a basin somewhere under Marginis after all. On the mouseover image there are a number of italic letters "S". These are swirls - bright streaks like Reiner Gamma that are mysterious in origin. These streaks are the illusive L100 in the Lunar 100 list.
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