image by Jérôme Grenier, Paris, France.
Floor-fractured craters are the most interesting ones on the Moon because of their concentration of rilles, dark halo craters and uplifted centers. While some have easily visible rilles (Posidonius and Gassendi, for example) others have such narrow rilles that they are rarely imaged. Cleomedes is an example of the latter, and so is Mersenius, shown above. Mersenius has long been known for its convex floor, although I am unaware of any measures of its height other than Neison’s statement in his 1876 book, The Moon, that the centre is perhaps 1500 ft higher than the foot of the walls. Schmidt detected two rilles on the floor, but they have been difficult for observers to consistently depict. Jérôme’s image - only the second one to clearly show them - reveals most of the rilles known from Lunar Orbiter IV photography. The rilles crudely circle the floor, especially on the south, west and north, and a very faint one cuts across the nearly featureless southeast quadrant of the floor. Although it is not visible here, on the high Sun Clementine image four dark pyroclastic deposits are visible along the western edge of the floor. At least one dark spot is from a dark halo crater on a rille. A question that I don’t have a definite answer for is why do Mersenius and Cleomedes have such narrow rilles while most floor-fractured craters have broader ones? Presumably the amount of floor fracturing - and thus rille width - is related to the amount of bending from uplift of the floor. Do these two craters have less uplift than Gassendi, Posidonius and Alphonsus?
Technical Details:
21 Nov. 2007. Orion Optics (UK) OMC 12″ + barlow 2x + Dmk31 AF03 + IR filter.
Related Links:
Rükl chart 51
Lunar Orbiter IV view
Yesterday's LPOD: Not from Orbit
Tomorrow's LPOD: Another Lunar Atlas?
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