June 24, 2008
Unknown Volcanic Center
Lunar Orbiter IV-191H image from USGS Lunar Orbiter Digitization Project, processed by Jim Mosher
Using the recently re-digitized ancient Lunar Orbiter IV photograph, Jim Mosher has produced the best image in existence of Messala, a 125 km wide crater north of [/June+23%2C+2008 Geminus.] Messala is considered by the US Geologic Survey to be older than the Nectaris Basin, and as is typical of older craters, its walls and floor record many different events. The most interesting feature is the evidence for fluids on its floor. I could have said volcanism but was hesitant because the narrow rilles below the crater Messala J remind me of similar cracks on impact melt within the Orientale Basin. But since there is no plausible source of impact melt here it is likely the material really is volcanic. This is made more likely by the nearby lacii Speii and Temporis, except those are dark mare lavas and the materials in Messala are brighter. The northern part of the floor is relatively smooth with small crater chains and a hint of a mare ridge. These chains could be collapse pits or secondaries - the high-Sun Clementine image resolves the issue, by revealing pyroclastics around the chains - they are volcanic vents. The crater between J and K has a floor cut by thick troughs and seems to be a floor-fractured crater - one of the smallest on record. There is a rough pancake feature on the elongated floor of K - a wall slump or a viscous dome? On the southwest edge of the floor there is a knob that I had previously speculated might be a dome. It still might be, with a broader shallower section extending south. Another possible, but uncertain, dome is NNE of the crater B. This putative dome is about the same size as the other one and is cut by a rille/crater chain. This crater has many manifestations of volcanism, few recognized until this good image became available.
Chuck Wood
Technical Details
May 25, 1967; 09:47:05.51 UT -- from an altitude of 5503 km; remapped to a north-up aerial view centered on Messala
Related Links
Rükl plate 16
COMMENTS
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2. Are my eyes playing tricks, or is there a faint pattern of squares in the upper third of the crater surface between K and J, showing straight lines and right angles. (??)
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