September 20, 2009
Little Treats
Lunar Orbiter V-042 image from LPI Lunar Orbiter Photo Gallery
I wonder if anyone other than Danny Caes recognizes this area? Danny recently pointed out some Lunar Orbiter V images of the Secchi Rille, and in looking at them I came across this other LO-V view that illustrates a lot of small scale lunar landforms. For geologic context see Paolo Lazzarotti's great telescopic image. Pieces of the Secchi Rille - really it should be rilles - are visible at the bottom left corner, including a fascinating crater chain rille. This image is available at much higher resolution which shows that unlike the Hyginus Rille crater chain, these pits do not occur within a rille-like channel. Here the pits are the rille. These could be collapses into a lava tube, or perhaps blowholes above a tube. Looking diametrically across the image is the Messier Rille which has hints of within-rille pits but they could be a few random impacts. Also notice that the rille is not continuous - its middle section is offset about a rille width from the ends. When a linear geologic feature has such offsets it is said to have en echelon components. On Earth, faults at right angles to the linear feature cause the offset, but I am not sure here. In the middle of the image is less than half of a semi-circular mare ridge which looks to be an almost completely gone ghost crater. Below the ghost rim is a circular dark area with three elongated hills. These hills probably were the volcanic vents for pyroclastic eruptions that deposited the halo. A similar chain of vents with a more muted ash deposit is near the top center of the image. Finally, two more features to notice - the almost parallel bright streaks are the famous rays from Messier - see Paolo's image. There are a few other intriguing small features that I'll leave you to find.
Chuck Wood
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Rükl plate 48
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