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Old Man River (of Lava)
A tight turn in a sinuous rille › View larger image
A tight turn in a sinuous rille. Could you drive a SUV sized rover through it? Probably, it's flat on the bottom. Just watch out for house-sized boulders. Image number M157934151L, incidence angle 35°, image is 1.75 km across [NASA/GSFC/Arizona State University].

This sinuous rille is a small part of Vallis Schröteri, which is the largest rille on the Moon, and actually contains two rilles! The larger rille (155 km long) cuts through the Aristarchus Plateau, and the smaller, curvier inner rille (204 km long) cuts through the floor of the larger rille. These two rilles may have formed during two different volcanic events, or they could represent a change in volume from a single event over time. The larger rille formed when the flow of lava was higher in volume. The inner rille subsequently cut through the floor of the larger rille after the eruption volume waned. Geologists ask questions such as why did the inner rille flow farther than the larger rille? Did the magma flow faster in the case of the smaller rille? Scientists do not fully understand how rilles formed on the lunar surface, but there are two main theories. In general, rilles form when large volumes of very fluid (low viscosity) magma erupt and flow rapidly. The molten lava may carve a channel into the lunar surface through the erosive power of the flow of magma that then drains away, leaving only the empty lava river behind. An alternative theory proposes that the lava initially forms levees on the edges of the flow, which then confine the flow to just one channel, creating the river-like rille instead of spreading out over a large surface like a mare.

LROC WAC 100 m/px mosaic of Vallis Schröteri › View larger image
LROC WAC 100 m/px mosaic of Vallis Schröteri. The red box marks the region of the rille seen in the Featured Image [NASA/GSFC/Arizona State University].

Vallis Schröteri starts at a 6 km diameter crater to the north of Herodotus. Some people call the start of the rille the "Cobra's Head" since it resembles a snake. The rille has a maximum width of about 10 km. For a scale comparison, consider that the Grand Canyon on Earth (formed by water) has a width that ranges from about 6.4 to 29 km.

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