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Ina Pit Crater on the Moon: Origin as a Drained Lava Lake Modified by Seismic Sifting

James Head
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Abstract Text: 

The enigmatic Ina feature, a ~2 x 3 km D-shaped depression on the Moon discovered by Apollo 15 and consisting of a host of bleb-like mounds surrounded by flat unusually fresh hummocky and blocky terrain, has generated numerous hypotheses for its origin, including extrusive volcanic activity in the last ~33 million years of lunar history. Here we show that the size, location, morphology, topography, optical maturity and spectroscopic characteristics of Ina are consistent with a two-stage origin as: 1) a drained lava lake (forming the mounds as draped floor topography, and the floor as a void-rich subsided lava lake crust) followed by 2) differential regolith development (normal development on the mounds, and abnormal development on the floor as seismic sieving causes regolith particles to preferentially drain into the underlying voids, maintaining roughness, a hummocky texture, and optical immaturity, and exposing boulders). In this scenario, geologically very recent volcanic effusive or venting activity is not required: the Ina structure forms as a summit pit on a small shield volcano contemporaneous with the latest stages of its evolution ~3.5 billion years ago in a process very common in volcanic pit craters in Hawaii and elsewhere on Earth. Impact-induced seismic shaking and sieving of regolith particles into voids underlying the floor that remain after final lava lake drainage appears to be sufficient to maintain a level of optical immaturity in parts of the Ina floor for several billion years.

"Paul A. Abell (NASA JSC)...
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Recognizing that science and human exploration are mutually enabling, NASA created the Solar System Exploration Research Virtual Institute (SSERVI) to address basic and applied scientific questions fundamental to understanding the Moon, Near Earth Asteroids, the Martian moons Phobos and Deimos, and the near space environments of these target bodies. As a virtual institute, SSERVI funds investigators at a broad range of domestic institutions, bringing them together along with international partners via virtual technology to enable new scientific efforts."