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The Lavish Lobes of Necho R
Lunar mounds › View larger image
What geologic process created these mounds (5.72°S, 122.09°E)? NAC image number M106074338R; incidence angle 35°, Sun is from the west, north is up, image is ~600 meters across [NASA/GSFC/Arizona State University].

As space enthusiasts and students of the Solar System, we all know that the tremendous energies unleashed by a hypervelocity impact vaporize much of the impactor, fracture the target subsurface, and send cubic kilometers of pulverized and melted ejecta sailing over the horizon. When conditions are right, however, some impacts also produce strange and counter-intuitive phenomena. Dry ejecta can sometimes behave like a fluid and travel as a ground-hugging debris flow great distances from the impact site. The exact physics of this ground-hugging flow behavior is not well understood, but several theories exist. The most popular of these employs the idea of acoustic energy waves keeping particles in motion -- bouncing off each other at just the right frequencies to keep everything suspended much longer than one might anticipate. With friction reduced by this resonant internal energy, the flow can move great distances before finally losing momentum to become part of a distal ejecta deposit.

On Earth, this type of debris flow is known as a "sturzstrom," and is most often associated with avalanches. Evidence for similar behavior is observed on the Moon in the vicinity of relatively recent craters. The odd mounds shown here were caused by the Necho impact (see context image below), and are the result of such flows bunching up and overriding each other as they slowed and came to their final resting place in nearby Necho R crater upon reaching its floor and steep southwestern wall; hence the technical term "deceleration lobes" for these landforms.

A portion of the global WAC mosaic showing the Necho crater region of the farside lunar highlands › View larger image
A portion of the global WAC mosaic showing the Necho crater region of the farside lunar highlands; image is ~60 km across; north is up [NASA/GSFC/Arizona State University].

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