Saturday, September 18, 2010

The deepest spot on the Moon nearly wasn't

The 10 km crater at bottom center, home of the Moon's lowest mean elevation (-9,020 meters - JAXA/SELENE, 2008 ) appears to have once nearly disappeared. Even though it's high in latitude, at 70 degrees south, it is not close enough to the pole to be permanently shadowed. It's a fairly normal crater for it's size, generally too small for a central peak with a rubble-lined middle interior and steep sloping inner rim. It's outer rim, however, disappeared one day, as it's host-crater's deep interior flooded with lava. LROC Wide Angle Camera observation M118639861M, LRO orbit 2617, January 20, 2010; alt. 53.95, res. 76.145, phase angle 85.12. Field of view = 32.3 km [NASA/GSFC/Arizona State University].

Not that anyone on Earth would have noticed. This "high-water" mark of molten lava, in this case probably oozing up from underground, was tucked away on the Moon's far side. If the "Man in the Moon" were a real face, the crater where the event took place was on the nape of the neck, where the spine meets the skull. In fact, because all the astronauts who ever visited there orbited much closer to the Moon's equator, no one has yet really seen this spot. Not that we haven't visited by proxy, plenty of times, since the Soviet Union managed to return the first photographs of the Moon's far side in 1959.

On Earth, multicellular life forms may have taken form by then. The raft of the melt that began flooding the host crater, 143 km Antoniadi (69.7°S, 188°E), came very close to spilling over it's north rim before it cooled. Because this flood event almost certainly happened long after after the formation of Antoniadi itself suggests it arose from the depths below Antoniadi, perhaps in the transfer of tremendous kinetic energy following the creation of Mare Orientale.

Studies just published, based on data returned from the Lunar Reconnaissance Orbiter, inform us this and the other "ponds" of mare-like material found in many place around there, near the center of the oldest, widest and deepest impact basin on the Moon - South Pole-Aitken (SPA) basin -is not composed of the "pristine magma" of the kind that may have once covered the entire Moon. The Moon's global "Magma Ocean" now appears to have "differentiated" as it cooled even before the basin-forming impact happened that formed SPA around 4 billion years ago, when no life we know of existed on Earth.

With no plate tectonics or the kinds of swift flowing water and wind we generally think of when we think of weather or climate on the Moon, our companion world holds fast to its ancient record of the history of the part of our star system also occupied by Earth. Because our record of the oldest life on Earth is mostly erased, its possible - even likely - that one day the oldest fossil record of life on Earth will be found on "terrestrial meteors" found on the Moon.

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