What you're looking at here is not an impact crater. It's a large hole on the Moon's surface that formed when the ground above a lava tube collapsed. NASA believes these pits widen underground and contain tunnels — which would be very handy for the first wave of lunar colonists.
Since 2009, NASA's Lunar Reconnaissance Orbiter Camera has found over 200 pits on the Moon's surface. They range in size from about 16 feet (5 meters) across to more than 2,950 feet (900 m) in diameter. They tend to be located either in large craters with impact melt ponds (areas of lava that formed from the heat of the impact and later solidified) or in the lunar maria (dark areas on the moon that are extensive solidified lava flows hundreds of miles across). "Maria" is the Latin word for "seas," and are named as such because that's what the ancients thought they were.
The pits are strewn across the melt ponds of 29 craters, all of which are considered geologically young "Copernican" craters that are less than a billion years old. Eight are located in the lunar maria, three are known from previous images captured by the Japanese Kaguya orbiter, and two pits are in the highlands terrain. There are undoubtedly more, as only 40% of the lunar surface has been scanned by the LRO. The researchers working on the project suspect there may be at least two to three more mare pits and several dozen to over a hundred more impact melt pits.
According to Robert Wagner of Arizona State University, Tempe, Arizona, the pits likely formed when the roof of a void or cave collapsed, possibly from the shaking generated by a nearby meteorite impact. But it's difficult to tell from space-based pictures alone. Wagner developed a computer algorithm that identifies the pits.
It's also possible that the voids were created when molten rock flowed beneath the Moon's surface. And in fact, lava flows are known to form on Earth when magma flows under a solidified crust and later drains away. The same thing may have happened on the Moon — especially in a large impact crater where the interior takes an inordinate amount of time to cool, a process that could have taken upwards of hundreds of thousands of years. Following the formation of an impact crater, the sides droop under the forces of gravity, pushing up the crater's floor causing magma to flow under the surface. Voids would then form in those places where the magma drained away.
"They are likely due to melt flow within the pond from uplift after the surface has solidified, but before the interior has cooled," said Wagner in a NASA statement. "Exploring impact melt pits would help determine the magnitude of this uplift, and the amount of melt flow after the pond is in place." His work appears in the latest issue of Icarus.
He also says that the pits would be useful in a support role for human activity on the lunar surface.
"A habitat placed in a pit — ideally several dozen meters back under an overhang — would provide a very safe location for astronauts: no radiation, no micrometeorites, possibly very little dust, and no wild day-night temperature swings."
Wagner says that, in addition to more space-based scanning, the ideal follow-up would be to drop probes into one of two of these pits to get a proper look at what's down there.
"Pits, by their nature, cannot be explored very well from orbit — the lower walls and any floor-level caves simply cannot be seen from a good angle," he says. "Even a few pictures from ground-level would answer a lot of the outstanding questions about the nature of the voids that the pits collapsed into. We're currently in the very early design phases of a mission concept to do exactly this, exploring one of the largest mare pits."
Encouragingly, a NASA-funded endeavor is already working on this. Developer William 'Red' Whittaker and his team at Carnegie Mellon University in Pittsburgh, Pennsylvania are working on a robotic probe capable of crawling through caves and getting out of tight situations.
The probe would be lowered into the Moon pit to check the walls for openings. More ambitiously, it could jump down the hole or lower itself using a cable.
Read the entire study at Icarus: "Distribution, formation mechanisms, and significance of lunar pits".
Images: NASA/GSFC/Arizona State University
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