An unusual ancient magma pattern has been found on the Moon's largest dark spot, suggesting that the Moon was actually once much more geologically alive than previously thought.
Scientists have found a giant rectangular magma pattern several kilometers below Oceanus Procellarum, the largest dark spot on the Moon's near side. Thought to be the remnants of a geological plumbing system, this ancient feature would have spilled lava across the Moon's surface about 3.5 billion years ago.
The feature is very similar to rift valleys found on Earth - areas where the crust rips apart as it cools and contracts. This degree of geological activity is normally associated with larger bodies like planets, which are better able to retain the internal heat which drives these processes.
"We're realizing that the early moon was a much more dynamic place than we thought," says Jeffrey Andrews-Hanna, a planetary scientist at the Colorado School of Mines in Golden and lead author of a new study published in Nature.
Apart from lending evidence that the Moon was once more geologically active than previously believed, this new finding also puts the origin of the circular Procellarum region in question. For decades, it has been thought that Oceanus Procellarum was a basin, or giant crater, created when an asteroid struck the Moon.
The rectangular pattern was found by mapping density variations of the Moon's subsurface. Underneath known impact basins, the expected ringlike patterns were found, but underneath the Procellarum region, the rectangular pattern was seen. "It was a striking pattern that demanded an explanation," Andrews-Hanna says.
The cause of the rifting is still unclear, because the Moon is simply not large enough to have the same cooling processes that the Earth has, which typically results in the formation of rifts on our planet. Some counter-arguments have suggested that the formation is not, in fact, rectangular, and just an ordinary circular pattern consistent with an impact basin.
Check out the images for yourself - what do you think? Rectangle or circle? Rift or impact basin?