The side of the Moon that faces us is dominated by a series of mares, or seas. These aren't water-filled seas teeming with moonsharks, however. Instead, they're filled with darker rock, thought to be the product of impacts that unleashed a flood of magma onto the surface of the Moon.
The largest of these features was apparently too big to be a mare, and it has picked up the name Oceanus Procellarum. It truly is massive, at about 2,500 kilometers across (for contrast, the Moon is 3,500 km in diameter). It has been interpreted as the remains of an even larger impact basin, but data from the lunar GRAIL mission has now called that theory into question. The new information suggests that Oceanus Procellarum may be the one case where the Moon's own internal heat drove massive volcanic eruptions.
GRAIL involved two satellites orbiting the Moon, with the distance between them being constantly monitored. As they zoom above denser features in the Moon's crust, the leading satellite will accelerate slightly, increasing the separation. Over multiple orbits, this builds a gravity map of the Moon, revealing details of its inner structure that could otherwise only be inferred from surface features. (The mission was based on the successful GRACE satellites that orbit Earth, coming to an end in 2012.)
With data on the whole Moon available, the authors were able to focus on the Oceanus Procellarum. Existing data showed that it was a low-elevation region with thin crust and part of a specific lunar terrain called KREEP. KREEP gets its name from its high levels of potassium (K), phosphorus (P), and rare-earth elements. The potassium typically includes its radioactive form, and the region is also rich in uranium and thorium. The decay of these elements would have ensured that the region remained hotter for longer than other areas of the Moon, possibly contributing to the amount of molten rock present.
Looking at the gravity maps of the Procellarum region, it's clear that it looks little like the other mares, which are circular and have a fairly even gravity signature. Instead, the Procellarum has a relatively sharp and thin border, and its interior is anything but even. In addition, it's anything but circular. In a standard projection of the lunar surface, it appears as somewhat star-shaped. Once corrections are made for the distortion of the mapping (the authors reorient the map to place the center of the Procellarum under a pole), it actually looks like a square.
That's clearly not an impact event, but it's less clear what could be causing it. The authors propose that it's similar to rift zones seen on Earth, where linear fractures get filled with magma that cools and contracts. In fact, they suggest that these magma conduits acted as a feeder system for most of the mares that haven't been clearly associated with an impact basin. Thus, a single feature appears to have dominated the development of the near side of the Moon.
What could have driven the rifting? The authors build a model suggesting that, as the radiation-induced warming of the Procellarum faded, the resulting contraction of the crust could have produced a change of as much as 8 km in the surface of the Moon. That in turn could have created the rifts that allowed magma to the lunar surface. The math on this works out provided that the cooling involved a change of 600K.
Although this is all rock-focused, the authors note that similar-looking structures exist on the south pole of the icy moon Enceladus that orbits Saturn. So it's possible that the basic mechanics of the cooling and fracturing process may work with a variety of materials.
Nature, 2014. DOI: 10.1038/nature13697 (About DOIs).
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