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Moon's largest plain is not an impact crater

Gravity data suggest flats of volcanic basalt formed from tectonic stretching.

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  1. Oceanus Procellarum, a dark plain visible to the naked eye on the near side of the Moon, is surrounded by gravitational anomalies (superimposed in red) hidden under the surface. These reveal ancient rifts that do not have the round shape expected of a crater's rim.

    Kopernik Observatory/NASA/Colorado School of Mines/MIT/JPL/Goddard Space Flight Center

  2. How the rifts on the western border of Oceanus Procellarum might have looked more than 3.5 billion years ago, when they were active and filled with lava. This image and the next one combine data from two NASA missions, GRAIL and the Lunar Reconnaissance Orbiter.

    NASA/Colorado School of Mines/MIT/JPL/Goddard Space Flight Center

  3. Another reconstruction of a now-buried lunar rift, this one bordering the Mare Frigoris plain, north of Oceanus Procellarum.

    NASA/Colorado School of Mines/MIT/JPL/Goddard Space Flight Center

  4. The Moon as observed in visible light (left), topography (centre, where red is high and blue is low), and the gravity gradients measured by the two GRAIL probes (right).

    NASA/Colorado School of Mines/MIT/JPL/Goddard Space Flight Center

For years scientists have believed that a vast plain of solidified lava on the nearside of the Moon — known as Oceanus Procellarum — was shaped by an asteroid that slammed into the orb billions of years ago.

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Elizabeth Gibney asks Jeffrey Andrews-Hanna about the giant rectangular pattern he and his team discovered in a map of lunar gravity.

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But now an analysis of a gravitational map of the 3,200-kilometre-wide ancient volcanic region suggests that it is not a giant impact crater after all. A study that reveals features of the crust beneath the surface shows that the area is rimmed by structures that run in straight lines and intersect at angles, similarly to a rectangle. Such a shape is hard to attribute to a collision, which would be expected to produce a more circular rim, and could instead be the result of geological activity within the Moon's crust.

In the study, published in Nature1 on 1 October, the researchers used data from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission, which ended in late 2012. GRAIL’s twin spacecraft measured changes in the Moon’s gravitational field caused by variations in the density and topography of the crust below. The map revealed anomalies buried beneath the plains' basalt surface, which the authors interpret as valleys where the crust of the Moon has been stretched and thinned, a process that on Earth happens as tectonic plates move apart. On the Moon, these rift valleys were later filled in with lava flows.

The findings were unexpected, says Jeffrey Andrews-Hanna, a planetary scientist at the Colorado School of Mines in Golden who led the study and was a member of the GRAIL science team. "Rift zones aren’t something known on the Moon, although we see them on Earth, Venus and Mars,” he says.

Previously scientists had thought the Procellarum was an impact basin based on its low-elevation, distinctive composition -- which includes higher than average levels of elements such as uranium, thorium and potassium -- as well as fragments of surface structures that looked like the remnants of a rim. But Andrews-Hanna and colleagues have a new explanation. They say that the region's high concentrations of radioactive elements would have made the area hotter than its surroundings, and quicker to cool (see 'Gravity maps reveal why the dark side of the Moon is covered in craters'). As the region cooled and contracted, the crust at its edges would have stretched, forming valleys in the distinctive rectangular pattern, they say. The process is similar to the way fractures that form around a mud puddle as it dries, says Andrews-Hanna.

The pattern looks remarkably similar to one on Saturn’s moon Enceladus, Andrews-Hanna adds. Despite the many differences between the moons, similar physical processes may have occurred on them, he suggests.

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  1. Andrews-Hanna, J. C. et al. Nature 514, 6871 (2014).

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