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Brace for impact. A new count of the moon’s craters has turned up 33 per cent more than predicted. Bombardment by small meteors is continually churning up the surface of the moon, and the dust-up could pose hazards to future lunar settlements.

Small meteors regularly impact the moon and Earth. On Earth, they usually burn up in the atmosphere, or land in uninhabited areas where they go undetected. But, on the moon, they constantly form craters and impact basins.

Previous studies of craters and of Apollo samples helped paint a picture of past crater formation, but we’ve only recently begun to distinguish new craters amid the moon’s already-battered surface. It turns out some of them may be newer than we thought.

and colleagues at Arizona State University compared 14,000 “before and after” pictures of the same areas taken on different occasions by the high-resolution camera on board the Lunar Reconnaissance Orbiter (LRO). They counted 222 new impact craters more than 10 metres wide, or 33 per cent more craters than previous estimates predicted.

They also found 47,000 new splotches, which is Speyerer’s term for slight splatter-like changes in reflectance on the moon’s surface. The splotches are formed in secondary impacts of dust and rock that are thrown off in the initial crater-forming impact.

“That’s unique to the moon,” he says. “That’s forming from the primary impact throwing out this ejecta, and that’s what makes these images really unique.”

The largest new crater is 43 metres in diameter. The smallest were 10 metres, which is as small as LRO can see.

Gardening on the moon

One crater was spotted shortly after it formed on 17 March last year. Astronomers at Marshall Space Flight Center in Huntsville, Alabama, noticed a bright flash in a region called Mare Imbrium, and the LRO team was able to find the 18-metre crater shortly thereafter. An automated search for before-and-after images turned up a large number of splotches, too – some up to 30 kilometres away from the initial impact. That suggests lunar dust and rock can travel great distances after being kicked up.

“There might have been splotches that were further, but that’s as far as our data went,” Speyerer says. “If you are an astronaut sitting on the surface, you don’t necessarily have to worry about being directly hit by a meteorite, but you would have to worry about all these secondaries, that are coming from kilometres and kilometres away.”

All these impacts suggest the moon’s surface turns over more frequently than previously thought. The first 2 to 3 centimetres of moon dust probably churns over every 80,000 years, not every million years as we previously thought – faster by a factor of 100, Speyerer says.

The revised number of craters suggests the moon is pummeled by space rocks much more frequently than predicted, says of the Southwest Research Institute in San Antonio, Texas. It also suggests that the soil on the lunar surface is turning over so often that materials like water molecules could escape into space sooner than previously thought. That could have important implications for researchers trying to date rocks on the moon, or future initiatives hoping to mine resources out of the moon.

“I like it when theories are proven wrong, or exciting new things come up,” she says. “The Lunar Reconnaissance Orbiter is starting to show there’s a lot we don’t know about the moon.”

Journal ref: http://nature.com/articles/doi:10.1038/nature19829