Moon shrinks

Newly discovered cliffs suggest slight waning over past billion years

The moon has shrunk globally in the past billion years, new high-resolution pictures suggest. The evidence comes in the form of cliffs all over the moon that have formed over the last million millennia like wrinkles on the surface of a dried-out piece of fruit.

Scientists have studied 70 to 80 of these cliffs, called lobate scarps, since the Apollo missions in the ’70s. The scarps are generally tens of kilometers long and less than 100 meters high. Researchers had already hypothesized that the scarps were due to shrinkage, but the Apollo images showed only the moon’s equatorial region. Scientists weren’t sure whether the scarps spanned the entire surface of the moon.

MOON ROCKS At less than a billion years old, lobate scarps are some of the moon’s youngest landforms. Scientists believe they form as the moon’s interior cools and shrinks, forcing pieces of the crust together. Courtesy of NASA/GSFC/Arizona State University/Smithsonian

Now, images in the Aug. 20 Science from the Lunar Reconnaissance Orbiter, NASA’s unmanned moon satellite, reveal 14 new scarps from spots all over the moon. It’s the first definitive evidence that the scarps occur globally.

“This is the best observational evidence so far that the lunar cooling and contraction has persisted over the last billion years,” says planetary scientist Sean Solomon of the Carnegie Institution for Science in Washington, D.C.

Lobate scarps are formed when a planet or moon cools and contracts inside. The loss of volume squeezes portions of the outer crust together. Eventually, the crust breaks and some of it is pushed up, creating long cliffs that look like wrinkles.

The process is a little like what happens when an apple has gone past its prime, says Thomas Watters, a planetary scientist at the Smithsonian National Air and Space Museum who led the study. As the apple dries out, it loses volume, so the skin gets wrinkly.

Watters and his team looked at the pattern of craters over and underneath the newly observed scarps to determine their ages. Most were untouched by the larger craters produced by giant impacts early in the moon’s history. And the scarps tended to cut across younger, smaller craters produced by objects that pelted the surface more recently.

The evidence points to recent shrinkage of perhaps 100 meters or so in radius, says Watters. In lunar terms, “recent” means the past billion years. “There’s a general impression out there that the moon is geologically dead, that anything of geological importance happened billions of years ago,” Watters says. “The scarps’ very young appearance suggests this shrinking of the moon could have happened very recently, and in fact could be still going on.”

But moon gazers have nothing to fear; the change isn’t noticeable. Just 100 meters over a billion years isn’t much, Watters says.

By comparison, the scarps on Mercury are over a kilometer high and some span 1,000 kilometers, he says, suggesting shrinkage of up to a kilometer.

Watters says the small shrinkage adds support to the idea that the moon started out somewhat cool and only partially molten and shrank less than 1 kilometer over its entire lifetime. But other scientists, such as retired lunar and planetary scientist Alan Binder of the Lunar Research Institute in Tuscon, think the moon was completely molten at its birth and may have shrunk as much as 5 kilometers in its life.

I’m glad to see this, even though I don’t think it’s a great revelation,” Binder says. He is among those who first described the scarps 30 years ago and hypothesized about their implications. “Every new scarp is better, but it hasn’t changed the picture at all; it’s just adding a little more data.”

Next, the scientists will use the LRO to image the entire moon surface and find the total number and pattern of its scarps. Scientists can then better infer how the cliffs were formed and how much the moon has shrunk. And by comparing current images with those taken 30 to 40 years ago, they’ll be able to see if the scarps have changed since then.

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