The Moon Is Shrinking!

We are all familiar with the constant waxing and waning of the moon, the optical result of sunlight illuminating the moon as it orbits around the earth. But Dr. Thomas Watters, a senior scientist with the National Air and Space Museum’s Center for Earth and Planetary Studies, has discovered that, i…
An image from the Lunar Reconnaissance Orbiter camera that shows one of the many fault scarps on the moon. Courtesy of NASA/GSFC/Arizona State University/Smithsonian.

We are all familiar with the constant waxing and waning of the moon, the optical result of sunlight illuminating the moon as it orbits around the earth. But Dr. Thomas Watters, a senior scientist with the National Air and Space Museum's Center for Earth and Planetary Studies, has discovered that, in fact, the moon's radius is literally shrinking, and it's not growing back any time soon.

Watters has used the high-resolution Lunar Reconnaissance Orbiter Camera to snap pictures of the moon's surface. These pictures show fault scarps, cliff-like formations that resemble stretch marks from afar. Watters attributes the scarps to an internal cooling of the moon, which causes the moon to shrink and the moon's crust to shrivel into scarps. All of this happened within the past billion years—a relatively short time in the history of our solar system. And it may still be going on today.

I caught up with Watters to hear more about his discovery.

The moon is a constant in our lives. We depend on it for the tides and some even look to it for our moods and psyches. Is it possible that the shrinking of the moon could ever affect life on earth?

Well, I can't speak to the moods and psyches, but certainly it affects the tides. The tide is the most immediate and direct influence the moon has. And the fact is that the shrinking doesn’t matter, because what controls the tidal forces is the mass of the object, not its size. If we saw a plume of material being ejected from the moon, then we would have a concern about its mass changing, but we haven’t seen anything like that.

What did your study conclude, and can you tell me why its findings are significant?

We have found this young population of fault scarps that indicate that the moon’s crust has shrunk or contracted, and we now see that these faults are globally distributed on the moon. The most plausible process is interior cooling of the moon. But what is really intriguing to me is not so much that, but that it happened so recently. It’s the fact that these fault scarps appear to be so young (less than a billion years old), which suggests that the moon is still tectonically active today. It’s still cooling very recently and may still be cooling and contracting now. These scarps look so pristine, they look like they could have formed yesterday.

I think there’s this general perception out there that, geologically, the moon is a kind of dead object, that everything that happened to the moon of geologic importance happened billions of years ago, and I think people are a little surprised when they realize that that may not be true at all, that the moon may still be geologically active today.

Did this surprise you?

Yes. To be fair, we knew that some of these fault scarps were on the moon from high-resolution images taken by a camera that was flown on three of the Apollo missions. But those were very limited in their coverage of the moon. With the Lunar Reconnaissance Orbiter, we could start to image the entire moon. Today we still only have about 10 percent of the surface of the moon that’s been imaged at this very high resolution, but were showing up even in that amount of coverage, and that was really exciting. Now we can say, this isn’t just some local or regional force that has acted to cause these faults. This is happening everywhere on the moon.

Will these fault scarps still be around in another billion years?

That’s one of the really interesting questions. Are the fault scarps that we’re seeing still active today? One of the things we’re going to do to test that is to actually look at the Apollo images where we first saw these fault scarps, and re-image those with the Lunar Reconnaissance Orbitter Camera, so we can compare the two. We have roughly 40 years difference between those Apollo-era images and the Lunar Reconnaissance Orbitter images, so we can actually look at those very closely and see if there’s any indication of change on those fault scarps that might suggest that the faults are still actively growing. And if they are, not only will these fault scarps continue to develop and actually be around in another billion years, there will be more of them that have formed in that time frame.

How much has the moon shrunk already?

If you add up all the contraction that we’ve seen on the faults that we know about and you project that over the entire globe, it turns out to be about 100 meters decrease in the radius of the moon. That may sound like a lot, but the radius of the moon is about 1,738 km, so it’s a small change in the overall radius of the moon.

Are any other planetary bodies getting slimmer?

Actually, yes. Mercury, for example, has the same kind of fault scarps, except they are huge. Instead of being several kilometers in length, they can be hundreds of kilometers in length, and instead of being tens of meters in relief, the ones on Mercury can be well over a kilometer in relief. So Mercury seems to have undergone the same kind of contraction but in a much bigger way.

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