Science & Technology

The Moon Has Wrinkles Because It's Shrinking

Did you think the moon was a dead and boring world? In recent years, we've discovered our closest large neighbor has so much more than we thought: a tenuous atmosphere, water ice, and even levitating dust. The most recent surprise? The moon has wrinkles. Not because it's old, but because it's shrinking.

Turbulent Seas

We've known for quite some time that the moon had ancient geological activity. After all, a small telescope shows the vast lunar mountains, and the Apollo astronauts explored some of them up close in the 1960s. What surprised scientists recently, though, is they found geological activity in a spot that before, they thought was quiet. A study based on the research was published in the journal Icarus, led by Nathan Williams, a post-doctoral researcher at NASA's Jet Propulsion Laboratory.

This zone is a "mare" (pronounced mah-ray), an ancient lava sea. It's called Mare Frigoris and it was captured in images by NASA's Lunar Reconnaissance Orbiter. Inside the sea are trenches and hills that are being created as the moon loses heat from its formation and slowly shrinks. It's the first time these so-called "wrinkle ridges" have ever been seen on a mare.

What's more, these ridges are relatively young — just 40 million years old, by some estimates. It's a long time ago in human terms, but consider this: It was roughly 66 million years ago that dinosaurs went extinct on Earth. And before this study, scientists thought all the wrinkle ridges on the moon stopped forming about 1.2 billion years ago. That's quite the time difference.

Mare Frigoris

How Is the Moon Shrinking?

The moon has moonquakes just like we have earthquakes, but the process is different. On Earth, huge sections of crust called tectonic plates slide against each other and create vibrations. But the moon is different.

The moon likely formed after a large, Mars-sized body smashed into Earth roughly 4.5 billion years ago. Over the eons, the pieces coalesced into the moon. As you can imagine, that collision generated a lot of heat. But that ancient heat is cooling, and as the whole moon cools down, its interior shrinks and its surface buckles.

In Mare Frigoris, this creates huge wrinkle ridges. The longest ones are roughly 250 miles (400 kilometers) long, or more than the distance between New York City and Washington, D.C. They also rise about 1,000 feet (333 meters) off the surface.

We can estimate the age of these ridges using "crater counting," or simply adding up the number of craters in a particular region. We know roughly how often objects hit the moon to generate these craters, but tectonic activity can cover them up over time. That means that by comparing the estimated collision rate to the actual number of craters present, scientists can have a rough idea of how old a region is. We also can see signs of craters "aging" — collecting debris from other impacts, for example. The moon might be old, gray, and wrinkled, but it's not dead yet.

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Written by Elizabeth Howell May 16, 2019

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