Why Is the Moon Shrinking?

The Moon is shriveling like baboon's testicles in an ice tea glass, NASA scientists said after analyzing new images from the Lunar Reconnaissance Orbiter. Not with those exact words, mind you, but similar enough: It's shrinking because it's cooling down.

The Moon was hot at the beginning of its life. The most accepted theory—proved by simulations and analysis of its surface materials—is that it formed after a massive collision of a Mars-sized object with Earth, around 4.5 billion years ago. The material ejected in the impact accreted to form the Moon. After that, the lunar surface suffered a period of massive bombardment of asteroids and meteors, which combined with the decay of radioactive elements, kept it hot for a long while.

So long, in fact, that it's still probably cooling down today. Before, scientists believed the cooling and shrinking period ended early in its story. Now, new images by NASA's LRO show lobate scarps that are only one billion to one hundred million years old. This is very recent in geological terms, according to Dr. Thomas Watters of the Center for Earth and Planetary Studies at the Smithsonian's National Air and Space Museum, Washington:

Why Is the Moon Shrinking?

One of the remarkable aspects of the lunar scarps is their apparent young age. Relatively young, globally distributed thrust faults show recent contraction of the whole Moon, likely due to cooling of the lunar interior. [...] We estimate these cliffs formed less than a billion years ago, and they could be as young as a hundred million years.

Lobate scarps—called that way because many of them appear to be lobe-shaped—form as inner parts of the moon collapse, forcing a section of the crust to crack and bulge out over another.

The team formed by scientists at NASA Goddard, Arizona State University and the Smithsonian, believe that these scarps are "among the freshest features on the moon, in part because they cut across small craters." Mapping these thrust faults, they have been able to reconstruct the Moon's tectonic and thermal history during the past billion years.

But how much has it shrunk? Using data taken by the LRO, they believe the distance between the moon's center and its surface shrank about 300 feet in the last one billion years. [Shrinking Image]