Moon

The moon's radius has shrunk about 45 meters as its interior cools, study finds

Over the past 400 to 800 million years, the moon's radius has shrunk by roughly 148 feet (45 meters) as its interior continues to cool and contract, according to a study published October 14 in The Planetary Science Journal and reported by Space.com. Researchers analyzing images from the Lunar Reconnaissance Orbiter Camera identified more than 10,000 young faults, or cracks, across the lunar surface to reach the estimate.

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How researchers counted the faults#

Thomas Watters, a geologist and scientist emeritus at the Smithsonian's Center for Earth and Planetary Studies, led the study and said in a statement that the number of young faults found in the images was far greater than expected, adding that as the moon's interior keeps cooling and contracting, existing faults are expected to keep growing while new ones form, Space.com reports.

The Lunar Reconnaissance Orbiter has observed the moon for 17 years, the longest run of any lunar orbiter, following a near-polar orbit and mapping more than 98% of the surface at 100-meter resolution outside permanently shadowed polar craters, according to Universe Today. Its camera system, made up of a Narrow Angle Camera and a Wide Angle Camera, supplied most of the data behind the new fault count.

Earlier surveys had far less imagery available, according to The Brighter Side of News: Apollo-era photographs supported a catalog of just 71 scarps, and a later study using Lunar Reconnaissance Orbiter Camera images found roughly 3,200. To translate the larger fault count into a contraction figure, the team determined the vertical displacement of 42 representative scarps by constructing terrain models from pairs of camera images, then extended these measurements across the full set of mapped features.

Lobate scarps and the moon's cooling core#

The moon is thought to have formed roughly 4.5 billion years ago when a giant body struck the young Earth, and its interior has been cooling ever since, a process that causes the whole body to contract slowly, Space.com reports. That contraction pushes crustal material upward along faults, producing lobate scarps, long curved cliffs, and smaller wrinkle-like ridges called mare ridges across the dark volcanic plains.

Most of the scarps measured in the new survey rise tens of meters above the surface, though some exceed 150 meters, and they range in length from under a kilometer to tens of kilometers, according to The Brighter Side of News. The work also raised a question the researchers flagged themselves: why such faulting on the lunar surface emerged only during a relatively recent phase of the moon's history.

A possible hazard for lunar missions#

Universe Today reports that lunar contraction and faulting pose a potential hazard for missions landing on the surface, making the subject more than a matter of academic interest. The faults are most common in the lunar highlands, where the scarps are described as simple, positive-relief landforms formed by the thrust faulting of crustal material.

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