Moon's surprise stretch marks show it isn't dead

New evidence suggests that the moon, once thought to be geologically cold and dead, is still stretching and contracting on its surface.
An image from the Lunar Reconnaissance Orbiter shows the largest of the newly detected graben in highlands on the far side of the moon. (NASA/Goddard/Arizona State University/Smithsonian Institution)

New evidence suggests that the moon, once thought to be geologically cold and dead, is still stretching and contracting on its surface.

Images released this week from NASA's Lunar Reconnaissance Orbiter spacecraft show surprising narrow, trough-like features called graben that appear to be the result of the moon's surface being stretched.

"The moon is actually expanding or stretching and being pulled apart in some small areas and by a little bit," said Tom Watters, a researcher with the Smithsonian Institution's National Air and Space Centre for Earth and Planetary Studies.

The stretching causes the crust to break along two parallel faults. The area between them slides down, forming a small valley that is visible to the spacecraft, which can make out features less than a metre across.

Watters led a team that found graben systems in several areas of the moon's surface while analyzing the LRO images. Their study, published in Nature Geoscience, reports that the features are up to 500 metres wide and 20 metres deep.

Watters said it isn't known what caused the stretching, but one possibility is that it's the result of magma rising inside the moon's interior.

"It hasn't broken through and erupted as a lava flow," Watters told CBC's Quirks & Quarks in an interview that airs Saturday.  "It just gets close to the surface — close enough to pick the surface up and stretch it a little bit."

A graben forms when the surface stretches, producing two parallel faults, and then the area between them sinks, forming a valley. (Arizona State University/Smithsonian Institution)

Another possibility is that stretching is caused by bending of the surface that occurs when other parts of the lunar surface contract. Such contraction, thought to be caused by the cooling of the moon's interior, is known to be occurring because of the presence of small cliffs or steps called scarps on other parts of the moon's surface — equivalent to wrinkles on the skin of a fruit such as an orange as it dries out.

The new images, combined with other data from the LRO and a closer look at the seismic data from the Apollo missions several decades ago, suggest the moon may still have a hot interior with a liquid outer core.

Watters said it's clear that the graben are the result of very recent geological activity. Small and medium-sized meteorites are constantly hitting the surface of the moon, stirring up dust and soil so that features such as trenches are filled in over time.

That means the graben are no older than 50 million years and could have formed as recently as 50 years ago.

"The fact that have very young tectonic features really points to a geologically and tectonically active moon not in the recent past, but possibly today," Watters said. "And that's really exciting."