His colleagues had once attributed the grooves to asteroid impacts and similar events, but as they don’t seem to be localized around one specific crater, Hurford believes that the grooves come from tidal forces, or the gravitational push and pull of Mars and Phobos at work. The gravitational pull of the Earth and moon is responsible for the planet’s tides as well as the bulging of both entities into a slight egg shape.
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Initially, scientists had thought the grooves were created by the massive impact that made the Stickney crater, the huge impact impression to the lower right of the main image.
A new model by Hurford and associates supports the view that the grooves on the moon are “stretch marks” that occur as Phobos gets deformed by the gravitational pull – known as tidal forces – between the Mars and the moon. This would also explain why a few of the grooves are newer than others.
Yet, Hurford’s research suggests that the grooves are actually far more similar to stretch marks than to impact markings.
Long shallow grooves cut into the moon’s surface appear to be stress fractures, according to a study presented at the American Astronomical Society’s planetary sciences meeting in Maryland this week.
The larger of Mars’ two moons, Phobos is closer to the planet than any other in the solar system, orbiting at only 6,000 kilometres above the surface and being pulled closer by two metres every hundred years.
In this scenario, the reason for Phobos’ ultimate destruction is the mutual gravity between it and Mars.
The same explanation was proposed for the grooves decades ago, after the Viking spacecraft sent images of Phobos to Earth. At the time, however, Phobos was thought to be more-or-less solid all the way through.
Worse, astronomers now think the moon’s interior is nothing more than a bunch of space rubble, packaged under a 100 meter-thick pile of powdery regolith. Tidal forces wouldn’t be strong enough to distort an entirely solid planet, the thinking went.
The easily distorted interior forces the outer layer to readjust, which the researchers believe behaves elastically while building stress, and will eventually cause it to fail.
Besides Phobos, the surface of Neptune’s moon Triton is also fractured, and is headed on a similarly slow spiral into its parent planet.
Phobos is the closest moon to any planet in our solar system.
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Bottom line: The mysterious grooves on Phobos are likely early signs of structural failure.
