Mars' moon Phobos is on a slow path to destruction, as evidenced by long, shallow grooves lining its surface, according to NASA scientists. The lines are the first stages of structural failure caused by tidal forces between the moon and its parent planet. At a distance of 3,700 miles (6,000 km), Phobos is closer to Mars than any other moon and planet in the Solar System, which is what is responsible for its looming death.
Phobos is being drawn closer to Mars at a rate of 6.6 ft (2 m) every 100 years. But although its end is nigh in planetary terms, the moon won’t be pulled apart for another 30 to 50 million years.
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The oblong-shaped Phobos, which is the largest of Mars' two moons, is too small to be rounded through gravity. The parallel grooves that line its surface are typically around 98 ft (30 m) deep or less, 330 to 660 ft (100 to 200 m) wide, and up to 12 miles (20 km) long. It was originally believed they were fractures resulting from the impact that created the large Stickney crater. Another theory has that the lines formed from multiple smaller impacts.
But recent modeling by NASA researchers reveals the grooves to be more akin to stretch marks created from the tidal forces caused by the gravitational pull between Mars and Phobos. The same forces between Earth and our moon result in both being slightly oval and produces tides in the oceans.
The new groove-from-gravitation theory was first posited in the 1970s from images taken by the Viking space probe, but scientists then believed the moon was solid and able to handle the stresses that created the lines. Now it’s thought the moon’s interior is made of rubble, wrapped by a 330-ft (100-m) thick outer layer of powdery regolith.
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. This would also explain why some of the grooves are newer than others.
Besides Phobos, the surface of Neptune’s moon Triton is also fractured, and is headed on a similarly slow spiral into its parent planet.