According to NASA, the larger Martian moon, Phobos, is spiraling in toward the Red Planet and will eventually be destroyed in tens of millions of years, but it turns out that it may have a second career after its death. University of California, Berkeley Department of Earth and Planetary Science postdoctoral fellow Benjamin Black and graduate student Tushar Mittal have calculated that the doomed satellite will be so torn by tidal forces that its fragments will form a ring like those that encircle Saturn and the other gas giants of the outer Solar System.
Studies of the orbit of Phobos show that it's being drawn toward Mars at a rate of 6.6 ft (2 m) every 100 years, which will result in its inevitable destruction. However, this will not result in a dramatic impact on the Martian sands. This is because decades of study have revealed that Phobos is structurally unsound and riddled with fractures, pores, and rubble. According to Black, this makes it less like a solid moon and more like an orbital granola bar.
As Phobos moves closer to Mars, the tidal forces will increase, tugging at the moon in the same way as the Earth's Moon pulls at the oceans. Black and Mittal studied the fracturing properties of terrestrial rocks and of meteors with a composition similar to Phobos, and combined this with structural data about Phobos itself. This allowed them to estimate the strength of Phobos and predict that in 20 to 40 million years it will be ripped into debris and dust that will distribute itself about Mars to form a ring.
The scientists say that the ring will not be permanent, but will last for one million to 100 million years. The largest pieces of debris will spiral in and strike Mars to produce egg-shaped craters, while the smaller pieces will be drawn in more slowly to create regular episodes of meteor showers until the ring disappears and only the smaller moon Deimos remains. How long the ring lasts depends on Phobos' proximity to to Mars when it breaks up.
"If the moon broke apart at 1.2 Mars radii, about 680 kilometers above the surface, it would form a really narrow ring comparable in density to that of one of Saturn's most massive rings," says Mittal. "Over time it would spread out and get wider, reaching the top of the Martian atmosphere in a few million years, when it would start losing material because stuff would keep raining down on Mars."
According to Mittal, it's uncertain if the ring will be visible from Earth. Saturn's rings are very visible because they consist partly of ice, which is very reflective, but the Martian ring will be of rubble and dust. However, it could make Mars quite the tourist spot.
"Standing on the surface of Mars a few tens of millions of years from now, it would be pretty spectacular to watch," says Black.
Mittal says that the study of Phobos' fate is important because rings may have been a common sight in the early years of the Solar System with 20 to 30 percent of the planets having self-destructing moons that may account for many planetary surface features seen today.
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