Phobos and Deimos, the two oddly-shaped moons circling Mars, are widely thought to have once been asteroids that were ensnared by the Red Planet’s gravity, but new research presented at the Lunar and Planetary Science Conference in Texas last month concluded the two moons are the result of a massive collision between Mars and a Pluto-sized planetary body.
Those supporting the asteroid theory have had difficulty explaining the moons’ fairly circular orbit around Mars. If they were asteroids captured by Mars’s gravity, they would likely have irregular orbits, research has shown. One circular orbit might happen by chance, but two would be against the odds, study researcher Julien Salmon of the Southwest Research Institute told New Scientist.
Changing past moon formation models
In the past, models of a moon-forming Mars collision have shown the material tossed by a collision into an orbiting disc gradually comes back down to the planet, which means nothing would be left to form Phobos and Deimos.
In the new study, Salmon modified models used to analyze the formation of Earth’s moon, which could also be the consequence of a large collision. The models showed an impacting object with around 3 percent of the mass of Mars could generate the proper type of disc to form the two moons
The outcomes indicate that such an object, with approximately the same mass as Pluto, would toss around a thousandth of Mars’s mass into orbit, and the fringe of the disc would extend past the 15,000 mile orbit of Deimos, the outer moon.
Over time, the material closer to Mars would form into large bodies, but the planet’s gravity would ultimately pull them back down. However, the outer section of the disk would rotate quickly enough to remain it out of Mars’ gravity, and the material would coalesce into the Phobos and Deimos, according to the study.
“The idea is that Phobos and Deimos are the only two survivors of a once much larger population of satellites,” Salmon said.
The SwRI research added that an impact would also explain Mars’ relatively fast rotation and the large differences in surface heights between northern and southern hemispheres.
“It makes sense to think about a big impact for Mars,” he said.
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Image credit: NASA
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