Planets

Mars May Have Once Had Rings — and Could Sport Them Again

Mars' diminutive moon Phobos may be the latest in a series of increasingly smaller moons that formed from rings of debris that circled the Red Planet in its past, and likely will again in its future.

<p>Credit: <a href="https://www.flickr.com/photos/kevinmgill/16528411470/" target="_blank">Kevin Gill</a></p>

Mars, like modern-day Saturn and other planets in the outer solar system, may have once sparkled with rings, according to a new study.

Computer simulations suggest that the Red Planet's first-generation ring formed some 4.3 billion years ago when an asteroid or other large celestial body slammed into the young planet, sending a shower of debris into space. Over time, Mars' gravitational field sculpted the debris into a ring, which later coalesced into a moon.

The moon would have been far more massive than the diminutive Phobos, which measures 10 by 14 by 11 miles, but they share the same fate.

Phobos, currently in orbit just 3,700 miles above Mars - closer than any moon in the solar system - is being drawn toward its home planet at a rate of some 6.6 feet every 100 years.

Phobos will be ripped apart by Mars' gravity in another 50 million to 70 million years, computer simulations show. Most of the debris will rain down onto the planet, but about 20 percent will remain in orbit and form another ring and then perhaps another moon.

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The cycle has repeated three to seven times since the original impact 4.3 billion years ago, Purdue University researchers Andrew Hesselbrock and David Minton wrote in a paper published Monday in Nature Geoscience.

Mars' other moon, Deimos, which is about half the size of Phobos and in a more stable and more distant orbit, is likely an original moon, the research shows.

"Deimos, in our model, is a distant remnant of the original giant impact debris disk that was more or less untouched by what the Phobos precursors were doing," Minton wrote in an email to Seeker.

The research casts additional doubt on previous theories that Phobos and Deimos are captured asteroids.

"Although they somewhat resemble asteroids, it would be exceedingly difficult to capture them into the orbits that we see them in," Minton noted. "Captured moons tend to have orbits are very distant from their planets, very inclined relative to the equator of their planet, and could be traveling in any direction relative to the spin of the planet."

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"Phobos and Deimos are very close to Mars, orbit very near the equator, and are both going around the same direction as Mars' spin," he added. "This indicates strongly that they formed from a debris disk or ring around Mars."

Previous research has shown that Mars' Borealis basin, an asymmetrical region cutting across the planet's northern hemisphere, could be the impact site of a large moon.

More answers may come from a Japanese spacecraft, scheduled to launch in 2022, which will pick up samples from Phobos and fly them back to Earth. Europe and Russia are partnering on a similar venture two years later.

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