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Did Mars Steal its Moons From the Asteroid Belt?

Scientists still aren’t sure where the odd little moons came from, but they have ideas

The dark gray object lurking above the Herschel crater on Mars is the Martian moon Phobos, as seen by the Viking Orbiter 1 in 1977 ( NASA/JPL/Michael Benson/Kinetikon Pictures/Corbis)
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The two moons that orbit Mars are tiny, dark and lumpy like two burnt potatoes. These apparently un-moon-like characteristics led researchers to think that Phobos and Deimos were originally asteroids in the asteroid belt that had been nabbed by the Red Planet. But now researchers argue that they could have formed much more like Earth’s own Moon probably did — after a planet-smashing impact.

For Science, Ken Croswell reports that this impact hypothesis isn’t new, but that the researcher who proposed it decades ago was met with scorn. Croswell writes:

"It was embarrassing how badly people came after me," says geologist Robert Craddock of the Smithsonian Institution’s National Air and Space Museum in Washington, D.C., who presented the idea at a 1994 conference. "The general tone was this couldn't possibly work." He submitted a scientific paper to several journals, including Nature and Science, which, he says, all rejected it.

But now one research team’s new computer simulations and another’s calculations shows how Craddock’s idea does work. The scenario may have played out as follows.

An object 10 times the size of Ceres, the largest asteroid now in the belt, may have catapulted into ancient Mars sometime during the solar system’s formation. It spun the planet enough so that its rotation today is faster than scientists expected. It scarred the surface of Mars with one of its largest craters — the northern hemisphere’s Borealis basin, which covers 40 percent of the planet. And it sent enough debris up in to space to form a disk around the planet. This debris disk hugged close and eventually gave rise to three moons. 

So what happened to that third moon? It was larger than both Phobos and Deimos, though still only a tenth of the size of Earth’s Moon. In this version of cosmic history, that third moon was eventually pulled down to smashed into Mars. "It probably only lasted a few hundred million years," Robin Canup, one of the researchers who worked on the simulations, told Science. 

However, Croswell reports that other researchers are still skeptical. He writes:

Scott Murchie, a planetary scientist at Johns Hopkins University in Baltimore, Maryland, who has been studying the martian moons for more than a quarter-century, says the new work is valuable. But, he adds, "I think by far the interpretation most consistent with observable characteristics of the moons aside from their orbits—[their] density, spectral properties, [dark color]—would be that they're captured asteroids." The new simulations don't address the moons' composition—only their sizes and orbits.

One way to resolve the debate is to send landers to Mars’ moons to check their composition. But if they want to go that route, researchers better get on it soon: Phobos will crash into Mars one day, whereas Deimos will eventually drift out of orbit. Fortunately, they have a few million years at least.

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