Mars' unusual moons may have been created by collision with Pluto-sized object

Mars' moons are unusual in the Solar System - for their size, shape and colour from their parent planet. Where did they come from? We've got some clues to work with.

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Of the four terrestrial planets in the inner Solar System, Mars has most of the moons. Phobos and Deimos are unusual, however, or at least unusual to us because of how different they are to our own Moon. (Residents of the moons of Jupiter and Saturn would probably be nonplussed.)

They're both a lot smaller relative to Mars than the Moon is to Earth, and - in the case of Phobos in particular - much closer. Both are less spherical and more potato-shaped, because they aren't large enough to be rounded under the force of their own gravity. Deimos (on average, roughly 12km across) orbits Mars at a distance of around 24,000km; Phobos, which is an average of 22km across, orbits at a height of only 6,000km. In astronomical terms, that's extraordinary - it's closer than many artificial satellites orbit Earth. Phobos is so close, in fact, that it orbits Mars faster than Mars rotates. To anyone watching from the planet's surface, the moon will appear to rise and set three times a day. Oh, and they both contain gateways to Hell.

This setup has puzzled astronomers for the more than a century since the moons were first discovered, and, this being Mars, some truly fantastical theories have been put forward over the year. (The best of these, undoubtedly, was Soviet astronomer Iosif Samuilovich Shklovsky's belief that Phobos was a kind of Martian space station, though better data soon ruled this out. Sadly.) As we've sent more and more landers and orbiting probes to Mars, astronomers have managed to gather a significant amount of data on these two moons as well, leading to two main schools of thought: one that says that they're captured asteroids, and another that says that they're made up of the debris from a large impact many millions of years ago.

Phobos passes in front of Deimos, as seen from the surface of Mars by the Curiosity rover in August 2013. Image: Nasa

This second hypothesis is currently seen as the best explanation for how our own Moon formed - the proto-Earth would have been struck by another object roughly the size of Mars, blasting both bodies into pieces which reassembled into the Earth-Moon system we know today. (Though there are some slight issues that are still proving tricky to resolve, such as the differences in chemical composition between the two bodies.) And, according to a new paper published in Icarus, that kind of scenario also works for explaining the Martian system.

Planetary scientists Robert Citron of UC Berkeley, and Hidenori Genda and Shigeru Ida of the Tokyo Institute of Technology, performed some simulations of the debris patterns that would be expected from large impact events, and what that debris would subsequently be expected to do. The authors stress that it's still a preliminary study, but the results show that the impact theory seems relatively plausible.

Mars has a huge depression covering most of its northern hemisphere, known as the Vastita Borealis. When Mars was younger, and warmer, it's possible that it formed the basin for the planet's largest ocean, and illustrations of a wet Mars tend to show the Borealis as a huge polar ocean. The researchers note that the shape and formation of the Borealis is consistent with what would be expected with a large, ancient collision event, as well as the thickness of the planetary crust, so they primarily look into whether something of that scale would be enough to form both Phobos and Deimos, while adjusting variables like the collision object's mass, its speed, its angle of impact and so on to see what happened.

"Our simulations show that for Borealis-scale impacts, enough material is ejected into orbit to form accretion disks that could produce Martian satellites," the researchers write. They estimate that an object of 1.68 x 1022kg - or something roughly the size of Pluto - once smashed into Mars' polar region, ejecting a debris disc of roughly 5 x 1020kg. An event like this would have actually caused the creation of possibly hundreds of "moonlets", less than 100m across each, which would have either rained back down onto the Martian surface over time, or have been forcefully ejected fast enough to reach escape velocity, and left the Martian system altogether. This would explain many of the craters we've seen on Mars, which look "stretched" compared to, say, the neat ones we see on our own Moon - they're the result of orbiting moonlets falling to the surface at angles close to horizontal, like pebbles skimming across a pond.

This is still only theoretical though, of course - this paper is just an exploration of whether such an impact was a) possible, and b) would provide enough debris so that whatever wasn't lost to space or back to the surface would match the real masses of Phobos and Deimos. The conclusion is that "at least one" could have been formed this way, or maybe both. "While a Borealis-scale impact may generate sufficient debris to form both Phobos and Deimos, further studies of the debris disk evolution are necessary," they write. "Our results can serve as inputs for future studies of martian debris disk evolution."

Ian Steadman is a staff science and technology writer at the New Statesman. He is on Twitter as @iansteadman.