The mystery of where Mars’s two moons came from may finally be solved. If Phobos and Deimos were formed when an icy object smashed into Mars, it could explain their contradictory properties, which have long baffled researchers.
Measurements of the geological makeup of Phobos and Deimos, including tentative signs of water, have suggested that they are more similar to asteroids than to Mars itself, hinting that they may be captured asteroids. However, that explanation wouldn’t account for their circular orbits, which instead indicate another origin: they could have formed when a large object smashed into Mars, blowing material off the Red Planet to create a disc of debris that later coalesced into the two moons. But this process would have heated up all that material to thousands of degrees, which would melt away any water and much of the asteroid-like material found on their surfaces.
It has proven difficult to find a scenario that matches both the makeup and the orbits of the moons, but at the Tokyo Institute of Technology in Japan and their colleagues found one that may work. They presented their research at the Lunar and Planetary Science Conference in Texas on 11 March.
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The researchers performed simulations of a smash-up between Mars and a large icy object, rather than a rocky one. One strength of this scenario is that, as some of the ice vaporised in the collision, it would have absorbed heat. So the disc would only be heated up to hundreds of degrees rather than thousands, allowing some water to stick around.
Another is that with an icy impactor, the disc would have ended up being up to 80 per cent impactor material and only 20 per cent Mars material. “This is interesting because it means that the moons are mostly composed of impactor materials rather than Mars materials,” says Monchinski. This would explain why the makeup of Phobos and Deimos is so different from that of Mars.
Plus, because it is a giant impact scenario, it would still make sense for the moons to end up in their observed circular orbits.
The main problem with this idea is that it could result in huge amounts of water falling onto Mars. But Monchinski thinks a smaller impactor, maybe about the size of the asteroid Vesta or even smaller, wouldn’t have the same effect.
“I think the key issue here will be matching observational constraints,” says at the Southwest Research Institute in Colorado. Recent observations from the United Arab Emirates’s Hope orbiter should help narrow it down, he says.
