The two moons of Mars may once have been a single comet that was ensnared and split by the planet – and an upcoming mission could find out for certain.
How Mars got its two moons, Phobos and Deimos, is a bit of a mystery. They are small, 27 and 15 kilometres across respectively, and both orbit around the planet’s equator. Astronomers have suggested that they may have formed after a collision on Mars’s surface, similar to how Earth’s moon was formed, or be asteroids that were captured by the planet’s gravity.
at the Paris Observatory and her colleagues have a different idea. They propose that the moons may be one or two captured comets, similar to comet 67±Ę/°äłółÜ°ů˛âłÜłľ´Ç±ąâ€“G±đ°ů˛ą˛őľ±łľ±đ˛Ô°ě´Ç, which the European Space Agency (ESA) spacecraft Rosetta visited in 2014.
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The researchers studied data on Phobos from ESA’s Mars Express spacecraft, which has been orbiting Mars for more than 20 years. “We found a huge number of observations that were never published before,” says Fornasier.
The team’s analysis of Phobos shows that the moon has similar visual properties to a comet, reflecting an amount and type of light that puts it more in line with comets like 67P than it does a typical asteroid.
This suggests that Phobos, and by extension Deimos, could have been part of a binary comet pair that were captured by Mars, or a single comet that broke into two pieces.
“Dynamically, it’s very difficult to capture an asteroid and have two satellites in the equatorial plane of Mars,” says Fornasier. “What we are suggesting is maybe it’s a binary comet that was captured by Mars.”
An upcoming Japanese mission called , set to launch in 2026, will attempt to bring samples of Phobos back to Earth in 2031. Any abundance of volatile elements like carbon, oxygen or nitrogen in those samples would support the idea that Phobos was a comet. That could mean we get samples not just from a moon of Mars, but from a body that originated in the outer solar system. “It would be the first sample from a cometary nucleus,” says Fornasier.
Other planets may have captured comets too, such as Saturn, whose moon Phoebe seems to have originated in the outer solar system.
arXiv
