The origin story of Mars's moons is rather explosive. Phobos and Deimos came to be after a large asteroid slammed into the Red Planet early in its history.
Researchers haven't identified the location yet. But they have three candidate, all basins on the Martian surface: Hellas Planitia, Utopia Planitia, and Borealis Basin.
In a study published in the journal Science Advances, researchers suggest a large space rock the size of Ceres or Vesta, two of the largest objects in the asteroid belt, smashed into the surface. The crash sent debris into orbit, which eventually coalesced, leaving the two moons we see today.
Researchers now want to find out more about the composition of the two moons, lead author Robin Canup told Seeker. Japan's Mars Moons eXploration (MMX) mission is scheduled to launch in 2024. It will land on the surface of Phobos and return a sample to Earth in 2029. Canup said her team expects Phobos to be very dry, with little ice or other hydrated minerals, like water or hydrogen.
"When this impact ejected material, it also heated the material. Any ice would be vaporized and hydrated minerals would lose the hydrogen," Canup, who is an astrophysicist at the Southwest Research Institute in Colorado, said.
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The origins of Phobos and Deimos has fascinated researchers for decades. These are tiny asteroid-sized moons, making many people think that perhaps they were space rocks that passed by Mars and were caught in its gravity. But as Canup pointed out, simulations have shown this to be very unlikely. The orbits of the moons are circular, which suggests they weren’t captured by the planet’s gravity. Deimos and Phobos also orbit close to the Martian equator, which wouldn't necessarily happen if they started as roaming asteroids.
The so-called impact hypothesis is a familiar theory to astronomers, who say much the same thing about how the Earth's moon was formed. (The impactor on Earth was a Mars-sized object, so it left behind more debris that formed our much larger moon.)
The new Martian model included, for the first time, a more accurate simulation of how a disc of debris accumulates into satellites. The researchers modeled the mutual gravitational interactions between the moons as they grew larger. Here, they found that the mass of the impactor matters. Too large, and the impact would produce interior moons that would eat up Phobos and Deimos.
The researchers ran a simulation 150 times with an asteroid of about 2,000 km (1,250 miles) in diameter crashing into Mars. (That was the impactor size that other studies suggested, Canup said.) In every case, no Deimos and Phobos analogs occurred because the interior moons were too big, swallowing up any moons close to today's Martian satellites.
So the researchers tried running a model with an impactor no bigger than Ceres (950 km, 590 miles). They also modeled different scenarios for how far away the disc coalesced from the Martian surface. The magic number was somewhere between 5 and 7 Martian radiuses, when Deimos and Phobos lookalikes popped up.
For Canup, one of the most exciting parts of the research is that it will soon be testable. With the Japanese mission, she hopes to see if the moons are of Martian composition and are lacking water.
"I think that we can build a strong case that says, if these moons formed in an impact, they would be water and hydrogen poor," she said.