Icy Impactor Might Explain the Formation of Mars’s Moons

The origin of Mars’s two enigmatic moons, Phobos and Deimos, is a mystery. Their irregular shapes, dark surfaces, and peculiar orbits have fueled two main theories: They might be asteroids that strayed too close to Mars and were captured. Or perhaps the moons instead coalesced from debris launched to orbit after giant impactor struck the Red Planet.

New research presented at the Lunar and Planetary Science Conference in The Woodlands, Texas, adds a cool twist to the latter idea by suggesting that the impactor was made mostly of water ice. The idea, presented by Courteney Monchinski (Tokyo Institute of Technology, Japan), eliminates some of the problems that have plagued the giant-impact scenario.

Unknown Origin

Phobos and Deimos are among the darkest objects in the solar system. Their spectral signatures don’t have clear features, so their surface composition isn’t known. However, they seem to be porous, either hollow or filled with frozen water. These characteristics, along with their battered, irregular shapes, have led some scientists to think that they could be captured asteroids.

However, that appearance could also indicate they are made of basalts; sitting out in space and bombarded by micrometeorites until their spectral features become similar to that of asteroids. A flyby of Deimos by the Emirates Mars Mission spacecraft in early 2023 backs that idea. Additionally, the moons’ orbits defy the typical capture scenario. Captured asteroids usually have highly oval-shape (elliptical) and randomly oriented orbits, but Phobos and Deimos travel take remarkably circular paths directly above Mars' equator. These observations support a giant-impact scenario instead.

But this scenario has a few problems of its own. Simulations trying to recreate the moons’ formation by impact launch too much material into orbit, resulting in an overly massive debris disk around the planet. In these models, the moons coalesce from this disk, but there’s enough material to produce at least one larger moon inside Phobos’s current orbit that eventually breaks into pieces. In some models, Phobos comes together out of this secondary debris disk.

Another problem is that a rocky impactor would result in a hot debris disk, whose high temperatures would alter or destroy any primitive materials — including basalts.

Ice, Ice Baby

To water down these problems, Monchinski and collaborators propose a scenario in which an icy impactor about 3% the mass of Mars, made of at least 30% and up to 70% water ice. Any icy impactor would have put much less rock into space, opening the possibility of forming Phobos and Deimos directly from the impact, without having to invoke a long-lost larger moon that formed first.

An icy impactor also results in a lower temperature in the disk, since the vaporization of such large quantities of water absorbs a lot of energy. As a consequence, the composition of the rock that makes it into orbit is better preserved.

“The most interesting point of these results is that they have shown the possibility, with this low temperature in the disk, that we could preserve some of the chondritic composition of the impactor,” says Pascal Rosenblatt (University of Nantes, France).

In earlier work, Rosenblatt and others had proposed a similar scenario, considering an all-rock impactor roughly one-third the size of Mars. “That was not the case with the impactor that we had modeled,” he says, adding that the new model “opens new perspectives.”

A large amount of water in the disk would also alter the chemical composition of the rocks, Rosenblatt says. This would likely result in the formation of hydrated minerals called phyllosilicates. The European Space Agency’s Mars Express spacecraft has previously found hints of such rocks.

Water vapor could also have played a role in Deimos’s formation beyond the synchronous orbit of Mars, where objects orbit the planet at the same speed as the ground rotates. Beyond this distance, tides cause moons to slowly recede from the planet. The interaction between dust particles and water vapor could have helped spread disk material further outwards, allowing Deimos to form in its current, far-out location.

The Origin of an Icy Impactor

Why was there a giant icy body on a collision course with Mars to begin with? Researchers think that such “wet” bodies could have formed in the outer rim of the solar system, beyond the orbit of Saturn or Neptune. Then, instabilities brought on by the giant planets could have flung some of these bodies into the inner solar system.

Rosenblatt also points out that early Mars likely had water on its surface, so Mars could have contributed some of its native water, too, requiring a less-icy body.

A definitive measurement will likely have to wait for launch of the Japanese Aerospace Exploration Agency’s Martian Moons Exploration (MMX) mission in 2026, which will send a spacecraft to orbit Phobos, touch down on the moon, and return a piece of it to Earth.

“We are waiting for the sample,” Rosenblatt says. “I'm pretty sure that we will discover something new.”