The Martian moons Phobos and Deimos are tiny compared to our own or the great satellites of the outer Solar System. Nevertheless, with Venus and Mercury having no moons at all, astronomers have long pondered their origins. Now two astrophysicists are confident the answer lies in the collision between Mars and an object the size of the largest asteroids.
It's now generally believed our own Moon was formed from material thrown into orbit when an object the size of Mars slammed into the early Earth, although one theory proposes a series of many smaller events. Between them, Phobos and Deimos have just one-50-millionth of Mars' mass, in stark contrast to the Moon, which has around 1 percent of the mass of the Earth. With Mars being closer to the asteroid belt, many people have assumed the moons are captured former asteroids.
However, Dr Robin Canup and Dr Julien Salmon of the South-West Research Institute argue in Science Advances, “Intact capture is difficult to reconcile with the moons’ nearly circular and coplanar orbits, which, instead, suggest that they accreted from an equatorial disk around Mars.”
However, when people have tried to model the events that could have led to their creation, they've noted the large size of the Borealis Basin, proposed as a likely impact site, suggested whatever hit Mars had a mass at least a thousandth of that of the planet it ran into, and possibly as much as several percent of Mars' mass. This would have thrown up far more material than now makes up the moons, but the theory ran that instead of forming into one satellite, numerous smaller moons accreted, with the innermost ones spiraling down until they landed back on the planet.
Canup and Salmon have run what they claim is a more sophisticated model, incorporating interactions between the moons, rather than treating each as an independent body affected only by Mars. They also use a much finer mass resolution for their models' particles.
“The outer portions of the disk accumulate[d] into Phobos and Deimos, while the inner portions of the disk accumulate[d] into larger moons that eventually spiral inward and are assimilated into Mars,” Dr Salmon said in a statement.
To produce objects of the right size and location, the pair conclude an object between the masses of Vesta and Ceres was required (520-940 kilometers/330-590 miles), with the exact size depending on the impact angle and speed. Such an object might have produced one of Mars' smaller basins, rather than Borealis.
The Japanese Aerospace Exploration Agency plans to launch a mission to the two moons in 2024, bringing back a sample from Phobos, potentially revealing if Canup and Salmon are right.
Robin Canup Southwest Research Institute
