Vid A new study suggests the early history of Mars was incredibly violent and the planet's two small moons are the sole surviving remnants of what was once a shimmering halo.
Mars has two moons – Phobos and Deimos (from the Greek words for fear and dread respectively) – but they are tiny, misshapen planetoids, just 22 and 13km (14 and eight miles) across. It had been thought that these were asteroids that Mars had captured but the new research suggests they are instead the survivors of a colossal collision.
The research team, composed of astronomers, computer modelers, and mathematicians, propose that between four and four and a half billion years ago Mars hit a massive protoplanet about a third of its size. The North Polar Basin, a basin in the northern hemisphere of the Red Planet, could be the remnant of the massive impact.
Using computer simulations the team from the French Le Centre National de la Recherche Scientifique (CNRS) think that the impact would have ejected a huge volume of the planet into orbit. This would form into a disc encircling the planet, which would have formed into as many as 10 new moons.
The inner side of the disc would have contained the densest material and the simulation suggests a single massive moon – about 10 times the diameter of Phobos – formed there. This, in turn, would have influenced smaller moons to form further out from Mars, where the debris disc is made of lighter dusts and gases.
But why wouldn't Mars get one big moon like Earth, instead of lots of little ones? Planetary scientist Sébastien Charnoz, coauthor of the paper in Nature Geoscience, said that the different rotational speeds of the two planets at the time is key to understanding how the moons formed.
“Earth took less than 4 hours to spin on its axis whereas Mars rotated very slowly over a 24-hour period,” he said. As a result the Earth got one large moon moving outwards while the Martian moons that were drawn close enough fell back onto the planet.
The paper postulates that around five million years after its formation, the large moon orbiting Mars got too close to the planet and fractured, raining down as smaller chunks onto the surface. We already know that Phobos will suffer the same fate in a few million years, and is already cracking up under the strain.
Deimos, on the other hand, is far enough out that it won’t fall to a rocky death. Instead it's spiraling slowly away from the Martian surface.
There's a good chance we might be able to get proof behind the theory besides computer simulations. In 2022 the Japanese space agency (JAXA) will launch the Mars Moons Exploration mission, which will take a close look at the Phobos and Deimos and bring sample of the larger moon back to Earth.
If the JAXA mission succeeds then the moons should be made up of very fine-grained particles that would have come from Mars and the impacting planet. If so, the mystery of Mars' moons could be solved. ®