The explosive origin story of Mars' two moons

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Scientists have run simulations that support the theory that Mars' two moons, Phobos and Deimos, were formed as the result of a massive collision between the Red Planet and a protoplanet about 4.5 billion years ago

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Scientists believe that Phobos, one of Mars' two moons, is destined to be torn to shreds and form a ring around the Red Planet, and if so, it might be a fitting callback to how it got there in the first place. The mystery of the origins of the moons may have been solved by a new study that gives weight to the theory a huge collision between Mars and an ancient protoplanet resulted in the creation of Phobos and Deimos, as well as several other now-missing moons.

Two main hypotheses are debated regarding the birth of the Martian moons. One suggests that Phobos and Deimos were originally asteroids that became trapped in orbit about Mars. But a study, conducted by researchers in France, Belgium and Japan, suggests that the shape and orientation of their orbits, as well as the fine, grainy composition of the moons, makes that scenario unlikely.

There's a second, more explosive theory that Mars collided with a protoplanet about a third of its size between 4 and 4.5 billion years ago. The debris thrown up by that colossal impact formed a disk around the planet, which, over the course of millions of years, led to the formation of Phobos, Deimos, and other moons which no longer exist.

This hypothesis has been around for a while, but the recent study claims to provide new evidence, and fill in some gaps in the story, after creating complex models and running various types of simulations.

"A major difficulty has been to explain why a giant impact on Mars would have left two moons so different from our own Moon, a huge single mass, that also formed from Earth undergoing such an impact," says Sébastien Charnoz, one of the researchers on the study.

The team believes they've cracked the mystery, and the key factor is the different rotation speeds between the two planets. When our moon was born, Earth was spinning much faster than it is now – a day would only have been four hours long. This resulted in our moon being pushed further away, until it became locked in its current orbit.

Mars, however, was spinning much slower on its axis, meaning the debris clumped into several smaller moons, and one large one, 1,000 times bigger than Phobos. Due to tidal forces, most of these were pulled back to the surface of the planet over the next few million years – it is these same forces that are currently dragging Phobos to its doom. Deimos, meanwhile, is far enough away to avoid a similar fate, and is actually steadily moving away from its parent body.

While the team is confident in their scenario, more solid evidence might back them up in 10 years' time. The Japanese Aerospace Exploration Agency (JAXA) is launching a mission in 2022 to visit both moons, and bring back samples from Phobos. After the mission returns to Earth in 2026, the team behind this current study will be responsible for testing the samples to determine the moon's composition, and find out if it really is a mix of rock from Mars and the unknown body it collided with.

The research was published in the journal Natural Geoscience.

An animation of the process can be seen in the video below.

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