Phobos and Deimos. They are the sons of Ares, the Greek god of war. In Roman mythology, Ares is known as Mars, and we named a planet after him. In 1877, astronomer Asaph Hall finally succeeded in his quest to find a moon of Mars. He had been about to give up when his wife encouraged him to keep going. He found Deimos the next night, and Phobos six nights later, and he named the two tiny moons after those two sons of Ares. Their names mean ‘dread’ and ‘fear’, respectively, which is great for sons of a war god but a little scary for science.

Still, these names haven’t stopped astronomers from trying to understand the two moons. After all, they are very different from our own Moon. They’re not perfect little spheres; they’re more potato-shaped blobs. They’re tiny and among the smallest moons in our solar system. They orbit really close to Mars: Phobos goes around Mars three times a day, and Deimos takes a whopping 30 hours to orbit.

Phobos is spiralling inward toward Mars and is expected to crash or break up before it reaches the surface in about 50 million years. It might even form a ring. Both moons are tidally locked. If you stood on Phobos, Mars would fill the sky. There is talk, though, of using it for a Martian base, a place where astronauts could remain shielded from cosmic rays while observing the Red Planet and sending more robots to the surface.

But how did Mars wind up with these two misshapen lumps as moons? They could be captured asteroids that got trapped in Mars’ gravity field, except that captured objects tend to have highly eccentric orbits at random inclinations, and that isn’t the case with Phobos and Deimos. Their orbits are nearly circular and in the equatorial plane of Mars. So… what are they?

A new paper in Nature Astronomy seeks to answer this question. The team responsible used computer simulations to calculate changes in the orbits over time and take a mathematical look at the past. They found that the two moons may have crossed orbits. Co-author Amir Khan explains: This means that the moons were very likely in the same place and therefore have the same origin.

Simply put, lead author Amirhossein Bagheri says: Phobos and Deimos are the remainders of this lost moon.

The team used data from NASA’s InSight Mission to create their simulations. InSight has been collecting data on its seismometer, recording marsquakes and maybe meteorite impacts, which allows scientists to understand how energy is being dissipated within Mars. Add in tidal effects from Phobos (and even Deimos), and they can get a good picture of the interactions between the Red Planet and its moons.

Other probes have been used to find the density of the moons, which is incredibly low compared to Earth — around 2 g/cm3 compared to 5.5 g/cm3 — and means Phobos and Deimos are very porous. All of these numbers and interactions allowed the scientists to refine their theory on the tidal effects, and that gave them the data needed to create and run their simulations, looking backward in time to track the orbits of the moons. And somewhere between 1 and 2.7 billion years ago, Phobos and Deimos were born, likely from one body.

I mentioned that Phobos is moving closer and closer to Mars every year. Deimos, it turns out, is probably still in the original orbit of that larger body. We’ll have to wait for JAXA’s Martian Moons eXploration mission to launch and explore the moons to learn more at this point. That mission is slated for a 2024 launch and a sample return in 2029. Another spacecraft to look forward to, and we’ll keep you up to date here on Daily Space. 

More Information

ETH-Zürich press release

Mars Moons (NASA)

“Dynamical evidence for Phobos and Deimos as remnants of a disrupted common progenitor,” Amirhossein Bagheri et al., 2021 February 22, Nature Astronomy