Phobos and Deimos look like asteroids, but a new study suggests they may be the sole survivors of a giant impact that produced many moons no longer in orbit.
Mars’ two small moons, Phobos and Deimos, were initially thought to be captured asteroids. However, a study published in Nature suggests an alternate explanation: a giant impact that created many moons, most of which have since fallen out of orbit. We speak with the lead author, Pascal Rosenblatt of the Royal Observatory of Belgium, about his findings.
ResearchGate: What prompted you to simulate a giant impact on Mars?
Pascal Rosenblatt: When I started working on the Martian moons, many scientists—if not all—thought that Phobos and Deimos were asteroids captured by Mars. But their opinion was actually built on the basis of inconclusive results. The giant impact scenario had been proposed just once, and no one really paid attention to it. I thought it was time to make an update. So, I made the decision to take a deep look at this scenario in order to assess whether it was more or less conclusive than the capture scenario.
RG: What do your results tell you about the history of Martian moons?
Rosenblatt: Our results tell us that Phobos and Deimos could be formed by accretion of smaller pieces blasted into Mars’s orbit by a giant collision with a body about one third the size of Mars. Our scenario also tells us that other moons—the biggest 10 times larger than Phobos—formed from this giant collision and played the role of a “sheepdog,” gathering the smaller debris into Phobos- and Deimos-like bodies. Once this job was achieved, these moons crashed back to Mars, leaving Phobos and Deimos to slowly reach their present orbits.
RG: How did you decide what size and type of impact to simulate?
Rosenblatt: To work, our scenario needed the formation of large moons close to Mars in order to promote accretion of smaller debris at a further distance from Mars. The largest moon must have a mass of around 10**19 kg, which can be formed with an impactor size similar to the one used in a previous study published by an American team to simulate the formation of the huge Borealis basin in the northern hemisphere of Mars as shown. We hadn’t expected such an agreement between our study and theirs. It was quite a nice surprise.
RG: What are Mars’ current moons like?
Rosenblatt: Phobos and Deimos look like small asteroids. They are irregularly shaped, and they have low albedo and cratered surfaces. The largest diameter of Phobos is 22 km, for Deimos about half that. Both moons have near-circular and near-equatorial orbits. Phobos' orbit is very close to Mars at an altitude of 6000 km—it’s actually the closest satellite to its primary in the solar system—while Deimos is further away, at an altitude of 20,000 km. Their composition is not well known. Only remote sensing data are available, which seem to indicate a composition different from that of Mars.
RG: Why does Mars have two moons today, whereas Earth only has one?
Rosenblatt: The key parameters are the spin of the planet and tides. The spin rate defines the position of the geostationary orbit, also called the synchronous limit, because it’s the place where the orbital revolution of a moon is exactly the spin rate of the planet. The faster the spin rate, the closer to the planet the synchronous limit. Due to the tidal effects with the planet, any moon orbiting below the synchronous limit will recede toward the planet, and is ultimately unable to stay in orbit On the other hand, any moon orbiting beyond the synchronous limit will recede away from the planet and can stay in orbit forever.
When our Moon formed, the synchronous limit was very close to Earth, because the Earth’s spin was very fast: 4 hours instead of 24. Most of the moons formed around Earth stayed in orbit and eventually accreted together to form a larger single body: our Moon.
The spin of Mars was slower, so that most of the moons formed from the accretion disk were below the synchronous limit and could not stay in orbit. Only the lighter moons that formed farther away from Mars—like Phobos—could stay in orbit today.
RG: Is a giant impact like the one you simulated the only plausible origin for Phobos and Deimos?
Rosenblatt: The capture scenario has been accepted for more than 30 years, despite the fact that it didn’t explain the present orbit of Phobos and Deimos. This scenario strongly relies on their presumed non-Martian composition, which seems to indicate that small bodies formed elsewhere in the solar system, like the asteroid belt, entered the Martian system.
However, a non-Martian composition doesn’t contradict our scenario. Our simulations show that the accretion disk can be made of material from Mars and from the impactor. That is, the impactor could be the source of the moons’ non-Martian composition.
RG: Can you make predictions about the future of Martian moons?
Rosenblatt: The orbit of Phobos is spiraling toward Mars due to tides. When it’s close enough, the tidal forces will pull it apart, and it will form a swarm of debris around Mars. This is expected to occur in the next few tens of millions of years. Deimos’s orbit is receding away from Mars. It will stay in orbit around Mars five billion years, when it will probably be destroyed along with Mars when the sun becomes a red giant and expands into Mars’s orbit.
RG: Now that you’ve completed this mathematical simulation, what’s next?
Rosenblatt: Our scenario could be tested in the near future thanks to Phobos sample return missions. These missions aim to bring samples back to Earth for lab analyses to identify Phobos’s true composition. If it is made of a mixture of Martian and non-Martian material, that would support our scenario. If Phobos is made of asteroid material, that would support the capture scenario. Such a mission is already scheduled to be launched in 2022 by the Japanese space agency (JAXA). The European (ESA) and Russian (Roscosmos) agencies are also planning such a mission for 2024.
