It has been suggested that the two natural orbiting satellites of Mars — the larger inner moon, Phobos, and the smaller outer moon, Deimos — are captured asteroids. While images of these moons taken from spacecraft orbiting Mars have revealed cratered surfaces similar to those seen on typical asteroids, uncertainties about their origin remain. The present-day, near-circular, low-inclination orbits of the moons around Mars are difficult to explain dynamically through external capture, and some scientists have suggested that they formed instead from debris ejected from a giant impact into Mars.

Researchers led by Amirhossein Bagheri of ETH Zürich conducted a dynamical modeling study to gain insight into the origin and evolution of Phobos and Deimos. Bagheri and colleagues used geophysical and seismic data and laboratory-based viscoelastic models to guide their simulations of the tidal evolution of Phobos and Deimos. Results based on calculations of the tidal interaction between each satellite and Mars suggest that the orbits of Phobos and Deimos would have intersected 1 to 2.7 billion years ago. Therefore, Phobos and Deimos could have formed from the disintegration of a common parent body that was already orbiting Mars near the synchronous-orbit distance (the location where the orbital period about Mars equals the rotation period of Mars). The model shows that Deimos initially began to move inwards as its eccentricity damped following the possible parent-body disruption. Once its orbit was nearly circularized, but still beyond the synchronous orbit, it began to slowly evolve away from Mars to reach its present position. The model can also explain the current position of Phobos, which orbits closer to Mars than any other moon to its planet in the solar system, through the tidal damping of an eccentric orbit that left it inside the synchronous-orbit distance. Phobos will continue evolving closer to Mars. This could result in Phobos eventually impacting Mars, but more likely will first lead to its own disruption due to the strengthening tides as it gets closer to Mars over the next tens of millions of years.

To date, there have been no successful landed missions to either Phobos or Deimos. The Japan Aerospace Exploration Agency’s (JAXA) Martian Moons Exploration (MMX) mission, with a planned launch in 2024, will provide opportunities to study the moons of Mars in detail, including interior properties that are of great interest for the tidal-evolution studies. MMX will also attempt to collect and return material from Phobos to Earth. A better understanding of the interior and other physical properties of Phobos and Deimos (and Mars itself) may finally answer the question of whether the moons of Mars were captured, formed from a giant impact, or formed from a parent body that may have co-formed with Mars itself. READ MORE