The Mystery of the Martian moons
PHOBOS — THE INNERMOST OF MARS’ TWO MOONS — will provide a beautiful show to anyone lucky enough to be watching in 20 to 40 million years. The small satellite is losing altitude with each orbit, slowly inching towards Mars. But long before it crashes into the surface, tidal forces will shred it to tiny pieces. Its surface already shows signs of its imminent collapse: long faults that run for many kilometres across the landscape, which is covered by a thick layer of powdered rock. When Phobos finally fails, it will turn to rubble — dust, pebbles and boulders that will stretch out like a spaghetti noodle in a ring around Mars.
The cataclysmic finale of Phobos doesn’t puzzle scientists nearly as much as the mystery of its formation. American astronomer Asaph Hall discovered the Martian moons, Phobos and Deimos, in August 1877 and named them after two sons of Ares, the god of war in Greek mythology. Many theories aim to explain their origin, but none of them fully grasps how these small, battered bodies could end up orbiting Mars in perfectly circular orbits, right on the equatorial plane. Are these captured asteroids? Are they as old as Mars? Did they emerge after a giant impact like the one that formed Earth’s Moon? Recent research unveils a complex picture.
A robotic mission to the Martian moons could answer these questions, but technical mishaps have doomed the three attempts made so far by the Soviet and later Russian space programs. Now is the turn for the Japan Aerospace Exploration Agency (JAXA), which in 2024 will send a spacecraft to visit both moons and return samples from Phobos to Earth. Its success will reveal how the Martian moons formed, shed light into the chaotic history of the Solar System, and — for the first time — bring samples from the Martian system to our planet.
A strange pair
Phobos and Deimos are the first non-spherical Solar System bodies ever explored
