The origin of Mars' moons explained

Accretion of Phobos and Deimos in an extended debris disc stirred by transient moons

Published in Nature Geoscience, July 2016

Authors: Pascal Rosenblatt, Sébastien Charnoz, Kevin M. Dunseath, Mariko Terao-Dunseath, Antony Trinh, Ryuki Hyodo, Hidenori Genda & Stéven Toupin

Pascal Rosenblatt and Antony Trinh on the Radio Télévision Belge Francophone

Phobos - Photo credit: ESA Deimos - Photo credit: NASA

An international team, involving two researchers at the Institute of Physics of Rennes, has recently demonstrated how Phobos and Deimos, Mars' two small satellites, could have formed by accretion in an extended disc of debris following a giant impact on the planet, catalysed by orbital resonances with a large inner moon that subsequently crashed back onto Mars. 

Animation credit: Antony Trinh

Thick red circle: orbit of the transient moon

Dotted red circle, visible in the animation: Roche limit, external boundary of the inner disc

Thin red circles: orbits corresponding to the 2:1 and 3:2 mean motion orbital resonances of the transient moon

Thin blue circle: orbit of a debris in the outer accretion disc; thickness increases with mass

Due to coupling with the dense, inner accretion disc, the large moon spawn at the Roche limit migrates outwards. Debris in the outer disc are captured in the mean motion resonances rings with the transient moon and accrete. Several hundreds numerical simulations with varying initial conditions were run and in about one third of cases, one satellite the size of Phobos is formed just below the synchronous radius at 6 Mars' radii and one satellite the size of Phobos is formed at about 7 Mars' radii.

After several millions of years, the inner disc empties and the large moon falls back to Mars, leaving Phobos and Deimos slowly evolve to their present day orbit.

Three contributors are active in the Master's degree in Modelling and Computational Science of the University of Rennes 1: Mariko Dunseath is in charge of the first year and teaches Programming in C and in Fortran, Kevin Dunseath teaches Parallel programming in Master 2 and Pascal Rosenblatt has so far supervised 3 internships including the one of co-author Stéven Toupin who graduated in 2011.

The possibility of testing this new theory is exciting: the Japanese (JAXA), European (ESA) and Russian (Roscosmos) agencies are planning missions in 2024 to bring samples of Phobos back to Earth. The accretion scenario would be confirmed if analysis reveals a mixture of Martian and non-Martian material. Traces of the large, transient moon could also be found across the surface of Mars. The composition of Phobos has implications for its porosity and how it will evolve under tidal forces. In 20 to 40 million years, Phobos will break up and fall back to the surface of Mars.