Insights into the origins of Phobos and Deimos based on a spectral comparison with small bodies and Martian materials

¹ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris-Cité, CY Cergy Paris Université, CNRS, 5 place Jules Janssen, 92195 Meudon, France
² LATMOS, CNRS, Université Versailles St-Quentin, Université Paris-Saclay, Sorbonne Université, 11 Bvd d’Alembert, 78280 Guyancourt, France
³ INAF-Astrophysical Observatory of Arcetri, largo E. Fermi n.5, 50125 Firenze, Italy
⁴ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa 2525210, Japan
⁵ Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNRS, 91405 Orsay, France

^★ Corresponding author; antonin.wargnier@obspm.fr

Received: 20 November 2024
Accepted: 26 January 2025

Abstract

Context. The origins of Phobos and Deimos are highly debated, and several distinct hypotheses have been put forth. The two most widely accepted theories are that (1) the two moons were created by a giant impact in analogy to the Earth-Moon system, whereby a debris disk was formed that then formed the two moons by accretion; and that (2) the moons were captured by the gravitational attraction of Mars.

Aims. To address questions about the origins of the Martian moons, we conducted a systematic search for analogs of Phobos and Deimos among asteroids, Martian terrains, and laboratory data using spectroscopy in the visible, near-infrared, and mid-infrared wavelength ranges.

Methods. We analyzed our dataset using multivariate statistical analysis techniques, namely principal component analysis and t-distributed stochastic neighbor embedding, on the spectral slope derived in various wavelength ranges, and on the albedo. Additionally, a visual comparison of the mid-infrared spectra, focusing on key features such as the Christiansen feature and the Reststrahlen bands, was performed.

Results. The comparison of the spectra of Phobos and Deimos with those of primitive asteroids reveals that the Martian moons exhibit spectroscopic similarities to D- and Z-type asteroids, as well as to Jupiter Trojans, centaurs, and potentially extinct comets. The blue unit on Phobos, generally considered as fresher areas that are mostly seen around the Stickney crater, appears to be spectrally best matched by P-type asteroids. No Martian terrain shows similarities with Phobos and Deimos. It is also notable that the Martian moons exhibit a spectroscopic resemblance to some of the Martian Trojans.

Conclusions. The analysis of available spectral data for asteroids, Mars trojans, and Martian terrains provided a unique opportunity to explore the origins of Phobos and Deimos prior to the arrival of the JAXA/Martian Moon eXploration (MMX) in the Martian system, which is scheduled for 2027. In light of the similarities between the Martian moons and Z-type asteroids, we put forward the hypothesis that Phobos and Deimos may have originated from one or two captured asteroids from the inner main belt, rather than from the outer main belt or the Jupiter trojans, as is often postulated. The formation of Phobos and Deimos may also have occurred simultaneously with that of the Mars trojans. This is a plausible scenario if the formation of the trojans was caused by the impact that created the Borealis basin, although additional processes such as space weathering may be necessary to explain the spectroscopic differences.