Phobos 0.4–2.5 μm spectral analysis of the red and blue units: Exploiting the Mars Express/OMEGA dataset

¹ Istituto Nazionale di Astrofisica (INAF) – Osservatorio Astronomico di Padova (OAPd), Vicolo dell’Osservatorio n.5, Padova, Italy
² Center of Studies and Activities for Space, CISAS, G. Colombo, University of Padova, via Venezia 15, 35131 Padova, Italy
³ LESIA Observatoire de Paris, Université PSL, CNRS, Université Paris Cité, Sorbonne Université, 5 place Jules Janssen, 92190 Meudon, France
⁴ INAF Osservatorio Astronomico di Arcetri, largo E. Fermi n.5, 50125 Firenze, Italy

^★ Corresponding authors: maurizio.pajola@inaf.it; joel.beccarelli@studenti.unipd.it; joelbeccarelli1997@gmail.com. M. Pajola and J. Beccarelli equally contributed to the work.

Received: 3 February 2025
Accepted: 24 March 2025

Abstract

Aims. We provide a detailed analysis of different Phobos Mars Express/Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (MEX/OMEGA) visible and near-infrared (Vis-NIR) datacubes to understand the satellite blue-red spectral transition and its surface mineralogy while framing it within the debated satellite formation theories.

Methods. We exploited three datacubes acquired in the 2004–2010 time frame. This dataset covers Phobos red and blue units located in the sub-Mars hemisphere at a scale ranging from 101 to 971 m/px. The OMEGA cubes were geometrically and photometrically corrected. We extracted the spectra in the 0.4–2.5 μm wavelength range from 11 different regions of interest (ROIs) located inside the Stickney crater and its closest surroundings as well as from Limtoc and Reldresal craters.

Results. We detected a wide range of spectral variation within the Phobos surface not identified before. Besides the red and blue end-members, we observed a transitional colour change on the satellite sub-Mars hemisphere. We possibly identify a 2–7% absorption in the 0.7–1.3 μm spectral range only visible in the bluest area of the satellite. If real and not an instrument artefact, this would highlight the presence of a basaltic compound on the surface of Phobos. By comparing the OMEGA Vis-NIR extracted spectra with asteroid mean types, we were able to confirm the spectroscopic similarity between Phobos spectra and D-, T-type objects. Nevertheless, the reddest spectra observed on the Martian moon largely exceed even the D-type spectral slopes. Compared to several carbonaceous meteorites, the bluest satellite spectra show similar trends to the Cold Bokkeveld, Kaidun, and Murchison fusion crust. In contrast, the Vis-NIR trends of the Tagish Lake meteorite are more similar to some of the redder OMEGA spectra. As for the asteroid comparison, we find no match between the reddest Phobos spectra and primitive or dark meteorite samples.