Letter to the Editor

Spectral variability of Phobos and Deimos from TGO/CaSSIS multiband observations

¹ Centro di Studi Attività Spaziali – “Giuseppe Colombo” (CISAS/UniPD) Via Venezia 15 35131 Padova (PD), Italy
² Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Padova (INAF/OAPD), Vicolo dell’Osservatorio 5 35122 Padova (PD), Italy
³ Physikalisches Institut, University of Bern Sidlerstr. 5 CH-3012 Bern, Switzerland
⁴ INAF Osservatorio Astronomico di Arcetri largo E. Fermi n.5 I-50125 Firenze, Italy
⁵ Brown University, Department of Earth, Environmental and Planetary Sciences 180 Thayer St Providence RI 02912, USA
⁶ Institute for Earth and Space Exploration, University of Western Ontario, Dept. of Earth Sciences 1151 Richmond Street London Onatrio N6A 5B7, Canada
⁷ The SETI Institute, 339 Bernardo Ave Suite 200 Mountain View CA 94043, USA
⁸ School of Physical Sciences, The Open University Walton Hall Milton Keynes, UK

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 10 November 2025
Accepted: 4 February 2026

Abstract

Aims. We present a comparative visible and near-infrared multiband analysis of Phobos and Deimos aimed at characterising the compositional variability of the Martian moons.

Methods. From multiband observations acquired by the Colour and Surface Stereo Imaging System (CaSSIS) on board on the ESA/ExoMars Trace Gas Orbiter (TGO), we analysed spectral ratios tracing ferric and ferrous minerals and mapped them over the surfaces of the Martian moons. We identified regions of interest (ROIs) on both moons and compare their mean spectra and spectral slopes.

Results. We identified an overall similarity between the two Martian moons, whose variability can be explained by a different degree of ferric and ferrous mineralogy. In particular, the blue unit of Phobos can be explained by the presence of ferrous minerals, while the ferric minerals dominate in the red unit. We show that overall the Deimos surface matches the Phobos red units. On the contrary, the Deimos bright blue spots are spectrally similar to the Phobos transitional unit. We show the presence of a 1000 nm band only in the blue unit of Phobos.

Conclusions. Our comparative multiband analysis of Phobos and Deimos is consistent with a similar composition of the two Moons, suggested by the spectral similarity of their redder units. The detection of an absorption towards 1000 nm in the blue unit suggests an exogenous nature of the latter.