Phase reddening of Phobos and Deimos from TGO/CaSSIS observations

Aims. We study the phase-reddening effect (i.e. the increase in spectral slope with phase angle) of Phobos and Deimos, with the aim of characterising the origin and physical properties of the two Martian moon surfaces and spectral units.Methods. We analysed Phobos and Deimos four-filter observations at visible to near-infrared wavelengths acquired by the Colour and Surface Stereo Imaging System (CaSSIS) on board ESA’s ExoMars Trace Gas Orbiter (TGO) over a wide range of phase angles. From these observations, we derived the spatial distribution of the phase reddening and spectral slope over the sub-Mars hemispheres of Phobos and Deimos.Results. We present the first spatially resolved map of the Phobos phase reddening and the first estimate of the Deimos global phase reddening in the visible to near-infrared wavelengths.Conclusions. Our results suggest that (i) the surface of Phobos is characterised by variable phase reddening, (ii) the phase reddening of Deimos is similar to that of the redder units on Phobos, (iii) the amount of phase reddening is linked to regolith maturity and can be explained by space-weathering, (iv) the Phobos Blue unit post-dates the Stickney impact and may have an exogenous origin, and (v) the physical properties of the regolith on Phobos and Deimos are different from those of Martian regolith.