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The Spectral Reflectance of Phobos and Deimos as measured by NOMAD-UVIS

Presentation #214.02 in the session Phobos and Deimos (Poster + Lightning Talk)

Published onOct 23, 2023
The Spectral Reflectance of Phobos and Deimos as measured by NOMAD-UVIS

The origin of Phobos and Deimos is still unknown with the two leading hypotheses being as captured, possibly carbonaceous chondrite, asteroids that originated outside the Mars system, or from the re-accretion of ejecta from a giant impact on Mars. The difficulty in determining the origin of the Moons is largely a result of their relatively featureless reflectance spectra at UV, Visible and near-infrared wavelengths and some potential commonalities between the martian Noachian crust and carbonaceous chondrite asteroids. Here, we present the Phobos and Deimos observations by the NOMAD-UVIS Spectrometer aboard ExoMars TGO which has taken spectroscopic measurements of Phobos and Deimos across the UV and Visible wavelength range to search for absorption bands associated with various transition series elements, including spin-forbidden and charge-transfer bands in order to constrain possible surface mineralogy.

The UVIS observations of Phobos and Deimos confirm a mostly featureless, red-sloped spectrum with a possible broad and weak spectral feature centred near 0.46 µm consistent with a spin forbidden band associated with Fe3+ in tetrahedral coordination in minerals such as serpentinites and ferric iron sulfates. The similarity to D- and T-type asteroids, in particular a close spectral likeness to the Tagish Lake meteorite, considered a potential Phobos-like analogue supports the capture asteroid hypothesis. However, the UVIS observations also show that the Phobos reflectance spectrum has a likeness to the Mars meteorite NWA 2737 across UV and visible wavelengths. NWA 2737 is formed of an unusual shock-produced dark-brown olivine, with embedded nanophase iron particles, that exhibits a similar red slope spectrum and suppressed ferrous spectral bands. This similarity could support the alternative hypothesis of the moon having a Mars-derived origin, formed from the re-accretion within an impact-generated accretion disk.

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