¹J.P.Mason et al. (>10)
Journal of Geophysical Research (Planets)(in Press) Open Access Link to Article [https://doi.org/10.1029/2023JE008002]
¹School of Physical Sciences, The Open University, Milton Keynes, UK
Published by arrangement with John Wiley & Sons

Spectroscopic measurements are a powerful tool to investigate the surface composition of airless bodies and provide clues of their origin. The composition and origin of Phobos and Deimos are still unknown and are currently widely debated. We present spectroscopic measurements of Phobos and Deimos at ultraviolet and visible wavelengths (250–650 nm) made by the NOMAD-Ultraviolet and Visible Spectrometer (UVIS) on the ExoMars TGO mission. These new spectra cover multiple areas on Phobos and Deimos, and are of generally higher spectral resolution and signal-to-noise than previous spectra, and extend to lower wavelengths than most previous measurements. The UVIS spectra confirm a red-sloped spectrum lacking any strong absorption features; however, we confirm the presence of a previously identified absorption feature near 0.65 μm and tentative absorption near 0.45 μm. The observed Phobos and Deimos spectra are similar to D- and T-type asteroids, adding weight to the captured asteroid hypothesis for the moons’ origins. We also find, however, that the UVIS Phobos reflectance spectra of Phobos’ red unit is a relatively close match to the olivine-rich, highly shocked Mars meteorite NWA 2737, with a low overall reflectance, a red-sloped spectrum, and lack of olivine-associated absorption bands in the UVIS spectral range. This meteorite, however, exhibits spectral features at longer wavelengths that not observed in the Martian moon spectra, indicating a need for further investigation at longer wavelengths to interpret whether this material could inform our understanding of Phobos’ origin.

¹David Nesvorný,²Nicolas Dauphas,³David Vokrouhlický,¹Rogerio Deienno,⁴Timo Hopp
Earth and Planetary Science Letters 626, 118521 Link to Article [https://doi.org/10.1016/j.epsl.2023.118521]
¹Department of Space Studies, Southwest Research Institute, 1050 Walnut St., Suite 300, Boulder, CO 80302, United States
²Origins Laboratory, Department of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637, United States
³Institute of Astronomy, Charles University, V Holešovičkách 2, CZ-18000 Prague 8, Czech Republic
⁴Max Planck Institute for Solar System Research, Planetary Science Department, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
Copyright Elsevier

Recent analyses of samples from asteroid (162173) Ryugu returned by JAXA’s Hayabusa2 mission suggest that Ryugu and CI chondrites formed in the same region of the protoplanetary disk, in a reservoir that was isolated from the source regions of other carbonaceous (C-type) asteroids. Here we conduct N-body simulations in which CI planetesimals are assumed to have formed in the Uranus/Neptune zone at ∼15–25 au from the Sun. We show that CI planetesimals are scattered by giant planets toward the asteroid belt where their orbits can be circularized by aerodynamic gas drag. We find that the dynamical implantation of CI asteroids from ∼15–25 au is very efficient with ∼5% of ∼100-km planetesimals reaching stable orbits in the asteroid belt by the end of the protoplanetary gas disk lifetime. The efficiency is reduced when planetesimal ablation is accounted for. The implanted population subsequently evolved by collisions and was depleted by dynamical instabilities. The model can explain why CIs are isotopically distinct from other C-type asteroids which presumably formed at ∼5–10 au.