There’s a new paper in Space Science Reviews (vol. 221), and yes, it’s a review: long and detailed.
Pätzold, M., Andert, T.P., Cardesín-Moinelo, A. et al. Investigations of the Moon Phobos by Mars Express and Implications Towards Its Origin. art no. 41 s11214-025-01165-7
The authors are European and clearly, they write from the point of view of ESA’s Mars Express orbiter. But don’t let that bias you: since Mars Express is in an elliptical orbit, it reaches out to Phobos’ altitude. NASA missions, after Viking 1 and 2 way back in the ’70s, have circularized their orbits to better study the Martian surface, interior, the bulk atmosphere, etc. This made them poor vantage points to study Phobos, and basically forget about Deimos.
Since Viking 1 and 2, the questions have gotten deeper. The hypothesis of Phobos and Deimos as captured asteroids doesn’t make any more sense than when they were just dots of light in ground telescopes. Nor do their surface compositions give us any better answers: their dark surfaces (as natural satellites go) don’t really show any mineral signatures in our instruments. In any case, Phobos (and to an extent Deimos) have surface color variations, indicating some sort of mixed makeup.
Ultimately, Pätzold et al. tip a cap to the coming MMX mission by JAXA. Unlike Mars Express, MMX is specifically constructed to tackle the Phobos-Deimos questions (including returning a Phobos sample). Had MMX launched in 2024 as planned, we would now be seeing initial data come back. Due to a failure and investigation with the H3 rocket, the launch is delayed until next year.