Table 1 Origin hypotheses for Phobos and the predicted mineral abundances (Murchie et al. 2014)

1. Capture of organic- and water-rich outer solar system body

Abundant phyllosilicates; carbonates and organic phases; anhydrous silicate phases rare

2. Capture of organic- and water-poor outer solar system body

Anhydrous, med. Fe (20–40%) pyroxene + olivine; abundant amorphous carbon or graphite?

3. Capture of inner solar system body

Low carbonates, phyllosilicates; pyroxene, olivine probably in range of known meteorites

4. Co-accretion with Mars

Anhydrous silicates with Fe, Mg of bulk Mars; low abundance of C-bearing phases

5. Giant impact on Mars

Evolved, basaltic mineralogy consistent with many datasets for Mars