The moons of Mars may have been formed in an icy planetary collision

The mystery of where Mars’s two moons came from may finally be solved. If Phobos and Deimos were formed when an icy object smashed into Mars, it could explain their contradictory properties, which have long baffled researchers.

Measurements of the geological makeup of Phobos and Deimos, including tentative signs of water, have suggested that they are more similar to asteroids than to Mars itself, hinting that they may be captured asteroids. However, that explanation wouldn’t account for their circular orbits, which instead indicate another…