Where did Mars’ moons come from? It’s a question that is deceptively difficult to answer, with two competing theories: Either the moons were captured, or they came from Mars itself. Though the obvious answer remains elusive, new “old” evidence uncovered from within 20-year-old data indicates that Mars’ moon Phobos may have formed following an impact on the Red Planet.

The evidence, published September 24 in the Journal of Geophysical Research: Planets, comes from Mars Global Surveyor Therm