For a long time after their discovery in 1877, scientists assumed Mars’ two puny moons — Deimos and Phobos — were captured asteroids. This belief persisted until evidence revealed both moons formed at the same time as the Red Planet itself, and that the smaller one, Deimos, has a mysteriously tilted orbit. However, it wasn’t until 2017 that researchers put forth a new idea that could explain why Deimos' orbit is slanted by 2 degrees.
“The fact that Deimos’ orbit is not exactly in plane with Mars’ equator was considered unimportant,” said SETI Institute research scientist and lead author Matija Ćuk in a press release. “But once we had a big new idea and we looked at it with new eyes, Deimos’ orbital tilt revealed a big secret.”
