Rather than the two Moons we see today, a collision followed by a circumplanetary disk may have given rise to three moons of Mars, where only two survive today. Image credit: Labex UnivEarths / Université Paris Diderot.

Did Mars once have three moons?

Phobos and Deimos may have had another, inner, much larger companion!

“The larger inner moons fall back to Mars after about 5 million years due to the tidal pull of the planet, after which the two outer satellites evolve into Phobos- and Deimos-like orbits.” -Pascal Rosenblatt, et al.

Mars’ two moons, Phobos and Deimos, are small, irregular, but orbit in the same equatorial plane as the red planet. Although they’ve long been thought to be captured asteroids, those orbits would be supremely unlikely. Another possibility would have been if a massive impact created a debris disk, similar to how Earth’s Moon was formed. That alternative creates equatorial orbits, but normally produces at least one very large moon. However, a new simulation was performed, showing how an impact could create three moons around Mars, where the largest, inner one decays, creating Martian system we see today.

A large impact from an asteroid billions of years ago may have created the moons of Mars, including an inner, larger one that no longer exists today! Image credit: Illustration by Medialab, ESA 2001.

Our Moon may be what we grew up with, but it’s a cosmic oddity among the rocky planets. Of all the rocky worlds in our Solar System, it’s the only one with a large Moon. Mercury is moonless, Venus is moonless, and Mars only has the two very small…