Why we must investigate Phobos, the solar system’s strangest object

Next to Earth, Mars may be the most-studied world in our solar system, currently home to a fleet of orbiters, landers and rovers. But above the red sands on which the rovers trundle, a strange moon rises twice each day. And despite all the scrutiny that Mars itself receives, this moon, Phobos, remains shrouded in mystery.

Phobos and its smaller neighbouring moon, Deimos – both discovered in 1877 – are two of the most perplexing worlds in the solar system. “They’re the only objects at this stage, in the solar system, for which we have pretty much no idea what they are,” says Pascal Lee at the SETI Institute in California. “We know what other moons are. We know asteroids and comets. Phobos and Deimos? No idea.”

The Martian moons might be captured asteroids, or they could have formed from the same disc of primordial planet-stuff as Mars. Perhaps they were forged from a fiery cataclysm like the collision that crafted Earth’s moon. Or maybe their origin story is something else entirely. “What the heck are they?” asks Abigail Fraeman at NASA’s Jet Propulsion Laboratory in California. “I think this is one of the great mysteries of planetary science.”

Now, there is hope we might finally solve that puzzle, thanks to a new mission to Phobos that is in the works. Doing so would offer more than just a satisfying answer: it could also open a new window on the history of the inner solar system, and perhaps point to the source of life’s building blocks on Earth.

It…