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

Space

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

Mars's moon Phobos is so strange that no one knows how it formed. But a forthcoming mission could solve this mystery - and a host of other puzzles connected to the solar system's deep past

By Nadia Drake

3 February 2025

Mars and Phobos. Computer artwork of how Mars (left) and its tiny moon Phobos (upper centre) might appear from a distance of about 100 miles from the surface of Phobos. Phobos is the larger of Mars' two satellites (the other being Deimos), being about 16 miles long and about 11 miles across at its smallest dimension. It may be an asteroid long ago captured by Mars' gravity and does not have enough mass for gravity to pull it into a uniform sphere like the larger satellites and planets in the Solar System. — Walter Myers/Science Photo Library

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…

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