The doomed Martian moon Phobos, which is on a slow death spiral above the Red Planet, may hold some secrets about how Mars was formed.

The moon’s orbit of Mars — gradually degrading over the eons — brings it through charged molecules (ions) of oxygen, carbon, nitrogen and argon that Mars has been shedding from its atmosphere for billions of years. Some of those ions might remain on Phobos’ surface, a new study shows.