Runaway ions from Mars may prove or disprove the idea that Mars was once much more Earthlike, with a thick atmosphere and liquid water on its surface. There could have even been some sort of life. Whatever is left of the Red Planet’s atmosphere (if you could even call it that) has less than one percent of Earth’s atmospheric density. Phobos is thought to have collected quite a few ions from Mars because it orbits extremely close to the planet. Because it is tidally locked, like our Moon, so one side is always facing Mars. That side has been exposed to anywhere from 20 to 100 times more ions than the far side, so whatever relics are left of Mars’ atmosphere could be embedded in its surface.