We have confirmed with some degree of certainty that there was, at one point, liquid water on Mars. But did liquid water ever cover the surface of Mars, to the extent where we might one day see evidence that there was a diverse and enduring global network of aquatic ecosystems?
The jury’s still out, but two studies have underscored just how complicated the question of water on Mars really is. The first comes from Icarus, where a team of geologists studied Aram Chaos and discovered evidence of a massive underground ocean that burst to the surface in a single cataclysmic event. And wen I say underground ocean, I’m not just being cute—we’re talking about 93,000 cubic kilometers, which would make it significantly larger than the Caspian Sea (78,000 cubic kilometers) and slightly larger than all of the surface-level freshwater on Earth (91,000 cubic kilometers). That’s not just water on Mars; that’s a massive amount of water on Mars, all in one place.
The other, from Nature Geoscience, suggests that Mars was, for at least a very large chunk of its history, both extremely cold and extremely dry—conditions that do not generally suggest the presence of liquid surface water in large quantities.
The studies—both groundbreaking, both published in the same month—don’t contradict each other, but they do complicate each other and, when read together, seem to suggest that Mars (much like Enceladus, Europa, and, most likely, Earth) kept most of its water underground. It’s clear, in any case, that the question of water on Mars is not going to be one that we can answer with a single word. And this makes sense; on a geological scale, there is no obvious reason why the history of Mars should be any less complicated than the history of our own planet.