There's a new piece of evidence suggesting that Mars was once much wetter than it is today.
A new NASA study, published in the journal Science, argues that the planet was once home to an ocean's worth of water about 4 billion years ago. If that turns out to be true, the amount calculated would have covered nearly half of Mars' Northern hemisphere.
But it's still too soon to say for sure. Scientists have long debated exactly how wet Mars used to be, and this new study definitely isn't the final word. Most experts agree that the planet had some liquid water at some point, but it's still unclear how much was really present, and for how long.
So here's what the new study found — and what it means for our understanding of Mars.
The new study found Mars once had much more water
The authors of the new study detected chemical signatures in Mars' atmosphere that indicate the planet had a lot more water billions of years ago, which presumably evaporated out to space at some point since.
The research team — led by Geronimo Villanueva and Michael Mumma of NASA's Goddard Center for Astrobiology — used data collected by a number of telescopes on Earth over the course of six years. These telescopes measured the water vapor in Mars' atmosphere and ice caps.
The measurements were split into two categories: the amount of normal water molecules, and the amount of special molecules called heavy water (because they have an extra neutron in one of their hydrogen atoms). On Earth, there's generally a standard ratio between normal and heavy water molecules, but on Mars, the scientists detected about eight times more of the latter.
One explanation for this would be that Mars once had the same ratio as Earth — but over time, lots of water evaporated out to space. (Because normal water molecules evaporate more readily, that would lead to the imbalance.)
Based on the data, the researchers estimated that the planet should have once had enough to cover its entire surface in a 450-foot-deep layer of water. In reality, it would have probably pooled in Mars' lower-altitude Northern hemisphere, forming an ocean that covered an estimated 19 percent of the planet's surface.
Other research also indicates Mars was wetter — but for how long?
This doesn't mean Mars definitely had a 450-foot-deep ocean. But other sorts of evidence also indicate that, at least in some locations, Mars once had a lot more water than it does currently.
The Curiosity rover has previously found geologic evidence of an ancient lakebed in Gale Crater, as well as rocks that seem to have been eroded by flowing water. Atmospheric data, meanwhile, suggests that Mars may have once had a thicker atmosphere, which could have warmed the planet via the greenhouse effect, making it warm enough for liquid water.
But climate modeling and study of Mars' craters have indicated that Mars might've only been warm enough for liquid water for very brief periods of time — perhaps as short as a few years. And even though some scientists have long hypothesized the existence of a big ocean covering the planet's Northern hemisphere, we still haven't seen direct geologic evidence of it.
The timing of warm periods and extent of water coverage matter because of the main reason we're so interested in Mars' water — as a sign that the planet may have once had life.
If Mars really did have a big ocean that persisted for hundreds of millions of years, then it's plausible life could've evolved. But if the ocean existed only briefly — or if these new calculations are off, and there were just a few smaller lakes — then Martian life seems much less likely.