For decades if not centuries, we wondered whether Mars had water. Now, after a string of robotic expeditions to the Red Planet, we know the answer: It definitely used to.
All of the traces of water we’ve discovered, however, have been on the planet’s surface. Recently, though, the European Space Agency’s Mars Express satellite changed that, by .
It is tough to collect evidence of groundwater on Mars because that water is, well, underground. The rovers we’ve landed on the surface can make only surface-level observations, and obviously satellites can’t peer underground. And yet scientists working with the Mars Express satellite, which orbits the planet, figured out a way to look for visible evidence of groundwater.
The Mars Express research team examined two dozen craters on the planet that all lie below the Martian sea level. In each of those craters, the researchers found evidence that pools of water used to exist there many years ago. That data suggests that Mars once had a large supply of groundwater that used to pour into these craters.
Over time, the amount of groundwater on Mars decreased, and eventually water levels dropped so much those crater basins stopped filling up. Today, those basins are dry, but the signs of that water are present in the eroded soils. That’s what Mars Express discovered, and it proves that Mars once had underground lakes and aquifers.
What’s more, these groundwater sources might still be present somewhere beneath the planet’s surface. It’s true that Mars’ groundwater levels have been dropping for a very long time, but there’s a chance that some of it could still be left. Even if the water itself is gone, traces of life could still exist in the places where that water used to be. This discovery provides a roadmap for future missions to learn more about possible life on Mars.