As far as we know, water is a necessary component for life. That's why it's exciting that we’re now finding evidence of water outside of Earth, like on Jupiter’s moon Europa and even on Mars. But now that we know that water once existed on Mars, where was it? According to scientists at Arizona State University, at least some of it existed in a lake, now known as Gusev Crater.
Gusev Crater, which is 100 miles wide, got a lot of attention in 2004, when NASA’s Spirit rover landed there. NASA chose that site because it looked as if it might have once contained water, in the form of a lake, due to dry river channels on its southern end. However, once Spirit started hunting for sediments that indicated a lake had once existed there, the robot came up empty. Spirit then drove to the Columbia Hills, which lay two miles from the crater. There, it found rocks that showed evidence of water, but at the time, scientists thought the water might be more like volcanic hot springs, erupting from beneath the planet’s surface as opposed to a static lake.
Undeterred, scientists didn't give up on Lake Gusev. The ASU team studied more data from Spirit, focusing on a particular rock outcrop in the Columbia Hills. There, they discovered carbonate materials, which suggested the presence of liquid water at low temperatures in the area. This area was once volcanic ash, but the scientists believe that water often flooded through the river channel into Gusev. The carbonate minerals were actually once a briny liquid that gradually evaporated, while the lake more than likely filled and dried many times, leaving carbonates in the surrounding area.
The ASU team are hoping that NASA's Mars 2020 mission will consider revisiting Gusev Crater. That mission will collect rock samples for return to Earth, allowing for more close-up and complete study of them.