Long ago, Mars had a lake in a place called Jezero Crater. NASA's rover Perseverance has been studying it since 2021. Scientists looked at rocks near the old shore of the lake. Rocks in high places were not changed by water.

But rocks in lower places showed signs of water three times. These were water under the ground, lake water, and hot water. This makes the area a good place to look for signs of past life.

Mars is dry today, but Jezero Crater once held a lake fed by a river. NASA's Perseverance rover has been exploring the area since 2021. An international team led by Purdue University used data from SuperCam, an instrument that fires a laser at rocks to study their chemistry.

They examined rocks along what may have been the lake's shore. Rocks at higher elevations had cooled slowly from molten rock, and water had hardly changed them. Rocks below an old lake level recorded three separate episodes of water.

First came carbon-rich groundwater. Next came lake water or changing groundwater. Later, hot fluids moved through the rocks. The study makes no claim about life. What it shows is that water came back to the old shore at least three times.

Shortly after landing in 2021, NASA's Perseverance rover confirmed that Jezero Crater once contained a lake with a river delta. Along the crater's inner edge lies the Margin unit, an olivine-rich terrain that orbital images had suggested might mark an ancient shoreline. In a study published in Communications Earth & Environment, an international team led by Purdue University analyzed data from the rover's SuperCam instrument to compare rocks at different elevations.

Rocks above roughly minus 2,350 meters turned out to be slowly cooled, crystalline igneous rocks showing little alteration by water. Below the second ancient lake terrace, however, the rocks preserved a complex sequence of water-rock interactions. First, neutral to alkaline groundwater rich in carbon dioxide formed carbonate minerals. Next, exposure to lake water or changing groundwater remobilized the carbonates and deposited silica in pore spaces.

Finally, later hydrothermal fluids flowing through new fractures produced calcium sulfate veins containing fluorite. These interpretations rest on remote measurements, not returned samples, and they concern water, which is a long way from finding life. Even so, repeated and varied episodes of water could have stretched out the period of potentially habitable conditions, which makes the unit a promising target for astrobiology.