Was There Water on Mars

From orbit, you can see branching valleys: they form networks like river basins and run into craters. At their mouths lie deltas - fan-shaped sediment deposits that form where a flow slows down and drops everything it carried. In 2021, the Perseverance rover landed on one such delta in Jezero Crater: it drives along a former shoreline.

Landforms alone could be explained by wind or lava, but the chemistry of the rocks confirms them. Curiosity found rounded pebbles in Gale Crater - stones become smooth only when they roll in a current for a long time. Nearby are thin layers of clay, like those that build up on the bottom of a calm lake, and in 2011 the Opportunity rover came across a light-colored vein of gypsum: this mineral precipitates from water that has seeped through a crack.

The debate is not about whether there was water, but what the climate was like: steadily warm, or generally cold with long periods of thaw.

Mars is small, and its interior cooled faster than Earth's. About four billion years ago, the planet lost its global magnetic field - the shield that deflects the solar wind. After that, the stream of charged particles began stripping away the upper atmosphere year after year; the MAVEN probe is measuring this escape right now.

The air thinned, pressure fell, and water lost its ability to remain liquid. Water itself also escaped: light hydrogen left the planet more readily than heavy hydrogen, so Martian air now has about five to six times more heavy hydrogen than Earth's water. That is a sign of a very large loss.

But not all of it escaped. Scientists still debate how much water went into space and how much remains bound in the rocks of the crust - estimates vary widely. One thing is certain: there is water on Mars even now, just not in a form that is easy to look for.