Where did Mars's rivers and lakes go?

Mars's water went two ways: down into the ground, the rocks and the polar ice, and up and out into space. Billions of years ago, rivers and floods carved the surface. But the planet's air was gradually lost, and without a thick atmosphere liquid water cannot stay on the surface. The water that remains is locked in the poles and the ground.

The evidence for the old water is written in the landscape. In 1971 the Mariner 9 spacecraft sent back images of ancient river beds, and later the Viking orbiters and landers found huge outflow channels and branched valley networks, the sort of pattern rain can leave. Some floods are estimated at ten thousand times the flow of the Mississippi River.

How did the air go? Mars lost its internal magnetic field, and that left the atmosphere open to being stripped away by the solar wind that flows out from the Sun. The ratios of argon, nitrogen and carbon isotopes in Mars's air today are consistent with about 60 percent of those gases having been lost this way. Meteorite impacts also threw atmosphere into space.

Scientists still argue about how much water went where. Part of it is thought to have been lost to the deep subsurface and part to space, but the exact balance is poorly understood. A 2021 study, using ratios of deuterium to hydrogen, suggested that a considerable amount may be locked into the rocks and crust rather than lost to space. Today the planet's poles hold enough ice to cover the whole planet 30 metres deep, but the air holds hardly any: a layer no thicker than 10 micrometres.