Mars became dry mainly because its ancient surface water escaped into space.

Ancient Mars had rivers and lakes, and may once have held large seas. One long-standing explanation for where that water went focused on atmospheric escape: water molecules were broken apart, and lightweight hydrogen gradually escaped to space.

That process certainly happened. The question was whether it could explain most of the missing water. Any model also had to match Mars’s present deuterium-to-hydrogen ratio, geological estimates of its ancient water supply and the reservoirs that still exist today.

A 2021 study added a major sink that had often been underestimated: water becoming chemically bound in the Martian crust. When water reacts with rock to form hydrated minerals, it can remain trapped there. On Earth, plate tectonics can recycle some of that water back toward the surface. Mars does not have the same large-scale recycling system.

With crustal hydration included alongside volcanic degassing and atmospheric escape, the models could reproduce the available constraints while storing an ocean-scale quantity of water in the crust. The estimated range was wide, so it was not a precise accounting of how much went where.

Current reviews therefore treat Mars’s lost water as a problem with several answers. Some escaped to space, some remains as ice or in the subsurface, and a substantial amount may still be present in hydrated rock. Atmospheric escape remains important, but it is no longer a sufficient explanation on its own.