Mars once had surface water. Now it doesn't. The standard explanation involves solar wind stripping the atmosphere over billions of years — a slow, steady leak. But observations from ESA's Mars Express and ExoMars Trace Gas Orbiter revealed that the leak isn't steady. It's episodic, and dust storms are the trigger.
Published in Communications Earth & Environment (February 2026), the study tracked an unusual regional dust storm during Martian Year 37. The storm lofted water vapor to 60–80 kilometers altitude — heights where UV radiation splits water molecules into hydrogen and oxygen. Water vapor at those altitudes reached ten times normal levels. Hydrogen at the exobase — the boundary where atmosphere becomes space — rose to 2.5 times previous measurements for the same season. The hydrogen escapes. The oxygen either escapes or oxidizes surface minerals. The water is gone.
The surprise wasn't that global dust storms drive water loss — that was already known. The surprise was that a regional storm, far smaller than the planet-encircling events that occur every few Martian years, could produce the same effect. The threshold for significant water loss is lower than expected. Mars doesn't need a catastrophic event to lose water. A moderately large storm in the right season will do.
The implication for Mars's history is quantitative. If regional storms — which occur multiple times per Martian year — each contribute meaningful water loss, the cumulative total over 3.5 billion years is much larger than models based only on global storms predicted. The missing water isn't missing from the budget. It escaped through a channel that wasn't being counted because the individual events seemed too small to matter. The loss was real but distributed across events below the threshold of attention.