Liquid water once flowed across the martian surface. Today it is mostly locked up in the ice caps and buried underground, but water loss still occurs today. The ESA-Roscosmos ExoMars Trace Gas Orbiter is providing new data to learn more about water loss and thus the planet’s climate evolution. It is following the vertical path of water through the atmosphere and its changing isotopic composition, a metric used to estimate water loss on Mars, behaving like a water ‘chronometer’. The new data reveal that as water travels and rises to colder regions it condenses and its isotopic signature changes dramatically, impacting the local value of the water chronometer. Yet, when water is fully vapourised, it mostly displays a common isotopic enrichment and a D/H ratio six times greater than Earth’s across all reservoirs on Mars, confirming that large amounts of water have been lost over time.

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