An extreme escape of atomic hydrogen from Mars

Martian water loss to space is commonly attributed to the thermal escape of atomic hydrogen. However, when projecting the currently recognized escape rates over Mars' 4.5-billion-year history, it only accounts for less than one-tenth of the loss amount inferred from analyses of geological remnants and water isotopes. This significant gap indicates that the entire spectrum of potential escape rates remains unidentified due to limitations in datasets or methodology. Here, by analyzing the large-amplitude proton cyclotron waves during a fortuitous alignment of the Tianwen-1 and MAVEN missions, we identify an extreme escape of atomic hydrogen. The instantaneous escape rate was at least 10²⁸ hydrogen atoms per second, nearly one order of magnitude higher than previous estimations. This extreme escape was probably driven by a fast solar wind stream, within the context of seasonally enhanced atmospheric water vapor transport when Mars approached its perihelion. These results demonstrate that, in the history of Mars, under the external influence of a younger, more active Sun and with stronger internal transport of water vapor, the atomic hydrogen escape rate could have been much higher than it is today.