AbstractMars is believed to have lost much of its surface water 3.5 billion years ago, but the amounts that escaped into space and remain frozen in the crust today are not well known. Hydrogen atoms in the extended martian atmosphere, some of which escape the planet's gravity, can be imaged through scattered solar UV radiation. Hubble Space Telescope (HST) images of the ultraviolet H Ly α emission now indicate that the coronal H density steadily decreased by a factor of roughly 40% over 4 weeks, a far greater variation than had been expected. The leading candidate cause is a decrease in the source rate of water molecules from the lower atmosphere, consistent with seasonal changes and a recent global dust storm. This implies that the rate of escape of martian hydrogen (and thereby water) into space is strongly dependent on the lower atmospheric water content and distribution.
