The upper atmosphere of Mars, like that of Earth, is slowly leaking into space. Hydrogen, the lightest element, suffers the fastest loss, and carbon dioxide now makes up all but 4% of Mars’s tenuous atmosphere. Near-UV sunlight dissociates water vapor into constituent hydrogen and oxygen atoms that eventually diffuse into the upper atmosphere and, after subsequent chemical reactions and interactions with the solar wind, can escape the planet’s weak gravity.
Because deuterium is twice as massive as hydrogen, it is less likely to escape, and over geologic time the atmosphere’s D/H ratio has steadily risen. The ratio was first measured more than a quarter century ago from Doppler-shifted molecular absorption lines of heavy and light water, HDO and H2O, recorded atop Mauna Kea in Hawaii. Blue- and redshifted lines distinguish Mars’s water vapor from Earth’s as the planets approach each other and recede every couple of Earth years. The highly enriched ratio that was observed—roughly six times the D/H of Earth’s oceans, the Vienna Standard Mean Ocean Water (VSMOW)—is evidence that at least 80% of Mars’s surface water has been lost.
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