The evolution of hydrogen on Mars

Mars is slowly losing its atmosphere. Lighter water escapes faster than heavier water, and the large enrichment in the Martian atmosphere’s deuterium/hydrogen ratio—relative to that in Earth’s oceans—is evidence for part of the loss. The ratio for Mars was first measured from Doppler-shifted molecular absorption lines taken by the Keck Observatory in Hawaii more than a quarter century ago. Using data from a survey that ran from March 2008 through January 2014, a team led by NASA’s Geronimo Villanueva has created spatially and temporally resolved maps of D/H on Mars. The researchers adopted the same remote spectroscopic approach used to obtain the older results, but instead of measuring a hemispherically averaged value, they stepped the entrance slit of an IR spectrometer across the planet and sampled the spectrum for a few minutes at each position. The resulting maps, each captured during a two-hour span and with a resolution of 500 km, reveal seasonal variability in D/H and such effects as cloud formation and polar-ice sublimation. The figure shows one portion of the northern hemisphere in mid-spring. From the new maps, the team derived a sevenfold enrichment in the representative D/H of the atmosphere. Given the roughly 21-m global equivalent layer of ice in Mars’s polar reservoirs today, the implication is that the planet must have had a layer of at least 137 m in its early history. (G. L. Villanueva et al., Science, in press .)