PASS Planetarium Activities for Student Success Please send new material for this page to Alan Gould <agould @uclink.berkeley.edu> [space before "@" is antispam device; please remove it when sending e-mail] A Whiff of Vanished Martian Seas by J. KELLY BEATTY March, 2002, Sky & Telescope magazine, p. 18. FUTURE ASTRONAUTS ROAMING THE SURFACE OF MARS WILL BE hard-pressed to find sources of water, but the red planet was not always as arid as it is now. From minuscule gullies to giant floodplains, the face of Mars bears mute witness to eras when water must have gushed across its surface at least briefly. In fact, a recent study shows indirectly, but convincingly, that Mars may have formed with enough water to cover its entire globe to a depth of at least 1.25 kilometers (about 4,000 feet). The implication is that this dusty, arid world actually started out with more water, relative to its overall mass, than Earth did. The provocative evidence comes not from some robotic sentinel on Mars itself, but from the Far Ultraviolet Spectroscopic Explorer orbiting 760 km above Earth. Tladimir. A. Krasnopolsky (Catholic University of America) and Paul ID. Feldman (Johns Hopkins University) used FUSE to make the first-ever detection of hydrogen molecules (H2) in the upper Martian atmosphere, as they describe in the November 30, 2001, issue of Science. The hydrogen, present at just 15 parts per million, comes from water molecules that have been broken down by ultraviolet sunlight. Four years ago Krasnopolsky used the Hubble Space Telescope to determine the amount of deuterium ("heavy" hydrogen) in the Martian atmosphere. These two abundances provide important clues to unraveling water's history there. Today Mars's atmosphere has a deuterium-to-hydrogen (ID/H) ratio 5.5 times higher than Earth's. Yet Martian meteorites ejected from Mars's surface 3.5 billion years ago testify to a time when the DIN enrichment was only about 1.9. Sometime earlier than that, water vapor reacted with iron on the surface and released huge quantities of hydrogen, which escaped wholesale into space. When this process shut off, water continued to leak away, albeit gradually, as molecules at the top of the atmosphere broke into their component atoms. The lighter H escapes from a planet more readily than the heavier ID, so the deuterium becomes enriched over time. Knowing the H, and D abundances, Krasnopolsky has modeled the atmosphere's evolution and deduces that the D/H rise from 1.9 to 5.5 represents a loss of Martian water equivalent to a planetwide ocean about 30 meters deep. What little water remains today in the polar caps and hidden elsewhere is probably enough for a 20-meter-deep layer. Thus, 31/2 billion years ago the ocean would have been some 50 meters deep. Working hack in time, he calculates that rapid escape of hydrogen molecules likely robbed the planet of all but 4 percent of its original water inventory, implying that it started out with a layer of 1-1/4 km or more. Krasnopolsky's model assumes that Mars and Earth acquired their water the same way and thus began with equal 1)/H ratios. But these assumed ratios could easily have been upset by varying the proportion of incoming water-hearing comets, which are known to have high D/H ratios.

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