National Geographic : 1973 Feb
* Clouds hanging over Nix Olympica and other volcanoes are thought to be substantial ly water ice (page 248). The same is true of the clouds at the edges of the polar hoods. * Ultraviolet measurements from Mariner 9 indicate that the hydrogen escaping daily from the atmosphere of Mars amounts to the equivalent of that in 100,000 gallons of water. Dr. Charles A. Barth (page 242), who is in charge of Mariner's ultraviolet spectrometer, suggests that the hydrogen comes from water vapor high in the atmosphere that has been dissociated, or chemically split, by the effect of the sun's ultraviolet radiation. How much water is still on the surface of Mars remains something of a mystery. The vast quantities of carbon dioxide on Mars (at least 90 percent of the atmosphere and the bulk of the polar caps) suggest that large amounts of water in some form have in past eons been introduced to the surface and into the atmosphere. As Dr. Norman Horowitz of the California Institute of Technology explained to me: "No planet can make a lot of carbon dioxide with out producing even larger amounts of water. Remember that hydrogen is by far the most important element in the universe, so we should see much more oxygen combined with hydrogen [water] than with carbon [carbon dioxide]. But on Mars we don't." Has most of the hydrogen and oxygen been lost to space, or does more of it reside, unrec ognized, somewhere on or in the surface? Perhaps the polar caps hold great reservoirs of water. These caps act as dehumidifiers, or cold traps. When winds blow across them, the air condenses because of the extreme cold. It is just like ice forming on the walls of your home freezer.