When the Galileo spacecraft flew by Jupiter’s moon Ganymede last June, it detected a dense stream of atomic hydrogen escaping from the moon’s sparse atmosphere. That in itself is not so unusual: atomic hydrogen pours off the top of the atmospheres of Earth, Mars, and Venus too. In all those cases, however, the hydrogen comes from water vapor in the atmosphere. Energy from the sun breaks up the water vapor molecules into hydrogen and oxygen atoms. While some of the hydrogen escapes, the oxygen accumulates in the atmosphere. But Ganymede doesn’t have much of an atmosphere, far less water vapor. So where, researchers have wondered, is all that hydrogen coming from?
Charles Barth, a planetary scientist at the University of Colorado in Boulder, believes he has identified the source of Ganymede’s escaping hydrogen: it comes from ice that covers its surface. Yet the oxygen left behind doesn’t form an atmosphere. We think that it’s building up and being incorporated into the icy surface, says Barth. So there is ice with extra oxygen--hydrogen peroxide. I think there is a good possibility that all of the ice on the surface of Ganymede may be covered with extra oxygen.
Barth and his colleagues have calculated that solar energy breaks apart a nanometer (.00000004 inch) of ice from Ganymede’s surface each year. That is both the amount of water that disappears and, under our theory, the amount of oxygen that is left behind. In a thousand years, then, a layer of ice .00004 inch deep would disappear; in a billion years, the moon would lose over three feet of ice--and gain more than three feet of extra oxygen. That’s a substantial amount of oxygen, Barth says. Observations from the Hubble Space Telescope, which has spotted oxygen in Ganymede’s ice, support Barth’s theory.
The oxygen, says Barth, would not necessarily stay on the surface. It might, over time, have mixed down through the ice to react with the rock below. This is what happens on Mars: there is extra oxygen that gets left behind, and it has reacted with the rocks to turn the dull colors into red iron oxides that give Mars its distinctive hue, says Barth. That is not to say that Ganymede is also red under its ice. It depends on whether there is iron in the rock.