This means that water vapor subliming from the northern cap can't travel far in the atmosphere before it recondenses into ice crystals. Water vapor circulates from the north pole to the south at an altitude above 5 to 10 km, but very little rises this high before freezing. In contrast, the south pole has its summer at perihelion, when the atmosphere is warmer. Because of the higher temperature, vapor subliming from the south polar cap resides in significant quantities above the critical range of 5 to 10 km. Thus, water from the south gets pumped northward, but water from the north doesn't circulate south, accounting for the larger ice cap at the north pole, Clancy theorizes.
Ice covers Europa, one of Jupiter's larger moons.
Reasoning that where there's water, there should be oxygen, researchers had predicted that this frozen body might have a thin atmosphere of molecular oxygen. Now, Hubble's Goddard high-resolution spectrograph has found evidence to support this. Europa thus joins Earth, Mars, and Venus as the only bodies in the solar system known to have an oxygen-containing atmosphere. The spectrograph recorded far-ultraviolet emissions from atomic oxygen in Europa's atmosphere. In the Feb. 23 Nature, Doyle T. Hall of Johns Hopkins University in Baltimore and his colleagues argue that the atomic oxygen most likely comes from molecular oxygen split apart by collisions with electrons.