IF Jupiter鈥檚 moon Io were a teenager, it would be mortified: a large yellow blob resembling a zit has appeared smack-dab in the middle of its face. The bright mark, 320 kilometres in diameter, is the most striking change in Io鈥檚 appearance in at least 15 years, according to scientists who spotted the blemish with the Hubble Space Telescope.
The yellow spot does not appear in images taken by the two Voyager spacecraft in 1979 or in Hubble photographs taken as recently as March 1994. Just 16 months later, however, a July 1995 Hubble picture shows the well-developed zit.
The feature is perfectly centred on Ra Patera, a known volcano on Io. 鈥淪o we know what did it,鈥� says John Spencer of Lowell Observatory in Flagstaff, Arizona. But what happened is still unclear. The spot is much brighter and more yellow than any other feature on Io鈥檚 mottled surface, and its colour could be caused by a frozen deposit of a volatile, sulphur-containing gas, says Spencer. 鈥淧resumably, it鈥檚 such an unusual colour because it鈥檚 so fresh,鈥� he says. If so, the spot should fade as the gas evaporates.
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The astronomers plan to have Hubble keep an eye on the spot to see how it changes. And if the troubled Galileo space probe is still working when it flies past Io in December, it may reveal whether a lava flow or a spattering produced by an explosion is to blame.