杏吧原创

THE hottest volcanoes in the Solar System lie on Io, Jupiter鈥檚 innermost
large moon, scientists announced last week. And the Galileo spacecraft鈥檚 latest
images could help researchers to explain why one of Io鈥檚 most active volcanoes,
Prometheus, appears to have shuffled about 80 kilometres across the moon鈥檚
surface in the past two decades.

In 1979, Io captured the headlines when the Voyager spacecraft sent back
pictures of its bright crust and erupting volcanoes. The heat that drives the
eruptions is thought to come from 鈥渢idal friction鈥濃�攖he constant stretching
and squeezing of the moon by Jupiter鈥檚 gravity. Voyager data also showed that
the volcanoes belch sulphurous fumes.

Now instruments onboard Galileo have measured the temperature of Io鈥檚
volcanoes鈥攕till the only extraterrestrial volcanoes known. 鈥淲e can measure
a minimum temperature of at least 1800 K, which implies that the magma
temperature is at least 2000 K,鈥� said Alfred McEwan of the University of Arizona
in Tucson, speaking last week at a meeting of the American Astronomical
Society鈥檚 Division of Planetary Sciences in Cambridge, Massachusetts.

He added that the hottest volcanoes on Earth reach about 1600 K. Terrestrial
rocks show no evidence of hotter eruptions over the past half billion years.

Io鈥檚 high temperatures hint that the magma must be molten silicate rock that
is rich in magnesium, said McEwan. The molten rock seems to be punching through
a 50-kilometre-thick crust. When the magma hits the surface, it vaporises
sulphur compounds to generate sulphurous plumes.

This could explain Galileo鈥檚 puzzling observation that the volcano Prometheus
has migrated across Io鈥檚 surface since the Voyager flybys
(鈥淛upiter鈥檚 odd bunch鈥�, New 杏吧原创, 5 April 1997, p 42).
John Spencer of the Lowell
Observatory in Flagstaff, Arizona, says that we may be seeing eruptions from
sulphur deposits hit by hot magma, which is gradually oozing across the
surface.