Pale Blue Dot ( PDFDrive.com ) (1)

EVEN MORE UNEXPECTED than the great Martian volcanos or the magma-flooded surface of Venus is
what awaited us when the Voyager 1 spacecraft encountered lo, the innermost of the four
large Galilean moons of Jupiter, in March 1979. There we found a strange, small, multihued world
positively awash in volcanos. As we watched in astonishment, eight active plumes poured gas and
fine particles up into the sky. The largest, now called Pele—after the Hawaiian volcano goddess—
projected a fountain of material 250 kilometers into space, higher above the surface of Io than some
astronauts have ventured above the Earth. By the time Voyager 2 arrived at Io, four months later, Pele
had turned itself off, although six of the other plumes were still active, at least one new plume had
been discovered, and another caldera, named Surt, had changed its color
dramatically.
The colors of Io, even though exaggerated in NASA's color-enhanced images, are like none
elsewhere in the Solar System. The currently favored explanation is that the Ionian volcanos are
driven not by upwelling molten rock, as on the Earth, the Moon, Venus, and Mars, but by upwelling
sulfur dioxide and molten sulfur. The surface is covered with volcanic mountains, volcanic calderas,
vents, and lakes of molten sulfur. Various forms and compounds of sulfur have been detected on the
surface of Io and in nearby space—the volcanos blow some of the sulfur off Io
altogether.[*******] These findings have suggested to some an underground sea of liquid sulfur
that issues to the surface at points of weakness, generates a shallow volcanic mound, trickles
downhill, and freezes, its final color determined by its temperature on eruption.
On the Moon or Mars, you can find many places that have changed little in a billion years. On Io, in
a century, much of the surface should be reflooded, filled in or washed away by new volcanic flows.
Maps of Io will then quickly become obsolete, and cartography of Io will have become a growth
industry.
All this seems to follow readily enough from the Voyager observations. The rate at which the
surface is covered over by current volcanic flows implies massive changes in 50 or 100 years, a
prediction that luckily can be tested. The Voyager images of to can be compared with much poorer
images taken by ground-based telescopes 50 years earlier, and by the Hubble Space Telescope 13
years later. The surprising conclusion seems to be that the big surface markings on Io have hardly
changed at all. Clearly, we're missing something.
A VOLCANO in one sense represents the insides of a planet gushing out, a wound that eventually heals
itself by cooling, only to be replaced by new stigmata. Different worlds have different insides. The
discovery of liquid-sulfur vulcanism on Io was a little like finding that an old acquaintance, when cut,
bleeds green. You had no idea such differences were possible. He seemed so ordinary.
We are naturally eager to find additional signs of vulcanism on other worlds. On Europa, the
second of the Galilean moons of Jupiter and Io's neighbor, there are no volcanic mountains at all; but
molten ice—liquid water—seems to have gushed to the surface through an enormous number of