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 Published by the American Geological Institute
Newsmagazine of the Earth Sciences

 July 2000

News Notes
 Planetary Geology

Spouting off about Io
More than 100 volcanoes vent sulfurous material onto the surface of Jupiter’s moon Io. Recent observations have shed light on several of the volcanic phenomena, some of which have remained enigmatic since Voyager first visited the jovian system more than 20 years ago. 

The largest volcanic plume observed by Voyager rose 300 kilometers above the volcano Pele. Following observations made by the Galileo spacecraft and the Hubble Space Telescope in the mid- to late-1990s, scientists believed the plume contained sulfur dioxide gas. In the May 19 issue of Science, a team of researchers led by John R. Spencer of the Lowell Observatory in Flagstaff, Ariz., reported that they also observed gas containing a rare compound of sulfur (S2) emanating from Pele. 

Using images from Hubble, they isolated the spectral absorption of the sulfur gas against the background of Jupiter’s spectra. The Hubble discovery is only the third astronomical observation of the UV-radiation-sensitive S2, the others being from comets and the impact sites of Shoemaker-Levy 9 on Jupiter. 

     In this enhanced color composite of Io, deposits of sulfur dioxide 
     frost appear as white and gray hues. Other sulfurous materials 
     appear as yellow and brownish hues. Bright red materials, such as 
     the prominent ring surrounding Pele, and "black" spots with low
     brightness mark areas of volcanic activity.  NASA/JPL.
 The researchers also suggested that the sulfur may polymerize into reddish compounds that could give Io its distinctive coloration. Modeling currently underway by Julie Moses of the Lunar and Planetary Science Institute in Houston may help determine whether condensing S2 gas is the culprit behind the colored surface.

In the same issue of Science, a team of researchers led by Susan W. Kieffer of S.W. Kieffer Science Consulting Inc. in Ontario, Canada, presented a model to explain the wandering of the gaseous plume emanating from Io’s volcano Prometheus. The plume has wandered 85–95 kilometers from the point of its discovery by Voyager in 1979, to the location recently observed by the Galileo probe. “The observation team and I realized that we had a major challenge in explaining this wandering,” Kieffer says, “because we have never seen it on Earth.”  According to Kieffer, the goal was to devise a testable hypothesis for how the plume could remain active for 20 years and still maintain its constant shape and optical properties. The researchers show that the critical property is the plume’s mass flux, which must remain unchanged for the 20-year period.

The researchers propose that the wandering can be explained by the vaporization of a “snowfield” of sulfur compounds over which a lava flow is moving. The melted, sulfurous fluid at the base of the lava erupts to the surface much like a rootless conduit on Earth (an eruption conduit that is not attached to a magma chamber).

In the May 4 issue of Nature, Amara L. Graps and a team of researchers led by Eberhard Gruen of the Max-Planck-Institut für Kernphysik in Germany pinpointed Io’s volcanoes as the source for a far-reaching dust stream that emanates from the jovian system. Using data from Galileo’s Dust Detector System, the researchers were able to observe both the frequency of dust emission and the general sizes of the particles.

The team determined the strong likelihood of an Io volcanism source and eliminated other potential sources, such as Jupiter’s gossamer ring and Io’s impact ejecta. While the dust composition is most likely sulfurous, “it’s a difficult measurement,” Graps says. “People doing chemical analysis and interpretation of dust particles . . . are sometimes surprised with what their spectra look like. This is a new field.”

The analyses of the Io dust stream will hopefully come in December. That is when the Cassini space probe will join Galileo to observe Io, and the combination of its onboard Cosmic Dust Analyzer and Galileo’s Dust Detector System may allow researchers to determine an accurate composition.

Josh Chamot
Geotimes contributing writer

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