An american planetary scientist has cast doubt on a claim that parts
of Jupiter鈥檚 moon Io have water ice. The most peculiar of the giant planet鈥檚
16 moons, Io has a surface rather like that of a pizza because of active
volcanoes that spew out sulphur and sulphur dioxide.
Last year, a team led by Farid Salama and Jesse Bregman at NASA鈥檚 Ames
Research Center in California observed Io at infrared wavelengths. The astronomers
found an absorption line at 2.79 micrometres, which they attributed to a
tiny amount of water ice trapped in frozen sulphur dioxide (Bulletin of
the American Astronomical Society, vol 25, p 851).
Now Douglas Nash of the San Juan Capistrano Research Institute in California
has measured the spectrum of sulphur dioxide ice in his laboratory. He finds
that sulphur dioxide alone can create an absorption line at 2.79 micrometres,
so he says Salama鈥檚 line provides no evidence for water on Io.
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Salama notes, however, that the spectral line which his team detected
on Io was much wider than the one Nash measured in his laboratory. He suggests
that another substance, in addition to sulphur dioxide, is involved. From
his own laboratory data, Salama thinks this additional substance is water
ice on Io. Nash considers that unlikely, based on the evidence he has seen.
Nash and Salama鈥檚 team will publish their conflicting claims in the
February issue of the journal Icarus.