SwRI scientists John
Spencer and Kandis Lea Jessup, working with an
international team including astronomers from Texas, California,
France, and Spain, have developed a new technique for studying the
elusive atmosphere of Jupiter's volcanic moon Io. They describe
their surprising results in a paper
that has recently been accepted for publication in the journal
Icarus. Io's tenuous atmosphere, composed mostly of sulfur
dioxide (SO2) gas at roughly one billionth of Earth
atmospheric pressure, has always been difficult to study, requiring
either spacecraft such as the Hubble
Space Telescope, or large radio telescopes, to detect it at
all. But Spencer's team were able to detect the atmosphere using NASA's Infrared Telescope
Facility at Mauna Kea
Observatory, Hawaii, thanks to a sophisticated instrument called TEXES,
developed (not surprisingly) at the University of Texas. TEXES is
a high-resolution mid-infrared spectrograph, which splits the infrared
light gathered by the telescope into extremely fine wavelength
divisions. This precision is necessary to detect the signature of
SO2 gas which, for instance, absorbs light with a wavelength
of 18.8546 microns and ignores light with a wavelength of 18.8555
microns.
Spencer's team first looked at Io with TEXES in November 2001, when
Spencer (who moved to SwRI in early 2004) was still an astronomer at Lowell
Observatory. The TEXES spectra revealed numerous absorption lines
produced by SO2 gas, which absorbs the heat radiation coming
from Io's surface and volcanos at its characteristic wavelengths.
As Io rotated (its rotation period, synchronous with its rotation
around Jupiter, is just 1.8 days), they were surprised to see large
variations in the strength of the SO2 signature. The
absorption lines were much stronger on the side of Io that
faces away from Jupiter than on the Jupiter-facing side. To
convert the strength of the SO2 lines into an estimate of
the surface pressure on Io, the team incorporated other information
about the atmosphere, particularly their own and other researchers'
Hubble Space Telescope observations that indicated that the atmosphere
is concentrated near the equator and much thinner near the poles.
The analysis implies that atmosphere near the equator is ten times
denser on the side of Io that faces away from Jupiter (the
"anti-Jupiter" hemisphere), where the surface pressure is a few
nanobars, than on the side facing Jupiter, where the pressure is a few
tenths of a nanobar. Such wide disparities are possible on Io
because the atmosphere can freeze out on the surface before it has a
chance to flow around to the other side to equalize the pressure.
The team is still not sure about what causes the excess atmosphere on
the anti-Jupiter side in the first place, however- it could be due to
evaporation of sulfur dioxide frost, which is more abundant on the
anti-Jupiter side, a greater supply of gas from volcanic plumes, which
are also more abundant on that side, or a combination of the two.
Since they first discovered the infrared absorption lines in 2001,
Spencer's team have been using TEXES and the NASA Infrared Telescope to
check in on the atmosphere once a year. The atmospheric pressure
on each side of Io has been remarkably stable since 2001, which is odd
regardless of where the SO2 gas comes from. If
it is from volcanos, they would expect the atmosphere to come and go
with the vagaries of Io's highly variable volcanic activity. If
the gas comes from evaporation of frost, they would expect the
atmosphere to be freezing onto the surface. This is because Io
has been receding from the sun since 2001, due to Jupiter's eccentric
orbit, so the frost should be cooling down.
Spencer and his team will continue to keep an eye on Io's atmosphere to see
if this puzzling stability continues, and will be working on improved
models to explain it.