A huge
cloud of gas, spewed from volcanos on one of Jupiter’s
moons, extends into space to a distance that is almost equal
to that of the earth from the sun, says a new report
published in the journal Nature.
A sensor on board the Cassini spacecraft built by scientists at the University of Maryland and one developed
by the Applied Physics Laboratory of Johns Hopkins
University detected ionized and non-ionized atoms of this
cloud during Cassini’s recent flyby of Jupiter. The authors
write that information from these sensors suggest that
volcanic gases from Io – one of Jupiter’s largest moons and
the most volcanically active body in the solar system –
escape Jupiter’s magnetic field and populate the
interplanetary environment around the planet.
“The University of Maryland-built CHEMS sensor detected ions of this cloud out to a distance of almost one
astronomical unit [the distance between the Earth and the
Sun],” said co-author Douglas C. Hamilton, a professor of
physics at the University of Maryland and leader of the
space physics team that designed and built the CHEMS (CHarge
Energy Mass Spectrometer) sensor.
“CHEMS, which provides detailed measurements of the composition of ions, was also able to show that a
significant portion of the particles in the cloud were
sulfur and oxygen with sulfur dioxide likely present as
well,” Hamilton said. “Sulfur dioxide is the chief gas
emitted by volcanos, indicating Io as the likely origin for
much of the gas cloud that Cassini detected.”
From Atom to Ion and Back Again
According to Hamilton the cloud is the result of various
processes in the region around Jupiter that change some
atoms derived from volcanic gas, first into ions, then back
to atoms and, in some cases, back to ions again. Ions are
atoms that have lost one or more electrons.
The first step of a volcanic gas atom’s journey from Io
to interplanetary space is when it becomes ionized and
energized in Jupiter’s magnetosphere. This ion now has the
speed it needs to fly away from Jupiter, but because of its
electrical charge, it remains held within the magnetosphere
by the planet’s magnetic field. However, such energetic
ions can pick up electrons from other atoms or molecules and
once again become “normal” or electromagnetically neutral
atoms. These energetic neutral atoms are no longer bound
within Jupiter’s magnetosphere and can zoom into
interplanetary space.
Once in space some of these neutral particles are again
converted into ions, this time through photoionization, a
process by which electrons are knocked away from atoms by
the impact of high energy ultraviolet photons from the sun.
The gas cloud extending far out beyond Jupiter’s
magnetosphere is thus a mixture of energetic atoms and ions
that are flowing away from the planet.
On board Cassini, Maryland’s CHEMS sensor detects ions,
while atoms are imaged by the APL-developed INCA sensor.
INCA stands for Ion and Neutral Camera. During the flyby of
Jupiter, INCA provided images showing some structural detail
of Jupiter’s magnetosphere, while also detecting atoms in
the nebulous cloud that stretches away from the planet.
Detection of ions and determination of specific types of
particles in the gas cloud were made by the CHEMS sensor.
On to Saturn Of course the primary target for Cassini is
Saturn, which it will reach in 2004. Cassini, which is
carrying the European Space Agency’s Huygens Probe
spacecraft, is the best-instrumented mission ever sent to
another planet.
On Cassini, INCA and CHEMS are linked together by a
central computer “brain” together with the LEMMS (Low Energy
Magnetospheric Measurement System) sensor. The three
sensors and their computer form Cassini’s Magnetospheric
Imaging Instrument, known as MIMI. MIMI is one of one of 12
science instruments on the main Cassini spacecraft and one
of six instruments designed primarily to investigate the
space environments around Saturn and its satellites. The
Huygens probe will investigate Saturn’s largest moon, Titan.
According to Maryland’s Hamilton, the flyby of Jupiter
was important scientifically because it has revealed new
information about Jupiter’s magnetosphere, its interaction
with the solar wind and its surrounding nebula. However, it
was also an important step on the road to Saturn. “This
flyby has provided us an excellent test of the capabilities
of MIMI and has allowed us to make important refinements to
some of the software running on MIMI’s micro-processing
unit,” he said. “A nebula of gases from Io surrounding Jupiter” by
Stamatios M Krimigis et al. (Applied Physics Lab, Johns
Hopkins University), Douglas C Hamilton and George Gloeckler
(Department of Physics, University of Maryland), Jannis
Dandouras (CNES), Thomas P. Armstrong (Fundamental
Technologies), Scott J Bolton (Jet Propulsion Laboratory), K
C Hsieh (Department of Physics, University of Arizona),
Norbert Krupp et al. (MPI fur Astronomie), Louis J
Lanzerotti (Bell Labs), Nature vol. 415, no. 994-996.
Related NASA web links:
http://www.jpl.nasa.gov/releases/2002/release_2002_46.html
http://www.jpl.nasa.gov/images/jupiter/index.html
