June 30th 06, 07:06 PM
http://saturn.jpl.nasa.gov/news/features/feature20060629.cfm
Enceladus Exudes an Air of Mystery
Jet Propulsion Laboratory
June 29, 2006
Atomic oxygen could never be confused with expensive perfume. But just
as a fragrance lingering in the air of an empty room offers hints about
a previous occupant, the cloud of oxygen the Cassini spacecraft
encountered as it first approached Saturn turned out to be a calling
card from another celestial presence, the tiny moon Enceladus.
The oxygen was the first clue that much more is going on beneath
Enceladus' icy surface than it first appeared. It took a while for
scientists to get the message. Tracking down the oxygen's source led
them to a most unusual spot in the solar system, a place that may
possess one of the rarest and most sought-after substances in the
universe - liquid water.
Now You See It, Now You Don't
Cassini's ultraviolet imaging spectrograph made its initial survey of
Saturn's atmosphere late in December 2003 as the spacecraft neared the
end of its seven-year voyage to the ringed planet. Along with the
expected hydrogen, long known to be the gas giant's major component,
the
spectrograph detected the presence of oxygen. "We were the first to
discover the oxygen in the Saturn system," says Dr. Larry Esposito of
the University of Colorado, principal investigator for the instrument.
Since Saturn's rings are made mostly of water ice, it wasn't surprising
to find one ingredient of water, oxygen, in Saturn's atmosphere, he
explains. The oxygen they observed was in the form of single oxygen
atoms, called atomic oxygen, like that in water, H2O.
In January 2004, when it looked again at Saturn's atmosphere, it found
a
massive bubble of oxygen near Saturn's outermost ring, the E ring. Then
just a few months later, much of the oxygen was gone. "What was
surprising was the amount of oxygen we saw and how it changed," says
Esposito. "This was the first sign that something unusual was going
on."
"Our first idea was that the collisions between small moons in the
rings
would release small puffs of ice and that ice would be broken down into
its atoms, one of which would be atomic oxygen," Esposito explains. "We
put out this hypothesis, but the idea didn't seem to be working out."
What did work out were a host of observations in spring 2005 showing
that something very odd was taking place on Enceladus, which just
happens to orbit within the E ring, where the mysterious cloud of
oxygen
was first discovered. Cassini's magnetometer showed that the moon had
an
atmosphere. Too small to have enough gravity to hold on to an
atmosphere
for very long, Enceladus had to have a steady source of gas, such as
geysers, to keep an atmosphere going. The cosmic dust analyzer detected
a stream of particles around Enceladus. The scientists wondered whether
these particles could be coming from the moon or from the E ring and
if,
perhaps, the moon itself might be the source of ice particles for the E
ring.
The Cassini science teams were eager to get a closer look. Mission
navigators re-designed the spacecraft's trajectory to bring it closer
to
Enceladus than had been previously planned. "This is a big advantage of
a mission like Cassini," according to Esposito. "You don't just fly by
once, you can follow-up when something seems promising."
Ready for a Close-Up
In July 2005, Cassini cruised only 175 kilometers (109 miles) from
Enceladus. The spacecraft's instruments revealed that the large dark
cracks, dubbed "tiger stripes," on the moon's south pole were warm and
spewing out water vapor and ice particles.
The flyby occurred just when a star was moving behind the moon's
southern pole. "As we watched the moon extinguish the light from the
star, the spectrometer identified oxygen," says Esposito. "The
starlight
showed water molecules between us -- the spacecraft -- and the star. It
showed a localized cloud of water near Enceladus."
"We were able to measure the shape of the cloud, estimate the amount of
water it contained and the rate it would be destroyed and produce
oxygen," says Esposito. The amount of water they saw, about a million
tons, was exactly that needed to provide a cloud of oxygen like the one
they had first observed near the E ring more than a year earlier. "This
was a most pleasing result," says Esposito. "We measured two new
distinct phenomena and found that they fit together."
The mystery of the atomic oxygen was solved. At the same time, its
source, the diminutive Enceladus revealed itself to be completely
different than the cold, dead icy moon it should have been. Small as it
is, it has an internal heat source and is geologically active. Its
geysers throw out enough water vapor and ice to maintain the moon's
atmosphere, feed the vast E ring, and decompose into clouds of oxygen
like the one first spotted by Cassini on its way to Saturn.
Not far beneath the moon's snowy white surface, scientists say, may be
large pools of liquid water, warmed by the same heat source that powers
its geysers. Its potential for water puts Enceladus into an elite group
of places where life could exist. The Cassini spacecraft will swing
back
in 2008 for another look.
To learn more about Enceladus, visit
http://saturn.jpl.nasa.gov/science/moons/moonDetails.cfm?pageID=83
Enceladus Exudes an Air of Mystery
Jet Propulsion Laboratory
June 29, 2006
Atomic oxygen could never be confused with expensive perfume. But just
as a fragrance lingering in the air of an empty room offers hints about
a previous occupant, the cloud of oxygen the Cassini spacecraft
encountered as it first approached Saturn turned out to be a calling
card from another celestial presence, the tiny moon Enceladus.
The oxygen was the first clue that much more is going on beneath
Enceladus' icy surface than it first appeared. It took a while for
scientists to get the message. Tracking down the oxygen's source led
them to a most unusual spot in the solar system, a place that may
possess one of the rarest and most sought-after substances in the
universe - liquid water.
Now You See It, Now You Don't
Cassini's ultraviolet imaging spectrograph made its initial survey of
Saturn's atmosphere late in December 2003 as the spacecraft neared the
end of its seven-year voyage to the ringed planet. Along with the
expected hydrogen, long known to be the gas giant's major component,
the
spectrograph detected the presence of oxygen. "We were the first to
discover the oxygen in the Saturn system," says Dr. Larry Esposito of
the University of Colorado, principal investigator for the instrument.
Since Saturn's rings are made mostly of water ice, it wasn't surprising
to find one ingredient of water, oxygen, in Saturn's atmosphere, he
explains. The oxygen they observed was in the form of single oxygen
atoms, called atomic oxygen, like that in water, H2O.
In January 2004, when it looked again at Saturn's atmosphere, it found
a
massive bubble of oxygen near Saturn's outermost ring, the E ring. Then
just a few months later, much of the oxygen was gone. "What was
surprising was the amount of oxygen we saw and how it changed," says
Esposito. "This was the first sign that something unusual was going
on."
"Our first idea was that the collisions between small moons in the
rings
would release small puffs of ice and that ice would be broken down into
its atoms, one of which would be atomic oxygen," Esposito explains. "We
put out this hypothesis, but the idea didn't seem to be working out."
What did work out were a host of observations in spring 2005 showing
that something very odd was taking place on Enceladus, which just
happens to orbit within the E ring, where the mysterious cloud of
oxygen
was first discovered. Cassini's magnetometer showed that the moon had
an
atmosphere. Too small to have enough gravity to hold on to an
atmosphere
for very long, Enceladus had to have a steady source of gas, such as
geysers, to keep an atmosphere going. The cosmic dust analyzer detected
a stream of particles around Enceladus. The scientists wondered whether
these particles could be coming from the moon or from the E ring and
if,
perhaps, the moon itself might be the source of ice particles for the E
ring.
The Cassini science teams were eager to get a closer look. Mission
navigators re-designed the spacecraft's trajectory to bring it closer
to
Enceladus than had been previously planned. "This is a big advantage of
a mission like Cassini," according to Esposito. "You don't just fly by
once, you can follow-up when something seems promising."
Ready for a Close-Up
In July 2005, Cassini cruised only 175 kilometers (109 miles) from
Enceladus. The spacecraft's instruments revealed that the large dark
cracks, dubbed "tiger stripes," on the moon's south pole were warm and
spewing out water vapor and ice particles.
The flyby occurred just when a star was moving behind the moon's
southern pole. "As we watched the moon extinguish the light from the
star, the spectrometer identified oxygen," says Esposito. "The
starlight
showed water molecules between us -- the spacecraft -- and the star. It
showed a localized cloud of water near Enceladus."
"We were able to measure the shape of the cloud, estimate the amount of
water it contained and the rate it would be destroyed and produce
oxygen," says Esposito. The amount of water they saw, about a million
tons, was exactly that needed to provide a cloud of oxygen like the one
they had first observed near the E ring more than a year earlier. "This
was a most pleasing result," says Esposito. "We measured two new
distinct phenomena and found that they fit together."
The mystery of the atomic oxygen was solved. At the same time, its
source, the diminutive Enceladus revealed itself to be completely
different than the cold, dead icy moon it should have been. Small as it
is, it has an internal heat source and is geologically active. Its
geysers throw out enough water vapor and ice to maintain the moon's
atmosphere, feed the vast E ring, and decompose into clouds of oxygen
like the one first spotted by Cassini on its way to Saturn.
Not far beneath the moon's snowy white surface, scientists say, may be
large pools of liquid water, warmed by the same heat source that powers
its geysers. Its potential for water puts Enceladus into an elite group
of places where life could exist. The Cassini spacecraft will swing
back
in 2008 for another look.
To learn more about Enceladus, visit
http://saturn.jpl.nasa.gov/science/moons/moonDetails.cfm?pageID=83