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The gravitational pull of Saturn changes the
amount of particles spraying from the south pole of Saturn's active moon
Enceladus at different points in its orbit. More particles make the
plume appear much brighter in the infrared image at left. Credit:
NASA/JPL-Caltech/University of Arizona/Cornell/SSI.› Full image and caption
Enceladus Jets: Surprises in Starlight
The Enceladus plume towers above the icy moon's south pole, reaching hundreds of miles into space. Scientists wanted to know if observed large increases in the plume's icy particle output were driven by a similarly large increase in water vapor. The latest finding is that no such increase is seen. Credit: NASA/JPL/Space Science Institute.› Full image and caption
Narrow jets of gas and icy particles erupt from
the south polar region of Enceladus, contributing to the moon's giant
plume. A cycle of activity in these small-scale jets may be periodically
lofting extra particles into space, causing the overall plume to
brighten dramatically. Credit: NASA/JPL/Space Science Institute.› Full image and caption
During a recent stargazing session, NASA's Cassini spacecraft watched
a bright star pass behind the plume of gas and dust that spews from
Saturn's icy moon Enceladus. At first, the data from that observation
had scientists scratching their heads. What they saw didn't fit their
The observation has led to a surprising new clue about the remarkable
geologic activity on Enceladus: It appears that at least some of the
narrow jets that erupt from the moon's surface blast with increased fury
when the moon is farther from Saturn in its orbit.
Exactly how or why that's happening is far from clear, but the
observation gives theorists new possibilities to ponder about the twists
and turns in the "plumbing" under the moon's frozen surface. Scientists
are eager for such clues because, beneath its frozen shell of ice,
Enceladus is an ocean world that might have the ingredients for life.
Its gas, man
During its first few years after arriving at Saturn in 2004, Cassini
discovered that Enceladus continuously spews a broad plume of gas and
dust-sized ice grains from the region around its south pole. This plume
extends hundreds of miles into space, and is several times the width of
the small moon itself. Scores of narrow jets burst from the surface
along great fractures known as "tiger stripes" and contribute to the
plume. The activity is understood to originate from the moon's
subsurface ocean of salty liquid water, which is venting into space.
Cassini has shown that more than 90 percent of the material in the
plume is water vapor. This gas lofts dust grains into space where
sunlight scatters off them, making them visible to the spacecraft's
cameras. Cassini has even collected some of the particles being blasted
off Enceladus and analyzed their composition.
Not the Obvious Explanation
Previous Cassini observations saw the eruptions spraying three times
as much icy dust into space when Enceladus neared the farthest point in
its elliptical orbit around Saturn. But until now, scientists hadn't had
an opportunity to see if the gas part of the eruptions -- which makes
up the majority of the plume's mass -- also increased at this time.
So on March 11, 2016, during a carefully planned observing run,
Cassini set its gaze on Epsilon
Orionis, the central star in Orion's
belt. At the appointed time, Enceladus and its erupting plume glided in
front of the star. Cassini's ultraviolet imaging spectrometer (or UVIS)
measured how water vapor in the plume dimmed the star's ultraviolet
light, revealing how much gas the plume contained. Since lots of extra
dust appears at this point in the moon's orbit, scientists expected to
measure a lot more gas in the plume, pushing the dust into space.
But instead of the expected huge increase in water vapor output, the
UVIS instrument only saw a slight bump -- just a 20 percent increase in
the total amount of gas.
Cassini scientist Candy Hansen quickly set to work trying to figure
out what might be going on. Hansen, a UVIS team member at the Planetary
Science Institute in Tucson, led the planning of the observation. "We
went after the most obvious explanation first, but the data told us we
needed to look deeper," she said. As it turned out, looking deeper meant
paying attention to what was happening closer to the moon's surface.
Hansen and her colleagues focused their attention on one jet known
informally as "Baghdad I." The researchers found that while the amount
of gas in the overall plume didn't change much, this particular jet was
four times more active than at other times in Enceladus' orbit. Instead
of supplying just 2 percent of the plume's total water vapor, as Cassini
previously observed, it was now supplying 8 percent of the plume's gas.
Call a Plumber
This insight revealed something subtle, but important, according to
Larry Esposito, UVIS team lead at the University of Colorado at Boulder.
"We had thought the amount of water vapor in the overall plume, across
the whole south polar area, was being strongly affected by tidal forces
from Saturn. Instead we find that the small-scale jets are what's
changing." This increase in the jets' activity is what causes more icy
dust grains to be lofted into space, where Cassini's cameras can see
them, Esposito said.
The new observations provide helpful constraints on what could be
going on with the underground plumbing -- cracks and fissures through
which water from the moon's potentially habitable subsurface ocean is
making its way into space.
With the new Cassini data, Hansen is ready to toss the ball to the
theoreticians. "Since we can only see what's going on above the surface,
at the end of the day, it's up to the modelers to take this data and
figure out what's going on underground."
The Cassini-Huygens mission is a cooperative project of NASA, ESA
(European Space Agency) and the Italian Space Agency. JPL, a division of
Caltech in Pasadena, manages the mission for NASA's Science Mission
Directorate in Washington. The ultraviolet imaging spectrograph was
designed and built at the University of Colorado, Boulder, where the
team is based.