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This diagram depicts conditions observed by NASA's Cassini spacecraft
during a flyby in Dec. 2013, when Saturn's magnetosphere was highly
compressed, exposing Titan to the full force of the solar wind. Image
credit: NASA/JPL-Caltech.› Full image and caption
Researchers studying data from NASA's Cassini mission have observed
that Saturn's largest moon, Titan, behaves much like Venus, Mars or a
comet when exposed to the raw power of the solar wind. The observations
suggest that unmagnetized bodies like Titan might interact with the
solar wind in the same basic ways, regardless of their nature or
distance from the sun.
Titan is large enough that it could be considered a planet if it
orbited the sun on its own, and a flyby of the giant moon in Dec. 2013
simulated that scenario, from Cassini's vantage point. The encounter was
unique within Cassini's mission, as it was the only time the spacecraft
has observed Titan in a pristine state, outside the region of space
dominated by Saturn's magnetic field, called its magnetosphere.
"We observed that Titan interacts with the solar wind very much like
Mars, if you moved it to the distance of Saturn," said Cesar Bertucci of
the Institute of Astronomy and Space Physics in Buenos Aires, who led
the research with colleagues from the Cassini mission. "We thought Titan
in this state would look different. We certainly were surprised," he
The solar wind is a fast-flowing gale of charged particles that
continually streams outward from the sun, flowing around the planets
like islands in a river. Studying the effects of the solar wind at other
planets helps scientists understand how the sun's activity affects
their atmospheres. These effects can include modification of an
atmosphere's chemistry as well as its gradual loss to space.
Titan spends about 95 percent of the time within Saturn's
magnetosphere. But during a Cassini flyby on Dec. 1, 2013, the giant
moon happened to be on the sunward side of Saturn when a powerful
outburst of solar activity reached the planet. The strong surge in the
solar wind so compressed the sun-facing side of Saturn's magnetosphere
that the bubble's outer edge was pushed inside the orbit of Titan. This
left the moon exposed to, and unprotected from, the raging stream of
energetic solar particles.
Using its magnetometer instrument, which is akin to an equisitely
sensitive compass, Cassini has observed Titan many times during the
mission's decade in the Saturn system, but always within Saturn's
magnetosphere. The spacecraft has not been able to detect a magnetic
field coming from Titan itself. In its usual state, Titan is cloaked in
Saturn's magnetic field.
This time the influence of Saturn was not present, allowing Cassini's
magnetometer to observe Titan as it interacted directly with the solar
wind. The special circumstance allowed Bertucci and colleagues to study
the shockwave that formed around Titan where the full-force solar wind
rammed into the moon's atmosphere.
At Earth, our planet's powerful magnetic field acts as a shield
against the solar wind, helping to protect our atmosphere from being
stripped away. In the case of Venus, Mars and comets -- none of which is
protected by a global magnetic field -- the solar wind drapes around
the objects themselves, interacting directly with their atmospheres (or
in the comet's case, its coma). Cassini saw the same thing at Titan.
Researchers thought they would have to treat Titan's response to the
solar wind with a unique approach because the chemistry of the hazy
moon's dense atmosphere is highly complex. But Cassini's observations of
a naked Titan hinted at a more elegant solution. "This could mean we
can use the same tools to study how vastly different worlds, in
different parts of the solar system, interact with the wind from the
sun," Bertucci said.
Bertucci noted that the list of similarly unmagnetized bodies might
include the dwarf planet Pluto, to be visited this year for the first
time by NASA's New Horizons spacecraft.
"After nearly a decade in orbit, the Cassini mission has revealed
once again that the Saturn system is full of surprises," said Michele
Dougherty, principal investigator of the Cassini magnetometer at
Imperial College, London. "After more than a hundred flybys, we have
finally encountered Titan out in the solar wind, which will allow us to
better understand how such moons maintain or lose their atmospheres."
The new research is published today in the journal Geophysical Review Letters.
The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. NASA's Jet
Propulsion Laboratory, a division of the California Institute of
Technology in Pasadena, manages the mission for NASA's Science Mission
Directorate, Washington, D.C. JPL designed, developed and assembled the
Cassini orbiter. The magnetometer team is based at Imperial College,