At the Edge of an Alien World: NASA Cassini Arrives at Saturn

When the Cassini spacecraft launched into space nearly seven years ago,
its destination – Saturn – appeared as a speck in a sea of stars. More
than a billion miles later, Cassini has at last arrived at the ringed
planet and discovered a dynamic world full of surprises.

"Our ideas about the rings have been expanded tremendously," said Dr.
Linda Spilker, deputy project scientist for the Cassini-Huygens mission,
NASA’s Jet Propulsion Laboratory, Pasadena Calif.

Only a week into the international Cassini-Huygens mission, new findings
are already changing what we know about Saturn, its majestic rings,
magnetosphere and many moons. Those discoveries include a mysterious
material dubbed "dirt" in the gaps between Saturn’s rings, a ringside
cloud of escaping oxygen, and a puzzling picture of the surface and
atmosphere of the planet’s largest moon, Titan.

"We have had a continuous stream of discoveries ever since we flew by
the small moon Phoebe, which turned out to be a geologist’s delight.
Saturn’s magnetosphere was bigger than expected, and Titan is indeed
exotic and challenges the Cassini-Huygens scientists to explain its
surface and atmosphere," said Dr. Dennis Matson, project scientist for
the Cassini-Huygens mission. "We have just scratched the surface of this
system. There is much more to see and measure."

In addition to Titan, Cassini recently captured images of the moons
Mimas, Tethys, Rhea and Iapetus. These and other new pictures from
Saturn can be found as raw images at
http://saturn.jpl.nasa.gov/multimedia/images/raw/index.cfm.

Cassini is presently orbiting Saturn on the opposite side of the Sun
from Earth. It will remain out of communication until July 12, 2004,
when it reappears from behind the Sun.

Cassini’s Wild Ring Ride

At 9:12 p.m. PDT on June 30, 2004, cheers and applause broke out at
NASA’s Jet Propulsion Laboratory when flight controllers announced that
Cassini and its piggybacked Huygens probe had successfully become the
first spacecraft to orbit Saturn. The spacecraft had passed through a
gap between Saturn’s F and G rings and executed a 96-minute engine burn
before passing back out through the rings and swinging into orbit around
Saturn.

That ride into Saturn’s orbit brought Cassini closer to the rings than
it will ever be again and resulted in the most detailed pictures of the
rings ever seen. The images, taken by the spacecraft’s narrow angle
camera, show density waves that resemble ripples on a pond and scalloped
ring edges that look like textbook drawings. "The images are
mind-boggling, just mind-boggling," said Dr. Carolyn Porco, Cassini
imaging team leader, Space Science Institute, Boulder, Colo. "I’ve been
working on this mission for 14 years and I shouldn’t be surprised, but
it is remarkable how startling it is to see these images for the first
time."

The sounds of Cassini’s trip through the rings were recorded by the
spacecraft’s radio and plasma wave science instrument. Tiny dust
particles, about 700 per second, rained down on Cassini as it crossed
the ring plane, producing sounds like those of a hailstorm on a tin roof.

An analysis of the rings with the spacecraft’s visual and infrared
mapping spectrometer led to the discovery of an unidentified material
that scientists are calling "dirt" until they can characterize it
better. Saturn’s rings are mostly made up of chunks of water ice, like
snowballs, but the new data show that some of the gaps between the
rings, as well as the F ring, contain an unknown impurity. This material
appears remarkably similar to what Cassini previously observed on
Saturn’s moon, Phoebe, suggesting that perhaps the rings formed from an
outer solar system object like Phoebe.

The Invisible Made Visible

Beyond the rings lies Saturn’s magnetosphere, a magnetic bubble of
charged particles like the one that envelopes Earth. Magnetospheres are
invisible to the human eye, but Cassini’s magnetospheric imaging
instrument was able to see Saturn’s for the first time. The image shows
a huge, blob-like cloud of gas sweeping along with the moon Titan in
orbit around Saturn. Scientists believe that this cloud is the result of
highly energetic particles from Saturn’s radiation belt, which bombard
Titan and excite neutral gas permitting it to be imaged.

Cassini also applied its ultraviolet eyes to Saturn on approach and
noticed a dramatic loss of oxygen atoms coming from one side of the
rings. The finding led scientists to hypothesize that something may have
collided within the main rings, producing excess oxygen. These data were
acquired by Cassini’s ultraviolet imaging spectrograph.

First Peek Through Titan’s Veil

On July 2, 2004, Cassini cruised past the planet-sized moon, Titan, at a
distance of 339,000 kilometers (210,600 miles). Scientists are
interested in this frozen, foggy world because it may preserve in deep
freeze many of the chemical compounds that preceded life on Earth. Using
a combination of instruments onboard Cassini, the spacecraft was able to
see through the veil of clouds that enshroud Titan.

Images from Cassini’s cameras show a large crater in addition to linear,
circular and curvilinear features that imply geologic activity. The
apparent lack of a multitude of craters also suggests a dynamic moon
surface. "We’re seeing a totally alien surface," said Dr. Elizabeth
Turtle of the University of Arizona, Tuscon. "We’ve got some exciting
work cut out for us."

Cassini’s visible and infrared mapping spectrometer found that Titan’s
dark surface regions contain relatively pure water ice, while bright
areas have higher concentrations of non-ice materials, such as
hydrocarbons. This is the opposite of what scientists had predicted, and
suggests that the dark regions are not methane lakes as previously
believed. In addition, methane lakes would glisten like oceans on Earth
and this telltale sign was not observed.

While these are tantalizing and surprising results, scientists say that
it is probably premature to draw any major conclusions about methane and
other hydrocarbons on Titan. There will be 44 more Titan encounters in
the prime mission, some at distances as great as 30 times closer to the
surface than this recent flyby.

Titan is the only moon in our solar system with a thick atmosphere. In
December of this year, Cassini will kick off its Huygens probe, which
will descend to Titan for a short-lived study of its enigmatic surface
and atmosphere.

The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. The Jet Propulsion
Laboratory, a division of the California Institute of Technology in
Pasadena, manages the Cassini-Huygens mission for NASA’s Office of Space
Science, Washington, D.C. JPL designed, developed and assembled the
Cassini orbiter.

For the latest images and more information about the Cassini-Huygens
mission, visit http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini.