December 16, 2004 —
A massive eruption of atomic oxygen from Saturn’s outer rings, seen by Cassini’s
ultraviolet camera as the spacecraft neared its destination, may be an indication
that the planet’s wispy E ring is eroding so fast that it could disappear within
100 million years if not replenished.
Saturn's peaceful beauty awaited the Cassini spacecraft as it approached in June
2004. NASA/JPL/Space Science Institute image.
Cassini’s Ultraviolet Imaging Spectrograph (UVIS) detected the oxygen atoms spewing
into a huge cloud on the dark side of Saturn’s rings as Cassini prepared to enter
orbit around Saturn in January 2004, said Donald Shemansky, professor of aerospace
and mechanical engineering in the University of Southern California Viterbi School
of Engineering and a co-investigator on the Cassini ultraviolet imaging team. Data indicated that about 275 million pounds (125 million kilograms) of oxygen
was abruptly released in a short period of time.
“This was our first surprise in the ultraviolet,” said Shemansky, who will analyze
ultraviolet data during Cassini’s four-year tour of Saturn and its rings with
Janet Hallett, a postdoctoral aerospace research associate in the USC Viterbi
“We aren’t sure yet whether this was a transient event or part of a routine recycling
process in Saturn’s magnetosphere,” he said. “Right now scientists are speculating
that the oxygen eruption may have been caused by a collision of ice particles
from the planet’s distant E ring with material in one of the main ring systems,
A, B or C. Or it could have been a meteorite collision or an eruption of icy
slush on Enceladus, a moon that orbits in the E ring.”
Shemansky and the 16-member Cassini ultraviolet imaging team reported their findings
in the Dec. 16, 2004 issue of Science Express (see http://www.sciencexpress.orgmagazine).
Despite Saturn’s placid appearance from Earth, the planet is anything but that. The first detailed UV images from the Cassini mission show that Saturn commands
a dynamic world of complex, braided ice rings, cannibalistic moons, 1,100 mile-per-hour
planetary winds and electrifying auroral displays high in the night skies.
Saturn, its moons and highly structured rings live inside a huge cavity in the
solar wind created by the planet’s strong magnetic field. The magnetosphere is
a bubble of particles including electrons, various species of ions, neutral atoms
and molecules, several populations of very energetic charged particles like those
found in Earth’s Van Allen Belts, and charged dust grains. These ionized (electrically
charged) gases are called plasmas. However, unlike Jupiter’s magnetosphere, Shemansky
said Saturn’s magnetic cocoon, which is smaller, is filled primarily with neutral
gas rather than ions.
“Saturn’s magnetosphere is turning out to be very different from Jupiter’s,”
he said. “It’s dominated by neutral gas and water-rich ingredients produced by
its rings, as icy moon debris collides, or by the more energetic collisions of
incoming meteorites. It doesn’t have nearly as many charged particles, and many
of them are absorbed by the rings, so the plasma processes we are observing are
Two months after his initial observations, Shemansky and his ultraviolet team
reported that the large cloud of escaping oxygen atoms had dissipated just as
rapidly as it had appeared. Shemansky discounted theories that the rapid loss
of material could be explained by “satellite sweeping,” a process whereby tiny
shepherding moons gobble up debris or deflect it as they clean out gaps between
“The rate at which we saw material escaping from Saturn’s outer rings implies
that mass equivalent to the entire E ring, even including larger fragments and
parent bodies, would be consumed in a period of about 100 million years if no
replenishment processes are at work,” he said.
Saturn's main ring system is so vast that it would barely fit in the space between
Earth and its Moon. NASA/JPL/Space Science Institute image.
The rings of gas giants are made up of rocky debris from moons that have been
torn apart by tidal waves or by an asteroid or comet collision during heavy bombardment
periods. Rings are considered ephemeral and thought to disappear over time spans
of billions of years.
But Saturn’s colorful rings appear to be younger. Scientists think the rings
are much younger than the planet itself – perhaps only 100 million years old –
which is young in cosmological time. They also suspect that Saturn has had several
ring systems in its history, although they have never had direct evidence on which
to base their assumptions.
“These observations are a first in solar system exploration,” Shemansky said. “We have direct evidence now that the rings are made up of pure ice and that
are shaped by processes that happen fast,” he added. “They aren’t the same processes
that shaped our solar system 4.5 billion years ago.
“Given the fact that the outer rings are present at this time means that the
system is being replenished by interactive plasma processes,” Shemansky continued. “Clearly, the fact that something is eating up micron-sized grains in the outer
ring zones at a high rate tells us that some sort of recycling process must be
going on to rebuild them.”
Cassini’s UV imaging spectrograph has made other important observations. In the
ultraviolet, scientists were able to see dust on the rings. Data showed variations
in the amount of water-ice contained in the surfaces of ring particles, suggesting
that darkened portions had been dusted with powder from pulverized moons or incoming
The Cassini UVIS team also obtained ultraviolet images of Phoebe, Saturn’s most
distant large moon, during the inbound flight to Saturn. Data showed the absorption
lines of water-ice on Phoebe’s dark surface, which gave scientists more clues
about its origins.
A closeup shot of Phoebe, Saturn's outermost moon, shows its heavily cratered
The only moon in the Saturnian system to orbit in a retrograde, or backward,
direction, Phoebe is similar to a common C-type carbonaceous asteroid. Scientists
theorize that it was flung out of the Kuiper Belt, a region well beyond Neptune’s
orbit where thousands of small, icy comets reside, and sucked up by Saturn’s strong
gravitational field, but no one is absolutely sure of its origin.
The Cassini-Huygens spacecraft and science instruments are part of an international
mission by NASA, the European Space Agency and the Italian Space Agency to explore
Saturn and its many moons and rings.