Cassini Spacecraft Continues Making
New Saturn Discoveries
By Carolina Martinez JPL, and Dolores Beasley
NASA’s Cassini spacecraft continues making new and exciting discoveries.
New findings include wandering and rubble-pile moons; new and
clumpy Saturn rings; splintering storms and a dynamic magnetosphere.
the last seven months it has been a nonstop, science-packed mission.
It has been a whirlwind, and already we have many new results,"
said Dr. Dennis Matson, Cassini project scientist at NASA’s Jet
Propulsion Laboratory, Pasadena, Calif.
density waves caused in Saturn’s rings by the small moons Atlas
and Pan have yielded more reliable calculations of their masses.
The masses imply the moons are very porous, perhaps constructed
like rubble piles. They are similar to the moons that shepherd
Saturn’s F ring, Prometheus and Pandora.
was a tiny moon, about 5 kilometers (3 miles) across, recently
named Polydeuces. Polydeuces is a companion, or "Trojan"
moon of Dione. Trojan moons are found near gravitationally stable
points ahead or behind a larger moon. Saturn is the only planet
known to have moons with companion Trojan moons.
The new findings,
published in this week’s edition of the journal Science, include
refinements in the orbits of several of Saturn’s small satellites.
One intriguing result is the eccentric and slightly inclined orbit
of Pan in Saturn’s A ring. The orbit’s shape is significant, as
it indicates the type of interaction the moon has with the ring
material surrounding it. If Pan’s orbit remains eccentric due
to this interaction, then planets growing in a disc of material
surrounding a star may also have eccentric orbits. This may help
explain the eccentric paths of planets orbiting other stars.
Saturn rings have been discovered in Cassini images. Some lie
in various gaps in the rings and may indicate the presence of
tiny embedded moons acting as shepherds. Several of the rings
are kinked, likely evidence of nearby moons.
also found Saturn’s winds change with altitude, and small storms
emerge out of large ones. For the first time, Cassini images captured
possible evidence of processes that may maintain the winds on
Saturn. The observations offer a glimpse into the process which
transfers energy by convection from Saturn’s interior to help
sustain strong winds.
improve the understanding of Saturn’s complex magnetic environment.
"Saturn’s magnetosphere is truly unique. It’s dynamically
similar to Jupiter’s, but in places it chemically resembles water-based
plasmas surrounding comets," said Dr. David Young. Young
is Cassini principal investigator for the plasma spectrometer
instrument from the Southwest Research Institute, San Antonio.
find was made by the ion and neutral mass spectrometer instrument,
which measured molecular oxygen ions above Saturn’s ring plane.
"This is at first surprising since the rings are made of
water ice," said Dr. Hunter Waite, principal investigator
for the spectrometer from the University of Michigan, Ann Arbor.
"This may have important consequences for the identification
of spectral features to use in the search for life on extrasolar
terrestrial planet systems."
of molecular oxygen on Earth is uniquely tied to biology. But
these new measurements at Saturn suggest there are lifeless processes
associated with cold icy surfaces that may produce an independent
pathway for the formation of molecular oxygen in atmospheres.
by NASA / JPL / Space Science Institute
A large, bright and complex convective storm that appeared in
Saturn’s southern hemisphere in mid-September 2004 was the key
in solving a long-standing mystery about the ringed planet.
and its rings are shown here in a false color composite made from
Cassini images taken in near infrared light through filters that
sense different amounts of methane gas. Portions of the atmosphere
with a large abundance of methane above the clouds are red, indicating
clouds that are deep in the atmosphere. Grey indicates high clouds,
and brown indicates clouds at intermediate altitudes. The rings
are bright blue because there is no methane gas between the ring
particles and the camera.
feature with arms and secondary extensions just above and to the
right of center is called the Dragon Storm. It lies in a region
of the southern hemisphere referred to as "storm alley"
by imaging scientists because of the high level of storm activity
observed there by Cassini in the last year.
Storm was a powerful source of radio emissions during July and
September of 2004. The radio waves from the storm resemble the
short bursts of static generated by lightning on Earth. Cassini
detected the bursts only when the storm was rising over the horizon
on the night side of the planet as seen from the spacecraft; the
bursts stopped when the storm moved into sunlight. This on/off
pattern repeated for many Saturn rotations over a period of several
weeks, and it was the clock-like repeatability that indicated
the storm and the radio bursts are related. Scientists have concluded
that the Dragon Storm is a giant thunderstorm whose precipitation
generates electricity as it does on Earth. The storm may be deriving
its energy from Saturn’s deep atmosphere.
is why the radio bursts start while the Dragon Storm is below
the horizon on the night side and end when the storm is on the
day side, still in full view of the Cassini spacecraft. A possible
explanation is that the lightning source lies to the east of the
visible cloud, perhaps because it is deeper where the currents
are eastward relative to those at cloud top levels. If this were
the case, the lightning source would come up over the night side
horizon and would sink down below the day side horizon before
the visible cloud. This would explain the timing of the visible
storm relative to the radio bursts.
Storm is of great interest for another reason. In examining images
taken of Saturn’s atmosphere over many months, imaging scientists
found that the Dragon Storm arose in the same part of Saturn’s
atmosphere that had earlier produced large bright convective storms.
In other words, the Dragon Storm appears to be a long-lived storm
deep in the atmosphere that periodically flares up to produce
dramatic bright white plumes which subside over time. One earlier
sighting, in July 2004, was also associated with strong radio
bursts. And another, observed in March 2004 and captured in a
movie created from images of the atmosphere (http://photojournal.jpl.nasa.gov/catalog/PIA06082
and http://photojournal.jpl.nasa.gov/catalog/PIA06083) spawned
three little dark oval storms that broke off from the arms of
the main storm. Two of these subsequently merged with each other;
the current to the north carried the third one off to the west,
and Cassini lost track of it. Small dark storms like these generally
get stretched out until they merge with the opposing currents
to the north and south.
storms are the food that sustains the larger atmospheric features,
including the larger ovals and the eastward and westward currents.
If the little storms come from the giant thunderstorms, then together
they form a food chain that harvests the energy of the deep atmosphere
and helps maintain the powerful currents.
many more chances to observe future flare-ups of the Dragon Storm,
and others like it over the course of the mission. It is likely
that scientists will come to solve the mystery of the radio bursts
and observe storm creation and merging in the next 2 or 3 years.
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 mission for NASA’s Science Mission Directorate, Washington,
D.C. The Cassini orbiter and its two onboard cameras were designed,
developed and assembled at JPL. The imaging team is based at the
Space Science Institute, Boulder, Colo.