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.
Saturn's atmosphere 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.
The complex 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.
The Dragon 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.
One mystery 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.
The Dragon 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 (PIA06082 and 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.
These little 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.
Cassini has 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.
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 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.
For more information about the Cassini-Huygens mission, visit
http://saturn.jpl.nasa.gov and the Cassini imaging team home page,
http://ciclops.org.