image: As the Cassini spacecraft gets closer to Jupiter, details heretofore not noticed are becoming obvious. The Great Red Spot, which only a week or two earlier appeared to lack features, is now revealing internal structure in this color composite, high pass filtered frame made from narrow angle images taken on December 1, 2000 from a distance of 28.6 million km, with a resolution equal to 170 km/pixel. The edges of the Red Spot are cloudier with ammonia haze than is the central region, a characteristic seen previously in Voyager and Galileo images. The filamentary structure in the interior of the Red Spot appears to spiral outward toward the edge. The Galileo spacecraft observed the outer edges of the Red Spot to be rotating counterclockwise at speeds up to 150 meters/second, but the inner darker portion was found to rotate weakly in the opposite direction. Whether the same is true now will be answered as we get closer to Jupiter and interior cloud features become sharper.
The Red Spot region has changed in one notable way over the years: In Voyager and Galileo imagery, the area surrounding the Red Spot is dark, indicating relatively cloud-free conditions. Now, some bright white ammonia clouds have filled in the clearings. This appears to be part of a general brightening of Jupiter cloud features that has taken place over the past two decades. The tops of the Red Spot clouds are somewhat higher than those in most other locations on Jupiter, and the red colors are suggestive of jovian "air" being brought upward into view from greater depths. Elsewhere on the planet, though, it is less obvious where the atmosphere is moving up and where it is moving down, and the stratigraphic relationships are unclear. Dark brownish or light pink vortices, such as that just south of the Red Spot, are not obviously lower or higher in altitude than the white ovals. The periodically spaced large greyish regions in the zone north of the belt containing the Red Spot (and seen too in last week's image) are 'hot spots' where Jupiter is relatively clear and we see to greater depths. Their relatively even spacing suggests an origin in a planetary-scale wave oscillation girdling the planet at that latitude.
What might it be like to live in the Jupiter system? Sky-gazers floating in the atmosphere of Jupiter would have a more interesting life than we do on Earth. Earthlings have only one moon to gaze upon, sometimes visible to us, sometimes not. Jovian citizens have their choice of 4 major satellites and an array of tiny ones on which to train their telescopes at night. At the time of last week's picture, Red Spot floaters could see Ganymede. In this picture, Jupiter's closest moon, Io, is visible. The white and reddish colors on Io's surface are due to the presence of different sulfurous materials while the black areas are due to silicate rocks.
Like the other Galilean satellites, Io always keeps the same hemisphere facing Jupiter, called the sub-Jupiter hemisphere. The opposite side, much of which we see here, is the anti-Jupiter hemisphere. If you were a creature that could survive on Io and lived always on the anti-Jupiter hemisphere, you would never see colossal Jupiter in the sky. However, you would experience tides and earthquakes on a 42-hour cycle as Io orbits Jupiter. And the colorful aurorae in the night sky would vary on a 13 hour cycle as Jupiter's rotation carries its tilted magnetosphere passed orbiting Io. Perhaps a brilliant Ionian scientist could deduce from these observations that there must be a massive body on the other side of her world. Imagine how astonished anti-Jovian side explorers would be when they traveled east or west and first saw the massive, ornately attired sphere of Jupiter rise over the horizon. Such explorations would undoubtedly be extremely hazardous: Io has more than 100 active volcanoes spewing lava at extemely high temperatures -- around 1900K -- and giant plumes of gas and dust rising up to 400 km high. The biggest plume on Io, named Pele, is located near the bottom left edge of Io's disk as seen here.
An Ionian living on the sub-Jupiter side would always see the dominating, ever-changing face of Jupiter in the sky. Seen from orbiting Io, Jupiter would make one complete rotation every 13 hours. And once every 42 hours, Jupiter would block the sun for 2 to 3 hours in a solar eclipse. Eclipse observers on both Io and Jupiter, working in the dark, would be treated to enhanced views of glowing, hot lava on Io's surface, the colorful changing auroral displays in its atmosphere and plumes, and dramatic lightning flashes in the giant thunderstorms of Jupiter.
The Cassini cameras will soon be making all these observations -- Io's active volcanic eruptions and auroral displays, and lightning flashes from Jupiter's giant thunderstorms -- as the spacecraft makes its way towards closest approach on December 30.
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. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.
Credit: Cassini Imaging Team & NASA/JPL/University of Arizona