NASA's Jet Propulsion Laboratory and the University of Arizona have released two movies of Jupiter's swirling atmosphere taken by the Cassini probe as it speeds towards a flyby of the giant planet next month.

The Great Red Spot
The first movie, made from narrow angle blue filter images taken during seven separate Jupiter rotations between October 1 and October 5, 2000, shows the counterclockwise atmospheric motions around the Great Red Spot. The smallest features are about 500 km across.

Jupiter's atmosphere swirls around the planet's most prominent feature, the Great Red Spot. Imagery: NASA/JPL/University of Arizona. See the animation: QuickTime (300x300 pixels, 43k file) Animated GIF (250x250 pixels, 55k file)

The spacing of the movie frames in real time is not uniform; some consecutive frames are separated by two Jupiter rotations, and some by one. Each frame consists of multiple Cassini images which have been re-projected and mosaicked together using a simple cylindrical map projection. The movie depicts the motions across a latitudinal band extending from 50 degrees north to 50 degrees south and a longitudinal region 100 degrees in extent -- about one quarter of Jupiter's circumference.

Because the Red Spot is in the southern hemisphere, the direction of atmospheric motion around it indicates that it is a high-pressure center. The eastward and westward motion of the zonal jets, which circle the planet on lines of constant latitude, are also easily seen. As far as can be determined from both Earth-based and spacecraft measurements, the positions and speeds of the jets have not changed for 100 years. Since Jupiter is a fluid planet without a solid boundary, the jet speeds are measured relative to the tilted magnetic field, which rotates, wobbling like a top, every 9 hours 55.5 minutes. The movie shows motions in the magnetic reference frame, so winds to the west correspond to features that are rotating a little slower than the magnetic field, and winds to the east correspond to features that are rotating a little faster.

Small bright clouds appear suddenly to the west of the Great Red Spot. Based on data from the Galileo spacecraft, scientists suspect that these small white features are lightning storms, where falling raindrops create electrical charge. The lightning storms eventually merge with the Red Spot and surrounding jets, and may be the main energy source for these large-scale features. Imaging observations of the dark side of the planet, which are planned for the period of time after closest approach, will search for lightning storms like these.

Atmospheric bands
The second movie is a time-interpolated, evenly spaced sequence of frames showing motions in Jupiter's atmosphere over the course of five days, from October 1 to October 5, 2000. The smallest features are about 500 km across.

Jupiter's atmospheric bands in motion. Imagery: NASA/JPL/University of Arizona. See the animation: QuickTime (Full version, 300x300 pixels, 100k file) Animated GIF (Edited version, 200x200 pixels, 96k file)

Beginning with narrow angle camera blue filter images taken from the same seven unevenly spaced Jupiter rotations shown in the movie above, this sequence was made using the zonal atmospheric wind profile derived from the real Cassini Jupiter images to create evenly spaced timesteps throughout. The final result is a smooth movie sequence consisting of both real and false frames. The region shown is a latitudinal band extending from 50 degrees north to 50 degrees south, and 100 degrees in longitudinal extent -- about one quarter of Jupiter's circumference -- on the side of the planet opposite to that depicted in the movie above. Towards the end of the sequence, the shadow of Europa appears.

The movie shows the remains of the historic merger that took place several years ago, when the three white ovals, which had existed for 60 years, rapidly merged into one. The resulting oval is visible in the lower left portion of the movie. Like the Great Red Spot, it is a high-pressure center in the southern hemisphere, but it is only half as large. The color difference between the white oval and the Red Spot is not well understood, but it is undoubtedly related to the updrafts and downwdrafts that carry chemicals to different heights in the two structures. The movie also shows the zonal jets that circle the planet on constant latitude. As in the movie above, winds to the west correspond to features that are rotating a little slower than the magnetic field, and winds to the east correspond to features that are rotating a little faster.