NASA/JPL/University of Arizona

The Io torus is a doughnut-shaped ring of gas surrounding Jupiter. The gas originates from the many volcanoes on Io, Jupiter's closest large satellite. The volcanoes on Io spew out sulfur and sulfur dioxide. These particles are ejected into space and are stripped of their electrons (or ionized). The particles (ions) then become trapped in Jupiter's magnetic field. Because the field rotates with the planet, the particles make a complete circle around the planet every ten hours, the rotational period of Jupiter.

About the images

This video clip was compiled from 235 exposures taken in November 2000 by the Cassini spacecraft. The exposures were made using the ultraviolet imaging spectrograph while Cassini was about 50 million kilometers (30 million miles) from Jupiter. The moving dot above and below the image indicates the rotation of the planet. As the planet rotates, the torus appears to wobble because Jupiter's magnetic field is tilted relative to its poles.

Ellen Cohen

NASA/JPL/University of Arizona

In this movie, the inner-most satellite of Jupiter, Io, is in the eclipse shadow of Jupiter (Jupiter is blocking the Sun for Io). This allowed NASA's spacecraft Cassini to image the active moon in darkness. These images were taken over a two-hour period and nearly capture the entire eclipse which took place on January 1, 2001. The spacecraft was over 10 million kilometers (6.3 million miles) away, yielding a resolution of 61 kilometers (40 miles) per pixel on these images.

Glows on Io

While Io is enshrouded in darkness, several glows are revealed to us. The bright points of light are hot lava from the active volcanoes on Io. The brightest of these is the volcano Pele which appears to be erupting constantly. To the right and slightly above Pele is a pair of bright spots from the volcano Pillan, the source of a major eruption in 1997. A second source of glow on Io is the faint, diffuse emission of atmospheric aurorae. Similar to the aurora borealis (northern lights) on Earth, the aurorae on Io result from collisions of charged particles (from the electrical currents that flow between Jupiter and Io) with gases in Io's tenuous atmosphere. The aurorae are seen here as a faint glow around Io's equator.

Ellen Cohen

NASA/JPL/University of Arizona

This brief animation shows the motion of the clouds in the Jovian atmosphere. Most notable is the dark, oval-shaped Great Red Spot. The animation was compiled from blue filter images taken using the narrow-angle camera on NASA's Cassini spacecraft during seven rotations of Jupiter between October 1 and October 5, 2000. The images reveal an area on Jupiter centered on the equator that extends about 50° north and south and covers 100° east-west (about a quarter of Jupiter's circumference).

Video: 428 kB, Animated GIF

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The dynamics of the Jovian atmosphere

Jupiter's atmosphere is driven by strong zonal winds, analogous to the jet stream on Earth. However, unlike Earth, the atmosphere is made up of an array of belts and zones which rotate in an east-west direction. The light zones are regions of upwelling air, forming ammonia cirrus clouds. The dark belts are regions where the cooler the circulation moves downward. Because there are no ammonia clouds, we can see deeper into the atmosphere in these regions. The speed of the main equatorial jet is about 300 km/hr and these speeds have not changed for hundreds of years.

The Great Red Spot

Jupiter's Great Red Spot is a storm that has been present for centuries, first observed by Galileo 400 years ago. The storm lies in the southern hemisphere and is about 30,000 kilometers by 14,000 kilometers, much larger than the planet Earth! It is an anticyclonic storm and has a high-pressure center (unlike storms on Earth that have low-pressure centers). Its winds rotate counterclockwise and have a period of about 6 days. Scientists do not know what drives the storm or if the storm will ever dissipate. One theory suggests the Great Red Spot is continually fueled by the smaller storms that merge with the giant storm.

Ellen Cohen