Cassini Imaging Diary: Jupiter

Cassini Images Jupiter

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The Galilean satellite, Ganymede, which orbits Jupiter between Europa and outermost Callisto, is captured here alongside the planet in a true color narrow angle composite from December 3, 2000, 00:41 UTC (spacecraft time.)

Ganymede is the largest satellite in the solar system, larger than the planet Mercury, and even larger than Saturn's largest satellite Titan. Both Ganymede and Titan have greater surface area than the entire Eurasian continent on our planet. The distance from the spacecraft to Ganymede is 26.5 million km. The smallest visible features are about 160 km across.

The bright area near the south (bottom) of Ganymede is Osiris, a large relatively new crater surrounded by bright icy material ejected by the impact which created it. Elsewhere on Ganymede, (for example, the area seen in the upper right) we see dark terrains that the Voyager and Galileo spacecraft have shown to be old and heavily cratered. The brighter terrains are younger and laced by grooves. Various kinds of grooved terrains have been seen on many icy satellites in the solar system. These are believed to the surface expressions of warm pristine water-rich materials that were once transported to the surface and froze.

Ganymede has proven to be a fascinating world, the only satellite known to have a magnetosphere produced by a convecting metal core. The interaction of the Ganymedian and Jovian magnetospheres may produce dazzling variations in the auroral glows in Ganymede's tenuous atmosphere of oxygen. The Cassini cameras will be used to search for these glows during the passage of Ganymede into Jupiter's shadow.

Credit: NASA/JPL/University of Arizona
Released: December 22, 2000
Image/Caption Information

In an ancient, orbital dance that has been ongoing for several billion years, the Galilean satellites Europa and Callisto are caught, under the watchful gaze of Jupiter, nearly perfectly aligned with each other, the Great Red Spot, and the planet's center in this true color frame made of narrow angle images taken on December 7, 2000 at 16:11 UTC (spacecraft event time). The distances here are deceiving. Europa (seen against Jupiter) is 600,000 km above the planet's cloud tops; Callisto (at lower left) is nearly three times that distance at 1.8 million km.

Europa is a bit smaller than our Moon and is one of the brightest objects in the solar system. Callisto is 50% bigger -- roughly the size of Saturn's largest satellite Titan -- and three times darker than Europa: its brightness had to be enhanced relative to that of Europa and Jupiter in order to see it in this image.

These objects, which have had very different geologic histories, share some surprising similarities. While they both have surfaces rich in water ice, Callisto has apparently not undergone major internal compositional stratification, whereas Europa's interior has differentiated into a rocky core and an outer layer of nearly pure water ice. Callisto's ancient surface is completely covered by large impact craters: the brightest features seen on Callisto in this image were discovered by the Voyager spacecraft in 1979 to be bright rayed craters, like those on our Moon. In contrast, Europa's young surface is covered by a wild tapestry of ridges and chaotic terrain and only a handful of large craters.

Surprisingly, data from the magnetometer carried by the Galileo spacecraft, which has been in orbit around Jupiter since 1995, indicate the presence of conducting fluid, most likely salty water, inside both worlds. (A similar Galileo discovery has been made of Ganymede, to be featured tomorrow.)

Does the surface of Saturn's Titan resemble that of Callisto or Europa, or will it be entirely different? We'll find out in 2004 when Cassini finally reaches its destination.

Credit: NASA/JPL/University of Arizona
Released: December 21, 2000
Image/Caption Information

Jupiter's 4 largest satellites have fascinated Earthlings ever since Galileo Galilei discovered them in 1610 in one of his first astronomical uses of the telescope. The images that will be released over the next several days capture each of the 4 Galilean satellites in their orbits around the giant planet.

This true color composite frame, made from narrow angle images taken on December 12, 2000 around 15 hours UTC (spacecraft event time) captures the innermost Galilean satellite, Io, and its shadow in transit again the disk of Jupiter. The distance of the spacecraft from Jupiter was 19.5 million km; the spacecraft latitude was 3 degrees above Jupiter's equator plane. The image scale is 117 km/pixel.

The entire body of Moon-sized Io is periodically flexed as it speeds around Jupiter and feels, as a result of its non-circular orbit, the periodically changing gravitational pull of the planet. The heat arising in Io's interior from this continual flexure makes it the most volcanically active body in the solar system, with more than 100 active volcanoes and plumes, like giant mushroom clouds, extending 100's of kilometers high. The white and reddish colors on its surface are due to the presence of different sulfurous materials; the black areas are silicate rocks.

Credit: NASA/JPL/University of Arizona
Released: December 20, 2000
Image/Caption Information

Jupiter's main ring, a narrow tenuous structure about 6000 km in width and encircling the planet at a planetocentric distance of approximately 126,000 km, was first imaged by the Cassini narrow angle camera on December 11, 2000 in a movie sequence that lasted 39.5 hours. (The orbital period of material in the ring is about 7 hours; Jupiter's magnetic field period is 10 hours.)

The distance of the spacecraft during this time ranged from 20.3 million km and 3.3 degrees above the ring plane to 19.0 million km and 2.98 degrees above the ring plane. The ring is about 100,000 times fainter than Jupiter, and during this movie sequence extended only about one sixth of a degree from Jupiter's edge. Accordingly, the pixels on the CCD detector were summed to enhance visibility of the faint ring against a severe scattered light background. The resulting resolution is ~230 km/pixel.

The 10 frames shown here are each a small section of a sum of 6 to 7 separate narrow angle images taken through the clear filter and spanning the entire 39.5 hour period. (The ring was also detected in the red region of the electromagnetic spectrum at ~ 757 nanometers, where the human eye is incapable of seeing.) The scattered light background has been removed, the images have been contrast stretched to enhance the ring, and the contours in the image, as well as the small variations in brightness of the ring from one frame to the next, are a result of the image processing and background removal. The ring clearly grows in angular extent, from upper left to lower right, as the spacecraft approaches the planet.

The Sun-ring-spacecraft, or `phase', angle reaches below 1 degree during this movie sequence. From this viewing perspective, the visible material tends to be grain size and larger. During the next month, the Cassini view of the Jovian ring system will grow in phase angle until the spacecraft is looking back from a phase angle of 120 degrees, at which point fine dust material should be more readily visible. Images of the ring system will be taken in a variety of spectral and polarimetric filters during this swingby. From this information, together with data collected by Galileo, will emerge the most complete picture of the Jovian ring system yet obtained.

Credit: NASA/JPL/University of Arizona
Released: December 19, 2000
Image/Caption Information

The familiar banded appearance of Jupiter at low and mid-latitudes gradually gives way to a more mottled appearance at high latitudes in this striking true color, contrast-enhanced, edge-sharpened frame composited from narrow angle images taken on December 13, 2000 at 4:26 UTC (spacecraft event time). The solar illumination on Jupiter is almost full phase: the Sun-Jupiter-spacecraft angle is less than 1 degree. The images were captured from a distance of 19.0 million kilometers and a resolution of 114 km/pixel. The details seen in Jupiter images have now surpassed those in even the highest resolution Hubble Space Telescope Planetary Camera images.

The intricate structures seen in the polar region are clouds of different chemical composition, different height, and different thicknesses. Clouds are organized by winds, and the mottled appearance in the polar regions suggests more vortex-type motion, and winds of less vigor, at higher latitudes. The cause of this difference is not understood. One possible contributor: the horizontal component of the Coriolis force -- which arises from the planet's rotation and is responsible for curving the trajectories of ocean currents and winds on Earth -- has its greatest effect at high latitudes and vanishes at the equator. This tends to create small intense vortices at high latitudes on Jupiter. Another possibility may lie in that fact that Jupiter overall emits nearly as much of its own heat as it absorbs from the Sun, and this internal heat flux is very likely greater at the poles. This condition could lead to enhanced convection at the poles and more vortex-type structures. Further analysis of Cassini images, including analysis of temporal sequences, should help us understand the cause of the equator to pole difference in cloud organization and evolution.

Credit: NASA/JPL/University of Arizona
Released: December 18, 2000
Image/Caption Information

The evolution of Jupiter's high altitude clouds can be seen in this brief movie made from narrow angle frames taken through a filter sensitive to methane gas absorption. The movie covers the same period of time (between October 1 and October 5, 2000), the same range of longitude (100 degrees) and latitude (50 degrees north to 50 degrees south), and is made in the same manner as the blue filter Red Spot movie released on November 20. (See below.) However, this is the first time a movie sequence of Jupiter has ever been made which illustrates the motions of the high altitude clouds (sensed in the methane filter) on a global scale.

(The still frame is one of the 7 map-projected timesteps in the movie.)

Dark locales are places of strong methane absorption, relatively free of high clouds. The bright areas are places with high, thick clouds which shield the methane below. Jupiter's equator and Red Spot are covered with high altitude, hazy clouds. Unlike their appearance in the blue filter movie, which sees to deeper levels, these places in the methane movie appear diffuse and lack sharp details. Interestingly, ovals which spin opposite in direction to the Red Spot are free of high hazes, and appear dark. Two such ovals, located in the lower left part of the methane movie, are barely visible in the blue filter movie.

A number of interesting atmospheric features can be seen in this methane movie. Some are stationary like the Great Red Spot, some move toward the east or west, and most maintain a constant brightness while a few show brightness fluctuations. (Motion is referred to the rotation of Jupiter's magnetic field. `Stationary' motion implies rotation equal to that of the field.)

Among the stationary or nearly stationary features are the Red Spot and a number of bright ovals at mid-latitudes in both hemispheres. These are the familiar anticyclonic (counter-clockwise rotating) ovals which are bright in the methane band because of their high clouds associated with rising gas. The dynamical behavior of these anticyclonic spots is different than that of terrestrial cyclones which swirl in the opposite sense. The mechanism which makes the Red Spot and similar spots stable apparently has no similarity to the mechanism which feeds terrestrial cyclones.

A stationary undulation can be seen in the darkest band a little north of the bright equatorial region. This may be tied to a wave-like temperature variation across the planet that is seen when one observes the planet in light sensitive to the heat emanating from the stratosphere. If confirmed, this would be the first time such large-scale stratospheric temperature waves have been seen to be related to variations in the haze thickness.

Some of the small-scale features in both movies are fascinating because of the brightness fluctuations they display. Such fluctuations observed in the methane band are probably caused by strong vertical motions which form clouds rapidly as in thunderstorms. Very obvious in the blue filter Red Spot movie below is a turbulent region which extends to the west (left) of the Great Red Spot most of the way toward the left edge of the frame. About one-third of the way in from the left edge is a cloud whose brightness changes quickly in both movies. At about the same longitude in the northern hemisphere, a number of smaller clouds appear to circulate counterclockwise around a dark spot, and these clouds exhibit brightness fluctuations, too, in both movies. These regions are candidates for lightning storms.

Credit: NASA/JPL/University of Arizona
Released: December 11, 2000
Image/Caption Information

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.

Credit: NASA/JPL/University of Arizona
Released: December 11, 2000
Image/Caption Information

Jupiter casts a baleful eye on wayward Ganymede in this frame, color-composited from narrow angle images taken on November 18 and high-pass filtered and contrast-enhanced to bring out details not readily seen otherwise. The smallest features in this image are 240 km across.

Jupiter's `eye', the Great Red Spot, was captured just before disappearing over the eastern limb of the planet. The furrowed eyebrow above and to the left of the Spot is a turbulent wake region caused by westward flow deflected to the north and around the Red Spot. (An animation of ISS images from early October, beautifully illustrating this flow, was released on November 20 and can be viewed below.) Within the band south of the Red Spot are seen a trio of white ovals, small high pressure counter-clockwise rotating regions that are dynamically similar to the Red Spot. The dark filamentary features interspersed between the white ovals are probably cyclonic circulations, similar to those seen by Galileo, and, unlike the ovals, are rotating clockwise.

Jupiter's Equatorial Zone stretching across the planet to the north of the Spot appears bright white, with gigantic plume clouds spreading out from the equator both to the northeast and to the southest. in a chevron pattern. This zone looks distinctly different than it did during the Voyager flyby 21 years ago when its color was predominantly brown, and only SW/NE-trending white plumes north of the equator were conspicous against the darker material beneath. The bluish gray regions near the equator, noted in earlier releases, are regions where the clouds have cleared and, except for partial obscuration by thin upper level hazes, we can see to great depth. The darker, brownish North Equatorial belt north of the Equatorial Zone is also quite turbulent. (See accompanying release below.)

Ganymede is Jupiter's largest moon, about 50% larger than our own Moon and larger than the planet Mercury. Like the Moon and Mercury, Ganymede has no atmosphere; the visible details seen in this image are different geological terrains on the satellite's surface. Dark areas tend to be older and heavily cratered; the brighter locales are younger and more sparsely cratered. The latter are the famed grooved terrains first seen by Voyager and imaged at high resolution by Galileo. Cassini images of Ganymede and the other Galilean satellites taken near closest approach on December 30 will have resolutions of ~60 km/pixel, four times better than that seen here.

Credit: NASA/JPL/University of Arizona
Released: December 4, 2000
Image/Caption Information

This 4-panel frame shows a section of Jupiter's North Equatorial Belt viewed by Cassini at 4 different wavelengths on November 27, when the resolution had improved to 192 km/pixel, surpassing the Hubble Space Telescope Wide Field Camera in resolving power. The images have been contrast-enhanced for the purpose of illustration. The upper panel is an image taken in the near-infrared at a wavelength inaccessible to the human eye. The gases in the atmosphere are relatively non-absorbing, allowing sunlight to penetrate deeply into the atmosphere and be reflected back out, thus giving us a direct view of the deeper regions of the troposphere. (On Earth, the troposphere is the portion of the atmosphere closest to the surface, in which most of the atmospheric water vapor resides.)

The second panel is taken in the blue at shorter wavelengths detected by the human eye. At these wavelengths, gases in the atmosphere scatter a modest amount of sunlight, so the clouds we see tend to be at somewhat higher altitudes than in the uppermost panel.

The third panel shows near-infrared reflected sunlight at a wavelength where the gas methane, an important constituent of Jupiter's atmosphere, absorbs strongly. In this image, dark places are locations of strong absorption (i.e., regions without high-level clouds and consequently large amounts of methane accessible to sunlight), and bright regions are locations with high (upper troposphere) clouds shielding the methane below.

The bottom-most panel is an image taken in the ultraviolet. At these very short wavelengths, the clear atmosphere scatters sunlight, and stratospheric hazes absorb sunlight, both very efficiently, making it difficult to see into the troposphere at all. So bright regions are generally free of high stratospheric hazes.

The fascinating aspect of these 4 images is the small bright spot that can be seen in the center of each one. Bright spots similar to this were seen in turbulent regions by the Galileo cameras, and they appear to be very energetic convective storms that move heat from the interior of Jupiter to higher altitudes. These storms are expected to penetrate to great heights, and so it is not surprising to see the storm in the first three images, which probe atmospheric altitudes from the lower to the upper troposphere. What is surprising is the appearance of the spot in the ultraviolet image. This may in fact be a `monster' thunderstorm, penetrating all the way into the stratosphere, as do some summer thunderstorms in the midwestern United States. Higher resolution, time-lapse images to be captured in the coming weeks will shed more light on these spectacular features.

Credit: NASA/JPL/University of Arizona
Released: December 4, 2000
Image/Caption Information

This brief movie, made from narrow angle blue filter images taken during 7 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. (Images from this time period were released by CICLOPS on October 5 and October 9. See below.)

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.

(The still frame is one of the 7 map-projected timesteps in the movie.)

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.

Credit: NASA/JPL/University of Arizona
Released: November 20, 2000
Image/Caption Information

This movie is a time-interpolated, evenly spaced sequence of frames showing motions in Jupiter's atmosphere over the course of 5 days, from October 1 to October 5, 2000. The smallest features are about 500 km across. (Images from this time period were released by CICLOPS on October 5 and October 9. See below.)

Beginning with narrow angle camera blue filter images taken from the same 7 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 still frame is one of the timesteps in the interpolated movie.)

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.

Credit: NASA/JPL/University of Arizona
Released: November 20, 2000
Image/Caption Information

This sequence of nine true color narrow angle images shows the varying appearance of Jupiter as it underwent more than a complete 360 degree rotation. The images composing this sequence were taken between October 22 23:17 and October 23 12:38 UTC/SCET (spacecraft event time) from a phase angle of 20 degrees and an angular distance of 3.3 degrees above the Jovian equator plane. The smallest features seen in this sequence are no bigger than about 380 km.

Rotating more than twice as fast as Earth, Jupiter completes one rotation in about 10 hours. From image to image (proceeding left to right across each row and then down to the next row), cloud features on Jupiter move from left to right before disappearing over the edge onto the nightside of the planet. The most obvious Jovian feature is the Great Red Spot, which can be seen moving onto the dayside in the third frame (below and to the left of the center of the planet). In the fourth frame, taken about 1 hour and 40 minutes later, the Red Spot has been carried by the planet's rotation to the east and does not appear again until the final frame, which was taken one complete rotation after the third frame.

Unlike weather systems on Earth, which change markedly from day to day, large cloud systems in Jupiter's colder, thicker atmosphere are long-lived, so the two frames taken one rotation apart have a very similar appearance. However, when this sequence of images is eventually animated, strong winds blowing eastward at some latitudes and westward at other latitudes will be readily apparent. The results of such differential motions can be seen even in the still frames shown here. For example, the clouds of the Great Red Spot rotate counterclockwise. The strong westward winds northeast of the Red Spot are deflected around the Spot and form a wake of turbulent clouds downstream (visible in the fourth image), just as a rock in a rapidly flowing river deflects the fluid around it.

The equatorial zone on Jupiter is currently bright white, indicating the presence of clouds much like cirrus clouds on Earth but made of ammonia instead of water ice. This is very different from Jupiter's appearance twenty years ago, when the equatorial zone was more of a brownish cast similar to the region just to its north. At the northern edge of the equatorial zone, local regions colored a dark grayish-blue are places where the ammonia clouds have cleared and we can see to deeper levels on Jupiter. Interrupting these relatively clear regions is a series of bright arrow-shaped equatorial plumes; the most obvious one is visible just above and to the right of center in the third and ninth frames. These plumes resemble the `anvil' clouds that accompany common summer thunderstorms on Earth, with the exception that the Jovian plumes are much bigger, and their somewhat regular spacing around the planet suggests an association with a planetary-scale wave motion. The southwest-northeast tilt of these plumes suggests that the winds in this region act to help maintain the eastward winds at this latitude.

In the dark belt north of the equatorial zone, a turbulent region with a white filamentary cloud is visible in the sixth frame, indicating rapidly changing wind direction. Several white ovals are visible at higher southern latitudes (toward the bottom of the fourth, fifth, and sixth frames, for example). These ovals, like the Great Red Spot, rotate counterclockwise and are similar in some respects to high pressure systems on Earth.

Credit: NASA/JPL/University of Arizona
Released: November 6, 2000
Image/Caption Information

These two images were taken through the narrow angle camera in the near-infrared region of the electromagnetic spectrum centered at a wavelength of 727 nanometers. The upper image was taken from a distance of 69.9 million km on October 17, 2000; the lower from a distance of 65.1 million km on October 22, 2000. In both cases, the spacecraft was about 3.3 degrees above Jupiter's equator plane; the Sun-Jupiter-spacecraft angle was about 20 degrees. The 727 nanometer filter accepts only a narrow spectral range centered on a relatively strong absorption feature due to methane gas. In this spectral region, the amount of light reflected by Jupiter's clouds is only half that reflected in a nearby spectral region outside the methane band.

The features that are brightest in these images are the highest and thickest clouds, like the Great Red Spot and the band of clouds girding the equator, as these scatter sunlight back to space before it has a chance to be absorbed by the methane gas in the atmosphere. This stratigraphic effect can be seen even more prominently in the image released on October 23, 2000, taken in the stronger methane band at 889 nanometers, in which the only bright features are the highest hazes over the equator, the poles and the Great Red Spot. By comparing images taken in the 727 nanometer filter, with those taken in the stronger methane band at 889 nanometers and a weaker one at 619 nanometers, we will be able to probe the heights and thickness of clouds in Jupiter's atmosphere.

Both images capture the Galilean satellite Europa, a Moon-sized icy satellite of Jupiter, at different phases in its orbit around Jupiter. (The upper image also captures Ganymede, larger than the planet Mercury and already showing distinct brightness variations across its surface.) In the upper image, Europa is caught entering Jupiter's shadow, and hence appears as a bright crescent; in the lower image, it is seen about 1.5 orbits later, in transit across the face of the planet. Because there is neither methane nor any strong absorber in this spectral region on the surface of Europa, it appears strikingly white and bright compared to Jupiter. Imaging observations of Europa (and Io and Ganymede) entering and passing through Jupiter's shadow are planned for the two week period surrounding closest approach on December 30, 2000. The purpose of these eclipse observations is to detect and measure the temporal variability of the emissions that arise from the interaction of a tenuous satellite atmosphere with the charged particles trapped in Jupiter's magnetic field.

Credit: NASA/JPL/University of Arizona
Released: November 6, 2000
Image/Caption Information

This true color image of Jupiter is composed of three images taken in the blue, green and red regions of the spectrum. All images were taken from a distance of 77.6 million km on October 8, 2000, around 16:57 UTC/SCET (spacecraft event time). The resolution is 466 km. Different chemical compositions of the cloud particles lead to different colors; the cloud patterns reflect different physical conditions -- updrafts and downdrafts -- in which the clouds form. The bluish areas are believed to be regions devoid of clouds and covered by high haze. The Great Red Spot (below and to the right of center) is a giant atmospheric storm about 2 Earths across and over 300 years old, with peripheral winds of 300 miles per hour. This image shows that it is trailed to the west by a turbulent region, caused by an atmospheric flow around the northern perimeter of the Spot. The small very white features in this region are lightning storms, which were seen by the Galileo spacecraft when it photographed the night side of Jupiter. Cassini will track these lightning storms and measure their lifetimes and motions when it passes Jupiter in late December and looks back at the dark side of the planet.

Credit: NASA/JPL/University of Arizona
Released: October 23, 2000
Image/Caption Information

These three images of Jupiter, taken through the narrow angle camera from a distance of 77.6 million km on October 8, 2000 and reduced by a factor of 2 in size from a full-resolution image, reveal more than is apparent to the naked eye through a telescope. The image on the left was taken at 16:58 UTC/SCET through the blue filter (centered at 445 nanometers in wavelength), within the part of the electromagnetic spectrum detectable by the human eye. The appearance of Jupiter in this image is consequently, very familiar: the Great Red Spot (a little below and to the right of center) and the planet's well-known banded cloud lanes are obvious. The brighter bands of clouds are called zones and are probably composed of ammonia ice particles. The darker bands are called belts and are made dark by particles intermixed with the ammonia ice. The composition of the latter is unknown. Jupiter's appearance changes dramatically in the images to the right, which were taken at 16:55 UTC/SCET in the ultraviolet at 255 nanometers (middle) and in the near infrared at 889 nanometers (extreme right). These images are near negatives of each other and illustrate the way in which observations in different wavelength regions can reveal different physical regimes on the planet.

All gases scatter sunlight efficiently at short wavelengths; this is why the sky appears blue on Earth. The effect is even more pronounced in the ultraviolet. The gases in Jupiter's atmosphere, above the clouds, are no different: they scatter strongly in the ultraviolet, making the deep banded cloud layers invisible in the middle image. Only the very high altitude haze appears dark against the bright background. The contrast is reversed in the near infrared, where methane gas, abundant on Jupiter but not on Earth, is strongly absorbing and therefore appears dark. Again the deep clouds are invisible, but now the high altitude haze appears relatively bright against the dark background. High altitude haze is seen over the poles and the equator. The Great Red Spot, prominent in all images, is obviously a feature whose influence extends high in the atmosphere.

As the Cassini cameras continue to return images of Jupiter throughout the encounter, it will be possible to construct a three-dimensional picture of how clouds form and evolve by watching the changing appearance of Jupiter in different spectral regions.

Credit: NASA/JPL/University of Arizona
Released: October 23, 2000
Image/Caption Information

This color image of Jupiter is a composite made of 3 images taken by the narrow angle camera on October 4 from a distance of 81.3 million kilometers from the planet. It is composed of images taken in the blue, green, and red regions of the spectrum and is therefore close to the true color of Jupiter that one would see through an Earth-based telescope. The image is strikingly similar to those taken by the Voyager 1 and 2 spacecraft more than 21 years ago, illustrating the remarkable stability of Jupiter's weather patterns. The parallel dark and bright bands and many other large-scale features are quasi-permanent structures that survive despite the intense small-scale activity ongoing in the atmosphere. The longevity of the large-scale features is an intrinsic property of the atmospheric flows on a gaseous planet, like Jupiter, having no solid surface; but smaller features, like those in the dark bands to the north and south of the equator, are observed to form and disappear in a few days. Similar behavior was observed during the Voyager era.

Everything visible on the planet is a cloud. Unlike Earth, where only water condenses to form clouds, Jupiter has several cloud-forming substances in its atmosphere. The updrafts and downdrafts bring different mixtures of these substances up from below, leading to clouds of different colors. The bluish features just north of the equator are regions of reduced cloud cover, similar to the place where the Galileo atmospheric probe entered in 1995. They are called "hot spots" because the reduced cloud cover allows heat to escape from warmer, deeper levels in the atmosphere.

The Galilean satellite Europa is seen at the right, casting a shadow on the planet. It is this satellite which scientists believe holds promise of a liquid ocean beneath its surface.

Credit: NASA/JPL/University of Arizona
Released: October 9, 2000 (Happy 60th Birthday, John Lennon)
Image/Caption Information

This image of Jupiter was taken by the Cassini Imaging Science narrow angle camera through the blue filter (centered at 445 nanometers) on October 1, 2000, 15:26 UTC at a distance of 84.1 million km from Jupiter. The smallest features that can be seen are 500 kilometers across. The contrast between bright and dark features in this region of the spectrum is determined by the different light absorbing properties of the particles composing Jupiter's clouds. Ammonia ice particles are white, reflecting all light that falls on them. But some particles are red, and absorb mostly blue light. The composition of these red particles and the processes which determine their distribution are two of the long-standing mysteries of jovian meteorology and chemistry. Note that the Great Red Spot contains a dark core of absorbing particles.

Credit: NASA/JPL/University of Arizona
Released: October 5, 2000
Image/Caption Information

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