Cassini Imaging Diary: Jupiter


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Cassini Images Jupiter

Compared with the Voyager 1 and 2 flybys of the planet in the early 1980s, the Cassini Jupiter flyby is leisurely and slow and nearly equatorial.

Data collection begins with the spacecraft 3.8 degrees above Jupiter's equator plane and approaching the planet from a phase (Sun-Jupiter-spacecraft) angle of 20 deg and a distance of 84.7 million kilometers. From this viewing geometry, Jupiter looks only slightly different than it does from Earth. By the middle of November, the phase angle drops to 18 deg, and the distance decreases to the point where 4 narrow angle camera images will be required to cover the planet. All throughout this period we will be making repeated observations of the atmosphere, and searching for previously undiscovered satellites in the region around Jupiter containing the Galilean satellites.

By the middle of December, the phase angle drops to zero, repeated monitoring of the atmosphere ceases, and we begin our observations of the rings and satellites. On December 18, we make our closest approach to Himalia, a small outer satellite of Jupiter. As Cassini sweeps through a large range of phase angle during the rest of the encounter, we monitor the light scattering behavior of the rings and Galilean satellites in a suite of spectral and polarimetric filters. (For a brief time surrounding closest approach, Jupiter is large enough to require 9 images to cover the planet.) And we will be on the lookout for time-variability -- in the rings and in the expected diffuse glows from the tenuous atmospheres of Io, Europa and Ganymede as they pass into Jupiter's shadow. (The Galileo spacecraft first observed such glows, as well as high temperature hot spots, from volcanically active Io.)

Ring, satellite and occasional atmospheric observations continue through closest approach and out to January 15, at which point the spacecraft is looking back on a crescent Jupiter from a distance of 18 million km and 3 degrees below the equatorplane. At this time, we return to repeated imaging of the planet as we depart. The last Jupiter images are taken on March 22, 2001.

The closest approach distance to the planet is not very close: 136 RJ, or 9.72 million kilometers. Thus, Cassini images will not have the exquisitely high resolution of either Voyager or Galileo images. But the slow pace of the flyby, the large data collection and downlinking capability of the spacecraft, and the wide spectral range and fine photometric precision of the Imaging Science System (ISS), make it possible to acquire high quality time-lapse CCD imagery of Jupiter's ever changing atmosphere extending over several months in a large suite of atmosphere-probing wavelengths, and to search for time-variability in other Jovian targets ... something no previous Jupiter-bound spacecraft has ever done before.

(For previous releases...go to Page 2, Page 3, Page 4)

Unexpected dynamics in Jupiter's upper atmosphere, or stratosphere, including the birth and motion of a dark vortex wider than Earth, appear in a movie clip shown here spanning 11 weeks of ultraviolet imaging by the Cassini narrow-angle camera.

The development of the vortex resembles development of ozone holes in Earth's stratosphere in that both processes appear to occur only within confined masses of high-altitude polar air. That similarity may help scientists understand both processes better.

The movie is the first from any spacecraft to examine the planet's churning atmosphere in ultraviolet wavelengths. Hydrocarbons in Jupiter's stratosphere are transparent at the longer wavelengths of visible light and infrared light, but appear as haze in ultraviolet light.
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Cassini's narrow-angle camera took images from a near-equatorial perspective as the spacecraft approached Jupiter from Oct. 1, 2000, to Dec. 15, 2000. The images acquired over each 10 hour Jupiter rotation during this period were mosaicked together to form a cylindrical map (one of which is reproduced below) showing all 360 degrees of Jupiter's longitude. The top edge is at 60 degrees north latitude; the bottom at 60 degrees south latitude.

Haze in Jupiter's upper atmosphere, or stratosphere, scatters and reflects ultraviolet wavelengths, but is transparent in the visible-light portion of the spectrum. Hence, wave patterns at high latitudes, plus the famous Great Red Spot, dominate this ultraviolet map of Jupiter which is missing the low latitude horizontal stripes of dark and light bands of clouds that dominate the familiar visible-light views of Jupiter.

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The maps were re-projected into a movie and displayed as if one were looking down at Jupiter's north pole and rotating with its magnetic field. Prograde (westerly) jets move in a counter-clockwise direction in this view. Contrast was enhanced to reveal faint features. The view extends south to the equator at the corners of the frame. The black area at the pole is where no presentable data were acquired due to Cassini's viewing angle. For reference, a circle of 60 degrees latitude is superimposed in white, and an oval where Jupiter has a persistent aurora is superimposed in blue. The aurora itself, comparable to Earth's Northern Lights, is not visible here.

Energetic auroras heat the stratosphere and stimulate the formation of complex hydrocarbons from the breakup of methane molecules. A dark patch appears and within two weeks becomes a well-defined oval about the same size and shape as Jupiter's southern hemisphere Great Red Spot. While this dark vortex is nestled inside the auroral oval, its outer edge begins to circulate in a clockwise direction as it simultaneously develops a small, brighter, inner core. It eventually moves out of the auroral region and deforms by flattening in latitude and growing in longitude. Near the end of the movie, a second, smaller, dark oval appears nearer to the pole and deforms in the wind shear.

A series of wave features rings the planet south of (outside of) the latitude-60 circle. These make visible some of the dynamics of how haze generated in the confined polar stratosphere mixes eventually into the winds farther south.

Comparison of this ultraviolet movie with a near-infrared movie that was produced the same way and released previously on July 16, 2001 reveals many differences. Instead of the waves and large vortex seen in the ultraviolet, the infrared imaging shows a multitude of small storms and parallel wind bands at a lower elevation in the atmosphere.

Cassini made its closest pass to Jupiter, about 10 million kilometers (6 million miles), on Dec. 30, 2000. It is now proceeding toward its ultimate destination, Saturn.

Released: March 13, 2002
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Bands of eastward and westward winds on Jupiter appear as concentric rotating circles in a movie composed of Cassini images which have been reprojected to appear as if the viewer were floating over Jupiter's north pole. The sequence covers 70 days, from October 1 to December 9, 2000. Cassini's narrow-angle camera cap tured the images of Jupiter's atmosphere in near-infrared light.

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What is surprising in this view is the coherent nature of the high latitude flows, despite the very chaotic, mottled and non-banded appearance of the planet's polar regions. This is the first extended movie sequence to show the coherence of the circumpolar winds and the features blown around the planet by them.

Jupiter's alternating eastward and westward jet streams flow in concentric rings around the pole, with equatorial motions visible in the corners. The large dark features flowing counterclockwise near the equator are "hot spots" where cloud cover is relatively thin.

Cassini collected images of Jupiter for months before and after its closest approach to the planet on December 30, 2000. Six images of the planet in each of several spectral filters were taken at evenly spaced intervals over the course of Jupiter's 10-hour rotation period. The entire spectral sequence was then repeated generally every second Jupiter rotation, yielding views of every sector of the planet at least once every 20 hours. The images used for the movies shown here were only those taken 20 hours apart and through a filter centered at 751 nanometers. The six images covering each rotation were mosaicked together to form a cylindrical map extending from 75 degrees North to 75 degrees South in latitude (vertical direction) and covering 360 degrees in circumference.

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The cylindrical movie, consisting of eighty-four such maps, spanning 70 Earth days in time or 168 Jupiter rotations, is displayed here also.

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Transforming the cylindrical maps into polar stereographic projections produces a movie of what Jupiter would look like if viewed from the pole. A zoom of the upper left hand quadrant of the polar movie clearly shows thousands of spots, each an active storm similar in size to the largest of storms on Earth. Terrestrial storms usually last a week before they dissolve and are replaced by other storms. But many of the Jovian storms seen here, while occasionally changing latitude or merging with each other, persist for the entire 70 days. Until now, the lifetime of these high-latitude features was unknown. Their longevity is a mystery of Jovian weather.

The movie seems to support the conclusion that one explanation for the circulation on Jupiter is incomplete at best, and possibly wrong. The model in question explains Jupiter's alternating bands of east-west winds as the exposed edges of deeper closely-packed rotating cylinders that extend north-south through the planet. In this laboratory-tested model, many such cylinders sit side-by-side, girdling the planet like rings of narrow solid-rockets strapped to the outside of a larger rocket. At the planet's surface, one would see only east and west winds, alternating with latitude and symmetric about the equator. However, the east-west winds that the movie shows in the polar regions don't fit that model. Jupiter's wind pattern may involve a mix of rotation-on-cylinders near the equator and some other circulation mechanism near the poles.

 Cosmos Studios
Released: July 16, 2001
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On January 15, 2001, Cassini resumed repeated imaging of Jupiter as it began its departure from the Jovian system. At this time, and throughout the departure phase, only a planetary crescent was visible. The image at left is a color mosaic taken around 18:30 UTC (spacecraft time) on January 15, 2001 when the spacecraft was at 120 degrees solar phase, 2.75 degrees below the equator plane and 18.3 million km from the planet. The smallest features are roughly 110 kilometers across. A crescent Io appears over the limb of Jupiter.

Imaging data were collected in the months following this observation. The last Jupiter images were taken on March 22, 2001. At that point, Cassini began the final leg of its journey to Saturn. Anticipated arrival: July 1, 2004.

Credit: NASA/JPL/University of Arizona
Released: May 31, 2001
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During the eclipse of the moon, Io, on January 1, 2001, Cassini recorded images in several colors ranging from the near-ultraviolet to the near-infrared. Two of these colors have been added to the clear-filter "movie" of this eclipse (released on February 5, 2001) to make visual the evidence used by imaging scientists in determining the source of Io's auroral glows. The color pictures were taken at lower resolution (120 versus 60 kilometers per pixel) and less frequently than the clear filter images, though they still span the entire two hour duration of the movie. The white dots near the equator are volcanoes, the brightest being Pele, that are often much brighter than the faint atmospheric glows.

Emissions in the 595 to 645 nanometer wavelength range likely arise from a tenuous atmosphere of atomic oxygen; they would appear red to the eye and are consequently colored red in the movie. Emissions in the near-ultraviolet, between 300 and 380 nanometers, correspond in wavelength to the bright blue visible glows one would expect from molecular sulfur dioxide; they have been colored blue in the movie. The blue glows are restricted to areas deep down in the atmosphere near the surface of Io, whereas the red glows are much more extensive (reaching heights of up to 900 km). This would be expected if the blue glows are produced by sulfur dioxide, which is heavier than atomic oxygen and more closely bound to the surface by the moon's gravity. The prominent blue and red regions near the equator of Io dance across the moon with the changing orientation of Jupiter's magnetic field, dramatically illustrating the relationship between Io's aurorae and the magnetic-field-aligned electrical currents which excite them.

A faint but localized blue emission is visible near the north pole of Io, possibly due to a volcanic plume erupting from the volcano Tvashtar at high northern latitude on the side of Io opposite Cassini. This eruption, observed by both Galileo and Cassini, left an enormous red ring around Tvashtar. (See the images released on March 29, 2001).

Credit: NASA/JPL/University of Arizona
Released: May 31, 2001
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Two tall volcanic plumes and the rings of red material they have deposited onto surrounding surface areas appear in images taken of Jupiter's moon Io by the Galileo and Cassini spacecraft in late December 2000 and early January 2001.

A plume near Io's equator comes from the volcano Pele. It has been active for at least four years, and has been far larger than any other plume seen on Io, until now. The other, nearer to Io's north pole, is a Pele-sized plume that had never been seen before, a fresh eruption from the Tvashtar Catena volcanic area.

The observations were made during joint studies of the Jupiter system while Cassini was passing Jupiter on its way to Saturn. The two craft offered complementary advantages for observing Io, the most volcanically active body in the solar system. Galileo passed closer to Io for higher-resolution images, and Cassini acquired images at ultraviolet wavelengths, better for detecting active volcanic plumes.

The Cassini ultraviolet images, upper right, reveal two gigantic, actively erupting plumes of gas and dust. Near the equator, just the top of Pele's plume is visible where it projects into sunlight. None of it would be illuminated if it were less than 240 kilometers (150 miles) high.These images indicate a total height for Pele of 390 kilometers (242 miles). The Cassini image at far right shows a bright spot over Pele's vent. Although the Pele hot spot has a high temperature, silicate lava cannot be hot enough to explain a bright spot in the ultraviolet, so the origin of this bright spot is a mystery, but it may indicate that Pele was unusually active.

Also visible is a plume near Io's north pole. Although 15 active plumes over Io's equatorial regions have been detected in hundreds of images from the Voyager and Galileo spacecraft, this is the first image ever acquired of an active plume over a polar region of Io. The plume projects about 150 kilometers (about 90 miles) over the limb, the edge of the globe. If it were erupting from a point on the limb, it would be only slightly larger than a typical Ionian plume. Unfortunately, the image does not reveal whether the source is actually at the limb or beyond it, out of view.

A distinctive feature in Galileo images since 1997 has been a giant red ring of Pele plume deposits about 1,400 kilometers (870 miles) in diameter. The Pele ring is seen again in one of the new Galileo images, lower left. When the new Galileo images were returned in March, 2001, scientists were astonished to see a second giant red ring on Io, centered around Tvashtar Catena at 63 degrees north latitude. (To see a comparison prior to the ring's depostion, see Galileo PIA-01604 or Galileo PIA-02309.) Tvashtar was the site of an active curtain of high-temperature silicate lava imaged by Galileo in November 1999 and February 2000 (image Galileo PIA-02584). The new ring shows that Tvashtar must be the vent for the north polar plume imaged by Cassini from the other side of Io, indicating a plume height about 385 kilometers (239 miles) high, just like Pele. The uncertainty in estimating the height is about 30 kilometers (19 miles), so the plume could be anywhere from 355 to 415 kilometers (221 to 259 miles) high.

If this new plume deposit is just one millimeter (four one-hundredths of an inch) thick, then the eruption produced more ash than the 1980 eruption of Mount St. Helens in Washington.

It has been said that Io is the heartbeat of the jovian magnetosphere. The two giant plumes evidenced in these images may have had significant effects on the types, density and distribution of neutral and charged particles in the Jupiter system during the joint observations of the system by Galileo and Cassini from November 2000 to March 2001.

The Cassini images were acquired on Jan. 2, 2001, except for the frame at the far right, which was acquired a day earlier. The Galileo images were acquired on Dec. 30 and 31, 2000. Cassini was about 10 million kilometers (6 million miles) from Io, ten times farther than Galileo.

NASA recently approved a third extension of the Galileo mission, including a pass over Io's north pole in August 2001. The spacecraft's trajectory will pass directly over Tvashtar at an altitude of 200 kilometers (124 miles). Will Galileo fly through an active plume? That depends on whether this eruption is long-lived, like Pele, or brief, and it also depends on how high the plume is next August. Two Pele-sized plumes are inferred to have erupted in 1979 during the four months between Voyager 1 and Voyager 2 flybys, as evidenced by Pele-sized rings found in Voyager 2 images but not seen in Voyager 1 images. Those eruptions, both from high-latitude locations, were shorter-lived than Pele, but their actual durations are unknown. Before its August flyby, Galileo will get another more-distant look at Tvashtar in May, 2001.

Credit: NASA/JPL/University of Arizona
Released: March 29, 2001
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This movie clip of 48 frames shows Jupiter's moon Io in the darkness of the giant planet's shadow. The sequence was recorded over a two hour interval that spanned nearly an entire eclipse on January 1, 2001. By the end of the clip, Io is emerging from shadow. The original images were taken through the clear filter of the narrow angle camera from a distance of over 10 million kilometers (6.3 million miles), with a spatial resolution of 61 kilometers/pixel. They have been cropped and processed to remove scattered light and artifacts.

Although no sunlight shines on the moon during an eclipse, two types of glows can be seen in this movie. The bright points of light are the glows of hot lava from actively erupting volcanoes. The brightest is the volcano Pele, which appears to be erupting steadily despite its great intensity.

To the right of Pele and slightly above it is a pair of bright spots associated with the volcano Pillan, the source of a major eruption in 1997 that was observed, both by the Galileo spacecraft and the Earth-based telescopes, to dwarf that of its energetic neighbor Pele. Pillan's eruption has waned over the past 30 months to the pair of small hot spots seen here. Another, newly discovered volcano, seen below Pillan and below and to the right of Pele, varies on a time scale of days. This sequence of images illustrates the great variations in intensity and longevity of Io's volcanic eruptions. The movement of these eruptions towards the right limb of Io during the course of the movie is due to the Io's rotation.

The second type of glow seen on Io during eclipse is a set of faint, diffuse emissions due to atmospheric aurorae. Similar to the Aurora Borealis on Earth, Io's aurorae are caused by the collisions of charged particles with gases in Io's tenuous atmosphere. A faint ring encircles the satellite, while brighter glows are concentrated on both sides of the moon's equator. These equatorial glows are seen gradually shifting in location as the eclipse progresses, due to the changing orientation of Jupiter's magnetic field. This is a new detection which confirms that these visible aurorae, like their counterparts seen at ultraviolet wavelengths, are caused by electrical currents that flow between Io and Jupiter.

Credit: NASA/JPL/University of Arizona
Released: February 5, 2001
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Jupiter's auroral ovals on the planet's night side were captured by the Cassini narrow angle camera on January 13, 2001 when the spacecraft was approximately 16.5 million kilometers from the planet and about 2.5 degrees below the equator; the smallest features are about 100 kilometers across. The images, taken 13 hours apart through a narrow spectral filter centered on an emission line of hydrogen known as `H alpha', have been processed to remove scattered light from the overexposed crescent of the planet. (Hydrogen is a major constituent of the Jovian atmosphere.) The Jovian magnetic rotation axis is tilted relative to the rotation axis and offset from the center of the planet. The result is that the magnetic pole is offset from the rotational pole in the north but more closely coincident with the rotational pole in the south. Energetic magnetospheric particles are constantly streaming towards Jupiter on magnetic field lines which intersect the planet's atmosphere on rings centered around the magnetic poles, causing the emission of light, or aurorae. In the north (upper) image, the rotational pole is located at the convergence of the blue longitude lines and falls to the left and out of the frame. The auroral oval encircling the north magnetic pole consequently appears like a draped necklace crossing lines of constant latitude on the planet. In the south (lower image), no significant offet is visible since the magnetic and rotational poles are closer there. The cusp of Jupiter's crescent, saturated in this frame, is seen on the left.

These Cassini imaging observations are the first to capture Jupiter's southern aurora on the planet's night side. Unlike Earth's aurorae, which are driven by solar activity, Jupiter's aurorae are caused by processes inside the Jovian magnetosphere. It is not understood why the auroral oval rings are so thin. Cassini's many images of this phenomenon will help unravel what brings about the narrow nature and other features of the aurorae, such as the break in the northern oval visible in the upper image.

Credit: NASA/JPL/University of Arizona
Released: February 5, 2001
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The brightest of Jupiter's outer satellites, Himalia, was captured and resolved, for the first time, in a series of narrow angle images taken on December 19, 2000 from a distance of 4.4 million kilometers during the brief period when Cassini's attitude was stabilized by thrusters instead of reaction wheels. This particular 1.0 second exposure was one of the sharpest, with a resolution of ~ 27 km/pixel, and was taken through a near-infrared spectral filter at 1:07 UTC (spacecraft time). The arrow indicates Himalia. North is up. The inset shows the satellite magnified by a factor of 10 and a graphic indicating Himalia's size and phase (the sunlight is coming from the left). It is likely that Himalia is not spherical: it is believed to be a body captured into orbit around Jupiter and as such, is likely to be an irregularly shaped asteroid. At the time this image was acquired, the side of Himalia facing the cameras was roughly 160 km across in the up/down direction.

Credit: NASA/JPL/University of Arizona
Released: January 22, 2001
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These color composite frames of the mid-section of Jupiter were of narrow angle images acquired on December 31, 2000, a day after Cassini's closest approach to the planet. The smallest features in these frames are roughly ~ 60 kilometers. The left is natural color, composited to yield the color that Jupiter would have if seen by the naked eye. The right frame is composed of 3 images: two were taken through narrow band filters centered on regions of the spectrum where the gaseous methane in Jupiter's atmosphere absorbs light, and the third was taken in a red continuum region of the spectrum, where Jupiter has no absorptions. The combination yields an image whose colors denote the height of the clouds. Red regions are deep water clouds, bright blue regions are high haze (like the blue covering the Great Red Spot). Small, intensely bright white spots are energetic lightning storms which have penetrated high into the atmosphere where there is no opportunity for absorption of light: these high cloud systems reflect all light equally. The darkest blue regions -- for example, the long linear regions which border the northern part of the equatorial zone, are the very deep `hot spots', seen in earlier images, from which Jovian thermal emission is free to escape to space. This is the first time that global images of Jupiter in all the methane and attendant continuum filters have been acquired by a spacecraft. From images like these, the stratigraphy of Jupiter's dynamic atmosphere will be determined.

Credit: NASA/JPL/University of Arizona
Released: January 22, 2001
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Night (left) and day side (right) narrow angle images taken on January 1, 2001 illustrating lightning flashes captured on the night side and their sources regions seen on the day side approximately 2 hours earlier. The images have been enhanced in contrast. The lightning locations on the night side images are coincident with bright, high clouds: when the effects of the planet's curvature are accounted for in these images, the upper pair of lightning flashes in the dark side image can be traced to the medium-bright cloud patch along the upper right edge of the day side image. The brighter pair of small clouds to the right of upper center in the day side image is probably also generating lightning, though no lightning was captured in this location by the camera when the night side image was taken. The cluster of lightning flashes in the lower left originates within the core of the swirling storm in the lower left of the day side image.

The storms occur at 34 degrees and 23.5 degrees North latitude, within one degree of the latitudes at which similar lightning features were detected by the Galileo spacecraft.

Credit: NASA/JPL/University of Arizona
Released: January 22, 2001
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The Galilean satellite Io floats above the cloudtops of Jupiter in this image captured on the dawn of the new millennium, January 1, 2001 10:00 UTC (spacecraft time), two days after Cassini's closest approach. The image is deceiving: there are 350,000 kilometers -- roughly 2.5 Jupiters -- between Io and Jupiter's clouds. Io is the size of our Moon, and Jupiter is very big.

Credit: NASA/JPL/University of Arizona
Released: January 22, 2001
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These images taken through the wide angle camera near closest approach in the deep near-infrared methane band, combined with filters which sense electromagnetic radiation of orthogonal polarization, show that the light from the poles is polarized. That is, the poles appear bright in one image, and dark in the other. Polarized light is most readily scattered by aerosols. These images indicate that the aerosol particles at Jupiter's poles are small and likely consist of aggregates of even smaller particles, whereas the particles at the equator and covering the Great Red Spot are larger. Images like these will allow scientists to ascertain the distribution, size and shape of aerosols, and consequently, the distribution of heat, in Jupiter's atmosphere.

Credit: NASA/JPL/University of Arizona
Released: January 22, 2001
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