Spaceflight Now: Breaking News

Cassini sees Jupiter lightning storms, small moon
Posted: January 24, 2001

A new batch of Jupiter images from NASA's Cassini spacecraft demonstrates some of the ways scientists are using Cassini's camera to learn more than what first meets the eye, such as determining particle sizes in clouds and identifying which storms produce lightning. One new picture is the best yet taken of the small moon Himalia, and is the first ever to show one of Jupiter's outer moons as more than a star-like dot.

One pair of frames shows the same portion of the planet both in daylight then after it had rotated to the night side, showing that only certain small areas were producing lightning.

Cassini made its closest pass to Jupiter on Dec. 30, 2000, gaining a gravitational boost for reaching its main destination, Saturn, in 2004. It will continue to make observations and measurements of the Jupiter system through March 2001.

Jupiter clouds
True and false
Jupiter clouds in true Color and false to show heights. Photo: NASA/JPL/University of Arizona SEE FULL PAGE IMAGE
These two frames of composited images taken by Cassini show the same cloud patterns on Jupiter both in natural color (left) and in a false-color combination (right) selected to show differences in clouds' height. The white spots in the right frame are storms high in the atmosphere.

Each frame is a combination of images taken by Cassini's narrow-angle camera through different filters on Dec. 31, 2000, one day after Cassini's closest approach to the planet. The smallest features are roughly 60 kilometers (40 miles) across.

The left frame shows the colors Jupiter would have if seen by the naked eye. The right frame is composed of three images. Two were taken through 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, bright white spots are energetic lightning storms that have risen to a height where there is no opportunity for absorption of light, so the clouds reflect all light equally. The darkest blue regions, such as the long linear regions bordering the northern part of the equatorial zone, are the very deep 'hot spots' 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 appropriate continuum filters have been acquired by a spacecraft. From images like these, the dynamics of the layers within Jupiter's atmosphere will be determined.

Lightning storms
Jupiter lightning storms in day and night. Photo: NASA/JPL/University of Arizona
Images taken of Jupiter's day and night sides by Cassini on Jan. 1, 2001 show that storms visible on the day side are the sources of visible lightning when viewed on the night side.

The two day-side occurrences of high clouds, in the upper and lower parts of the image, are coincident with lightning storms seen on the dark side. The storms occur at 34.5 degrees and 23.5 degrees North latitude, within one degree of the latitudes at which similar lightning features were detected by the Galileo spacecraft. The storms' longitudinal separation changes from one image to the next because the winds carrying them blow at different speeds at the two latitudes. The images have been enhanced in contrast.

Io above Jupiter
Io above clouds. Photo: NASA/JPL/University of Arizona SEE FULL PAGE IMAGE
The moon Io floats above the cloudtops of Jupiter in this image captured by Cassini on the dawn of the new millennium, Jan. 1, 2001, two days after Cassini's closest approach to Jupiter. The image is deceiving: There is room for two and a half Jupiters between Io and Jupiter's clouds. Io is the size of our Moon.

Jupiter's poles
Polarized light from Jupiter's poles. Photo: NASA/JPL/University of Arizona
These frames taken by Cassini show that infrared light from Jupiter's north and south poles is polarized, which indicates that the size of cloud aerosol particles in those regions is likely smaller than the particles found in clouds near the equator.

Cassini's wide-angle camera took these images with a near-infrared filter useful for seeing methane, combined with filters that sense whether the light is polarized. The pole appears bright in one image and dark in the other, showing that the light from those regions is polarized. 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.

Jupiter's moon Himalia
Himalia, a small moon of Jupiter. Photo: NASA/JPL/University of Arizona SEE FULL PAGE IMAGE
NASA's Cassini spacecraft captured images of Himalia, the brightest of Jupiter's outer moons, on Dec. 19, 2000, from a distance of 4.4 million kilometers (2.7 million miles).

This near-infrared image, with a resolution of about 27 kilometers (17 miles) per pixel, indicates that the side of Himalia facing the spacecraft is roughly 160 kilometers (100 miles) in the up-down direction. Himalia probably has a non-spherical shape. Scientists believe it is a body captured into orbit around Jupiter, most likely an irregularly shaped asteroid.

In the main frame, an arrow indicates Himalia. North is up. The inset shows the little moon magnified by a factor of 10, plus a graphic indicating Himalia's size and the direction of lighting (with sunlight coming from the left). Cassini's pictures of Himalia were taken during a brief period when Cassini's attitude was stabilized by thrusters instead of by a steadier reaction-wheel system. No spacecraft or telescope had previously shown any of Jupiter's outer moons as more than a star-like single dot.
