Press Release

New Cassini Images Online 22 Jan 2001

By SpaceRef Editor
January 22, 2001
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New Cassini Images
January 22, 2001
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 dimensions of the side of Himalia facing the cameras is roughly 160 km in the up/down direction.
Credit: NASA/JPL/University of Arizona
Released: January 22, 2001
Io Transit 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.
True/False Color 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.
Day and night side narrow angle images taken on January 1, 2001
illustrating storms visible on the day side which are the sources of visible lightning when viewed on the night side. The images have been enhanced in contrast. Note 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 images were taken at different times. The storms’ longitudinal separation changes from one image to the next because the winds carrying them blow at different speeds at the two latitudes.
Methane polarization
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.

SpaceRef staff editor.