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Neon Saturn : unlit side of Saturn's rings

Flying over the unlit side of Saturn's rings, the Cassini spacecraft captures Saturn's glow, represented in brilliant shades of electric blue, sapphire and mint green, while the planet's shadow casts a wide net on the rings.

This striking false-color mosaic was created from 25 images taken by Cassini's visual and infrared mapping spectrometer over a period of 13 hours, and captures Saturn in nighttime and daytime conditions. The visual and infrared mapping spectrometer acquires data simultaneously at 352 different wavelengths, or spectral channels. Data at wavelengths of 2.3, 3.0 and 5.1 microns were combined in the blue, green and red channels of a standard color image, respectively, to make this false-color mosaic.

This image was acquired on Feb. 24, 2007, while the spacecraft was 1.58 million kilometers (1 million miles) from the planet and 34.6 degrees above the ring plane. The solar phase angle was 69.5 degrees. In this view, Cassini was looking down on the northern, unlit side of the rings, which are rendered visible by sunlight filtering through from the sunlit, southern face.

On the night side (right side of image), with no sunlight, Saturn's own thermal radiation lights things up. This light at 5.1 microns wavelength (some seven times the longest wavelength visible to the human eye) is generated deep within Saturn, and works its way upward, eventually escaping into space. Thick clouds deep in the atmosphere block that light. An amazing array of dark streaks, spots, and globe-encircling bands is visible instead. Saturn's strong thermal glow at 5.1 microns even allows these deep clouds to be seen on portions of the dayside (left side), especially where overlying hazes are thin and the glint of the sun off of them is minimal. These deep clouds are likely made of ammonium hydrosulfide and cannot be seen in reflected light on the dayside, since the glint of the sun on overlying hazes and ammonia clouds blocks the view of this level.

A pronounced difference in the brightness between the northern and southern hemispheres is apparent. The northern hemisphere is about twice as bright as the southern hemisphere. This is because high-level, fine particles are about half as prevalent in the northern hemisphere as in the south. These particles block Saturn's glow more strongly, making Saturn look brighter in the north.

At 2.3 microns (shown in blue), the icy ring particles are highly reflecting, while methane gas in Saturn's atmosphere strongly absorbs sunlight and renders the planet very dark. At 3.0 microns (shown in green), the situation is reversed: water ice in the rings is strongly absorbing, while the planet's sunlit hemisphere is bright. Thus the rings appear blue in this representation, while the sunlit side of Saturn is greenish-yellow in color. Within the rings, the most opaque parts appear dark, while the more translucent regions are brighter. In particular, the opaque, normally-bright B ring appears here as a broad, dark band separating the brighter A (outer) and C (inner) rings.

At 5.1 microns (shown in red), reflected sunlight is weak and thus light from the planet is dominated by thermal (i.e., heat) radiation that wells up from the planet's deep atmosphere. This thermal emission dominates Saturn's dark side as well as the north polar region (where the hexagon is again visible) and the shadow cast by the A and B rings. Variable amounts of clouds in the planet's upper atmosphere block the thermal radiation, leading to a speckled and banded appearance, which is ever-shifting due to the planet's strong winds.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL. The Visual and Infrared Mapping Spectrometer team is based at the University of Arizona, where this image was produced

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SPACECRAFT - Cassini Orbiter Instruments - VIMS
There are a lot of things that our eyes cannot see; just ask those who operate a spectrometer -- an optical instrument that splits the light received from objects into its component wavelengths. Since each chemical has a unique spectral signature, they can be identified this way.

The Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft is made up of two cameras in one: one is used to measure visible wavelengths, the other infrared. Combined, the two cameras gather a lot of information on the composition of moon surfaces, the rings, and the atmospheres of Saturn and Titan.

"Our instrument measures the characteristic colors of the many materials that make up planetary surfaces and atmospheres: rocks, ices -- such as water, ammonia, methane -- and organic compounds," says Dr. Bonnie J. Buratti, VIMS Investigation Scientist. "VIMS data makes it possible to know what surfaces and atmospheres are made up of."

The instrument detects signals from a large range of wavelengths -- three octaves -- and it gathers 99 percent of radiation that is reflected from the Sun to a surface. Soon after Cassini reached Saturn, the instrument sent home valuable information.

"Data from VIMS allowed us to discover an ice volcano on Titan and the presence of fresh ice at the 'tiger stripes' on the moon Enceladus," Buratti says. "We are looking forward to the many upcoming flybys of Titan and of the moon Rhea. I am excited about Rhea because I'd like to see if it's an older version of Enceladus -- like Dione -- or something completely different."

The instrument is designed to measure reflected and emitted radiation from atmospheres, rings and surfaces over 352 contiguous wavelengths from 0.35 to 5.1 micrometers. It will also help determine the compositions, temperatures and structures of these objects.

With VIMS, scientists are also conducting a long-term study of cloud movement and morphology in the Saturn system to determine the planet's weather patterns. VIMS also observes the sunlight and starlight that pass through the rings to learn more about ring structure.

The VIMS team is spread across the Atlantic.

"We are very international! Most of our members are from Europe -- France, Italy and Germany," Buratti says

In The Images-
1.Neon Saturn
2.Map of Titan in Infrared
3.Infrared Image of Titan Volcano
4.Geologic Map of Titan Volcano

Release link: http://www.nasa.gov/mission_pages/cassini/multimedia/pia09212.html