Color view of Saturn's rings.

This view of Saturn's rings in the ultraviolet indicates there is more ice toward the outer part of the rings, than in the inner part, hinting at the origins of the rings and their evolution.

How It Works

The Ultraviolet Imaging Spectrograph creates pictures by observing ultraviolet light. In ultraviolet wavelengths of light, gases that the human eye can’t see become observable. The spectrograph can determine what those gases are made of by splitting the light into its component wavelengths, or "colors," much like a rainbow.

Saturn's Auroras
Saturn's auroral emissions are similar to Earth's Northern Lights. These dual Images were taken in 2005 with Cassini's ultraviolet imaging spectrograph.

How We Use It

Scientists use this instrument to study Saturn’s rings and atmosphere, along with features of the planet's many moons. The instrument can study the composition of atmospheres on the night-sides of the planet and its moon Titan, and also looks at how light from the sun and stars passes through atmospheres and rings in the Saturn system. Light is altered when it passes through gas or dust, and those changes tell scientists about the density and composition of the material through which the light passed.

The Ultraviolet Imaging Spectrograph (known to the science team as UVIS) is a box of four telescopes that can see ultraviolet light, which is invisible to the human eye. On Earth, ultraviolet light causes sunburns, but with a space-based spectrograph, ultraviolet light shows researchers what no other type of light can show.

The cool thing about UVIS is that we can see things that are invisible to other instruments.
- Amanda Hendrix, UVIS team member

"The cool thing about UVIS is that we can see things that are invisible to other instruments," said Amanda Hendrix, a planetary scientist at the Planetary Science Institute and a UVIS team member who analyzes icy satellite data. "For example, UVIS can 'see' gases in the Saturn system that aren't seen by Cassini’s visible-light camera. The instrument can also see the dark, night side of the moons of Saturn, which stand out against the sky because they are bright at ultraviolet wavelengths."

When the instrument collects ultraviolet light, the light contains patterns that scientists recognize as tell-tale signs of certain chemical elements and compounds. So from a million miles away, the spectrograph can tell what some materials are made of.

The science team has used the instrument to study the composition of Titan’s upper atmosphere, and helped determine that the plume of material erupting near Enceladus’ south pole is made largely of water. Elsewhere in the Saturn system, the instrument has observed acetylene, ethane, hydrogen, methane and oxygen, among other materials.

Catching Saturn's Ring Waves
This false color image of two density waves in Saturn's A ring was made from the stellar occultation observed by Cassini's ultraviolet imaging spectrograph when the spacecraft was 6.3 million kilometers (4 million miles) from Saturn. Bright areas indicate the denser regions of the rings.

Though ultraviolet light is invisible to the human eye, scientists use measurements from Cassini's ultraviolet spectrograph to produce pictures we can see. By studying the images, scientists can watch certain gases come and go over time in a world’s upper atmosphere, or they can compare their images to data from Cassini’s other instruments to figure out the causes-and-effects of phenomena on Saturn’s moons or in the rings.

In addition to looking at the night side of Saturn or its moons, or at the sunlight reflected off of gases and ring particles, the UV spectrograph science team also observes features of the Saturn system by looking a them when they’re backlit by the sun or stars. Cassini is constantly moving, so it can lock onto a bright star and record how the star’s light changes as Saturn’s rings or an atmosphere intrude upon the instrument’s line of sight. The process is called an “occultation” because the object in the background (in this case a star) is “occulted” (hidden) by an object in the foreground such as the rings or a moon.

The occultation technique has allowed the spectrograph team to map features of Saturn’s rings that are 10 times smaller than Cassini’s visible-light cameras can see.

Ultraviolet Imaging Spectrograph

At a Glance

Built by the Laboratory for Atmospheric and Space Physics (LASP) located in the Research Park of the University of Colorado in Boulder, UVIS is a Remote Sensing Instrument (think sight) that captures images of the ultraviolet light reflected off an object. Designed to measure ultraviolet light over wavelengths from 55.8 to 190 nanometers, this instrument is also a valuable tool to help determine the composition, distribution, aerosol particle content and temperatures of their atmospheres. UVIS includes a two-channel, far- and extreme-ultraviolet imaging spectrograph that studies light over wavelengths from 55.8 to 190 nanometers.

  • Mass (current best estimate) = 14.46 kg
  • Peak Operating Power (current best estimate) = 11.83 W
  • Peak Data Rate (current best estimate) = 32.096 kilobits/sec
  • Dimensions (approximate) = 48 cm x 30 cm x 23 cm

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