Visual and Infrared Mapping Spectrometer
So what exactly is a Visual and Infrared Mapping Spectrometer?
VIMS is, in essence, a color camera mounted on the Cassini spacecraft bound for Saturn. But its a very special camera because of the KIND of color it captures. When the human eye looks at an object, the cones in the retina are able to discern the amount of light that hits them at 3 different wavelengths, which are interpreted as colors. Light with a wavelenth of around 420 nm (nanometers, or billionths of a meter) looks blue, while light at 534 nm looks yellow and 564 nm looks red. Colors other than red, yellow, and blue are the result of the eye receiving different amounts of light at each wavelength at the same time. Cassini VIMS takes pictures in 352 different colors at the same time, with wavelengths between 300 and 5100 nm. Thus the color range of VIMS's vision is greater than that of the human eye (300 - 5100 nm as opposed to 380 - 620 nm) and VIMS is far more accurate in determining the wavelength of the light that strikes it than the eye as well.
Why is all that color information important?
The range of wavelenghts that the human eye can see is called the visible region of the spectrum. VIMS is sensitive beyond this range. In addition to covering the visible spectrum, VIMS extends its capabilities into what is called the infrared part of the spectrum, which has wavelengths longer than the reddest our eyes can see. This range, coupled with the ability to discern different wavelengths (called spectral resolution), allows the VIMS instrument to be able to very accurately quantify the light it detects.
On the many icy moons of Saturn this will mean the identification of the composition and properties of their surfaces. For instance, the moon Iapetus is one of the strangest in the solar system because one half of it is darkly colored, while the other side is bright. Ever since the Voyager encounters discovered this anomaly, planetary scientists have been trying to determine what causes it. Data from VIMS will help us find out. For instance, it has been proposed that the dark half sweeps up dark dust-like material that was thrown off of Phoebe, another moon of Saturn, by micrometeorite impacts (which is much less farfetched now that such impacts have been shown to be the source of some of the tenuous rings of Jupiter from the Galileo spacecraft). A spectral match between the dark side of Iapetus and Phoebe would lend credence to this hypothesis.
Another aspect of the Saturnian system VIMS will shed light on is the rings. The composition and nature of the rings are still debated, and VIMS will be able to not only determine what the rings are made of, but also how large the particles that make them up are. As an example, in the image to the left you'll notice that you can see through parts of the rings. Particles of different sizes scatter light differently. Dust, for instance, transmits more light at longer (redder) wavelengths (this is why sunsets appear red), thus an analysis of the amount of light you can see through the rings at different wavelengths will reveal the fraction of the ring that is made up of dust as opposed to larger objects.
On Saturn itself VIMS will be able to look at many different layers of the atmosphere simultaneously. Clouds have different optical properties at different wavelenths. Taking advantage of this, at the wavelenghts where the upper cloud decks are transparent, pictures of the lower atmosphere can be obtained and at wavelengths where they absorb or reflect the upper atmosphere is seen. By studying precisely the wavelengths where the atmosphere absorbs, reflects, and emits, the composition of the atmosphere can also be inferred.
Saturn's moon Titan, the only moon in the solar system with an apreciable atmosphere, is covered by a layer of smog think enough that the surface is invisible at optical wavelengths. When Voyager 1 flew by Titan in 1980, its pictures revealed a nearly featureless orange ball (left image). However, the Hubble Space Telescope was able to see through the haze by operating near 1000nm, in the infrared. Although extremely rough due to being taken from such an extreme distance, the HST infrared images revealed a large high reflectivity area about the size of Australia on the surface of Titan. What these brightness differences mean will be something VIMS and other Cassini instruments, including the Huygens probe, will investigate.