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Watch Saturn's storms spin with VIMS

Posted By Emily Lakdawalla

2011/04/28 01:15 CDT

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A lot of attention has been paid recently to a storm in Saturn's northern hemisphere that is large and bright enough to be visible from Earth, but Saturn's atmosphere actually features lots more swirling storms. They can be hard to see, at least in visible wavelengths. Saturn usually looks much more bland than Jupiter, but there actually are Jupiter-like belts and storms just underneath the visible cloud tops. Cassini's Visual and Infrared Mapping Spectrometer (VIMS) can peer through the methane in Saturn's uppermost atmosphere to reveal Saturn in all its stormy detail. Here's an animation of Saturn's southern hemispheric storms rotating with Saturn, produced from archived VIMS data by Daniel Macháček.

VIMS data, like the data from Cassini's main cameras, is freely available to the public, but it's a bit harder to understand. Imaging spectrometers split incoming light into its component wavelengths, and measure the intensity of each point in each wavelength. A mapping spectrometer can do that task for many points simultaneously, building up an image that is incredibly rich with color information. VIMS images contain information on 352 different wavelengths of light from ultraviolet to the mid infrared, captured simultaneously. This is 15 or 20 times higher spectral resolution than the cameras, but the spatial resolution of VIMS is roughly 100 times lower than the resolution of the narrow-angle camera. The VIMS field of view is about half that of the wide-angle camera, or five times that of the narrow-angle camera.

VIMS produces what scientists refer to as "image cubes." A monochrome image is two-dimensional: it has information in two spatial directions (x and y, or north-south and east-west). A color image adds a third dimension: wavelength. But ordinary color images only have information along three points along the wavelength dimension. VIMS has a lot more depth, 352 points along the wavelength dimension. As a result, VIMS "images" are truly three-dimensional data entities.

Daniel's animation is cool but it doesn't take advantage of VIMS' awesome spectral depth; the color is based on only three wavelengths from the 352 in a VIMS cube. The images of Saturn below are based on much more of the VIMS data; Gordan Ugarkovic convolved all the data from visible wavelengths with a model of how the human eye responds to light at different wavelengths to produce these views of Saturn as the human eye would perceive them.

NASA / JPL / UA / color composite by Gordan Ugarkovic

Saturn as seen from VIMS

Several natural color mosaics of Saturn from Cassini's Visual and Infrared Mapping Spectrometer.
While the instrument lacks spatial resolution (the above imagery was magnified 4x times), it compensates by being able to split each of the 64x64 pixels into 352 separate wavelengths. Among other things, this enables more accurate color representation than what is possible by using filtered images from Cassini's ISS cameras, so it serves as a reality check on ISS color images.

The top image is a mosaic of 9 VIMS data "cubes", with gapfill at the left ring ansa provided by a lower resolution image. Two dim specks below the rings at right are Enceladus (brighter) and Mimas (above Enceladus, barely noticeable).

The bottom row shows Saturn as viewed between 2006 and 2008 at varying phase angles and seasonal period (note the rightmost image is oldest and has ring shadows high up in the north compared to other images). As the ring shadows move south, the blue color of the northern hemisphere faded, turning greenish and then yellow.