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Astronomical Observatory: Cool Images


Images: Two Views of Saturn

Saturn in true color
Saturn in red-shifted colors
Contents: These two images of Saturn were made 11 December 2001. The top one is a normal, full-color view, made by combining separate exposures through blue, green, and red filters. The bottom one might be called a red-shifted, full-color view, made by combining green, red, and infrared images. It shows how the planet would appear if our eyes were sensitive to somewhat longer wavelengths of light.

Saturn's southern hemisphere is exposed in this image as the planet is tilted 25.9 degrees viewed from the Earth. Like the Earth, Saturn experiences seasons, only with longer duration due to its orbital period of 29 years. Saturn will have its maximum tilt toward the Sun (26.7 degrees) in October 2002 (its winter solstice).

A bright stripe can be seen across the equator (where the planet meets the rings at the top) around which highly reflective clouds mark the equatorial zone of trade winds. Zonal winds are faster on Saturn than on any other planet in part due to its rapid rotation rate (10.2 hours). The rapid rotation also flattens the polar diameter to just 80% of the equatorial diameter.

The yellow color of the planet's atmosphere arises from the scattering of blue light by molecules in the atmosphere. Similarly, the turqouise color in the "red-shifted" image arises from absorption by methane of infrared light in the atmosphere.

Unlike the gaseous atmosphere, the rings consist of solid particles (primarily water ice) that reflect light well from infrared through ultraviolet. Also contrasting with the three dimensional planet, the ring particles reside in a nearly two dimensional disk: 274 million meters across but only a few meters thick! In these images the rings pass in front of the planet at the top and behind it at the bottom.

The portion of the rings to the left and right of the planet are known as ansae (one ansa, two ansae), from the Latin word for "handle" (like the handle on a coffee mug). With poor angular resolution (and no infrared sensitivity for color discrimination), many decades passed from the first telescopic observation of Saturn by Galileo until Christiaan Huygens recognized in 1659 that the rings truly encircled the planet without touching it (rather than being appendages like handles). This is one of the few topics in which Galileo publicly acknowledged his earliest theories were just plain wrong.

Processing: The normal color image was made by combining three separate monochrome images (one each using the Johnson B, V, and R filters, which pass blue, green, and red light, respectively). The red-shifted image was made by combining images with Johnson V, R, and I filters, which pass green, red, and infrared light, respectively. In turn the individual filter images were made by combining a set of 0.2 second exposures (7 at B, 4 at V, 4 at R, and 5 at I band). Short exposures were necessary to avoid atmospheric blurring. Averaging was needed to improve signal-to-noise ratio in the final images. Image sharpness was enhanced using unsharp masking. All images were obtained between 11:39 and 11:58 pm on December 11, 2001. The relative color brightness was set to make the rings neutral or gray in color. All images were made and reduced by Prof. Molnar.

Orientation and scale: North is up and East is to the left. Each image is 90 arcseconds across.

Content updated 8/1/02



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