Volume: 03, Issue: 16 08/10/2005 
This false color image of Saturn's moon Mimas reveals variation in either the composition or texture across its surface. Image courtesy NASA/JPL/Space Science Institute.
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This image shows the first look at the infrared (heat) radiation from the south pole of Saturn's moon Enceladus. Image courtesy NASA/JPL/GSFC.
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This artist concept shows the detection of a dynamic atmosphere on Saturn's icy moon Enceladus. Image courtesy NASA/JPL.
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The Cassini spacecraft has obtained new images of Saturn's auroral emissions, which are similar to Earthís Northern Lights. Image courtesy NASA/JPL/University of Colorado.
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Other Articles in This Issue:
Discovery Returns to Earth
Scientists Discover Tenth Planet
Spitzer Finds Hungry Black Holes
Put Your Astronomy Skills Into Practice

Cassini Continues to Explore the Unknown

The Cassini spacecraft has proven to be an invaluable tool in exploring the mysteries of the ringed planet and its satellites. Recent discoveries have shed light on intriguing moons Mimas and Enceladus, as well as auroral activity on Saturn itself.

Cassini flew by tortured moon Mimas on Aug. 2, 2005, revealing a battered and bruised satellite with a surface that may be the most heavily cratered in the Saturn system. The flyby returned eye-catching images of its most distinctive feature, the spectacular 87-mile-wide, landslide-filled Hershel crater. Numerous rounded and worn-out craters, craters within other craters, and long grooves reminiscent of those seen on asteroids are also seen in the new images.

Scientists hope analysis of the images will tell them how many crater-causing impactors have coursed through the Saturn system and where those objects might have originated. They also suspect the moonís grooves, first discovered by NASA's Voyager spacecraft, are related to the giant impact that formed Herschel on the opposite side of the moon.

New discoveries have also been made as scientists decode data from the July 14 flyby of the tiny, icy moon Enceladus. The moon ought to be cold and dead, but it instead displays evidence for active ice volcanism. Cassini found a huge cloud of water vapor over the moon's south pole and warm fractures where evaporating ice probably supplies the vapor cloud.

"Enceladus is the smallest body so far found that seems to have active volcanism," said Cassini imaging-team member Dr. Torrence Johnson. "Enceladus' localized water vapor atmosphere is reminiscent of comets. 'Warm spots' in its icy and cracked surface are probably the result of heat from tidal energy like the volcanoes on Jupiter's moon Io. And its geologically young surface of water ice, softened by heat from below, resembles areas on Jupiter's moons Europa and Ganymede."

Flyby data also confirms an extended and dynamic atmosphere at Enceladus. Water vapor comprises about 65 percent of the atmosphere, with molecular hydrogen at about 20 percent. The rest is mostly carbon dioxide and some combination of molecular nitrogen and carbon monoxide. The variation of water vapor density with altitude indicates the water vapor may come from a localized source comparable to a geothermal hot spot. Ultraviolet results strongly suggest a local vapor cloud. The fact the atmosphere persists on this low-gravity world, instead of instantly escaping into space, implies the moon is geologically active enough to replenish the water vapor at a slow continuous rate.

Images also indicate the south pole has an even younger and more fractured appearance than the rest of Enceladus. It features icy boulders the size of large houses and long, bluish cracks or faults dubbed "tiger stripes."

In addition, data shows the south pole is warmer than anticipated. Temperatures near the equator were found to reach a frigid 80 degrees Kelvin (minus 316 Fahrenheit), as expected. The poles should be even colder because the sun shines so obliquely there. South polar average temperatures, however, reached 85 Kelvin (minus 307 Fahrenheit) -- much warmer than expected. Small areas of the pole, concentrated near the tiger stripe fractures, are even warmer: well over 110 Kelvin (minus 261 Fahrenheit) in some places.

"This is as astonishing as if we'd flown past Earth and found that Antarctica was warmer than the Sahara," said Dr. John Spencer of the Southwest Research Institute, team member of the composite infrared spectrometer.

Scientists find the temperatures difficult to explain if sunlight is the only heat source. More likely, a portion of the polar region, including the tiger stripe fractures, is warmed by heat escaping from the interior. Evaporation of this warm ice at several locations within the region could explain the density of the water vapor cloud detected by other instruments. How a 500-kilometer (310-mile) diameter moon can generate this much internal heat and why it is concentrated at the south pole is still a mystery.

Cassini also confirmed Enceladus is the major source of Saturn's largest ring, the E-ring. Cassini's cosmic dust analyzer detected a large increase in the number of particles near Enceladus. Scientists think micrometeoroids blast the particles off, forming a steady, icy, dust cloud around Enceladus. Other particles escape, forming the bulk of the E ring.

Besides these discoveries about Saturnís satellites, new information has been exposed regarding auroras on Saturn. New images obtained by a team led by the University of Colorado at Boulder show auroral emissions at the planetís poles similar to Earth's Northern Lights.

Taken with Cassiniís Ultraviolet Imaging Spectrograph, the two UV images are the first from the Cassini-Huygens mission to capture the entire "oval" of the auroral emissions at Saturn's south pole. They also show similar emissions at Saturn's north pole, according to UVIS principal investigator Larry Esposito and UVIS team member Wayne Pryor. The images show the aurora lights at the polar regions respond rapidly to changes in the solar wind.

Major changes in the emissions inside the Saturn south-pole aurora are evident by comparing the two images, which were taken about one hour apart. The brightest spot in the left aurora fades, and a bright spot appears in the middle of the aurora in the second image.

The images also contain more than 2,000 wavelengths of spectral information within each picture element. Researchers will use the wavelength information to study Saturn's auroras, gases, and hazes and their changing distributions.

Additional information and images from the Cassini-Huygens mission are available on the Web:

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