* Astronomy

On Nov. 9, 2006, Cassini's composite infrared spectrometer captured its first view of the infrared heat radiation emanating from the "tiger stripe" fractures at the south pole of Saturn's moon Enceladus (right) since the discovery of the hot spot 16 months earlier (left).
The original discovery was made just before a close flyby of Enceladus on July 14, 2005, and coincided with the discovery of plumes of water-rich gas and ice particles jetting out of the tiger stripes. However, the spacecraft's orbit did not provide any good views of the south pole for follow-up observations until November 2006. The new observations were made from a range of 110,000 kilometres, slightly more distant than the 80,000-kilometer range of the original observations.
Comparison of the two images shows that the south polar region continues to be active, and the distribution of temperatures there has changed little in 16 months. The distribution of heat radiation suggests that most or all of the south polar heat comes from the tiger stripes themselves, though the individual stripes are not resolved at the approximate 30-kilometer spatial resolution of these images.


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Credit: NASA/JPL/GSFC/Southwest Research Institute

The images show the intensity of heat radiation in the 10- to 16-micron wavelength range, translated into temperature and displayed in false colour. Peak south polar temperature on both dates reached about 85 Kelvin, averaged over the 30-kilometer spatial resolution of the data. However, the variation in brightness with wavelength, which is also measured by the composite infrared spectrometer, reveals that the warm region includes small areas, possibly zones a few 100 meters wide along the length of the tiger stripes, that are at higher temperatures, reaching at least 130 Kelvin and perhaps much warmer still. While the south polar tiger stripes are almost certainly heated by energy from the moon's interior, daytime regions at low latitudes are warmed by sunlight to temperatures in the high 70s Kelvin.
The white numbers on the images show west longitudes on Enceladus, which is 500 kilometres in diameter. The dashed line shows the terminator, the boundary between day and night. The blotchy appearance of the cooler regions away from the south pole, and of the sky beyond the globe of Enceladus, is an artefact resulting from the fact that apart from the polar hot spot, the composite infrared spectrometer can barely detect the very faint heat radiation from this very cold moon.

New research casts doubt on the existence of water near the surface of a tiny Saturn moon — a finding that, if confirmed, could mark a reversal in the hunt for extraterrestrial life.
Earlier this year, the international Cassini spacecraft orbiting Saturn caused a stir when it spied what appeared to be Yellowstone-style geysers spouting from the south pole of Enceladus. Scientists speculated the eruptions were driven by shallow pools of water lurking just below the icy surface.
In an alternative view published in Friday’s issue of the journal Science, other researchers propose that buried ice clathrates — not liquid water — are responsible for releasing the towering plumes through a sudden tectonic shift in the crust that causes cracks in the ice and gas to vent.

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A new (PDF) map of Enceladus showing all IAU-approved names has been posted on the Gazetteer of Planetary Nomenclature.


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Looking down onto the northern hemisphere of geologically complex Enceladus, the Cassini spacecraft spies softened, or "relaxed," craters and east-west trending fractures and faults.
The anti-Saturn hemisphere of Enceladus (505 kilometres across) is lit here.



The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Oct. 12, 2006 at a distance of approximately 521,000 kilometres from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 101 degrees. Image scale is 3 kilometres per pixel.