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New Detail on Cometary Jets Seen By Stardust.

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Stardust: Image credit: NASA/JPL.

When NASA's Stardust spacecraft flew through the tail of Comet Wild-2, it took some of the most detailed images ever seen of a comet. Analysts have taken a short exposure of the comet's surface and overlaid it onto a longer exposure that showed the comet's jets. This combined image allows scientists to understand which surface features are creating the jets, to better understand why Wild-2 is so incredibly pockmarked with craters, rifts and holes. Stardust will return to Earth in 2006 with its precious cargo of cometary particles.

On 2 January 2004, NASA's Stardust spacecraft successfully survived flying through the coma (dust and gas cloud) surrounding Comet 81P/Wild 2, captured thousands of fresh cometary dust particles released from the surface just hours before, and is now on its way home for Earth return set for January 2006.

During the flyby, the highest resolution images ever taken of a comet's nucleus were obtained and have been the subject of intense study since the flyby. A short exposure image showing tremendous surface detail was overlain on a long exposure image taken just 10 seconds later showing jets.

"This spectacular composite image shows a surface feature unlike any other planetary surface see to date in our solar system", says Prof Donald Brownlee, the Stardust Principal Investigator from the University of Washington. "Other than our sun, this is currently the most active planetary surface in our solar system, jetting dust and gas streams into space and leaving a trail millions of kilometers long."

"The overall shape of the nucleus resembles a thick hamburger patty with a few bites taken out", says Thomas Duxbury, the Stardust Project Manager from JPL. "The surface has significant relief on top of this overall shape that reflects billions of years of resurfacing from crater impacts and out gassing".

One mystery from the close-views of Wild-2 was its pockmarks. "I looked at the images in stereo view," said Brownlee. "One large depression has a bottom that is flat, with very steep walls [400-500 meters deep]. While any scientific evidence is only two days old," most impact craters are expected to be bowl-shaped with much shallower aspect ratios (0.1-0.2), meaning they are five times wider than they are deep. Some of these depressions are not round, but scalloped and much deeper (aspect ratio, 0.4).

"I am from Washington state", said Brownlee, "and when the Comet is viewed in stereo pairs like that, it reminds me of Grand Cooley, with its steep cliffs and run-out areas at the bottom. Like flood areas from the Columbia River, if you were standing at the bottom of one of these Comet depressions. But the floor of these Comet depressions are incredibly complicated, like balls of clay have been mashed together and then etched."

"The mission scientists with Deep Space I," which flew by Comet Borrelly, found surprising "mesas", said Brownlee. "They speculated that these walls can sometimes face sunwards, and volatiles like ice and methane may evaporate or etch that surface. But on Wild-2, we see pits, not mesas. The two comets are quite different. We may have [with Wild-2] a young Comet that evolves towards Borrelly, or vice versa."

Three large Comet jets registered on one of Stardust's instruments, its dust counter. Three distinct peaks appeared with thousands of particle strikes each. Slightly less than an ounce of Comet dust, or about a thimbleful, were collected over the spacecraft's 12 minute pass through these large jets. "The secret of our mission is that we sample only the volatile material, that which is evaporating into space," said Brownlee. "That's the way we avoid any contaminants that might have left those impact-like marks on the comet's surface. So it was better in this case to fly-through the lighter dust stream, than to land on this comet. We'd have to drive around a bit to find just the Comet stuff." In just such a science-fiction scenario of landing on a comet, the European mission, called Rosetta, will launch next month and travel to Comet Churyumov-Gerasimenko in November 2014.

Preliminary scientific results obtained from the Wild 2 encounter are being presented at the Lunar and Planetary Science Conference in Houston, Texas by the Stardust science team. Stardust will bring samples of Comet dust back to Earth in January 2006 to help answer fundamental questions about the origins of the solar system.

Original Source: NASA Astrobiology Magazine

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