"Other than the Sun, this is the most active planetary surface in our solar system," said Donald Brownlee, principle investigator of the comet study.
The surface of the Wild 2 (pronounced "Vilt 2") has no significant atmosphere and is rife with strange, flat-bottomed depressions with sheer walls, possibly caused craters or gas vents. At least 10 jets were active when NASA's Stardust spacecraft swung by the comet in January, collecting data for Brownlee's study. Astronomers believe that number may double after more data analysis.
Comet Wild 2 also sports crisp cliffs unlike anything seen in close-up views of the past icy bodies like the famed comet Halley and more recent, smooth-surfaced comet Borrelly, which was visited by Deep Space 1 in 2001. Surface features range from the very small on up to 1.2 miles (two kilometers) in size, almost half the entire diameter of the 3.1-mile (5-kilometers) comet.
"The overall shape of the nucleus resembles a thick hamburger patty with a few bites taken out," said Thomas Duxbury, Stardust project manager. "The surface has significant relief on top of this overall shape that reflects billions of years of resurfacing from crater impacts and outgassing."
Comets, once thought to be pristine and primordial, are now known to have had mostly rough lives.
Astronomers believe Wild 2 is a rather fresh comet whose latest swing past the Sun was only the fifth in its present orbit. The comet spent most of its 4.5 billion-year lifetime -- it's thought to have formed just after the Sun developed -- following a distant and more circular solar orbit. It was perturbed into its closer approach and a more elliptical path when it wandered too close to Jupiter in 1974.
Stardust took 72 close-up shots of the icy body.
During its high-speed flyby of Wild 2, Stardust also studied the comet's dusty, gassy coma, finding it less dense than expected. While the spacecraft did pick up some dust, it was at levels 100 times less than anticipated, which suggests that Stardust caught Wild 2 at a lull in its activity. It is also possible that the spacecraft's Cometary and Interstellar Dust Analyzer (CIDA) was shadowed from the dust in some way due Stardust's position.
Data from a second dust instrument, the Dust Flux Monitoring Instrument, is still being evaluated, though it did detect thousands of small particles, some of which were large enough to penetrate the first layer of Stardust's protective dust shield.
Project scientists also lauded the success of Stardust to physically retrieve some Wild 2 material using an aerogel dust trap, which will be returned to Earth in 2006. The dust sample is expected to give astronomers an unprecedented laboratory look into the composition of comets, whose contents are not known with certainty.
Stardust scientists presented their preliminary findings during the Lunar and Planetary Science Conference in Houston, Texas this week.
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