June 18, 2004 | David F. Coppedge

Comet Surface Wild and Crazy

“Completely unexpected,” was the reaction of Donald Brownlee, principal investigator of the Stardust mission, to the photos revealed by the spacecraft that flew into the tail of Comet Wild-2 last January (see 01/02/2004 headline), reports a University of Washington press release.  The comet mission is the cover story in the June 18 issue of Science, with four scientific papers and two reviews.  Photos and information have also been released at the Jet Propulsion Laboratory Stardust website.
    Scientists expected to find a dirty, fluffy snowball of loose material, but found instead a surface unlike anything else in the solar system.  Deep flat-floored pits, craters with steep walls and pinnacles more than three hundred feet high indicate that the comet material is rigid and cohesive enough, despite the low gravity, to hold together under impacts.  Planetary scientists thought that most comets were like rubble piles loosely held together by gravity, easily torn apart by gravitational perturbations.  Maybe some are, but the sharp edges, angular shapes and steep cliffs on Wild-2 make it look brittle and very unlike an asteroid, and different than the previously-visited comets Halley and Borrelly.  Perhaps there is more variety among comets than expected.
    The comet jets on Wild-2 (pronounced vilt-2), also unexpectedly, emerge at high velocity from numerous places around the roughly circular body, rather than being sublimated off the outer surface volatiles.  Some of them appear to be collimated like the jets from a fire hose, suggesting that they emerge from pits deep in the interior.  Stardust was hit with two wallops as high-velocity dust particles from particularly strong jets pummeled its shields at hundreds of kilometers per hour on the way through the dust tail.  The spacecraft succeeded admirably, despite the hazards, in taking pictures and collecting dust particles in its aerogel collector for return to earth.
    Delighted, but perplexed, describes the mood of scientists over the mission so far.  “New in situ observations of a comet are demonstrating once again how little we understand about these dark and mysterious planetesimals,” remarked Harold Weaver in Science.1  Brownlee et al.2 claim that the surface reveals a “juxtaposition of features that are young and old” on an object thought to be a primordial relic of the formation of the solar system.  Hinting that these findings are putting theories in turmoil, an editorial in the same issue3 hopes that the return of the particles in January 2006 will “clear up any nightmares about the origin of the solar system and the dynamics of comets.”


1Harold A. Weaver, “Not a Rubble Pile?” Science, Vol 304, Issue 5678, 1760-1762, 18 June 2004 [DOI: 10.1126/science.1100581].
2Brownlee et al., “Surface of Young Jupiter Family Comet 81P/Wild 2: View from the Stardust Spacecraft,” Science, Vol 304, Issue 5678, 1764-1769, 18 June 2004 [DOI: 10.1126/science.1097899].
3“Sweet Dreams Are Made of These,” Science, Vol 304, Issue 5678, 1760, 18 June 2004 [DOI: 10.1126/science.304.5678.1760a].

Everyone should be thrilled at the success of this mission of discovery, but it does point out a lesson about scientists.  Since scientists know so little about things they can observe, and since they often find contradictions to their expectations, why should we trust any confident-sounding pronouncements about things they can’t observe?  When they talk about this comet having formed billions of years ago, how can they possibly know that?
    Brownlee’s paper says, “Pinnacles were not anticipated land forms on primitive bodies, and their origin on Wild 2 is a mystery.”  He thinks the jets, pinnacles and impact craters are young, but the rest of the comet is old, only because current theories require the solar system to be 4.6 billion years old, and comets had to form near the beginning.  But then how does he keep the comet from dissipating away completely long ago?  (See 03/27/2003 headline.)  He tells an ad hoc story to get the theory to fit the observations.  He claims Wild-2 may have repeatedly come inside Jupiter’s orbit and back out again.  But then how did the comet escape complete break-up by a collision in the planetary shooting gallery, or avoid getting ejected out of the solar system entirely, during one of those excursions?
    The nightmares may not go away entirely in January 2006.  If history is any guide, the comet dust samples will answer some questions but raise many others.  For a sweet dream, however, imagine yourself standing on the surface of the three-mile wide comet.  The gravity is so low, you could jump and launch yourself into orbit.  Cool.

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Categories: Solar System

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