March 29, 2010 | David F. Coppedge

Comets Are Cracking Up

An amateur astronomer observed a comet splitting in two, reported (PhysOrg), but it’s not just the comets that are breaking up.  Theories about them have undergone a revolution at revelations they are not all they were cracked up to be.  They used to be pristine remnants of the formation of the solar system.  Analysis of actual cometary material has changed all that.
    Last month, a headline heralded the first dating of comet stuff (see Science Daily).  Beneath the surface, though, the article recognized a paradigm shift:

The NASA Stardust mission to comet Wild 2, which launched in 1999, was designed around the premise that comets preserve pristine remnants of materials that helped form the solar system.  In 2006, Stardust returned with the first samples from a comet.
    Though the mission was expected to provide a unique glimpse into the early solar system by returning a mix of solar system condensates, amorphous grains from the interstellar medium and true stardust (crystalline grains originating in distant stars), the initial results painted a different picture.  Instead, the comet materials consisted of high-temperature materials including calcium-aluminum rich inclusions (CAIs), the oldest objects formed in the solar nebula.  These objects form in the inner regions of the solar nebula and are common in meteorites.
    The presence of CAIs in comet Wild 2 indicates that the formation of the solar system included mixing over radial distances much greater than has been recognized by scientists in the past.

The date and location of CAIs, though, is dependent on theories of the formation of the solar system.  Something is clearly wrong.  To salvage the theory about where and when high-temperature materials form, the scientists now have to invoke “radial transport of material over large distances in the early solar nebula.”  Is that even probable?  Could it be just a theory-rescuing device at work?  A scientist at Lawrence Livermore recognized the problem: “These findings also raise key questions regarding the timescale of the formation of comets and the relationship between Wild 2 and other primitive solar nebula objects.”  Those are quotes from the paper in Science Express posted Feb 25, 2010.1  The paper claimed a date of 1.7 million years for the particle studied.  Is that kind of precision plausible, given the following explanation?

This observation in turn requires transport of inner solar system material to the outer reaches of the solar system at distances exceeding 30 AU and incorporation into cometary bodies over an extended period of at least several million years.  Outward transport of Coki [the name they gave to the particle] to the Kuiper belt must have occurred as late as (if not later than) the time over which chondritic meteorites and the oldest differentiated meteorites formed [see (30)].  The age constraint derived from Coki indicates that the transport mechanisms which supplied high-temperature inner solar system material to the outer reaches of the solar nebula, whether by lofting above the disk in an X-wind model (31) or via mixing processes within the solar nebula [e.g., (32, 33)], operated over a >2 million year timescale as solids settled to the midplane and the disk evolved.

In other words, this particle had to be cooked in the inner solar system, then fly outward to 30 times the earth-sun distance to get to the assumed comet-forming region.  Giving these implausible stories names like “transport mechanisms” and “mixing processes” seems a cover for ignorance. recognized the ignorance: “How the material in Coki got transported to the outer solar system, whether by lofting above the solar system disk or mixing processes within it, isn’t yet known, but it likely occurred during a time period of more than two million years, the researchers say.”  If one cannot describe how it happened, it seems presumptuous to claim to know when it happened.
    Meanwhile, we do know from direct observation that comets don’t last forever. and National Geographic reported another SOHO observation of a sun-grazing comet making a death plunge into the sun, where it was seen to vaporize and vanish.  That’s the thousandth one.  Before SOHO, the article said, only 16 sun-grazers were known.  New Scientist reported that “Three years ago, the comet 17P/Holmes exploded with a blast comparable to a small nuclear bomb.”  And National Geographic reported in January a “strange” comet that may have formed from the collision of asteroids.  These observations, combined with the breakup observed by the amateur astronomer, show destructive processes at work – not comet formation processes.  The comets we see are cracking, vaporizing, exploding, and fizzling out.  They form only in theory.
    National Geographic fell back on the old paradigm: “It’s believed most comets come from the cold, distant reaches of the solar system and travel on long, elliptical orbits, which keep the icy bodies far from the sun most of the time.”  That must be – or else they would be young.  The collision was not observed.  At the tail end of the article, scientists admitted they didn’t even know what an asteroid collision would look like.  Jim Scotti [U of Arizona] said, “We have some ideas, but I’m not sure anyone has really sat down and modeled the size and velocity of the debris, or where all that debris goes and how long it would remain potentially observable.”  It seems premature, therefore, to say that a collision formed this particular strange comet.
    Another detail came out at the end of the article: scientists were puzzled this comet had any ice left after billions of years:

For now, scientists can only wait and watch to see if P/2010 A2 (LINEAR) slowly dissipates, like debris from an explosion, or continues to act like a comet—which would pose a new round of puzzling questions.
    A rare handful of comet-like bodies are known to orbit in the main asteroid belt.  But if P/2010 A2 (LINEAR) is actually a comet, how did it conserve its water ice so close to the sun for some 4.5 billion years—roughly the age of the solar system—only to begin releasing gases now due to some unseen event?
    “That’s a long time to bake an object,” Scotti said.
    “It’s hard to imagine how an object would maintain a reservoir of volatiles that it could use to suddenly start producing a tail.  But you know, stranger things have happened.

Comet science marches on.  Science Daily reminded us that the same Stardust spacecraft that collected Coki and her friends is honing in on a rendezvous with Tempel 1, the comet that Deep Impact blasted with a projectile five years ago.  Also, the WISE infrared orbiting telescope got a nice infrared image of a comet, reported National Geographic.  At least now we know not to worry about comets as bad omens (unless they impact the earth).

1.  Matzel et al, “Constraints on the Formation Age of Cometary Material from the NASA Stardust Mission,” Science Express, Published Online February 25, 2010, Science DOI: 10.1126/science.1184741.

Kids!  Now you can give a scientific excuse to dad.  Next time he asks you who broke the window with a baseball, you can just shrug your shoulders and say “some unseen event” did it.  Tell him you don’t know how long it was “potentially observable.”  If he persists, just say, “It’s hard to imagine” how an object could do that, but “stranger things have happened.”  This has the all the authority of the Lunar and Planetary Lab of the University of Arizona behind it.  Try it!  For extra fun, claim that the ball came from the Kuiper Belt millions of years ago!
    In most careers, if you were as wrong as astronomers have been about comets, you would lose your job.  What if astronomers’ jobs depended on the correctness of their predictions when observations come in?  It should be noted that puzzlement is a function of expectations.  Comets themselves are indifferent to what humans think about them.  They just exist and go about their orbital business.  The scientists were puzzled because they expected to see cold, pristine material from the outer reaches of the solar system.  They taught that idea for years.  They were wrong.  This shows the value of better observations.  They can put the lie to evolutionary theories.  More power to Stardust, Deep Impact and other missions that gather data to replace speculation.  It’s a little too late now for the false prophets to make up stories after the fact and tell the public, who pays much of their salaries through federal grants and tuition, that “stranger things have happened.”

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Categories: Solar System

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