Crisis in Comet Formation Theories
Results from the Stardust mission last week (12/15/2006) are causing quite a stir. Detailed analysis of comet dust particles from Comet Wild 2, published in Science Dec 15, reveal the wrong stuff. Scientists found olivine, pyroxene and osbornite – minerals said to form at high temperatures – instead of the cold volatiles expected for an object from the outer solar system. According to an article on EurekAlert, the head of the Lawrence Livermore Stardust team, John Bradley, said that osbornite only forms at 3,000 degrees Kelvin. “If we found it in the comet, then how the heck did it get out there?” he asked.
It has been textbook orthodoxy for decades that comets contain pristine, unaltered material from the cold outer disk of the solar nebula, and have been preserved in deep freeze since before the planets formed. Yet “virtually no grains that pre-date the Sun were seen” in the dust, said Joanne Baker in the introductory article to the cover story of Science.1 Michael F. A’Hearn (U of Maryland), principal investigator of last year’s Deep Impact mission, wrote a Perspectives article in the Science cover story.2 He said that “These missions [Stardust and Deep Impact], coupled with recent dynamical studies, have caused a major rethinking of the origin of comets.” Comets and their interiors can no longer be neatly arranged into groups; they have been mixed up, with stark differences between the comets that have so far been visited by spacecraft. He said, “we now have clear evidence that this mixing must be taken into account in any theory of our solar system.” With no indication that he thinks we are near to a solution, he added, “Stardust has certainly brought us plenty of food for thought.”
Don Burnett (Caltech) summarized the findings in his article in Science,3 noting that the majority of the material is identical to rocks in the inner solar system.
“The most abundant minerals are the crystalline silicate minerals, olivine and pyroxene, along with troilite (FeS). These are very stable phases, common in planetary materials; however, finding them here is somewhat surprising because many expected that cometary material would be similar to interstellar material, in which most silicates are believed to be amorphous.
The expected cometary amorphous material was “rare or nonexistent” in the samples analyzed so far. The abstract to the lead paper in the issue4 by principal investigator Donald Brownlee (U of Washington) et al states,
The Stardust spacecraft collected thousands of particles from comet 81P/Wild 2 and returned them to Earth for laboratory study. The preliminary examination of these samples shows that the nonvolatile portion of the comet is an unequilibrated assortment of materials that have both presolar and solar system origin. The comet contains an abundance of silicate grains that are much larger than predictions of interstellar grain models, and many of these are high-temperature minerals that appear to have formed in the inner regions of the solar nebula. Their presence in a comet proves that the formation of the solar system included mixing on the grandest scales.
The paper on organics5 mentioned that in addition to methylamine and ethylamine, possible glycine was also detected. To explain the mixture of high-temperature silicates with organics and volatiles, the researchers are trying to imagine ways some of the minerals formed near the hot sun then got transported radially outward for tens of astronomical units. There has been a hypothesis floating around that bipolar outflows from a young star could launch inner particles to the outer regions of the disk. This “X-wind” hypothesis would have had to transport particles as large as 20 microns out to beyond the orbit of Neptune, and also explain how the material got incorporated into the comet during its formation (11/20/2002, 10/11/2002), raising as many questions as it answers (03/14/2006). “It’s shaking up our view of the solar system condensation process,” another of the Livermore Stardust researchers said. “It’s been pretty intense. It opens up a whole bunch of new questions.”
Popular articles on these results can be found online: a press release from the European Synchrotron Radiation Facility that helped analyze some of the comet dust, National Geographic, BBC News, and Space.com. Most of these emphasize the teaser about organics and life which was only briefly mentioned in one of the scientific papers (see 12/15/2006). The Planetary Society’s report quotes Don Brownlee as saying, “Truthfully, we didn’t expect to find anything from the inner solar system among the Wild 2 samples.” A press release the from Carnegie Institution mentioned another puzzle: if carbon compounds from the inner solar system were transported out to where the comets formed, “How could such fragile material have survived capture at 6 km/sec collision velocity?” News@Nature said the findings bring confusion about Solar System modelling. Science Now was more dramatic, claiming this “hot, crazy start to the solar system” has “thrown the conventional solar system formation hypothesis on its head.”
Almost all the ten Science articles mentioned that the evidence was inescapable for an inner solar system origin of some of the ingredients. See also our 03/14/2006 story when this surprise was first mentioned last March.
1Joanne Baker, “Look into the Seeds of Time,” Science 15 December 2006: Vol. 314. no. 5806, p. 1707, DOI: 10.1126/science.314.5806.1707.
2Michael F. A’Hearn, “Whence Comets?”, Science, 15 December 2006: Vol. 314. no. 5806, pp. 1708 – 1709, DOI: 10.1126/science.1137083.
3Don S. Burnett, “NASA Returns Rocks from a Comet,” Science, 15 December 2006: Vol. 314. no. 5806, pp. 1709 – 1710, DOI: 10.1126/science.1137084.
4Brownlee et al, “Comet 81P/Wild 2 Under a Microscope,” Science, 15 December 2006: Vol. 314. no. 5806, pp. 1711 – 1716, DOI: 10.1126/science.1135840.
5Sandford et al, “Organics Captured from Comet 81P/Wild 2 by the Stardust Spacecraft,” Science, 15 December 2006: Vol. 314. no. 5806, pp. 1720 – 1724, DOI: 10.1126/science.1135841.
We love planetary scientists, but sometimes they deserve to get rattled for thinking inside the box. There’s a creation scientist who thought outside the box and predicted things that have now been confirmed from Stardust and Deep Impact. Read Walt Brown’s theory at CreationScience.com. It includes a detailed explanation of why the other theories don’t work. Dr. Brown has a PhD from MIT and taught mathematics and physics at the college level. His ideas will be too radical for some, but isn’t science supposed to be about making predictions that can be falsified? Look at the scoreboard and watch the secularists scratching their heads on this one before ruling out the mavericks. His article is also beneficial for its review of physics and celestial mechanics, and shares many interesting little-known facts about comets often glossed over in the textbooks.
For millennia, comets were seen as bad omens, striking fear and worry into observers. It’s amazing how history repeats itself in unusual ways.