Crystals and Comet McNaught
Published on Jan 9, 2008 at 1:59 pm.
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Filed under comets, conference blogging.
One year ago, C/2006 P1 (Comet McNaught) was making all sorts of news as it put on a wonderful naked eye performance. At the time, though, I was at the AAS Meeting in Seattle, where it was rainy the whole time (except for when it snowed!). I finally got to see the comet, though, from the window of the airplane on the plane ride home to Texas. That is where I took the photo below. That was the only time I saw the comet, though, as it was cloudy back in Texas from the time I arrived until the comet was no longer visible.
Earlier today, I was walking through the poster sessions at this year’s 211th Meeting of the American Astronomical Society, and I ran across an interesting bit of work by Michael Kelley, of the University of Central Florida. He was presenting spectra of Comet McNaught taken using the Spitzer Space Telescope.
Given the thrill of finally seeing the comet through the window of the aircraft, I was naturally interested when I saw some research being presented on C/2006 P1. And, it was quite interesting, too.
He presented data from the spectra of the dust tail of the comet taken some months after it made all the fuss in the news (Note that while the amateurs were taking photos at its brightest, professional astronomers don’t just go for the pretty pictures. There is still a lot of data that can, and should, be collected at times other than when the comet looks coolest to the eye!). He also had some comparison data there for other comets.
What was really interesting to me was that C/2006 P1 showed spectra of amorphous carbon and silicon rather than crystalline forms, such as olivine. Now, this is very important, because analysis of the dust kicked out by the Deep Impact probe when it hit Comet 9P/Tempel 1 shows a lot of olivine. And, the spectrum of Comet C/1995 Hale-Bopp shows a lot of crystalline olivine, too. In fact, these comets have most of their carbon and silicon in that form. Nearly 50 of the carbon and silicon (by mass) of Comet Hale-Bopp is in olivine, but less than 7% of Comet McNaught’s silicon and carbon is in that form. Note: much, if not most, of a comet is still frozen gas and ices, but here I a talking mainly about the silicon and carbon components. So, why the difference in these comets?
Well, Kelley speculated that what may be at work is one of two scenarios. First, he notes that Hale-Bopp and Tempel 1 are both periodic comets. Comet Hale-Bopp has a period of about 2300 years, and Tempel 1 has a period of about 6 years. Both have been by the Sun numerous times. This latest Comet McNaught, though, was on a hyperbolic orbit. That means that it may be the first time that it had dived in near the Sun from the Oort Cloud, and it likely will never come back. So, he speculates that perhaps, over eons in the Oort Cloud, the olivine in Comet McNaught was degraded by cosmic ray damage. That would have happened with the other comets, too, of course. However, the cosmic ray damage may be confined to the outer layers of the comet. So, over time, sublimation and the associated shedding of material when a comet passes close enough to the Sun removes some of that degraded material. Both of the periodic comets he mentioned have been by enough times for the degraded olivine to be lost. But, this is the first time that Comet McNaught has been by, so the outers layer may still be composed more of amorphous carbon and silicon rather than olivine crystals. If the comet were to come by several times more, then the situation may be different, but this is the only time that it is passing by close to the Sun.
Another possibility that he mentioned was that perhaps there is something different about the region of the Solar System where Comet McNaught formed rather than where the other comets formed. Comet McNaught is an Oort Cloud comet, and the others are likely Kuiper Belt comets. The two likely have different origins, but that may be the subject of a later post.
Whichever scenario results in the differences in the comet compositions will require more study. But, it is interesting to see the work in progress.
-Astroprof
