Wednesday, July 14, 2010

Clockwork Mass Extinctions, Cosmic Rays, and Cycles

"Death Star Off the Hook for Mass Extinctions"
Wired Science (July 13, 2010)

"A massive extinction like the one that claimed the dinosaurs has hit the Earth like clockwork every 27 million years, a new fossil analysis confirms. But the study claims to rule out one controversial explanation: a dark stellar companion called Nemesis that sends a regular rain of deadly comets toward Earth.

" 'The main astronomical ideas you can come up with that could cause something like this just don't work,' said physicist Adrian Melott of the University of Kansas, a co-author of the new study.

"Nemesis was first suggested in 1984 as a way to explain an alarmingly regular series of extinctions in the marine fossil record, which was discovered by paleontologists David Raup and Jack Sepkoski. In light of the suggestion in 1980 that the dinosaurs were killed by a catastrophic impact, an invisible cosmic sniper lobbing comets at the inner solar system seemed like a plausible culprit...."

Judging by the Wired Science article, I think a reasonable summary of the scientific consensus about the 'Death Star' idea and what Adrian L. Melott and Richard K. Bambach published recently is: 'Maybe, then again maybe not.'

"...But now, Melott and co-author Richard Bambach of the National Museum of Natural History in Washington, D.C., say that's not actually what happens. The extinctions come almost exactly every 27 million years, they say, to a confidence interval of 99 percent.

" 'It's really too good, it's too sharp and fixed,' Melott said. 'It's like a clock.'..."

I think I see what Melott means by "like a clock," although this graph doesn't show quite the sort of unequivocal metronome beat that'd keep discussions of the new idea's validity from happening.

(from Adrian L. Melott and Richard K. Bambach, used w/o permission)
"Figure 2. A plot of extinction intensity (genera going extinct divided by total extant genera) at the substage level from the Sepkoski data. Extinction dates are taken as end dates of intervals as described in the text. Events meeting the criteria for 'mass extinctions' in the review of Bambach (2006) are circled. It is visually apparent that an large proportion of the circled points lie near the vertical lines which mark off a 27 My interval. In the text we show a probability of p≤0.01 that this can be a random alignment."

On the other hand, there are quite a few peaks at those multiples of 27,000,000 years. Can't say that I'm sorry that we'll have to wait a bit for the next one to come along.

If it turns out that Melott and Bambach found a recurring cycle - we've got time to figure out what's causing the mass extinctions, and maybe do something about it.

Cosmic Rays, Exploding Stars, and Mass Extinctions

The earlier paper that Melott co-authored, speculating that cosmic rays had something to do with the (semi?-) regular mass extinction that Earth's experienced, is intriguing. I've run into suggestions that there's something that our Solar system runs into periodically, during its orbit around the Milky Way Galaxy, that makes life on our Earth a whole lot more exciting.

I think there's something to that speculation. Right now, we're sailing through one of those bright spiral arms of the Milky Way Galaxy. That makes for a somewhat higher concentration of big, hot, bright stars: which add sparkle to our night sky; and then explode.

Generally, as will probably be the case with Betelgeuse, whatever's on Earth gets a sort of cosmic light show. If the exploding star is close enough, though, the jolt of radiation could mess up our climate.

Mass Extinctions, Cosmic Rays, and We Still don't Know Everything

We're far from having anything close to a solid idea of what's been happening, apparently. One of the more skeptical scientists, according to the Wired article, says that the timing of those mass extinction events isn't all that precise.

On the other hand, I wouldn't be at all surprised to learn that there are natural cycles longer than Earth's seasons and the sunspot cycle of our star. It's a big universe we live in - and it's been around for a long time.

At the end of this post, I put links two papers co-authored by Melott. They're a bit on the technical side, but give a level of detail that you won't find in the Wired Science article.

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