Skeptophilia (skep-to-fil-i-a) (n.) - the love of logical thought, skepticism, and thinking critically. Being an exploration of the applications of skeptical thinking to the world at large, with periodic excursions into linguistics, music, politics, cryptozoology, and why people keep seeing the face of Jesus on grilled cheese sandwiches.

Friday, January 26, 2018

Life out of balance

As anyone who is a regular reader of Skeptophilia knows, I have a bit of an obsession with alien life.

My students know this, too, not because I bring it up in class, but because I have a poster from the Roswell UFO Museum, the Fox Mulder "I Want to Believe" poster, and a ceramic pot shaped like an alien head in my classroom.

I also have a lot of Bigfoot-related stuff, but we'll save that for another time.

Despite my obsession, I recognize the problem with the claim that extraterrestrial life has been detected, or (even more) that alien intelligence has made it here to Earth.  I've examined a lot of claims for both, and none of them have held up to scientific scrutiny.  It's too bad, really; I think one of the best moments I've ever seen in a science fiction movie was when Zefram Cochrane shakes hands with the Vulcan at the end of Star Trek: First Contact.

I've imagined myself many times in the position of being the first human to welcome a friendly alien intelligence to Earth.  The sad truth is that, without warp drive, the interstellar distances are simply too large.  Einstein's General Theory of Relativity, which sets the speed of light as the ultimate universal speed limit, has never been shown to have an exception -- nor that there's some kind of technological workaround.

Dilithium crystals notwithstanding.

So we're kind of stuck here, meaning that if we do detect life on other planets, it'll have to be remotely.  And now three scientists, from the University of Washington and the University of California-Riverside, have shown us how we might do that.

In a paper that came out in Science:Advances this week, titled, "Disequilibrium Biosignatures Over Earth History and Implications for Detecting Exoplanet Life," astronomers Joshua Krissansen-Totton, Stephanie Olson, and David C. Catling have developed a method of figuring out whether an exoplanet hosts life -- by simply analyzing the spectral lines from its atmosphere.

The idea here is that life keeps Earth's atmosphere out of balance -- more specifically, out of chemical equilibrium.  The most obvious example is the presence of diatomic oxygen, which is highly unstable (it is, unsurprisingly, a strong oxidizer).  If all life on Earth were to vanish, the amount of atmospheric oxygen would decline until it was very close to zero, as it interacted with (and was chemically bound up in) rocks and sediments.

As has been so often pointed out in seventh-grade life science classes, without photosynthesis, we'd be monumentally screwed.

So what Krissansen-Totton, Olson, and Catling did is to figure out how far out of chemical equilibrium an atmosphere would have to be to be a significant indicator for the presence of life.  The authors write:
Chemical disequilibrium in planetary atmospheres has been proposed as a generalized method for detecting life on exoplanets through remote spectroscopy.  Among solar system planets with substantial atmospheres, the modern Earth has the largest thermodynamic chemical disequilibrium due to the presence of life.  However, how this disequilibrium changed over time and, in particular, the biogenic disequilibria maintained in the anoxic Archean or less oxic Proterozoic eons are unknown.  We calculate the atmosphere-ocean disequilibrium in the Precambrian using conservative proxy- and model-based estimates of early atmospheric and oceanic compositions.  We omit crustal solids because subsurface composition is not detectable on exoplanets, unlike above-surface volatiles.  We find that (i) disequilibrium increased through time in step with the rise of oxygen; (ii) both the Proterozoic and Phanerozoic may have had remotely detectable biogenic disequilibria due to the coexistence of O2, N2, and liquid water; and (iii) the Archean had a biogenic disequilibrium caused by the coexistence of N2, CH4, CO2, and liquid water, which, for an exoplanet twin, may be remotely detectable.  On the basis of this disequilibrium, we argue that the simultaneous detection of abundant CH4 and COin a habitable exoplanet’s atmosphere is a potential biosignature.  Specifically, we show that methane mixing ratios greater than 10−3 are potentially biogenic, whereas those exceeding 10-2 are likely biogenic due to the difficulty in maintaining large abiotic methane fluxes to support high methane levels in anoxic atmospheres.  Biogenicity would be strengthened by the absence of abundant CO, which should not coexist in a biological scenario.
So that's what is known, in scientific circles, as "pretty freakin' cool."  The SETI Project and other programs designed to detect electromagnetic signals from ET are awesome, but the problem is, it tells us nothing about forms of life out there that for one reason or another might not use electromagnetic transmissions.  In fact, some scientists think that the era of using EM carriers for information might be fairly short -- for us here on Earth, it began about 120 years ago, and could well be drawing to a close already because of better technology.

The proposal by Krissansen-Totton et al. might give us a means of detecting life of all sorts -- not just intelligent life.  Which, as a biologist, I find tremendously exciting.  I mean, if a Vulcan ship isn't going to land in my back yard, I'll take what I can get, you know?

No comments:

Post a Comment