Thursday, May 05, 2005

The color of life

Astrobiologists would like to find life on other planets even if it's impractical to actually go there. At spaceref.com , Seager et al. suggest that a planet with photosynthetic organisms should show a spectral abnormality. That is, the "plants" are harvesting lots of solar energy in some wavelengths, and rejecting other wavelenghths, which creates a distortion in the spectrum of light returning from the surface.

In the case of Earth's chlorophyll , people on Earth see lots of reflected green; but the diagnostic signal from outer space would actually be a sharp "edge" in the red wavelengths:

CG5D

Earth plants use UV and bluish light for photosynthesis, and chlorophyll also absorbs far-red light. The high reflectance in the rest of the visible spectrum is thought to be an adaptation to prevent overheating. The result in this case is the sharp "red edge" between reflectance and strong absorption between 750 and 700 nm.

So my first thought was that the position of such a spectral edge is going to depend quite a lot on the exact chemistry of the chlorophyll equivalent; which in turn would probably have been fine-tuned via evolution to match whatever star's spectral output. Well, they're way ahead of me. I didn't realize this but people have been giving extraterrestrial photosynthesis quite a lot of thought, and the spectrum harvested by chlorophyll would also be useful for other earth-like planets. The main exception is a cool sun (reddish?) in which case you'd need a two-photon capture photosynthesis... you're best off reading the web page attachments! In any case, it looks like there are some universal constraints keeping the visible light reflectance high and therefore yielding a red edge.

To really nail it, you have to look for "seasonal" changes in this signature.

My own opinion is that you'd expect "chemical-eater" strategies to be easier, so that photosynthesis might only be in fewer places. However, I guarantee you that if there is enough plant life to sense remotely, there will for sure be plant-eaters, and carnivores too.

UPDATE: An older reference to this concept is here.

UPDATE, June'05: The "red edge" is expected to be harder to see on a planet dominated by waterborne slimes such as algae etc. The need to reflect thermal energy would be a lot less for water dwellers.

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