Many scientists are searching for signs of extraterrestrial life. Some of the methods they use include hunting for signs of technology (such as radio waves), and for environmental conditions thought to be required for life (such as water).
If one is searching for biological signatures of life, the signature should be unique for life, incapable of production by abiotic processes. Once such a signature is found, further studies can be prioritized for that planet.
William Sparks (Space Telescope Science Institute, Maryland) and coworkers have validated a new detection method for identifying life on other worlds. Their spectroscopy technique is designed to hunt for signatures of photosynthesis.
Why search for signs of photosynthesis?
Photosynthesis, the utilization of carbon dioxide and sunlight for the production of sugar molecules, arose early in our planet's history. It provided a huge evolutionary advantage for early microbes.
It is therefore reasonable to speculate that photosynthesis has originated on other worlds as well, at least on worlds that are capable of supporting life. Detecting photosynthesis should be a priority for detecting signs of extraterrestrial life.
How can photosynthesis be detected?
How can one detect photosynthesis from very large distances? It turns out that photosynthesis generates an obervable optical signature.
The light absorbed during photosynthesis exhibits a feature known as circular dichroism. This refers to the polarization of the light.
Natural light is generally unpolarized. This means that propagation of the light waves is equally probable in every direction.
In contrast, light is polarized when it travels in a specific direction. Light is circularly polarized when it is comprised of two waves, of equal wave oscillation magnitude, in which one wave is one-fourth of one wavelength ahead of the other wave.
A material exhibits circular dichroism if it inherently absorbs light of a given polarization more strongly than light of another polarization. Certain proteins in photosynthetic organisms exhibit this property.
Hence, if light reflected from a far-away world exhibits circular dichroism, it is reasonable to speculate that photosynthesis is going on. By inference, life is present there.
Implementation for detecting extraterrestrial life.
A problem for detecting circular dichroism on other worlds is that light tends to scatter. Consequently, light will no longer be polarized.
The challenge is therefore to determine if a balance can be elucidated between the expected amount of scattering versus the amount of circular polarization of light from photosynthetic organisms. The scientists set out to determine this balance in cultured microorganisms.
Detecting photosynthesis in bacteria.
The scientists used a polarimeter to measure scattered light reflection and transmission polarization spectra from microbes cultured in a small glass dish. Their goal was to see if they could be related to the absorption properties of the microorganisms.
For the bacteria the scientists studied, they found that the sign of the circular dichroism reverses at the maximum absorption of the absorbing photosynthetic proteins. No correlation was observed between any detectable circular polarization and absorbance for abiotic materials.
Thus, in principle, if the scientists train a specialized telescope on a distant planet, and the sign of the circular dichroism changes at a light wavelength expected to be utilized by photosynthetic organisms, it's a good bet that there's life on this planet. Abiotic processes will not give the same effect.
Is this polarization detectable with telescopes?
The scientists have shown that photosynthesis is detectable by correlating the circular dichroism of light with its absorbance. Can telescopes actually detect circular dichroism to a degree relevant for detecting extraterrestrial life?
The scientists note that astronomical polarimeters can detect light polarization as small one millionth of a wavelength. Since scattered light polarization from microorganisms is typically 100 to 1,000 times stronger than this, it should be detectable with telescopes, even with a large amount of light scattering.
Utilizing this protocol.
Current technological capabilites limit the use of this protocol to within our solar system, such as on Mars (no photosynthesis detected yet, as expected) and on comets. It will be far more challenging to detect photosynthesis on far-away planets.
This is due to difficulties in separating light reflected from the planet (weak) from light emitted from the star (much stronger). Incidentally, this is why it's currently so difficult to find small extrasolar planets at all.
Much technical development (and tax dollar infusion) needs to take place before scientists can put this protocol to practical use for detecting life on far-away planets. However, with such development, this discovery will possibly be a very useful tool for detecting signs of extraterrestrial life.
for more information:
Sparks, W. B., Hough, J., Germer, T. A., Chen, F., DasSarma, S., DasSarma, P., Robb, F. T., Manset, N., Kolokolova, L., Reid, N., Macchetto, F. D., & Martin, W. (2009). Detection of circular polarization in light scattered from photosynthetic microbes Proceedings of the National Academy of Sciences, 106 (19), 7816-7821 DOI: 10.1073/pnas.0810215106