One of the fundamental properties of light is its polarization which refers to the spatial orientation of the electric and magnetic fields constituting a light wave. There are many fascinating facts about polarized light which are of both academic and applied interest, but perhaps the most intriguing one is the ability of chiral or handed organic compounds to change the plane of polarization of circularly polarized light. This has proven to be an invaluable tool for chemists in detecting and assigning the structures of molecules, especially biological molecules like sugars and amino acids which tend to exist in only form (left or right handed) in nature.
This fact has now been put to good use by a team of Chilean, Spanish and British astronomers who, in a paper in this week's Nature, have come up with a novel way to detect biosignatures of life on planets. They demonstrate their method by detecting the polarization signatures of water, clouds and vegetation in Earthshine. Earthshine refers to the sunlight reflected by the earth which has been reflected back by the moon towards the earth. It turns out that earthshine contains polarized light whose polarization has been shaped by the earth's atmosphere and vegetation and their constituent molecules. Specific molecules polarize specific wavelengths of light so scanning the whole range of wavelengths is essential. Crucially, the presence of vegetation is manifested in the polarization of the light by chlorophyll. Chlorophyll is special because it absorbs light up to about 700 nm. Beyond 700 nm (in the infrared region) it sharply reflects it, leading to a spike in the spectrum known as the "red edge". That is why plants glow strongly in infrared light. The red edge is a major part of the earth's reflected light as detected in outer space. It's a remarkable phenomenon which could be put to good use to detect similar life-enabling pigments on other planets.
The team used the Very Large Telescope in Chile to analyze reflected earthshine during two months, April and June. The two different times were necessary to make observations of two different viewing faces which the earth presented to the moon; one face was predominantly covered by land and vegetation and the other mainly by water. The earthshine arising from the two faces would be characterized by different spectroscopic signatures, one belonging mainly to vegetation and the other to water. At each wavelength of light they observed peaks and discontinuities corresponding to oxygen, water vapor and chlorophyll. They then compared these observations to calculations from a model that contains as parameters varying proportions of vegetation and ocean. There is some uncertainty because of assumptions about cloud structure but overall there is good agreement. Remarkably, the vegetation is sensitive to even a 10% difference in vegetation.
The technique is fascinating and promises to be useful in being able to make gross detections of water, oxygen and vegetation on other earth-like planets, all of which are strong indicators of life. Yet it is clear that earthshine presents a relatively simple test case, mainly because of the proximity of the moon which is the source of the polarized light. By astronomical distances the moon is right next to the earth and there's very little in the intervening medium by way of dust, ice and other celestial bodies. The situation is going to be quite different for detecting polarized reflections from planets that are millions of light years away. A few thoughts and questions:
1. The authors note that the lunar surface partially depolarizes the light. Wouldn't this happen much more with light coming from very far that has hit multiple potentially depolarizing surfaces? Light could also be depolarized by dense atmospheres or by interstellar media like dust grains and ice grains. More interestingly, the polarization could also be reversed or affected by chiral compounds in outer space.
2. A related question: how intense does the light have to be when it reaches the detectors? Presumably light from worlds that are billions of light years away is going to strongly interact with surfaces and interstellar media and lose most of its intensity.
3. It's clear that chlorophyll is responsible for the signature of vegetation. Alien plants may not necessarily utilize chlorophyll as the light harvesting pigment, in fact they may well be equipped to use alternative wavelengths. There could also be life not dependent on sunlight. How we will be able to interpret signatures arising from other unknown pigments and constituents of life is an open question.
4. It is likely that advanced civilizations have discovered this method of detecting life. Could they be deliberately broadcasting polarized light to signal their presence? In the spirit of a past post, could they do this with specific molecules like amino acids, isotopically labeled molecules or stereoisomers? How sensitive is the polarization to molecular concentration? Any of these compounds would strongly suggest the presence of intelligent life which has developed the technology for the synthesis and purification of organic molecules.
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One of the fundamental properties of light is its polarization which refers to the spatial orientation of the electric and magnetic fields constituting a light wave. There are many fascinating facts about polarized light which are of both academic and applied interest, but perhaps the most intriguing one is the ability of chiral or handed organic compounds to change the plane of polarization of circularly polarized light. This has proven to be an invaluable tool for chemists in detecting and assigning the structures of molecules, especially biological molecules like sugars and amino acids which tend to exist in only form (left or right handed) in nature.
A cool paper and idea.
ReplyDeleteAs far as detecting life on exoplanets, isn't the most obvious problem that the light of the star is going to completely drown out the planetshine?
One of the coolest things I was exposed to at the Harvard Origins project was the work of David Charbonneau. One way to detect exoplanets is to use a telescope to look at a star and observe how its light dims cyclically (presumably, from a planet passing in transit). Charbonneau went one step further and used these events to gather spectra of the distant planets' atmospheres. Really cool.
ReplyDeleteYes, I am not sure how they would get around it; perhaps the polarization signatures somehow stand out from the rest of the noise. In fact the commentary in Nature proposes placing a telescope on the moon to fine-tune results, suggesting that even the earth's atmosphere poses some problems. I am aware of the method for detecting periodically dimmed starlight but will look up Charbonneau. I think Sara Seager at MIT has done similar studies.
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