In search for life through the twists of light

Finding Earth-like planets is common place now. What about detecting life on them?

Two centuries ago a French engineer noticed something special about light from the sun. As it reflected from the window and passed through a crystal of calcium carbonate, depending on the angle at which the crystal was placed, the image it created grew stronger or weaker. Étienne-Louis Malus had discovered a phenomenon called polarisation of light. The simplest example of this can be seen in the above images whereremoval of certain polarised light increases the contrast with clouds.

Sunlight is unpolarised which means that the electromagnetic waves that make up sunlight are not restricted in their spatial orientation. But when this light interacts with biological molecules like sugars, amino acids or chlorophyll it changes its spatial orientation, and, more importantly, we are able to detect the change and measure it.

This week researchers using the Very Large Telescope in Chile used this characteristic of light to show the presence of water, clouds, and vegetation in Earthshine – the sunlight that’s been reflected off of Earth to the dark portion of the Moon’s face and then back to our planet – through a method dubbed spectropolarimetry. Michael Sterzik, an astronomer at the European Southern Observatory in Santiago, Chile, said that the state of polarisation contains a lot of information that hasn’t been used very often.

Comparing their measurements of Earthshine with models of how various land and sea surfaces reflect polarised light, the researchers could discern which part of our planet was covered with oceans and which with land mass. They also identified the biosignature of chlorophyll which showed up when land masses on Earth were illuminated.

The upshot is that it might be possible to use this technique to spot the presence of water and other biological molecules on the many Earth-like planets that have been discovered recently. The techniques currently available can only detect the presence of water and other simpler molecules which is not enough to ascertain the existence of life. The occurrence of biological molecules on the other hand increases the probability of finding life by many factors.

But as these planets are usually many light years away, the light received from them is very faint. Researchers will have to wait for the next generation of telescopes, such as the European Extremely Large Telescope planned for 2022, to gather the required data. But possibly, within a decade, the twists of light will help us seal the fate of life beyond our planet.

First published on Science Oxford Online.

Reference: Sterzik, M., Bagnulo, S., & Palle, E. (2012). Biosignatures as revealed by spectropolarimetry of Earthshine Nature, 483 (7387), 64-66 DOI: 10.1038/nature10778


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