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A handy way to detect life

Photonics Spectra
Jun 2009
Hank Hogan, hank@hankhogan.com

BALTIMORE – The next time the soil of an alien world is examined for life, scientists may already have a good idea where to dig. Thanks to a new technique, they might be able to remotely detect life from the comfort of Earth, pinpointing promising locations for future landers to target.

The scheme works because, when light bounces off living organisms, the scattered reflections are preferentially circularly polarized. That signature makes it possible to spot organic clusters via a polarization measurement. The technique could conceivably be used to determine whether planets outside our solar system harbor life.

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Measuring the chirality of circularly polarized light reflected off distant moons and planets – such as Jupiter’s moon Europa – might help astronomers find extraterrestrial life without sending spacecraft.

“If an extrasolar planet had extensive oceans filled with photosynthetic microbes, even if different from terrestrial, we might be able to detect that,” said William B. Sparks, an astronomer with the Baltimore-based Space Telescope Science Institute. He added, however, that it’s a long way from a demonstration in a lab to this ultimate goal.

Sparks led a team of researchers that reported on the technique in the Proceedings of the National Academy of Sciences in April. The measurements were done at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md.

Chirality

Many molecules, organic and otherwise, exhibit chirality, or handedness. This leads to circular polarization of scattered light. Inorganic materials are a jumble of left- or right-handedness, and the resulting aggregate circular polarization is almost nonexistent.

That’s not the case for living things, however. Life is self-replicating, Sparks said. That leads to selection pressure for either left- or right-handed versions of a particular biomolecule. All known life on Earth, for example, uses only left-handed amino acids in proteins and right-handed sugars in nucleic acids.

A second important consideration that makes scattered light an attractive probe is that any life likely will try to exploit light from the local sun because capturing that energy provides an advantage. That propensity dictates certain optical characteristics of alien life, explained NIST physicist and research team member Thomas A. Germer. “It’s going to be trying to harvest light near the peak wavelength of the star, and all that light-absorbing pigment is going to be near the surface of the planet.”

To evaluate the potential of the technique, the researchers measured the polarization of light scattered by photosynthetic marine cyanobacteria using a polarimeter from Hinds Instruments Inc. of Hillsboro, Ore. They showed that the circular polarization arose from the optical properties of the photosynthetic molecules, demonstrating that they could detect life at a distance.

Life: past and present

More work remains to be done before the technique can be deployed. For one thing, the small amount of circularly polarized light arising from biomolecules might be swamped by light reflected from rocks, waves, clouds or other objects. The scientists plan to test that in the next phase of the research.

Assuming success, the technique could be used to examine the ocean of the Jovian moon Europa and other places in the solar system that might harbor life. Doing the same for extrasolar planets would be a challenge because of the need to separate the light reflected off the planet from that of the nearby – and much brighter – star.

The technique also could be used to indicate where life had once existed, Sparks said. “The timescale for chirality to disappear can sometimes be very long.”


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