A circular argument?
[Here’s the pre-edited version of my latest Muse for Nature News. I have a book review of the stimulating reference 8 appearing shortly in Nature Physics.]

A new proposal for the signature of life on alien worlds resurrects an old idea linking light and life.

The difficulty of saying with scientific rigour what constitutes ‘life’ brings to mind Justice Potter Stewart’s famous description of pornography in 1964: it is hard to define, but we know it when we see it.

Yet do we really? Astrobiologists are haunted by the suspicion of terracentricity: we imagine that life on other planets will look like life here, and bias our searches for extraterrestrials accordingly.

Some of this community struggle nobly to free themselves from such prejudice, questioning for example the complacent conviction that life depends on water. Others seek very general signatures that make no assumptions about biochemical specifics.

One of the first such proposals, made by James Lovelock in the context of lander explorations of Mars [1], argued that sustained chemical disequilibrium in the planetary environment should be a telltale sign. This proposal had the virtue that it could rely on surveying a planet’s atmosphere alone. On Earth, the high proportion of oxygen, along with the presence of other trace gases, should be a giveaway, relying as it does on the operation of photosynthesis to prevent oxygen from getting locked into minerals.

One of the most ingenious ideas is that life affects the topography of a planet, for example by mediating chemical reactions that erode rock, by forming soil and protecting it from erosion, and by dictating climate. Geomorphologists William Dietrich and J. Taylor Perron have argued [2] that the types and distributions of landforms on Earth probably carry an imprint of life’s influence, and that a better understanding of their formation processes might lead to a clear distinction between the contours of planets with and without life.

In 1993, the idea of ‘remote sensing’ of the fingerprints of life from space was explored experimentally when Carl Sagan and coworkers used the data from a planned flyby of the Earth by NASA’s Galileo spacecraft to investigate our planet as though it were an unknown world [3]. From the chemistry of Earth’s atmosphere, images of the planetary surface, and detection of radio-wave emissions, the researchers inferred – somewhat reassuringly – that the presence of water-based life, probably of an intelligent kind, seems highly likely.

Now a new kind of fingerprint has been proposed by William Sparks of the Space Telescope Science Institute in Baltimore and his coworkers. They suggest we search for a characteristic signature of life in the light scattered from the surfaces of extrasolar planets [4]. They say that living organisms will be likely to make the light circularly polarized, meaning that the plane of its oscillating electromagnetic fields is not random but has a characteristic twist to it, either to the left or the right. This shouldn’t happen if the light simply bounces off inorganic surfaces.

Circular polarization is a feature of light scattered from organisms on Earth, where it has its origin in the ‘handedness’ or chirality of the building blocks of biological molecules. All natural proteins are made from amino acids that have a ‘left-handed’ molecular shape – the mirror-image right-handed amino acids can’t be used by cells to build proteins. And all nucleic acids use only ‘right-handed’ sugar molecules in their backbones. This molecular-scale twist means, for example, that light circularly polarized to the left or the right is absorbed to different degrees by the photosynthetic molecular apparatus of bacteria and plants, creating a net circular polarization in the scattered light.

It’s not obvious that this will be evident when the light is measured from afar, however, because the scattering process is complicated. Light rays that are scattered many times tend to have their polarization randomized, and scattering from reflective surfaces reverses the polarization. That’s why the researchers needed to check the light that bounces off cultures of marine photosynthetic bacteria, to make sure that the signature remains evident. It does – as indeed they found also for reflected light from a maple leaf. In contrast, light scattered from inorganic iron oxide shows no significant circular polarization.

Whether this method will work in real exoplanet searches is another matter. It depends on how much surface scattering would come from living organisms as opposed to inorganic substances, and on whether this light can be distinguished clearly enough from that of the parent star. Some of the planned astronomical instruments that might conduct planet searches could have sufficient resolution for this, however – probably not NASA’s Terrestrial Planet Finder (which is currently postponed indefinitely in any case), say Sparks and colleagues, but perhaps the ground-based European Extremely Large Telescope, which might begin operating around 2018. The Hubble Space Telescope has already revealed some of the chemical ingredients of an extrasolar planet [5,6]

But who says life must have a chiral molecular basis? do. ‘Homochirality’, say Sparks and colleagues, ‘is thought to be generic to all forms of biochemical life as a necessity for self-replication.’ This statement relies on the work of astrobiologist Radu Popa of Portland State University in Oregon [7]. But what Popa offers is a plausibility argument based on the idea that homochirality simplifies polymer structure in a way that promotes the efficiency of copying information. This doesn’t imply that homochirality is essential, but only that it might help. And we know that life does not always do things in the most efficient way.

However, the notion that sparks and colleagues are invoking actually goes back much further. An intimate association between life, chirality and light polarization was made in the nineteenth century, first by the French scientist Jean-Baptiste Biot and then by Louis Pasteur, who sought Biot’s advice on his seminal discovery of handedness in organic molecules. Biot, a pioneer in the study of optics and polarization, coined the term ‘optical activity’ to describe a substance that rotates the plane of polarized light, and it was no coincidence that this in itself suggested the operation of some vital, ‘active’ agent, rather than lifeless passive matter. Biot came to believe that optical activity was ‘the sole means in man’s possession of confronting the otherwise indefinable limit between life and nonlife on the molecular level’ [8]. Pasteur became a staunch advocate of this view, to the extent that (contrary to the popular view) he developed something of an anti-materialist, vitalist stance on what life is: he felt that optical rotation must result from ‘the play of vital forces’.

We now know, partly through Pasteur’s own work, that he and Biot were wrong. Sparks and colleagues are on sounder ground, but their idea could be seen to support the suspicion that life is everywhere built in our own image.

References

1. Lovelock, J. E. Nature 207, 568-570 (1965).
2. Dietrich, W. E. & Perron, J. T. Nature 439, 411-418 (2006).
3. Sagan, C. et al., Nature 365, 715-721 (1993).
4. Sparks, W. B. et al., Proc. Natl Acad. Sci. USA doi: 10.1073/pnas.0810215106 (2009).
5. Charbonneau, D. et al., Astrophys. J. 568, 377–384 (2002).
6. Vidal-Madjar, A. et al., Astrophys J. 604, L69–L72 )2004).
7. Popa, R. Between Necessity and Probability: Searching for the Definition and Origin of Life (Springer, Berlin, 2004).
8. Levitt, T. The Shadow of Enlightenment (Oxford University Press, Oxford, 2009).