Editors note: This story is part of a Feature "The Color of Plants on Other Worlds" from the April 2008 issue of Scientific American.

Aside from colors reflected by plants, these other features could be signs of life:

Oxygen (O₂) plus water (H₂O). Even on a lifeless world, light from the parent star naturally produces a small amount of oxygen in a planet’s atmosphere by splitting water vapor. But the gas is quickly rained out, as well as consumed through oxidation of rocks and volcanic gases. Therefore, if a planet with liquid water has abundant oxygen, some additional source must be producing the gas. Oxygenic photosynthesis is the leading candidate.

Ozone (O₃). In Earth’s stratosphere, radiation splits apart oxygen, which then recombines to form ozone. Together with liquid water, ozone is a strong biosignature. Whereas oxygen can be detected at visible wavelengths, ozone can be detected at infrared wavelengths, which is easier for some telescopes.

Methane (CH₄) plus oxygen or seasonal cycles. Oxygen and methane are an awkward chemical combination that is hard to achieve without photosynthesis. A seasonal cycle of rising and falling methane concentrations is also a good sign of life. On a dead planet, methane levels are fairly constant, declining slightly over the long run as starlight splits the molecules.

Methyl chloride (CH₃Cl). On Earth this gas results from the burning of vegetation (mainly forest fires) and from the action of sunlight on plankton and seawater chlorine. Oxidation destroys it. But an M star’s relatively weak radiation might allow the gas to build up to detectable amounts.

Nitrous oxide (N₂O). When plant matter decays, it releases nitrogen in the form of nitrous oxide. Abiotic sources of this gas, such as lightning, are negligible.

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