Recognizing a living planet

Imagine yourself aboard a space ship beyond the Solar System but still able to see its planets. The ship has a wide-range spectrometer, with which you can discover the gaseous composition of each planet’s atmosphere in turn.

The bar charts illustrate the read-out of the spectrometer for three of the planets: Mars, Venus, and (lowest set) Earth. The ship’s on-board computer has also calculated the likely read-out for Earth, were life absent. For convenience, the gases are displayed according to their chemical behaviour, in three separate classes: oxidizing (oxygen and carbon dioxide), reducing methane and hydrogen), and inert (nitrogen and argon).

The information read-out provides conclusive evidence of life on Earth, and equally conclusive evidence of its absence on Mars and Venus. On Earth, the atmosphere is in a state of great chemical disequilibrium, with reducing and oxidizing gases both present in a highly reactive mixture. Mars and Venus, by contrast, have atmospheres close to the equilibrium state, with oxidizing and inert gases only. In fact, no other planet in the Solar System has oxidizing and reducing gases mixed as on Earth. Nor would the Earth itself if life were absent. Instead it would, as the read-outs show, have an atmosphere with a composition midway between those of Mars and Venus, close to chemical equilibrium, and dominated by the exhaust gas carbon dioxide.