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.


The Earth’s improbable atmosphere

The air we breathe is an unlikely cocktail of reactive oxidizing and reducing gases. Just how unlikely is demonstrated by the coexistence of oxygen, at 21 per cent, and methane at a fairly constant level of 1.7ppm. Methane reacts with oxygen in sunlight to form carbon dioxide and water. To remain at its constant level, it must be replenished (by methane-producing organisms) at a rate of some 500 million tons a year.

If life on Earth were suddenly to cease, all the hundred-plus elements that make up the surface, oceans and atmosphere would react together until no more reactions were possible, and a state close to chemical equilibrium was reached. The planet would become a hot, waterless, and inhospitable place.