Water and ultraviolet light could create the gas without help from organisms
The first sign of extraterrestrial life probably won’t be a spaceship landing in a cornfield or a radio transmission from deep space. Most likely, the announcement will be encoded in the chemistry of a distant planet’s atmosphere. On Earth, oxygen betrays life’s presence. But oxygen in an exoplanet’s atmosphere wouldn’t necessarily point to alien shrubbery. Two researchers argue that an ocean-bearing planet zapped by its sun’s ultraviolet light could masquerade as a living world.
Plant photosynthesis produces Earth’s oxygen. But Robin Wordsworth and Raymond Pierrehumbert, geophysicists at the University of Chicago, wondered whether there was another way for a rocky planet to have an abundance of the gas in its atmosphere. The pair considered a deceptively simple scenario: a planet devoid of certain other gases, such as nitrogen. One of nitrogen’s roles on Earth is to help form a low-temperature layer in the atmosphere that water