In recent years the discovery of extremophiles, bacteria that live in nuclear reactors, hot ocean vents and other unlikely places, and of exoplanets has spurred new work and ideas about habitable planets. If Mars can have microfossils, why not Venus?
Moreover, Dr. Grinspoon said, new studies of Venus have led to the conclusion that the planet might have lost its oceans rather recently, only 700 million years ago, allowing plenty of time since the formation of the planet for life to have evolved and then escaped to the clouds.
What kind of life would that be? In 2004, Dirk Schulze-Makuch, an astronomer at the Technical University Berlin, in Germany, and his colleagues suggested that microbes floating in the clouds could be coated with a compound called cyclooctasulfur that would act as a sunscreen and convert ultraviolet light into visible wavelengths for photosynthesis.
Earlier this year, Dr. Seager and her colleagues expanded on this idea and sketched out an entire possible life cycle for such organisms. The microbes could inhabit droplets of sulfuric acid in the clouds, they proposed; as the droplets collided and merged, more and more microbes would be enclosed together, metabolize and divide.
Eventually the drops would grow too heavy and rain down from the clouds, but they would evaporate before hitting the ground, causing the microbes to dry out and go dormant.
Dr. Seager noted that Venus is known to have a layer of haze. “It’s very stable, and people don’t know what the particles there are, but they remain suspended for a very long time,” she said. “So I postulated that some of those particles, not all of them, but some of those particles might actually be dried-out life — spores.”
These spores would be light enough to drift back up to the clouds on currents called gravity waves, where they would serve as seeds for new droplets to condense around, restarting the whole cycle.