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How could a toxic gas be a sign of life of Venus?

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Scientists recently announced that they had found possible signs of life in the clouds of Venus. We probably should have suspected as much all along.

Venus is a natural place to look for life beyond Earth. It is Earth’s twin — almost the same size and structure — and closer to us than Mars, the current favorite of astronomers looking for life elsewhere in the solar system. Venus is also closer to the Sun, which provides the warmth necessary for life as we know it. In the past, a few scientists have suggested that Venus was a source of primordial life that was later seeded on Earth. That theory, lithopanspermia, never gained popularity because current conditions on Venus seemed very inhospitable to life. The high concentration of carbon dioxide in the atmosphere of Venus ensures that the planet has a runaway greenhouse effect that makes its surface incredibly hot, way hotter than your oven, which kills off microorganisms. And the clouds in its atmosphere are acidic. So scientists turned their attention elsewhere, to Mars and the moons of Saturn. So far, no definitive signs of life have been found on any of them.

But the latest announcement about Venus is a tantalizing one. Astronomers haven’t actually seen life on Venus. Instead, they have observed evidence of a gas called phosphine in the planet’s clouds. What could phosphine have to do with extraterrestrial life?

Phosphine is a highly toxic gas formed when one atom of phosphorus combines with three atoms of hydrogen. Giant planets such as Jupiter have a lot of hydrogen in them and in their atmospheres, and are known to produce phosphine. But on Venus and Earth, there is very little hydrogen in the atmosphere. So, the thinking goes, any phosphine detected is likely associated with life, because someone