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LIKE a hapless cartoon character, Saturn has its own personal “rain cloud” that follows it around. Water vapour spewing from its moon Enceladus appears to drizzle onto the giant planet from space.
When water was discovered in Saturn’s cloud tops in 1997, the source was a mystery. It is too cold there for water to linger – it should condense and rain down to lower altitudes.
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But now Europe’s Herschel space observatory has an answer: space rain. It spotted the infrared signature of water vapour in a vast ring around the planet (). The most likely source is Enceladus’s geysers, which shoot 900 tonnes of water into space per second from the moon’s south pole.
Models suggest that up to 5 per cent of that water may rain down onto Saturn, making Enceladus the only moon known to affect the chemical make-up of its planet.
“There is no analogy to this behaviour on Earth,” says Paul Hartogh of the Max-Planck Institute for Solar System Research in Katlenburg-Lindau, Germany.
of the University of Oxford says the space rain offers “a rare chance to glimpse the processes through which a planet’s stratosphere is determined”.
