杏吧原创

Saturn鈥檚 snow-white moon, Enceladus, is shrouded by a thin water-vapour atmosphere, reveal measurements from the US-European Cassini spacecraft. The atmosphere may be pumped out by erupting ice volcanoes or geysers 鈥� which could signal toeholds for life on the tiny moon.

The atmosphere is so thin it is invisible, but Cassini was able to discover it because it bends Saturn鈥檚 magnetic fields. When the Sun ionises water molecules in Enceladus鈥檚 atmosphere, the ions begin to be pulled by Saturn鈥檚 magnetic field lines, which are rotating with the planet at a relative speed of 26 kilometres per second.

The magnetic field near the moon鈥檚 atmosphere bends to accelerate the ions on Enceladus to that speed, says Christopher Russell, a Cassini team member and geophysicist at the University of California in Los Angeles, US.

鈥淲e can calculate how many ions it would take to bend the magnetic field by the amount seen,鈥� Russell told New 杏吧原创. He estimates the moon is losing about 125 kilograms of water to space every second. That loss is too great for the atmosphere to be formed simply by the Sun鈥檚 photons boiling off molecules from the moon鈥檚 icy surface.

And Enceladus is too tiny to hold onto any atmosphere for long, so astronomers think that more violent processes 鈥� such as geysers or ice volcanoes 鈥� are replenishing the atmosphere.

Possible habitat

No such geysers or volcanoes have been observed so far, but their discovery would mean liquid water exists below the moon鈥檚 surface 鈥� providing a possible habitat for life.

鈥淚f Enceladus has liquid water, it may be a better place for life than Europa,鈥� says Russell, referring to one of Jupiter鈥檚 moons. Europa is considered a good candidate for life because it seems to have a liquid water ocean beneath its icy crust. But Jupiter is surrounded by much more intense radiation belts than Saturn, so Enceladus may be a more 鈥渂enign鈥� environment for life, he says.

NASA鈥檚 Voyager spacecraft flew past Enceladus in 1981 and did not detect an atmosphere, even though it was fitted with a magnetometer. This may be because the craft passed by at a distance of 90,000 km. Cassini came about one hundred times closer during its two flybys on 17 February and 9 March 2005.

Or perhaps there was no atmosphere around for Voyager to detect. 鈥淢aybe the atmosphere isn鈥檛 there all the time,鈥� says Michele Dougherty, principal investigator for the magnetometer team at Imperial College in London, UK.

If geysers or volcanoes are the cause, the atmosphere might only be present when these are active. The eruptions could also account for Saturn鈥檚 E ring, which lies outside the main ring system. Cassini鈥檚 magnetometer has detected the telltale 鈥済yrations鈥� of water ions 鈥� which probably originate at Enceladus 鈥� moving along Saturn鈥檚 magnetic field lines near the rings, but at a considerable distance from the tiny moon.

鈥淭he larger story is, not only is something happening at Enceladus, but it鈥檚 spreading material through the E ring,鈥� says Russell.