Four fissures in the south pole of Saturn鈥檚 moon Enceladus are spewing out a plume hundreds of kilometres high, the Cassini probe has revealed, and the ejecta is leaving a vapour trail that rings Saturn.
快猫短视频s are shocked by this volcanic activity on what should be a small, quiet moon. 鈥淚t is a stunning surprise,鈥� said Dennis Matson, from NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California, US. But researchers are beginning to develop theories about what is going on.
Matson and other members of the Cassini spacecraft team revealed the latest data on Enceladus in London, UK, on Tuesday. Cassini snapped an image of the fissures, nicknamed 鈥渢iger stripes鈥�, when it flew past Enceladus on 14 July 2005, skimming within just 173 kilometres of the moon鈥檚 surface.
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Meanwhile, Cassini鈥檚 Composite Infrared Spectrometer picked up unexpectedly strong infrared radiation (heat) from the south pole. 鈥淚t鈥檚 like flying by Earth and discovering that Antarctica is warmer than the equator,鈥� says John Spencer of the Southwest Research Institute in Boulder, Colorado, US. Zooming in, CIRS found that the fissures are at least 90掳 kelvin (-183掳C), 15掳 warmer than most of the moon鈥檚 surface.
Ice bombs
The tiger stripes are strange in other ways too, showing the spectral signatures of organic molecules and a form of ice that can only exist at relatively high temperatures.
Other instruments on Cassini sampled a vast plume of water vapour towering above the south pole, almost certainly coming from the hot fissures. 快猫短视频s have speculated before that Enceladus might supply material for one of Saturn鈥檚 rings, the E-ring, and the new observations seem to confirm it. Water is pouring out at a rate of half a tonne per second 鈥� enough to keep the E-ring topped up.
Cassini has also seen 20-metre boulders near the moon鈥檚 south pole. Could these have been blown out of the fissures, like giant, icy lava bombs? 鈥淭hey are awfully large鈥� to have been ejected, says Torrence Johnson of the Cassini imaging team, 鈥渂ut Enceladus鈥� gravity is weak, so it doesn鈥檛 take much to lift stuff off the surface鈥�.
Tidal friction
Internal heat must be driving all this activity, but the source of the heat remains a big puzzle. Natural radioactive decay in the moon鈥檚 rocky core might warm the interior just enough to produce a sludgy plume of water and ammonia. This could heat the surface ice just enough to allow water to evaporate slowly.
But Cassini also detected dust and whole ice grains in the plume, implying that the material is squirted out of Enceladus with some force. That would need a lot of heat 鈥� far too much to come from the core.
An alternative is the tidal pull of Saturn鈥檚 gravity, which makes the moon flex and produce heat by internal friction. But initial calculations put that at only 1% of the heat from the core.
Johnson speculates that thousands of years ago the orbit of Enceladus may have been different, producing much more severe tidal heating. Today, researchers just see leftover heat escaping.
Or perhaps all the tidal stresses on Enceladus are focused on those four fissures, rubbing the surfaces together to melt the ice. 鈥淪omehow Enceladus is doing it, so we鈥檙e going to have to figure out how,鈥� says Johnson.
