Giant Water Plume Spews From Saturn's Moon
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Giant water plume spews from Saturns moon
The four long fissures straddle the south pole of Enceladus, running for more than 100 kilometres from south-west to north-east the circular grid marks 60° South (Image: NASA)
Four fissures in the south pole of Saturn's 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.
Scientists are shocked by this volcanic activity on what should be a small, quiet moon. "It is a stunning surprise," said Dennis Matson, from NASA's 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 "tiger stripes", when it flew past Enceladus on 14 July 2005, skimming within just 173 kilometres of the moon's surface.
Meanwhile, Cassinis Composite Infrared Spectrometer picked up unexpectedly strong infrared radiation (heat) from the south pole. "Its like flying by Earth and discovering that Antarctica is warmer than the equator," says John Spencer of the Southwest Research Institute in Boulder, Colorado, US. Zooming in, CIRS found that the fissures are at least 90° kelvin (-183°C), 15° warmer than most of the moon's surface.
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. Scientists have speculated before that Enceladus might supply material for one of Saturn's 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 - enough to keep the E-ring topped up.
Cassini has also seen 20-metre boulders near the moon's south pole. Could these have been blown out of the fissures, like giant, icy lava bombs? "They are awfully large" to have been ejected, says Torrence Johnson of the Cassini imaging team, "but Enceladus' gravity is weak, so it doesn't take much to lift stuff off the surface".
Internal heat must be driving all this activity, but the source of the heat remains a big puzzle. Natural radioactive decay in the moon's 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 far too much to come from the core.
An alternative is the tidal pull of Saturn's 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. "Somehow Enceladus is doing it, so we're going to have to figure out how," says Johnson.
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