新聞來源: http://tinyurl.com/lle87v 9news- Saturn moon may have an ocean: study 2009.06.25 By Marlowe Hood Huge geysers on Saturn's moon Enceladus may be fed by a salty sea below its surface, boosting the odds of extraterrestrial life in our own solar system, a study shows. Researchers in Europe detected salt particles in the volcanic vapour-and-ice jets that shoot hundreds of kilometres into space, the strongest evidence to date of a liquid ocean under this moon's icy crust. Scientists already knew that tiny Enceladus, only 500 kilometres across, had two of the three essential ingredients for the emergence of life. One is an energy source produced in this case by "tidal warming" driven by the shifting gravitational tug of its parent planet during the moon's lopsided orbit and perhaps by other forces too. The Cassini spacecraft circling Saturn since 2004 has also found a potentially life-sustaining mix of organic chemicals in Enceladus' plumes, ejected from a quartet of 120-kilometre long fractures - known as "tiger stripes" - aligned on the moon's south pole. That left the third critical ingredient: liquid water. Since their discovery in 2005, the giant geysers have fuelled intense speculation on the presence of a subterranean ocean, and the new discovery goes a long way towards resolving one of the most hotly debated topics in planetary science. A team led by Frank Postberg of the University of Heidelberg studied data from Cassini's Cosmic Dust Analyser, and tested their findings in laboratory experiments. Their results, published in the British journal Nature, show that ice grains in the Enceladus plumes contain substantial quantities of sodium salts, and that the moon's hidden sea - if there is one - could be as salty as earth's oceans. "The abundance of various salt components in the particles ... exhibit a compelling similarity to the predicted composition of a subsurface Enceladus ocean in contact with its rock core," the researchers concluded. "Individual plume sources stay active for years, implying outflow from a large reservoir." Sodium is a good telltale tracer of possible liquid water for two reasons, according to John Spencer of the Southwest Research Institute in Boulder, Colorado. It is highly soluble: "so any Enceladan water that has prolonged contact with the moon's silicate core should be rich in sodium salts, like Earth's oceans," he noted in a commentary for Nature. Sodium also scatters sunlight efficiently in the orange-yellow range of the spectrum and so is easy to detect, even in minute quantities. In a second study in Nature, another team led by Nicholas Schneider of Colorado University also looked for salts in Enceladus' plumes, this time using spectrographs on earth-bound telescopes. That it failed to detect any would seem to challenge Postberg's findings, but the earth-based observations - combined with the Cassini data - may in fact give us additional clues as to how they may be true, said Spencer in his commentary. It tells us, for example, that the plumes could not have been formed by boiling salty water spewing directly out of Enceladus' tiger stripes, otherwise the sodium would be so abundant as to be observable from Earth. Instead, the plumes could come from salty water distilling into fresh water vapours, not through evaporation as happens over Earth's oceans, but rather in pressurised chambers under the moon's surface. Cassini is scheduled to make four additional close fly-bys of Enceladus before mid-2010, and another dozen in the next five years if its mission is extended. So lingering doubts on the moon's hidden seas may soon be put to rest. --

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