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Antifreeze on Saturn's
moon Enceladus

KEITH COOPER
ASTRONOMY NOW

Posted: July 23, 2009


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When NASA's Cassini spacecraft skimmed the surface of Saturn's icy moon Enceladus in 2008, it revealed the presence of ammonia for the first time, according to new results published in the 23 July issue of the journal Nature. This discovery adds another twist to our topsy-turvy understanding of the plumes of water vapour that emanate from the 498 kilometre wide moon's southern hemisphere.

If there is an underground ocean on Enceladus, then ammonia would be vital to it. Ammonia acts as an antifreeze, allowing water to remain liquid at temperatures as low as –97 degrees Celsius (176 kelvin). Recorded temperatures in the ‘tiger stripes’ – the long fractures in ice from which the plumes originate – have reached as low as –93 degrees Celsius (180 kelvin).

Ammonia has been discovered coming from inside Enceladus, strengthening the case for an underground ocean. Image: NASA/JPL/Space Science Institute.

“We think we have an excellent argument for a liquid water interior,” says Hunter Waite of the South-west Research Institute in Texas. Waite is the lead scientist on Cassini’s Ion and Neutral Mass Spectrometer, which ‘tasted’ the plumes during the fly-bys in July and October 2008, when it flew as close as 50 and 25 kilometres above the surface during the respective close encounters.

“Ammonia is sort of a holy grail for icy volcanism,” adds William McKinnon of Washington University. “This is the first time we’ve found [ammonia] for sure on an icy satellite of a giant planet. It is probably everywhere in the Saturn system.”

The discovery of ammonia adds more fuel to the fire of the debate as to whether there is an underground ocean or not. Last month, astronomers from the University of Leicester and the Max Planck Institute in Germany announced the detection of sodium chloride – in simple terms, salt – in Saturn’s E-ring, which is composed of material ejected by Saturn’s plumes (read our story here). However, another team using one of the ten-metre Keck telescopes in Hawaii could find no spectral evidence for individual sodium atoms in the E-ring.

The Keck team doesn’t entirely rule out an underground ocean, but says that if there is an ocean then the material is not erupting violently, but perhaps evaporating into “misty caverns” just below the tiger stripes. The Leicester/Max Planck Institute team, who used Cassini’s Cosmic Dust Analyser to detect the salt, argue that most of the sodium atoms would be bound up in sodium chloride molecules from an ocean, meaning that there would be very few sodium atoms for Keck to detect. Their analysis relies on the existence of an ocean, and the discovery of ammonia certainly backs that hypothesis up.