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New Theory Sheds Light On Space Enigma

UK and German scientists explain intriguing phenomenon on Saturn’s moon
An enormous plume of dust and water spurts violently into space from the south pole of Enceladus, Saturn's sixth-largest moon. This raging eruption has intrigued scientists ever since the Cassini spacecraft provided dramatic images of the phenomenon.

Now, physicist Nikolai Brilliantov, at the University of Leicester, and colleagues in Germany, have revealed why the dust particles in the plume emerge more slowly than the water vapour escaping from the moon's icy crust.

Enceladus orbits in Saturn's outermost "E" ring. It is one of only three outer solar system bodies that produce active eruptions of dust and water vapour. Moreover, aside from the Earth, Mars, and Jupiter's moon Europa, it is one of the only places in the solar system for which astronomers have direct evidence of the presence of water.

The erupting plume on Enceladus is ejected by geyser-like volcanic eruptions from deep, "tiger-stripe" cracks within the moon's south pole. Some astronomers have suggested that the myriad tiny grains of dust from these eruptions could be the actual source of Saturn's E-ring. However, the dynamics and the origin of the plume itself have remained a mystery.

Now, Brilliantov, who is also on the faculty at the University of Potsdam, Germany and Moscow State University, working with Juergen Schmidt and Frank Spahn of Potsdam and Sascha Kempf of the Max Planck Institute for Nuclear Physics in Heidelberg, and the Technical University of Braunschweig, Germany have developed a new theory to explain the formation of these dust particles and to explain why they are ejected into space.

The researchers point out that once ejected the dust particles, which are in fact icy grains, and water vapour are too dilute to interact with each other and so the water vapour cannot be the cause of the dusty slowdown. Instead, the team suggests that the shift in speed must occur below the moon's surface before ejection.

The numerous cracks through which the plume material escapes from the moon's icy surface, and which can be hundreds of metres deep, are narrower at some points along their length. At these points temperature and pressure of vapour drop drastically down, causing condensation of vapour into icy grains and hence to formation of the dust-vapour mixture. The required vapour density to accelerate the grains to the observed speeds implies temperatures where liquid water can exist in equilibrium with solid ice and water vapour within the moon's frozen crust.

These peculiar conditions allow the water vapour to erupt rapidly carrying with it the dust particles. However, these particles undergo countless frequent collisions with the inside of the channel walls which causes friction that slows them down before final ejection. The larger the particle the slower the ejection speed. This effect, quantified by the new theory, explains the structure of the plume and eventually the particle size distribution of the E-ring of Saturn.

The existence of liquid water is a prerequisite for life and, while not suggesting there is life on Enceladus, offers another extraterrestrial place that might be searched.

The scientists published details of their findings in the journal Nature.

For More information, please contact Nikolai Brilliantov on 0116 252 2521

The paper appeared in:

Nature, Vol 451, p. 685-688, |7 February 2008|doi:10.1038/nature06491

- A member of the 1994 Group of universities that share a commitment to research excellence, high quality teaching and an outstanding student experience.

Ranked top for student satisfaction in England (jointly with Oxford) among mainstream universities (average score of 4.4 out of 5 for overall satisfaction)

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Founded in 1921, the University of Leicester has 19,000 students from 136 countries. Teaching in 18 subject areas has been graded Excellent by the Quality Assurance Agency- including 14 successive scores - a consistent run of success matched by just one other UK University. Leicester is world renowned for the invention of DNA Fingerprinting by Professor Sir Alec Jeffreys and houses Europe's biggest academic Space Research Centre. 90% of staff are actively engaged in high quality research and 13 subject areas have been awarded the highest rating of 5* and 5 for research quality, demonstrating excellence at an international level. The University's research grant income places it among the top 20 UK research universities. The University employs over 3,000 people, has an annual turnover of £184m, covers an estate of 94 hectares and is engaged in a £300m investment programme- among the biggest of any UK university.

Ather Mirza | University of Leicester
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