Published online 26 November 2008 | Nature | doi:10.1038/news.2008.1254

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Enceladus shoots supersonic jets of water

Saturn's icy moon spouts water vapour from its cracks.

EnceladusDoes Enceladus harbour liquid water beneath its surface?Cassini Imaging Team, SSI, JPL, ESA, NASA

Four supersonic jets of water vapour have been detected within the enormous geyser of gas and dust that spurts from the south polar region of Saturn's icy moon Enceladus.

The observations, made by NASA's Cassini orbiter, add weight to the idea that liquid from a lake not too far beneath the satellite's surface is erupting through warm cracks to form the giant plume.

The probe spotted the gas streams on 24 October 2007 during one of its flybys of the moon, recording the light from a star that was passing behind the Enceladus geyser using its Ultraviolet Imaging Spectrograph.

In 2005, a similar procedure had allowed Cassini to confirm1 the existence of water vapour within the geyser. But in the later observations, the star's light cut horizontally through the plume, revealing the four distinct jets of water vapour within it, says planetary scientist Candice Hansen of the Jet Propulsion Laboratory in Pasadena, California, and lead author of the new study. Her team's findings appear in Nature2.

The four jets of gas also overlap with previously imaged3 jets of ice grains emerging from four, roughly parallel, fissures named Alexandria, Baghdad, Cairo and Damascus. The two most prominent vapour jets are each one-eighth the width of the moon's entire icy plume. The researchers were also able to calculate that vapour in the two most prominent gas jets rockets at supersonic speeds out of Enceladus's cracks.

Testing time

Juergen Schmidt of the University of Potsdam in Germany is "particularly excited" about the observed supersonic gas streams because they're consistent with a model of the moon's plume that he and his colleagues proposed4 earlier this year. Schmidt's team modelled the hot fissures as long underground nozzles funnelling water from an underground reservoir to the surface, where it shoots into space. Friction means that some of the water in the nozzle cools to form ice grains that cannot reach the supersonic speeds the vapour does — a prediction now confirmed by Cassini.

Map of gas jets on EnceladusIn this image of Enceladus’s south pole, gas jets correspond with the locations of the previously discovered dust jets found along the moon’s cracks. The blue line shows the direction of starlight used to resolve the jets.Nature

But geologist Susan Kieffer of the University of Illinois in Urbana-Champaign disagrees that the new research supports the idea of an underground reservoir of liquid water. She says that gas streaming through a nozzle under pressure gradients such as those on Enceladus would go supersonic whether there's liquid water present or not, she says.

Kieffer has proposed5 that the jets are produced by gasses trapped in a reservoir of clathrates that lies under the water ice cap in the south polar region. In her model, the clathrates - a mixture of gas and ice - break up explosively and release water vapour and other gases when exposed to the near-vacuum conditions at the fractures. The process would not require the surface temperatures on the moon to be as high as those in the model proposed by Schmidt's team.

Stretch and squeeze

The new analysis also tests predictions of how the density of the giant plume should vary with time as Enceladus orbits Saturn, which stretches and squeezes its satellite. Planetary scientist Terry Hurford of NASA's Goddard Space Flight Center in Greenbelt, Maryland, and his colleagues modelled6 this effect and found that it should make the cracks in the moon's surface open and close over time.

Hansen's team, however, observed a small but significant increase in plume activity in 2007 – when the model predicts the fissures should be closed – over that seen in 2005, when they were predicted to be open. Hurford has since updated his model to include an inherent wobble in the rotation of Enceladus to account for the discrepancy.

Hansen, however, says that more observations are needed before scientists can know what's really going on beneath Enceladus' surface. 

  • References

    1. Hansen, C. J. et al. Science 311, 1422–1425 (2006).
    2. Hansen, C. J. et al. Nature 456, 477-479 (2008). | Article |
    3. Spitale, J. N. & Porco, C. C. Nature 449, 695-697 (2007). | Article | ChemPort |
    4. Schmidt, J., Brilliantov, N., Spahn, F. & Kempf, S. Nature 451, 685-688 (2008). | Article | ChemPort |
    5. Kieffer, S. W. et al. Science 314, 1764-1766 (2006). | Article | ChemPort |
    6. Hurford, T. A., Helfenstein, P., Hoppa, G. V., Greenberg, R. & Bills, B. G. Nature 447, 292-294 (2007). | Article | ChemPort |
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