A liquid plume is spewing from Saturn’s icy moon Enceladus — but is it coming from heated ice on the surface, or a liquid ocean underneath?
Analysis of the plume’s chemistry, detailed in the Cassini (CICLOPS) image above and reported in Nature this week, may put the debate to rest.
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Lead author Jack Hunter (J.H.) Waite, of the Southwest Research Institute in San Antonio, Texas and his colleagues say ammonia detected in the jets from Enceladus’ south pole provides the strongest evidence yet for the existence of liquid water beneath the surface.
A previous paper led by Frank Postberg of the University of Heidelberg in Germany, published in Nature just last month, reported the discovery of salts in E-ring particles derived from the plume, also suggestive of a liquid reservoir.
But Susan Kieffer of the University of Illinois at Urbana–Champaign and her colleagues proposed in a 2006 Science paper that warm ice is heated near the surface, causing dissociation of clathrate hydrates. And Nicholas Schneider, of the University of Colorado at Boulder, and his colleagues published a paper in the same Nature issue as Postberg’s team (June 24) — reporting that there’s not enough sodium in the plume to support a liquid ocean.
The ammonia may tip the scales, say the authors of the new paper.
“The presence of ammonia provides strong evidence for the existence of at least some liquid water, given that temperatures in excess of 180K have been measured near the fractures from which the jets emanate,” the authors write. “We conclude, from the overall composition of the material, that the plume derives from both a liquid reservoir (or from ice that in recent geological time has been in contact with such a reservoir) as well as from degassing, volatile-charged ice.”
Besides ammonia, the authors detected various organic compounds and deuterium — ‘heavy’ hydrogen abundant in the oceans of Earth. Ammonia, together with methanol and salts, acts as an antifreeze, allowing liquid water to exist at below-freezing temperatures. The authors suggest that preserving even a residual oceanic layer during cooling episodes would maintain conditions necessary for tidal heating and geologic activity.
Enceladus is one of only three moons in the Solar System known to be volcanically active. The plume of gas and particles is thought to make up Saturn’s outermost ‘E’ ring.
UT ran a story last month, when Nature ran two papers with different ideas about whether Enceladus harbors a liquid ocean. See that story here.