Ocean spray a billion kilometres away

The Cassini spacecraft has found the best evidence yet that a salty ocean lies beneath the surface of Saturn’s moon Enceladus. As previously reported, Cassini had analyzed the icy particles being ejected from the water vapour geysers at the south pole in 2008 and 2009, with very interesting results, but now has found that the ones furthest away from the moon contain little salt, but the ones closer to the moon’s surface have much more salt, sodium and potassium specifically, and have a composition very similar to ocean water on Earth. The new study is in this week’s issue of the journal Nature.

Credit: NASA/JPL/Space Science Institute

To quote the article:

“There currently is no plausible way to produce a steady outflow of salt-rich grains from solid ice across all the tiger stripes other than salt water under Enceladus’s icy surface,” said Frank Postberg, a Cassini team scientist at the University of Heidelberg, Germany, and the lead author on the paper. When water freezes, the salt is squeezed out, leaving pure water ice behind. If the plumes emanated from ice, they should have very little salt in them.”

“The data suggest a layer of water between the moon’s rocky core and its icy mantle, possibly as deep as about 50 miles (80 kilometers) beneath the surface. As this water washes against the rocks, it dissolves salt compounds and rises through fractures in the overlying ice to form reserves nearer the surface. If the outermost layer cracks open, the decrease in pressure from these reserves to space causes a plume to shoot out. Roughly 400 pounds (200 kilograms) of water vapor is lost every second in the plumes, with smaller amounts being lost as ice grains. The team calculates the water reserves must have large evaporating surfaces, or they would freeze easily and stop the plumes.”

The “tiger stripes” are the long fissures that the geysers emanate from. At its closest, Saturn is just over a billion kilometres from Earth. It was previously thought almost impossible that liquid water, never mind an ocean, could exist so far from the sun. But, as theorized, between tidal stress from Saturn and inner radioactive decay, that’s exactly what is on Enceladus (and Jupiter’s moon Europa and possibly others). And what about similar moons that may exist orbiting some of the many other gas giant planets being found in other solar systems? The geysers of Enceladus would appear to literally be ocean spray, a long way from home…