"Geyser" Moon Sprinkles Salt on Saturn's Rings
for National Geographic News
|June 24, 2009|
The icy, geyser-like plumes spewing from Saturn's moon Enceladus are sprinkling the planet's famous rings with sodium salts, a new study says.
The finding may mean that the moon, which is completely encrusted with ice, hides a liquid ocean deep beneath its surface.
Images from NASA's Cassini spacecraft first revealed the watery plumes on Enceladus in 2005.
The dramatic discharges shoot thousands of kilometers into space from relatively warm "tiger stripe" fissures near the moon's south pole.
Many astronomers think the geysers created and continue to feed the so-called E ring, Saturn's outermost ring.
New data from Cassini offer evidence of sodium salts in the icy particles of the E ring, which suggests the salts came from Enceladus's plumes.
"It's really hard to explain [salt in the E ring], other than [the theory] that there is a saltwater reservoir feeding the plumes. That's by far the most plausible explanation," said study co-author Frank Postberg, of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany.
In a paper appearing in this week's issue of the journal Nature, Postberg suggests that water exists in deep caverns under the moon's ice.
Salts are washed into the water from rocks on the moon's subterranean seafloor, in a process similar to that seen in Earth's oceans, Postberg thinks.
As the water slowly evaporates, most of the salt gets left behind. But some slightly salty bubbles of gas get propelled into space along the fissures, where they immediately freeze and form part of the massive plumes.
"Jury's Still Out"
A separate study, also published in Nature, may quash the theory of an ocean below Enceladus's surface.
Using telescopes in Hawaii and Australia, Nicholas Schneider and colleagues searched for salts in the plumes themselves. Even from Earth, the signature of sodium should be easy to see, he noted.
"Sodium is an incredibly visible element, the same one we use in streetlights," Schneider said.
But Schneider's team found no sodium in the water-vapor clouds near the moon.
While postberg believes the results of both studies could point to an ocean leaking out into space, Schneider is less certain.
The salts found in icy particles in the E ring are not enough to account for an underground ocean, at least not one near the moon's surface, Schneider said.
Instead he thinks there's a chance the plumes are created by freshwater reservoirs or even tectonic motion moving and warming the surface ice. (Get an overview of plate tectonics.)
"As much as I and many of my colleagues have a wish for there to be an ocean creating these plumes, I think the jury is still out," Schneider said.
Ocean or Puddles?
Astronomer John Spencer, of the Southwest Research Institute in Boulder, Colorado, thinks the discovery of salt in the E ring provides great evidence for liquid water below Enceladus's icy crust.
The water could be in the form of an ocean, said Spencer, who wrote a commentary for Nature discussing the two papers. Or the water could be in under-ice puddles near the moon's surface.
Around the warmer tiger stripes, slightly salty surface ice could be melting, creating puddles that become saltier as some of the liquid evaporates.
"But even if it was a liquid puddle, that's still pretty exciting," he noted. "We haven't found liquid puddles anywhere else [aside from Earth]."
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