Saturn Moon's Geysers Don't Need Liquid Water?
for National Geographic News
|August 17, 2007|
Unusual geysers on the south pole of Saturn's moon Enceladus have been drawing the attention of astronomers searching for liquid water—and maybe life—on other worlds.
But new research on the moon's geologic features suggests water need not be involved, and may put hopes for life on ice.
The icy pole boasts ridges, vents, and other formations that emit the geysers, and scientists have recorded heat transfer at the pole's surface.
Based on these observations, some experts have theorized that the moon has a warm, shifting interior similar to Earth's.
Our planet's hot insides are what power geysers like the famous Old Faithful in Yellowstone National Park.
On Enceledus, scientists think, the geysers form when heat from deep inside melts water ice just below the surface, forming shallow pockets of liquid water.
When tectonic movements crack the ice cap, liquid water bursts forth and quickly refreezes, creating the geysers.
Now a new model "shows that it's possible to put these three observational elements"—geysers, geologic features, and heat flow—"into one unified explanation without the need for water," said study co-author Gustavo Gioia of the University of Illinois at Urbana-Champaign.
Gioia and colleagues suggest that the ridges where the plumes emerge were formed from a single heating event that expanded and cracked the surface.
A shell made of icy compounds is exposed by the ridges, causing the compounds to decompress and absorb heat. The compounds then explosively split into smaller parts and send ice crystals and gases skyward.
"That doesn't mean that there is no [liquid] water," Gioia cautioned. "It just means that you can explain the whole thing without there being water there."
Enceladus's geysers were first imaged by NASA's Cassini-Huygens spacecraft in 2005.
(See more images of Saturn from the Cassini mission.)
The moon has since been dubbed a cosmic graffiti artist, because the huge geysers spew icy particles and vapor that cover its surface.
(Read: Saturn Moon's Ice Geysers Create 'Cosmic Graffiti' [February 8, 2007].)
The geysers sandblast nearby objects and likely created Saturn's massive, outermost ring.
Last year study co-author Susan Kieffer, also at the University of Illinois, suggested that the gases in the plume are produced from stiff compounds called clathrates that form a shell under the south pole's cap.
Clathrates are like icy lock boxes—outer shells of water ice are bonded to gases such as methane or carbon dioxide that are trapped inside.
When periodic tectonic cracks appear in the ice above, the clathrates may decompose explosively and send gassy plumes skyward.
In their latest version of this "Frigid Faithful" model, Kieffer, Gioia, and colleagues suggest that at one point in the moon's past an internal heat source warmed and stretched the clathrate-rich shell and the polar ice cap above.
"As a result the shell cracked, forming the four 130-kilometer-long [81-mile-long] fractures known as tiger stripes" where the geysers emerge, Gioia said in a press statement.
"The tiger stripes are analogous to the cracks that form in the glazing of a porcelain vessel when the vessel is filled with hot tea."
This would mean that the cracks where the geysers emerge might not be the result of plate tectonics.
What's more, the scientists say that gases from deep inside the moon may continue to rush upward carrying the heat that breaks down the clathrates.
This process, known as heat advection, doesn't require the surface temperature to be very different from the temperature where the gases originate.
In this case, the below-freezing surface temperatures might be the same as the bottom of the tiger stripe fissures: about 22 miles (35 kilometers).
And no extra heat near the surface would mean no pockets of liquid water.
Still a Chance
The research, published online on August 15 by the Proceedings of the National Academy of Sciences, is certain to fuel further debate about Enceladus's now famous plumes.
David Stevenson, a planetary scientist at the California Institute of Technology in Pasadena, said that the shallow heat source assumed by the new model isn't likely to account for the geysers.
"The primary heat source must be much deeper than where the plume originates (in their model) otherwise you cannot get enough heat," Stevenson said in an email.
"The search for water has not been dealt a blow (in my opinion). But neither is liquid water mandatory" for the geysers to form.
Carolyn Porco is a senior research scientist at the Space Science Institute in Boulder, Colorado, and the leader of the team that first suggested that the geysers are formed by shallow liquid water.
Porco said that there is still reason to suspect that Enceladus is a "wet" moon.
"There's still a very good chance there is liquid water deep beneath the surface on Enceladus," she said.
"What may be in serious question is whether or not the jets derive from near-surface pockets of liquid," she added.
"But down deep—say tens of kilometers—it's still very likely."
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