Cassini finds evidence for water on Enceladus
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: March 9, 2006
Pockets of liquid water may exist near the surface of Saturn's icy moon Enceladus, the apparent source of huge Yellowstone-type geysers seen erupting from the moon's south polar region by NASA's Cassini spacecraft, scientists reported today. If so, Enceladus would join a very short list of bodies in the solar system with environments that could, in theory at least, support life.
It is not yet clear what process might be generating the energy needed to thaw ice in the ultra cold realm, but it may be a combination of natural radioactive heating and tidal flexing caused by Saturn's gravity, coupled with an era of heating in the past.
While the heat source is not yet clear, the results are. High resolution images and data from other instruments aboard Cassini show towering plumes erupting from long fractures dubbed "tiger stripes" in the fractured terrain near the south pole. Based on the volume and size of the ice particles spewing out, scientists believe they may originate from near-surface reservoirs of water that have been warmed above the freezing point.
"This finding has substantially broadened the range of environments in the solar system that might support life," Porco said in an email. "And it doesn't get any more significant than that. I'd say that if we did nothing else at Saturn, this discovery alone and its possible biological implications would have made the Cassini mission worthwhile."
Carrying a suite of sophisticated instruments, Cassini braked into orbit around Saturn in July 2004 after a seven-year voyage from Earth. Since then, it's been looping around the planet in a series of ever-changing orbits, studying its spectacular ring system, its atmosphere, the space environment near Saturn and many of the planet's more interesting moons. Much of the early attention has been focused on Titan, a moon larger than Mercury that is shrouded in a thick atmosphere.
The much smaller Enceladus, just 300 miles or so across but one of the brightest moons in the solar system, has long been a target of interest because of earlier observations that hinted at an unusual surface. Cassini's observations have raised that interest to a new level.
"They also have yielded indications of extreme geologic youth in the SPT, morphological evidence of a change over time in surface stresses in the southern hemisphere, and possible evidence of an epoch of intense heating in the past."
During the November 2005 flyby, Cassini images revealed "many distinct near-surface jets, emanating from the surface in a variety of directions and supplying a much bigger, fainter plume towering over the south polar region by at least 435 km (270 miles)," the team wrote.
The source of the jets appears to be the so-called tiger stripes, a family of long, roughly parallel features that are typically 1,600 feet or so deep, about a mile wide and some 80 miles long. Most of the ice seen erupting in plumes falls back to the surface, but about 1 percent escapes and contributes to Saturn's E ring.
"At the moment, we interpret these observations to indicate that the south polar jets are the primary source of the E-ring," the science team writes. "The fact that almost all of the observed particles in the jets' extended plume are falling back to the surface may explain the extreme brightness of the interstripe plains as being due to freshly fallen snow."
As for the heat source, there are two possibilities: "either sublimating ice, above or below ground, or underground reservoirs of boiling liquid erupting through vents in the tiger stripes." The second option seems to fit the observations best and "the erupting mixture of vapor and liquid - or, in the case of Enceladus, vapor, liquid and ice particles - is like a cold Yellowstone geyser."
Heat from radioactive decay and tidal stress alone do not appear sufficient to produce the presumed reservoirs. But the moon may have undergone more extreme heating in the past due to changes in its orbital path around Saturn and the lingering traces of that heat, plus the ongoing radioactive and tidal processes, just might be enough to produce the necessary heating.
Based on data from NASA's Galileo Jupiter probe, researchers believe the jovian moon Europa and others may harbor subsurface reservoirs of water. But in those cases, the presumed water is at least several miles below the surface and no plumes like those seen rising from Enceladus have been obsesrved.
Porco said the reservoirs on Enceladus may be no more than a few tens of meters deep.
"We previously knew that Mars, the subsurface of Mars, might have water and therefore living organisms," Porco told CBS News. "We knew that moons like Europa around Jupiter ... might also be regions or bodies where we might find subterranean water. But now Enceladus has become and even more exciting target because this is a body where the water is closer to the surface. And also, it's not bathed in the intense radiation environment that Europa is."
Porco said the discovery could lead to "a redirection of our plans in exploring the solar system to focus on Enceladus as the next body we go to."
But not in the near term. NASA funding for space science is being cut back to help pay for the Bush administration's program to send astronauts back to the moon as a prelude for eventual manned flights to Mars.