ENCELADUS F4 jump caption....eddie:
TIGER STRIPES F4 jump caption.....eddie:
It's snowing on Enceladus.
That may not sound all that strange in April, but Enceladus isn't a ski mountain. It's a moon of Saturn, small enough to fit between San Diego and Mammoth Mountain. The finding, announced in the March 10 issue of the journal Science by Robert Brown of the University of Arizona's Lunar and Planetary Laboratory, is one of many recent discoveries that have converted this icy chunk of outer solar system real estate into one of the most exciting places space scientists have studied.
To begin with, Enceladus' snow appears to be creating one of Saturn's rings. And, as if that's not enough, scientists now think that the pools of water from which the moisture originates may be the solar system's best prospect for extraterrestrial life.
Until 25 years ago, scientists saw Enceladus as little more than an unappealing ice ball, renowned primarily for the brilliance of its snowy-white surface. Then, NASA started sending probes to Saturn, and planetary geologists got a chance to take a closer look.
The first thing they noticed was that Enceladus isn't simply a cue ball of ice. Some areas are heavily cratered, meaning they are old enough to have experienced eons of meteorite bombardment. Others are smoother, newer.
But the most interesting part of Enceladus was its south pole, which showed a pattern of odd bands reminiscent of tiger stripes.
This much has been known since 1981, when the Voyager 2 probe visited Saturn en route for interstellar space. But its instruments were crude by today's standards and it got only within 70,000 miles of Enceladus. Then, in 2004 the Cassini orbiter reached the Saturn system. The following year it did a series of flybys of Enceladus, including one that passed a mere 105 miles above the tiger stripes.
The findings were astounding. The stripes, scientists discovered, hid a warm spot that appears to be jetting water vapor into space. Some of that water is falling back, forming the fresh snow seen by Brown. But at least as much appears to be escaping from this tiny world whose gravity is only 1.2 percent of Earth's.
Enough of it is being emitted to have formed one of Saturn's most mysterious rings.
The E Ring
Saturn's rings are named “A,” “B,” “C,” “D”, etc., in order of discovery. Enceladus orbits in the heart of the E ring, which is so faint that it wasn't found until 1979. Unlike the other rings, which can have chunks as big as houses, the E ring is composed of extremely fine, dust-sized motes of ice.
From the moment it was discovered, scientists suspected a connection between the ring and Enceladus, but nobody knew what it might be. Then Cassini found the jets. In the first of three flybys, its instruments detected oddities in the way Enceladus interacts with Saturn's magnetic field.
The only plausible explanation was that Enceladus had a tenuous atmosphere containing ionized water, Carolyn Porco of the Space Science Institute in Boulder, Colo., explained at a December meeting of the American Geophysical Union.
Other instruments found water vapor extending 180 miles into space – but only above the south pole. Presence of a vapor plume was further confirmed by watching changes in the light of a star that passed behind it.
But seeing is believing. On Nov. 27, the Cassini team took a long-range photo of Enceladus, backlit by the sun. The angle of the light was perfect to highlight a spreading cloud of ice particles, condensed from water vapor leaving Enceladus' surface.
Months earlier, the scientists had thought they'd seen the same thing, but they'd been reluctant to announce it, unsure whether they were really seeing a plume, or merely lens flare, created by sunlight striking the camera lens.
The new image stilled their doubts, showing not only the main body of the plume, dissipating from the moon's south pole region, but also distinct jets marking its source. Although it's not possible to tell for certain from the photo, these jets probably emanate from the tiger stripes, Torrence Johnson of NASA's Jet Propulsion Laboratory (JPL) said in December.
Hot spot
But why would the tiger stripes be spewing dust and vapor into space? The answer appears to lie in one of the most surprising finds from the probe's closest flyby last July.
As Cassini swept across Enceladus' south pole, it trained infrared cameras on the surface zipping by beneath it. Such cameras can function as heat detectors, and what they found was a hot spot, precisely at the location of the stripe. That makes the pole, which should have been cold, the hottest place on Enceladus, Johnson said. The stripes, he added, are cracks, offering glimpses of “hot” ice underneath.
The source of the heat is anybody's guess. John Spencer of the Southwest Research Institute in Boulder, Colo., estimated that the total power output from the south polar region is between four and 12 gigawatts – a subterranean energy source equivalent to several large electrical power plants.
There are two possible sources for all that energy. One is the decay of radioactive elements in subsurface rocks. But producing that much heat would require an unusually radioactive ore body beneath the south pole, and nobody knows why that might be the case.
Alternatively, interactions with other moons might generate frictional heat by repeatedly flexing Enceladus, like a child squeezing a rubber ball.
Normally, such heat would be dispersed throughout the planet's interior, and there wouldn't be enough of it to produce the plume. Scientists therefore speculate that something in Enceladus' internal structure may cause much of that energy to be concentrated at the south pole.
“We're looking at some kind of focusing,” Porco said. “We can't say why it would be at the south pole.”
The same factors may explain why the young, uncratered terrain is in the south, while the older, heavily cratered surfaces are in the north.
“Perhaps that has something to do with the south being warmer and squishier,” Porco said. “Global symmetry is not what Enceladus is about.”
Boiling pools
That far from the sun, of course, “hot” is a relative term. Most of Enceladus' southern reaches are about minus 315 F. Spencer estimated that in order to produce the infrared signatures seen from space, the hot spots had to be at least minus 260 F. – a lot colder than Antarctica, but surprisingly warm for an airless world nearly 10 times farther than we are from the sun.
And that's the surface temperature. Farther down it's got to be warmer yet.How warm depends on what's producing the jets.
Initially, there were two theories. One theory allowed it still to be quite cold beneath the ice. At temperatures above about minus 100 F., ice undergoes a process called sublimation, in which it evaporates without ever melting.
This process occurs in cold, Earthly climes. North of the Arctic Circle in Greenland, villagers take advantage of it by hanging out wet laundry on dry winter days. First the laundry freezes. Then it dries, just as it would if hung on a clothesline in midsummer.
But while sublimation can produce water vapor, it's a slow process – too slow to produce the density of ice particles seen in the escaping plume.
“When we analyzed the images (of the plume) it became apparent that the abundance of particles was far more than can be explained by sublimating ice,” Porco says.
What that means, she says, is the plume is fed by pools of liquid water, boiling into space.
In other words, the jets are rapidly freezing steam from a geyserlike process that Susan Kieffer, a geologist at the University of Illinois, has dubbed “cold faithful.”
Ring food
JPL's Candice Hansen has calculated the rate at which Enceladus is venting water vapor, based on the amount of water Cassini's instruments have measured in the plume and the speed at which it appears to be moving. Her conclusion: Enceladus is blasting out about 100 gallons of water vapor per second – enough to fill a suburban swimming pool every couple of minutes. And that doesn't count the ice crystals, which can't be measured by her instruments.
That's exciting because other scientists calculate that the E ring is losing water at the rate of about a half-gallon per second, due to chemical reactions with sunlight and collisions between particles.
“So we have more than enough to keep the E ring from disappearing,” Hansen said.
In another observation reported in Science, Bonnie Buratti, also of JPL, reported that spectral analysis of the plume indicates that its composition is almost identical to the E ring.
“This is a smoking gun,” she said. “The plume is the source of the E ring.”
Hansen agrees.
“A decades-long mystery has been solved,” she said. But she has a better term than “smoking gun.”
“We've found the steaming gun,” she said
Some of Saturn's other moons dive through the E ring during portions of their orbits. One is Dione, which appears to be coated with fine grains of fresh ice on one side, but not the other.
“We're pretty sure that this is happening to the other inner satellites as well,” Buratti said. “Enceladus is responsible for the whole ecology of the inner (Saturn) system. It produces the plume and the E ring, and they coat the other satellites.”
But most dramatic of all are the implications for astrobiology. That's because the water pools that feed the geysers probably lie only a few dozen yards below the surface.
“Once you have liquid water, you have the potential for living organisms,” Porco said.
In fact, she said, all the building blocks of life seem to be present. Not only is there liquid water and heat, but signs of organic chemicals potentially useful to life also have been seen in the vicinity of the tiger stripes.
“Those are the three main ingredients,” Porco said, “which is why, at this stage, it's very provocative.”
If they exist, Enceladus' organisms would most likely be limited to microbes, and nobody expects to find them soon. An actual visit to Enceladus isn't probable for a long time, and an unmanned probe will no doubt be sent first. But if humans ever get there in person, it would certainly make for the solar system's most exotic ski trip.
Richard Lovett writes about science from Portland, Ore.
