From: Planet Pride
Date: Dec 21, 2007 3:49 PM
Hot spot on Enceladus causes plumes
The heat being generated on the moon's south pole at a hot spot is enough to eject plumes of ice and vapor above Enceladus. These plumes, according to William B. McKinnon, Ph.D., professor of earth and planetary sciences in Arts & Sciences at Washington University in St. Louis, are this moon's most intriguing feature.
"The plume particles are like smoke, ice smoke," McKinnon said. "If you were standing on Enceladus' surface you wouldn't even be able to see the plumes. The particles are just larger than the wavelength of light, about one-thousandths of a millimeter. Most icy bodies of this size are geologically inert, but this is a clear indication of geological activity. Cassini has found active venting of water vapor. This leads to scientifically intriguing speculations and questions."
One is: Is this active ice volcanism on Enceladus? If so, is it due to ice sublimating, in the manner of a comet, or to a different mechanism, like boiling water, as in Old Faithful at Yellowstone? The biggest question: If there is water on Enceladus, is there life?
"I don't think so," McKinnon said. "The strongest piece of evidence against that is measurements made from Earth of the plume don't show any sodium. If the source of the plumes were fresh water like on Earth, the plumes would contain enough detectable sodium. Fresh water flows through rocks and on streambeds, and so it picks up bits of mineral chemistry. The emerging view is that there's not obvious evidence for a subterranean ocean in contact with rock, no boiling or venting."
McKinnon said that the leading model for the cause of the plumes on Enceladus is that the moon's tides cause its crust to ratchet or rub back and forth in a set of faults near the south pole. This action generates just enough heat to vaporize the ice that makes the plumes.
Cassini, which has been passing through the plumes of Enceladus, makes its next pass in March of 2008. It will go deeper into the plume and take more pictures of the moon's features, the venting area in the infrared, impact craters, cracks and fissures, and make better measurements of gases and vapors.
McKinnon presented "Cold Fire: The Geology and Geophysics of Enceladus," Dec. 10, 2007, at the Fall Meeting of the American Geophysical Union in San Francisco.
The mythological Enceladus is buried beneath Mount Etna and is responsible for the mountain's tremors and volcanism. The moon Enceladus is only 500 kilometers wide -- roughly 300 miles wide, the distance between St. Louis and Chicago, and quite round for such a small body. Data from Cassini has revealed a rock-rich body, 55 to 60 percent rock by mass, with a surface of nearly pure water ice.
The temperature at the poles is some -220 degrees Celsius (C), but the hot spot is at least 100 degrees warmer. Enceladus is in a special relationship called dynamical resonance with another one of Saturn's moons, Dione. Every time Dione, in an exterior orbit around Saturn, circles Saturn, Enceladus goes around exactly twice. This resonance keeps Enceladus' orbit tidally pumped, maintaining an eccentric path that leads to a continuous squeezing under Saturn's gravity field.
This process makes a small part of the planet hot, relatively, for an icy satellite. It's the same mechanism that runs the tremendously hot silicate volcanism of Io and activates Europa, maintaining its ocean. Lo and Europa are two of Jupiter's moons.
"You only have to get so hot to make ice active," McKinnon said. "It doesn't have to get tremendously hot like it does on Lo. Ice volcanism requires an order of magnitude less energy for things to work out pretty well. The hot spots are -100 degrees C or possibly 'warmer'; the area around it is more than twice as cold. We still can't say how truly 'hot' the hot spots are. We'll probably learn this in March."
source: Saturn Today
Enceladus: Cold Moon With a Hot Spot
Written by Nancy Atkinson
The temperature at Enceladus' south pole is about -220 degrees Celsius, but the hot spot is at least 100 degrees warmer. The leading model for the cause of the plumes on Enceladus is that the moon’s tides cause its crust to ratchet, or rub back and forth, in a set of faults near the south pole. The forces between Enceladus, the big planet Saturn and another moon, Dione cause what’s called dynamical resonance, and Enceladus is continually squeezed under this gravity field. This process creates a small hot spot, in relative terms, for an icy satellite.
Cassini has actually flown through the plumes, giving scientists a glimpse of the plume’s make up.
“The plume particles are like smoke, ice smoke, “said William B. McKinnon, professor of earth and planetary sciences at Washington University in St. Louis. “If you were standing on Enceladus’ surface you wouldn’t even be able to see the plumes. The particles are just larger than the wavelength of light, about one-thousandths of a millimeter. Most icy bodies of this size are geologically inert, but this is a clear indication of geological activity. Cassini has found active venting of water vapor. This leads to scientifically intriguing speculations and questions.”
The scientists are pondering if Enceladus has active ice volcanism, and if so, is it due to ice sublimating, like a comet, or due to a different mechanism, like boiling water as in Old Faithful at Yellowstone. Even though there may be water on the moon, McKinnon doesn’t believe there is the possibility of life on Enceladus. This is because measurements made from Earth don’t indicate there is enough sodium present in the plumes to warrant the “life” question. “The emerging view is that there’s not obvious evidence for a subterranean ocean in contact with rock, no boiling or venting,” said McKinnon.
The Cassini science team has made Enceladus a major priority and there will be seven additional close fly-bys of the moon by the spacecraft through mid-2010 (provided the mission is extended to that period.) The next fly-by will be on March 8, 2008 and Cassini will approach Enceladus at an incredibly close 25 km in altitude at the low latitudes and fly over the south pole at 580 km altitude. The spacecraft will actually fly through the plumes and should be able to take high-phase images of the plumes, map the temperatures of that region, search for any activity at other latitudes as well as image other interesting features on Enceladus, such as “tiger-stripe” –like fissures found near the south pole.
“We still can’t say how truly ‘hot’ the hot spots are,” said McKinnon. “We’ll probably learn this in March.”
source: Universe Today
Is an ocean the source of Saturn moon's plumes?
David Shiga
Plumes of steam on Saturn's moon Enceladus are not spurting from a liquid water ocean beneath the icy surface as previously thought, a controversial new study argues. If the conclusion is borne out by future observations, it would be bad news for scientists hoping to use the plumes to probe an environment that could host life.
Scientists were surprised when the Cassini spacecraft discovered plumes of water vapour coming from icy Enceladus in 2005. Some suggested that the plumes were squirting up from an ocean of liquid water beneath the surface of the 500-kilometre-wide moon, raising hopes that life could be present there.
Jets of fine, icy particles erupt from the south polar region of Enceladus (Image: NASA/JPL/Space Science Institute)
But a new study led by Nick Schneider of the University of Colorado in Boulder, US, casts doubt on the idea that the plumes originate in an ocean of liquid water. Using the Keck Observatory in Hawaii and the Anglo-Australian Telescope in Australia to observe the vicinity around the moon, the researchers failed to detect light with the characteristic wavelength that sodium atoms emit.
Sodium is abundant in the solar system and should be present in rocky material inside Enceladus. If an ocean exists beneath Enceladus's surface, then it should contain a lot of sodium leached out of this material, the researchers say. Since no sodium was found in the plumes, the team argues that they do not spurt out of any ocean that may lie beneath the moon's surface.
Warm ice
The plumes may instead be vapour wafting off of relatively warm ice near Enceladus's surface, which, unlike ocean water, would not contain sodium dissolved out of rocks. The heat needed to warm the ice might be generated as Saturn's gravity squeezes and stretches Enceladus as it moves along its orbit.
Knowing the source of the plumes is important, says Carolyn Porco, chief imaging scientist for the Cassini spacecraft mission to Saturn at the Space Science Institute in Boulder, Colorado, US.
If the plumes are connected to an ocean, then studying them could allow scientists to probe the ocean environment for signs of biological activity. "You could land on the surface, look up and stick your tongue out and get some biological chemistry," she told New Scientist.
Vital signs
NASA is considering sending a mission to Enceladus for this purpose, but the idea is competing with missions to other destinations, such as Jupiter's moon Europawhich is thought to have a subsurface ocean of its own.
If Enceladus's plumes provide a sample of ocean material, this would make it much easier to sniff out chemical signs of life in the ocean. But if it turns out the plumes are not connected to an ocean, then any such ocean will be much harder to probe.
"You're in a situation analogous then with the situation with Europa, where they think there's a sea there, but it's so deep under the ice that it's not going to be accessible," Porco says.
But Porco says the results by Schneider's team do not rule out the possibility that the plumes are connected to an ocean. It is so cold at Saturn's distance from the Sun that any sodium in the plumes would condense to form solid particles, she says.
This could explain the lack of a sodium detection, since the method used by Schneider's team can only see sodium in gas form. "You could have solid sodium particles there and they would not have seen them," she says.
Fly through
Even if the plumes are not connected to a subsurface ocean or sea, there is another reason for suspecting that liquid water is present beneath the surface, Porco says – the large amount of heat energy seen coming off the moon's south pole.
If the heat comes from friction as Enceladus is squeezed and stretched by Saturn's gravity, the presence of liquid would lubricate the process. "We're seeing so much heat that I would say chances are excellent there's liquid water somewhere on Enceladus," Porco says. "I'm putting my bets on a sea under the south pole."
The Cassini team expects to learn more about the plumes when the spacecraft flies through themon 12 March 2008. Although the spacecraft's instruments are not capable of detecting sodium, they may be able to pin down the amount of ammonia in the plumes. The amount of ammonia is important, because it acts as an antifreeze, potentially allowing water to remain liquid far below 0° C.
The results of the sodium search were presented at a meeting of the American Geophysical Union in San Francisco, California, US.
source: New Scientist Space
