Saturn's tiny moon Enceladus is "absolutely a highlight" of theCassini mission and should be targeted in future searches for life,Robert H. Brown of The University of Arizona, leader of the Cassinivisual and infrared mapping spectrometer team, said last week.
Brown and other Cassini scientists attended a meeting in London lastweek and are at the 37th annual Division of Planetary Sciences meetingat Cambridge University this week.
"Enceladus is without a doubt one of the most spectacular thingsCassini has seen," Brown said in a phone interview Thursday. "It's oneof the biggest puzzles. It'll be a long time before anyone comes upwith a good explanation of how Enceladus does what it does, and for ascientist, that's pure, unmitigated fun. Solving the biggest puzzles isthe thrilling part of doing science."
Scientists got their first glimpse of Enceladus's geology whenVoyager 2 flew by the icy bright satellite in August 1981. They werecompletely baffled. Voyager photographed areas of young, smooth terrainthat told them that the moon must have been geologically active as lateas 100 million years ago.
But nothing explained how tiny Enceladus -- only 314 miles across --could get hot enough to melt. It seemingly doesn't have enough interiorrocks for radioactive heating, an eccentric enough orbit for tidalheating, or enough ammonia to lower its melting temperature. AfterVoyager, researchers shelved Enceladus as an unsolvable problem for awhile.
This year, Cassini turned its more powerful cameras and instrumentson Enceladus during Feb. 17, March 9 and July 14 flybys. Results havestunned and delighted.
The diminutive moon turns out to have a primarily water vaporatmosphere tinged with nitrogen, carbon dioxide and other simplecarbon-based molecules (organics) concentrated at its south pole. Itssouth pole is a hotspot, hovering at a relatively balmy minus -183degrees Celsius compared to the expected temperature of -203 degreesCelsius.
Enceladus's south pole is a hotbed of geological action. The southpole region is cut by parallel cracks roughly 81 miles long and 25miles apart. The cracks, dubbed "tiger stripes," vent vapor and fineice water particles that have crystallized on Enceladus's surface asrecently as 1,000 years to 10 years ago. The fine ice material isprobably the major source of particles that replenish Saturn'soutermost ring, its E ring.
"The kind of geophysical activity we see is quite likely beingdriven by liquid water below the surface," Brown said. Cassini hasn'tseen ice geysers or ice volcanoes, but the lack of ammonia, and thesheer volume of water vapor escaping suggests there's pure-watervolcanism on Enceladus, he added.
"We detected simple organics in the tiger stripes," Brown said. Thesimple organics include carbon dioxide andhydrogen-and-carbon-containing molecules like methane, ethane andethylene. "Methane (basically natural gas) has probably been locked upinside Enceladus since the solar system formed and is now bubbling upthrough the vents."
The visual and infrared mapping spectrometer can't detect nitrogen,but Cassini's ion neutral mass spectrometer may have found nitrogen inEnceladus's atmosphere. All other results from these two very differentinstruments are entirely consistent, which gives Cassini missionscientists confidence in their results, Brown said.
"So you've got subsurface liquid water, simple organics and watervapor welling up from below. Over time -- and Enceladus has been around4.5 billion years, just like Earth and the rest of the solar system --heating a cocktail of simple organics, water and nitrogen could formsome of the most basic building blocks of life," Brown said. "Whetherthat's happened at Enceladus is not clear, but Enceladus, much likeJupiter's moon Europa and the planet Mars, now has to be a place wherewe eventually search for life."
The $3.2 billion Cassini-Huygens mission is a joint venture betweenthe NASA, the European Space Agency and the Italian Space Agency. TheJet Propulsion Laboratory, a division of the California Institute ofTechnology in Pasadena, manages the mission for NASA's Science MissionDirectorate, Washington.
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