June 3, 2006

One of Saturn’s innermost moons, Enceladus, has been added to the small list of terrestrial bodies in our own solar system that may support life. Scientists are keenly observing interesting images of the moon that boasts icy Yellowstone-like geysers erupting from its surface. These frosty jets give out particles at high speed, just like the hot springs at Yellowstone National Park. The reason for these icy jets is thought to be liquid water under the surface of Enceladus.

The image sent back by Nasa’s Cassini spacecraft in March this year has prompted researchers to plan a more serious and detailed mission to the mysterious moon to learn more about the interesting plumes. At present, the only thing that experts are waiting for is Cassini’s closer look at Enceladus in the spring of 2008.

By that time, the spacecraft will be within 220 miles or 350 kilometers of the satellite for a last look. Its mission will end on June 30, 2008, after four years of study of the Saturnian system.

Although it is possible to extend the spacecraft’s mission, but that would depend entirely on the budgetary resources. The most interesting end to the mission and a great gift for astrobiologists on Earth would be to actually “dive” into the atmosphere to catch some of the stuff given out by the plumes.

“After what we’ve discovered with Cassini, if we don’t get an extended mission, then there’s no hope for any body,” says Carolyn Porco, the team’s imaging leader. The mission is a joint effort of the Nasa, the European Space Agency and the Italian Space Agency.

Presently, the Cassini group are not sure whether the mission will get the extension they are hoping for. The earliest they will know of this is a year from now. Then it will take them six months to a year to know if the money is available. The team, though, is hopeful that they will get the official approval and the funds required.

“In the meantime, we are planning… thinking about what it is we really want and need to do. Certainly, one of the cardinal goals of extending Cassini’s tour of the Saturnian environment would be further exploration of Enceladus,” says Porco.

The mystery

In order to delve deeper into the mystery of the jets shooting from the surface of this celestial body, close flybys will help in determining the interior structure of the moon. “We would aim to fly very close over the South Pole (of the moon) and through the jets and plume, in order to make more accurate measurements of the composition of the vapour and ice particles,” says Porco.

But how close the spacecraft will fly to the surface of Enceladus will only be determined after safety issues are considered. As to the examination of the particles gushing out, Porco says that it is still not clear whether Cassini has the means to determine if the ice crystals contain microbes.

“It may require a device with much greater compositional precision than we have, and so this may have to be left for a future mission,” the scientist adds. To find certain answers, Porco is of the opinion that remote sensing equipment will have to be more precise. Also, the thermal radiation at the south poles of the moon will have to be studied in detail to learn where exactly the warmest places are.

“We will want to do a better job of determining the composition of the ices comprising the fractures, which contain the simplest organic compounds not found anywhere else. And we will want to take higher resolution images of the jets to refine our estimates of particle abundance and size and ascertain in which locales the most vigorous geysers are occurring,” he explains.

Cassini is so well equipped that it could be a precursor mission for a future landed mission to the south pole of Enceladus. In other words, the scientists at Nasa could do in the next few years with Cassini at Enceladus what the next orbiter of Jupiter would do at Europa. Furthermore, Enceladus does not have an intense radiation field like Europa, and so there would be unlimited time for the spacecraft to operate and explore.

“Water closer to the surface and more time spent conducting scientific investigations… sounds like a winner to me,” says Porco.

As far as the mysterious plumes are concerned, Jonathan Lunine, professor of planetary sciences and physics at the University of Arizona, is of the opinion that Cassini is capable of studying their nature. “It’s possible but not certain, by the end of the Cassini mission, that there will be definitive answers as to whether the plume geyser activity requires liquid water in the Enceladus subsurface,” he said at the 37th Lunar and Planetary Science Conference in Houston, held in March.

In comparison to Europa, scientists are of the opinion that Enceladus is a better bet for finding water and biology. “What’s different here is that pockets of liquid water may be no more than tens of metres below the surface,” remarks Andrew Ingersoll, member of the Cassini imaging team and an atmospheric scientist, from the California Institute of Technology.

Europa, the other contender for liquid water in the solar system, is thought to have an ocean of liquid water miles below its surface of thick ice. Enceladus with its icy frigid geysers, is more inviting. The optimism is apparent in the scientific colony.

“You’ve got liquid water, and its liquid water interfacing with rock... and there’s energy. We’ve got the very most basic ingredients here and so that notches it up on the biological potential list,” says Candice Hansen-Koharcheck, Nasa’s Cassini scientist. The team led by Hansen-Koharcheck have come to the conclusion that the plumes have been jetting out of Enceladus’s surface continuously for 15 years and the activity is responsible for replenishing Saturn’s E-ring.

In a related article to the plume story on Enceladus, Jeffrey Kargel, researcher in the department of hydrology and water resources, at the University of Arizona, feels that Cassini is capable of detecting water below the icy surface but would not be able to determine a suitable habitat for life. “Any life that existed could not be luxuriant and would have to deal with low temperatures, feeble metabolic energy, and perhaps a severe chemical environment. Nevertheless, we cannot discount the possibility that Enceladus may be life’s distant outpost,” he writes.

In order to plan future missions successfully, it is most important that Cassini studies the Saturnian system in as much detail as possible, says Lunine. “One could even imagine an extended mission with enough emphasis on Titan and Enceladus, to constitute a kind of ‘Cassini Astrobiology Mission’.” He adds, “One of the lessons about this is that we cannot explore the outer solar system without well-instrumented flagship missions.”

Another eager scientist for Enceladus exploration is Chris McKay, who is a planetary scientist at the Nasa Ames research centre in Moffett Field, California. According to McKay, there are three main reasons why Enceladus holds great interest as an astrobiology target.

The moon’s activity points to the possibility of a subsurface energy source. This source seems to be concentrated in parts, leading to the assumption of subsurface liquid water and possibly even methane. “All together this makes Enceladus a key target for astrobiology,” says Lunine.

The scientist is hopeful of Cassini’s endeavour to study the geyser activity. “Key would be the detection of carbon- and nitrogen-containing particles,” he says. With this recent and puzzling discovery on the Saturnian satellite, Enceladus now joins the list of the foremost targets for astrobiology, which include Mars and Europa.

Without mentioning the latter two destinations, McKay said: “But clearly the Saturn system with organic rich Titan and water-active Enceladus is on the first rank of targets for astrobiology. We should start planning a combined Enceladus lander and Titan balloon mission.”

The extraterrestrial bodies containing liquid water are the best places to set sights on for future space forays and, according to Porco, Enceladus is the foremost place to explore.

“It has liquid water, two-and-a-half times more heat coming out of the South Polar Region per square metre than the Earth, and simple organics. It’s quite provocative, and is looking very good from the astrobiological point of view,” she says. She also states in an email interview with , “this finding has substantially broadened the range of environments in the solar system that might support living organisms, and it doesn’t get any more significant than that. I’d say we’ve just hit the ball right out of the park”. The surface temperature on Enceladus is only -201 degrees Centigrade or -330 degrees Fahrenheit.

Moreover, Porco’s team concludes that the buried pockets of liquid water are above 32 degrees Fahrenheit or 0 degrees Celsius. The source of heat at the hot spot is also undetermined though there are assumptions. In order to get all the answers, “We’re all going to have to patiently wait,” says Hansen-Koharcheck.

The writer is a freelance contri-butor