The discovery of apparent liquid water reservoirs erupting in Yellowstone-like geysers on Saturn’s moon Enceladus has produced a gusher of questions.
One leading unknown to solve: Could liquid water residing within a body so small and so cold provide comfort-level conditions suitable for living organisms?
NASA announced last month that high-resolution Cassini images of Enceladus show icy jets and lofty plumes that expel large quantities of particles at high speed. Scientists think the jets spout from near-surface pockets of liquid water, super-cold versions of the Old Faithful geyser in Yellowstone.
In the spring of 2008, Cassini is slated for another chance to look at Enceladus, flying within 220 miles (350 kilometers) of the perplexing moon. The end of Cassini’s "prime mission" is June 30, 2008, four years after arrival at Saturn.
The opportunity exists for placing Cassini in "extended mission" mode — but that’s only if financial resources allow.
Fueled by the spacecraft’s findings to date, attention is now turning to future observational roles of the interplanetary probe. One leading candidate is a sharper focus on astrobiology — even a "diving catch" to inspect in detail the makeup of those Enceladus plumes.
"After what we’ve discovered with Cassini, if we don’t get an extended mission, then there’s no hope for anybody," said Carolyn Porco, the Cassini imaging team leader based here at the Space Science Institute. The Cassini Imaging Central Laboratory for Operations, or CICLOPS, is the nerve center for the imaging team of the Cassini mission to Saturn.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency.
Cassini scientists should know a year from now whether or not Cassini will be given official approval for an extended mission. Then, within six months to a year after that, they will learn if the money is available to proceed beyond the nominal end of the mission, in mid-2008.
"In the meantime, we are planning for such an extension ... thinking about what it is we really want and need to do," Porco told Space.com. "Certainly, one of the cardinal goals of extending Cassini’s tour of the Saturnian environment would be further exploration of Enceladus," she said.
Microbes in the mix
Porco said Enceladus’ interior structure would be determined with close flybys designed to allow for such measurements.
"We would aim to fly very close over the south pole [of the moon] and through the jets and plume," Porco said, "in order to make more accurate measurements of the composition of the vapor and ice particles." Of course, how close Cassini can get to this action from Enceladus will be ultimately determined by spacecraft safety issues ... "but that would be the goal," she said.
And what about looking for microbes mixed in with those high-flying plumes?
"It’s not clear that Cassini has the means to determine if the ice crystals themselves contain microbes," Porco explained. "It may require a device with much greater compositional precision than we have, so that may have to be left for a future mission."
Sounds like a winner
In charting out further study of Enceladus, Porco said scientists would also want to refine their remote sensing measurements. They would map the thermal radiation emerging from the south polar region in much greater detail, to see exactly where the warmest places are on the Saturnian moon.
"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," Porco continued. "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."
In short, Porco said, "Cassini is so very well equipped that it could be the precursor mission for a future landed mission to the south pole of Enceladus. In other words, we could do in the next few years with Cassini at Enceladus what the next orbiter of Jupiter would do at Europa."
Unlike Jupiter’s Europa, Enceladus has no intense radiation field that can limit the available time for spacecraft operations. "So, water closer to the surface and more time spent conducting scientific investigations ... sounds like a winner to me," Porco said.
"It is very important for future missions to Titan and Enceladus that Cassini gather as much information as possible on these bodies," said Jonathan Lunine, professor of planetary sciences and physics at the University of Arizona in Tucson.
Exciting new discoveries will be made, and it will be possible to advance significantly the planning of possible future missions to Titan and Enceladus on the basis of what is learned from Cassini, Lunine told Space.com. "One could even imagine an extended mission with enough emphasis on Titan and Enceladus to constitute a kind of 'Cassini Astrobiology Mission,'" he said.
During last month’s 37th Lunar and Planetary Science Conference in Houston, Lunine also underscored another realization stemming from Cassini’s take on Enceladus.
"One of the lessons about this is that we cannot explore the outer solar system without well-instrumented flagship missions," Lunine told the LPSC gathering. Given Cassini’s range of science gear, the nature of an Enceladus-spewing plume can be studied, as can the mechanisms that form a plume, he said.
"It’s possible, but not certain, by the end of the [Cassini] mission that there will be a more definitive answer as to whether the plume geyser activity requires liquid water in the Enceladus subsurface," Lunine suggested.
There’s a trio of reasons why Enceladus is very interesting for astrobiology, said Chris McKay, a planetary scientist at NASA's Ames Research Center in Moffett Field, Calif.
The moon’s activity suggests that there must be some sort of subsurface energy source, possibly gravitational tidal effects, McKay told Space.com, and that this energy source is concentrated and not a global diffuse heating. This may imply liquid water below the surface, he said.
A concentrated energy source, liquid water, and perhaps evidence of CH4 — methane — being seen, McKay added it up this way: "All together this makes Enceladus a key target for astrobiology. Let’s go."
What additional can Cassini do?
McKay said that further observations of the plume and more detailed measurements of the dust coming from the plume would be ideal. "Key would be the detection of carbon- and nitrogen-containing particles," he added.
Best place to go?
McKay tries to avoid pitting Enceladus against Mars and Europa — other targets of keen astrobiology interest. "But clearly the Saturn system, with organic-rich Titan and water-active Enceladus, is on the first rank of targets for astrobiology," he noted. "We should start planning a combined Enceladus lander and Titan balloon mission."
Carolyn Porco explained that, in her mind, if the goal is to get up close and personal with extraterrestrial bodies of liquid water, then Enceladus appears to be at the moment the best place to go.
"It has liquid water, two and a half times more heat coming out of the south polar region per square meter than the earth, and simple organics," Porco concluded. "It’s quite provocative, and is looking very good from the astrobiological point of view."