It had been known for decades that there was probably some sort of geyser-like activity erupting material off the surface of Enceladus. Its surface is coated with dazzlingly white fresh frost; the long-distance Voyager photos of it showed that some patches had apparently had most of their craters obliterated; and Enceladus is located smack in the middle of Saturn's huge doughnut-shaped "E Ring" of microscopic ice particles. But no one expected Cassini to reveal that all of Enceladus' current-day plumes are erupting from a small area around its south pole, and are much bigger than expected.

Still less did they expect Cassini to reveal that -- instead of erupting a mixture of water and ammonia, which can stay liquid all the way down to minus 103 degrees C -- Enceladus' plumes seem to have almost no ammonia mixed with their water. That fact suggests that Enceladus' south polar region may have a local subsurface sea composed not of very cold liquid water/ammonia mix, but instead of plain water at room temperature!

It's hard to devise a mechanism by which such a big southern pocket of water could stay liquid beneath Enceladus' ice crust, for the extent to which it is heated by tidal flexing from the other moons is thought to be much less than the tidal flexing that heats Jupiter's moons Io and Europa.

One recent theory suggests that one pocket of Enceladus' large rocky core may have been initially melted into magma by a particularly high concentration of the intense radioisotope aluminum-26, which seems to have been injected into the very early Solar System in large amounts by a nearby supernova -- and that, even after the Al-26 had decayed, the greater friction resulting from the tidal forces working on such an already-gooey chamber of melted rock could generate enough heat to keep it molten up to today.

Even more important, however, is the possibility that -- if Enceladus really does have a subsurface liquid-water sea at one place -- it might be still another unexpected possible site in the Solar System for microscopic life to evolve. Cassini's onboard mass spectrometer was designed to anlyze the denser gases making up Titan's upper atmosphere, and so it has had trouble identifying trace constituents mixed in with the Enceladus plumes' water vapor -- but it has definitely located traces of nitrogen, carbon dioxide and methane, and seems to be detecting smaller traces of more complex hydrocarbons such as acetylene, propane, and/or ethylene.

The latter, in particular, suggest that Enceladus, at least at one time, had enough heat in its rocky core to both break down the little moon's initial frozen ammonia into nitrogen, and to synthesize methane and other hydrocarbons -- and that combination of warmth and organic synthesis just might be capable of converting nonliving organics into more complex forms and perhaps even into microscopic life. (As with the Solar System's other cold worlds, even the discovery that the development of life there had been shut off at the partway point would make it priceless as a preserved record of the unknown steps by which life first evolved out of organics on Earth.)

The trouble is that we are still not certain, at this point, that Enceladus' erupting plumes of water vapor and tiny ice particles really are erupting from a still-existing subsurface liquid-water sea -- they may be released by much lower excess internal heat that is breaking down near-surface "clathrates" (water ice with different types of gases frozen into its molecular structure), with Enceladus not having any subsurface liquid water at all anymore. Even in that case, however, the trace gases trapped in such clathrates would likely have been formed in the first place by heat-driven chemical processes during Enceladus' earliest, hotter days -- which raises the possibility that fossil microbes might be frozen into Enceladus' near-surface ice even if the little moon has been dead for billions of years.

And so Enceladus' sudden stunning new status as a major subject of actual biological interest in the Solar System has led to the possibility that a followup Enceladus mission may crash the previously-decided queue of big near-future Solar System missions. In its final 2006 report, the Roadmap Group clearly hadn't decided how to deal with this new problem -- the report only mutters vaguely about the possibility of an (undescribed) Enceladus Explorer, or perhaps of trying to enlarge the previously selected Titan Explorer mission to study Enceladus as well (again in some totally unspecified way). The logical way to carry out biological exploration of Enceladus would be with a soft lander, like the one that may ultimately be flown to Europa.

But -- while Enceladus has far weaker gravity than Europa, and Saturn lacks Jupiter's electronics-destroying super-intense radiation belts -- such an Enceladan lander would still be a big and difficult mission. Once a craft has entered orbit around Enceladus, actually landing there takes only 680 km/hour more braking delta-V -- but getting into orbit around Enceladus is particularly hard in the first place: the little moon races around Saturn so fast that even a craft already in an orbit stretching between the orbits of Titan and Enceladus would have to make a really huge braking maneuver just to match Enceladus' orbit; and it is so small and nonmassive that you can't use repeated gravity-assist flybys of Enceladus itself to help trim down its orbit (which is the technique that will be used for the Europa Explorer orbiter).

So -- as with Europa -- before we dispatch something as major as a soft-lander to Enceladus, we need to take a better advance look at the moon to see if it really is seriously promising as a possible abode of life. During the next 3 1/2 years of its mission, Cassini will make no less than eight more flybys of Enceladus -- and during three of them, it will skim only 30 km above its surface. This will allow its mass spectrometer to get a much stronger whiff of the plume gases to identify what they really include -- and it will also allow Cassini''s infrared instruments to find out just how narrow and how warm the plumes' vent sources really are. But even after that, another intermediate-scale Enceladus mission will likely be necessary before we decide to jump all the way to a lander. And we're very far from deciding just what form such a mission should take, or whether the future Titan Flagship mission could be augmented to study Enceladus as well.

Add to this the continuing fact that NASA will simply be seriously strapped for money to fly these big Flagship-class planetary missions, and a growing number of scientists are beginning to consider just how we might be able to economize in future Solar System exploration by designing and flying cheaper missions that could still answer a lot of the most urgent scientific questions. My next chapter will talk about these new discussions -- and I'll end by presenting several possible ideas for such cheaper but worthwhile missions.

Bruce Moomaw is our first "Space Blogger" at www.spaceblogger.com Feel free to create an account on SpaceBlogger and discuss this issue and more with Bruce and friends.

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