#37 July 1990
Section 220.127.116.11.037.of the Artemis Data Book
by Peter Kokh
Ever since the romantic pre-space-age vision of a planet-wide ocean on Venus was so cruelly burst by the radar detection of surface temperatures there in the 900xF range, and since our probes showed that the atmosphere on Mars was too thin to allow liquid water to subsist on its surface, we have had a growing appreciation of our water-laden home planet for the very special oasis which it is. But the revised popular notion that, in all the Solar System, Earth alone has appreciable reserves of liquid water, is mistaken.
On their fleeting passes through the Jovian system, the four Pioneer and Voyager probes revealed an ice-crusted Europa, with a surface that has been aptly described as "smooth as a billiard ball". Absent are any crater relics of the earlier epoch of wholesale bombardment by debris left over from planet-formation, an ordeal apparently experience in common by all the planets and their moons. Europa's brilliant white crust is crisscrossed by narrow brownish streaks that show no topographic relief (height or depth). Global elevation differences do not much exceed a token hundred meters or so.
Apparently, the ice crust of this moon is thin enough to fracture under internal pressure from time to time, letting a water-brine of some sort erupt out onto the surface, quickly freezing the fracture shut. Europa, it seems, has an ocean! Water and vacuum do not socialize. But ice and vacuum get along quite well. Thus a thick enough self-derived icy "firmament" can contain an ocean just as effectively as a thick atmosphere. However, some source of internal heat is needed to keep the ice crust from thickening until that's all there is. Europa, a little smaller than the Moon (1942 miles versus 2160 miles in diameter), is hardly big enough either to have long-retained any residual heat of formation, or to have undergone sufficient internal heating from radioactive materials in the rocky silicate crust that probably underlines its ocean - we know Europa's mass, hence its density, and can argue from that. Of the four great Jovian moons, Europa is the second closest-in, and that's near enough to provide continuous heating from the tidal stress that characterizes its location deep down the throat of Jupiter's massive gravity well.
The guesstimates I've seen are that the ice crust is no more than 2-3 miles thick and that the globe-enveloping ocean below could be 60 miles deep. Even considering Europa's smaller size, 1/4 th the diameter and 1/16 th the surface area of Earth, that still amounts to 1.8 times as many cubic miles of water as in all Earth's oceans (if you'll pardon a back-of-the-envelope calculation). Any hypothetical Europans would be amused, if not chagrined, to read of Earth's boast to be THE "water planet"!
How about a proper name for Europa's Ocean? In ancient mythology, Rhadamanthus was the son of Europa by Jupiter. So The Rhadamanthic would seem an especially appropriate choice. Europa and its ocean supplied the supporting theme of a major motion picture: Arthur C. Clarke's "2010". What might the conditions in this ocean be like? We would expect it to be devoid of dissolved oxygen. But any gases vented by submarine volcanoes, a plausible feature, once they reached a certain saturation point in the water, may keep collecting in gas pockets below the ice, eventually building up enough pressure to fracture it in the manner our probes have observed. There could also be some dissolved salts, not from any sediments washed down from eroding protruding continents, of course, but from ash vented by these same conjectured submarine volcanoes. Given the light Moon-like gravity, even at some sixty miles down, the water pressure would be only half that in the Marianas Trench in our own Western Pacific. Exciting? Conjure up story plots? Hold on! Enter another discovery 400 million miles sunward, down home in Earth's own ocean depths. Scientists have found thriving teeming oases of ocean-bottom life huddling around deep sulfur-rich hot water vents on the mid-ocean ridges. Here in the absence of appreciable dissolved oxygen, hitherto totally unsuspected well-diversified colonies of life-forms evidently descended from familiar varieties - yet strangely, grotesquely, and beautifully evolved - live out secluded lives feasting on their exotic dietary bonanza, in a darkness whose totality is sometimes punctuated by the orange glow of quickly cooling erupting lava. We had once thought that all food chains, must begin with photosynthesis. Evidently, hot sulfur-rich mineral-laden water jets provide nourishment and energy enough upon which to base whole independent ecosystems.
[For an amply-illustrated early account, look up the November 1979 issue of National Geographic.]
It is the safer assumption that life could not originate in Europa's ocean - or that it could not have gotten much further than sulfur eating bacteria. On Earth, even if the sulfur-eating organisms at the bottom of the hot-vent food chains are native to such sites, the diverse multi-cellular metazoan creatures such as the tube worms we now find there, are surely cousins to long-familiar varieties and co-descended from ancestors that had evolved in oxygen-rich, photo-plankton fed ecosystems in much shallower seas. Yet it seems equally plausible that we might someday successfully transplant some denizens of our own ocean bottom vent colonies there, at the bottom of the Rhadamanthic. We could then hang around for some centuries to watch their progress from stations on the ice-crust-firmament above, using telemetry and tele-operated benthic probes.
Where is the imagination of our planetary scientists that they have not yet brainstormed a mission to dispatch a drill-equipped probe to Europa? Well, drilling through some miles of ice is far more ambitious - as remote robotic missions go - than anything we've attempted to date, even skin-pricking comet penetrators. This is a mission for the next century, one that will generate a lot of suspense and expectation. Now that we have found relics of ancient, probably now extinct, native microbial life on Mars, shall we someday find evidence of existing microbial life in Europa's ocean? How far and how diversely will it have evolved?
[At the Midwest Space Development Conference recently held in Cleveland, Ohio in October 1996, Dr. Donald Palac, chief of the Advanced Program Development Branch at NASA Lewis Research Center, speaking on the early Galileo probe results, in response to my question on this very topic, said that he has been asked by the AIAA to formulate a Request For Proposals for a Europa Lander, aimed at University and College teams. These RFPs will go out by year's end (1996), with the best entry to receive AIAA sponsorship in the competition for the next annual funding slot in NASA's "smaller, faster, cheaper" Discovery Mission series. Currently (4/'00) NASA is working on Europa Orbiter to map the thickness of the ice crust and the ice/ocean interface. PK]
Some have suggested that Europa's larger sibling moons, Ganymede and Callisto, with 3.3 and 2.4 times as much surface area respectively, also may harbor vast oceans under their dirty ice crusts. But it is clear from the near saturation incidence of slumped craters on their surfaces that the ice crust on these moons must be substantially thicker, and that as a consequence, water from below is far less likely to work its way up. Both moons are too far removed from Jupiter to benefit from tidally-induced heating. So it seems more likely that any oceans there are long since frozen through.
Around Saturn, close-in Enceladus also sports a smooth bright icy surface apparently regenerated in recent times. Again, tidal heating and water venting are suspect. Whether this so much smaller world, only 310 miles across, still has a mini-sea under the visible surface is something else.
Whatever the case for Ganymede, Callisto, and Enceladus, in general, the conditions for the formation and maintenance of Europa-like moon worlds seem rather easy to meet in the vicinity of gas giant planets. And gas giants should be quite commonplace throughout the galaxy. It will matter little if the Jove-like primary of the candidate moon does not orbit a sun-like star. Might they not even circle rogue gas giants and isolated brown dwarfs in the sunless interstellar reaches? (Brown dwarfs are dud wanna-be stars with not quite enough mass to trigger or sustain nuclear ignition and thus become true "stars". The jury is still out on whether they are relatively rare, or far more abundant than all other star types.)
[As of 4/'00, similar sub-ice oceans are strongly suspected on both Jupiter's moon Callisto, and Pluto's moon Charon. - PK]
As Europa is one of a class of very special worlds that we might search for elsewhere we'll need a generic name for them. One possibility is "oceanids", pronounced oh-SEE-a-nids. In mythology, the oceanids were daughters of Oceanus and Tethys. But "europids" (yoo-ROH-pids ) would also work, taking their name from the first of the class to be discovered. What really gets the juices flowing is the possibility that Europa-like worlds are far more common out there than Earth-like ones, outnumbering them by perhaps a thousandfold, or more. We ought to make it a top priority in the next century of two to see what we can do with Europa - exercising all due environmental caution, of course. Farming the sub-glacial oceans of such moons could be an alternate ticket to the universe, one which many a star-faring civilization has adopted for its main thrust.
What would a humanoid culture in a Europa-type setting be like? How would it develop? Would they live like the fictional survivors of some sunken Atlantis - subject of several science fiction films and TV episodes - in pressure resistant glass and metal "bubbles" within which they breathe air - and live, work, and play much as we do? Hey, if you're looking for a fresh twist for your science fiction novel, why not brainstorm such a "stranger in a strange land" type of culture? What you come up with might be a truer caricature of the galactic "beyond-the-cradle mainstream" than is the world and civilization into which we've all been born.
Contents of this issue of Moon Miners' Manifesto