On September 25, 1513, Vasco Nunez de Balboa gazed out "from a peak in Darien" and saw the Pacific Ocean shining in the distance. Passing over his claim to discovery (which many others, from Peru to China, might have found a bit strange at the time), the discovery of an ocean is surely a rare moment in history. On Space Day, 21 May 1998, at a panel discussion at Caltech, four speakers (along with the writer Sir Arthur C. Clarke via internet hookup from Sri Lanka) discussed the evidence that a new ocean is even now being discovered on Europa. The organizers were wise to balance the panel with representatives of other disciplines, for remarkable and exciting as the results from Europa are in themselves, it was the convergence of the new information from space with other discoveries in earth science, oceanography, and biology that made the story electrifying. By the end of the evening it was clear that a truly monumental change in our understanding of the emergence of life, on Earth and elsewhere, is at stake.
Europa is the smallest of the four large "Galilean" satellites of Jupiter. Even in the Voyager images of 1979, its remarkably bright, smooth, icy surface, nearly devoid of craters, attracted immediate attention. The discovery at the same time of intense volcanic activity on Jupiter's innermost large moon, Io (orbital radius 422,000 km), demonstrated the importance of heating of the deep interiors of Jupiter's inner moons due to their strong, time-variable squeezing and stretching by tidal forces as they follow their slightly elliptical orbits. For Europa, next out at 671,000 km, the internal heating would be less, but still substantial. The implications from Voyager were not lost on Clarke, who, with characteristic energy, quickly produced a sequel, 2010, to his famous novel 2001, featuring an ocean of liquid water under the protective icy surface, in which primitive Europan life waited in the cold and dark for its chance to bloom. It is now clear that developments since 1979 have brought Clarke's charming fantasy into the scientific main line.
Since Galileo arrived in Jupiter orbit in December 1995, it has added greatly to our knowledge of Europa, and the evidence for an interior ocean has strengthened to the point that, with the end of the original Galileo mission late last year, it was decided to focus the remaining life and resources of the Project on a more detailed investigation of that one moon. Thus was born the "Galileo Europa Mission". Jim Klemaszewski, of the Arizona State University, described the evidence for an ocean based on data from the Galileo Imaging Team. First, the surface of Europa is confirmed to be remarkably young (by the absence of extensive cratering) and smooth. Many moderate-resolution images bear a striking resemblance to pictures of the Arctic Ocean on the Earth, with great white floes or rafts that appear to have split off and separated, then moved slightly or rotated, before being locked into place by the freezing of material in the cracks left between. Compared to the Earth's moon, where craters and impact scars hundreds of km in diameter are found, the larger craters on Europa hardly exceed 40 km in dia, and their profiles are quite different, being strangely slumped and flattened from the bowl-shape typical on Earth's Moon and on Callisto, Jupiter's heavily-cratered outermost Galilean satellite. In addition, there are many signs of more recent reprocessing due to activity driven from below, as expected if warm water was being driven up by the expected internal heat source. Older features, such as cracks and mysterious long straight grooves that extend for hundreds of km across the surface, are frequently interrupted by local upwelling, "chaotic terrain", domes, and astonishingly smooth little patches. As on Io we see volcanism characterized by magma and molten sulphur, on Europa we seem to see "volcanism" driven by heated water.
It is important to remember that the ability of Galileo to survey the entire surface of Europa, as was once hoped, has been severely curtailed by failure of the main antenna to deploy properly, greatly limiting the amount of data that can be returned. Thus it has been necessary to take wide area views at low resolution, pick a few areas for more detailed study, and then obtain moderate resolutions images of those. As a result, a truly comprehensive view of the surface must await some future mission. Nevertheless, the tentative conclusions are roughly as follows:
1. The age of the surface is no more than about 10-30 My, as estimated from crater counting. It would be surprising if liquid water were to be present for 99% of the age of the Jupiter system, and then vanish in the last 1%. Thus the implication is strong that an ocean exists below the crust to the present day.
2. Given estimates of the heat flow and other factors, the ice is probably about 15 km thick, but could be thinner in regions of local heating and recent activity.
3. Similarly, the ocean under the ice probably has a depth of 50-100 km. In this case the amount of water is comparable to, or possibly even larger than, the total in all the oceans of the Earth.
4. Given the key role of the tidal heat source, one expects physical conditions on the bottom of the Europan ocean to bear a strong resemblance to those in the volcanically active regions along the mid-ocean ridges of the Earth.
The most exciting thing about all this is that, within the last 20 years, exploration of the deep ocean floor of the Earth and molecular biology have together greatly changed our view of the likely environment in which life originated on Earth. Dr. John Delaney, of the University of Washington, described the ALVIN submarine and sea-floor volcano research in which he has been involved, and reviewed the implications for Europa. A vast and unexpected community of life exists around the vents of volcanically heated water, saturated with hydrogen sulfide and other minerals, which are found along the ocean spreading centers. The molecular biological evidence is that the bacteria-like organisms that thrive in these waters, the archaebacteria --an entire new kingdom of life, distinct from the eukaryotes and prokaryotes previously known -- may well be the closest living relatives of the earliest life on Earth. Based on undersea volcanic ejections of immense quantities of such bacteria, some lasting for over a year, it has very recently been realized that in addition to the immediate environment of the volcanic vents, very large populations of subterranean bacteria must be present, apparently all based on processes like the reduction of hydrogen sulfide for their energy metabolism. It has even been estimated that the entire biomass of the Earth and its associated carbon inventory may be dominated by these previously unsuspected communities of the abyss.
The similarity between the environment of these newly discovered kingdoms of life on Earth and that extrapolated to the bottom of the Europan ocean clearly has the potential to completely transform our view of the origin and distribution of life. We must now recognize that the origin of life may not necessarily even require the environment of a star.
Further investigation of Europa has thus become one of the Agency's highest priorities. A Europa Orbiter mission is planned for launch in 2003, which would arrive at Jupiter about 2.5 years later. It should prove the existence of the ocean beyond doubt, and with radar sounding map the thickness of the ice crust. Then a following lander mission which would send a melt-through probe into the ocean, and release a small submarine to conduct detailed investigation. Joan Horvath, manager of the "Europa Vostok Initiative" at JPL, described technology development work now underway to explore Lake Vostok, a large and very ancient lake sealed some 2 km below the ice near the Russian Antarctic station, to try out melt-through probes and miniature submarines that could be used as the basis for the exploration of the Ocean of Europa, within, she added, "my professional lifetime".