Rico's rants: Europan mysteries

Gizmodo has an article about things happening on Europa:

In recent years there has been considerable interest in Europa, one of the four Galilean moons of Jupiter. The Galileo Mission has provided encouraging evidence that Europa might have an ocean of liquid water under a layer of ice and this has stimulated speculation that life might possibly exist in such an environment. A future mission to send a spacecraft into orbit around Europa is still awaiting funding. The goal would be to determine definitively if such an ocean really exists. If the result turns out to be positive, then a subsequent mission will send some kind of robotic submarine to melt through the ice and explore the sea below. On this page I will try to trace the origin of the idea that an ocean might exist under Europa's icy crust. Then I will describe some of the early speculations about how life might have come to evolve in such an environment. Many of the articles, books, and lectures that will be mentioned discuss not just Europa, but also two other Galilean moons - Ganymede and Callisto, which in the 1970s were also thought to potentially possess an ocean of liquid water (still a real possibility). Just to give the flavor of the research and speculation into this topic, I will include copious quotes, letting the authors speak for themselves. The reader will also find numerous links to various related topics and to some of the beautiful photos that are available on the internet (indicated by an asterisk *).
Jupiter* has sixteen known moons. The four largest are Io*, Europa*, Ganymede*, and Callisto* which were discovered by Galileo in 1610. Simon Marius, who may have discovered them at the same time as Galileo, named them after Jupiter's illicit lovers in Greek and Roman mythology. They are usually called the Galilean satellites and referred to in many scientific papers as JI, JII, JIII, and JIV in the above order (which is the order of their distances to Jupiter). Until Pioneer 10 and 11 reached the Jupiter system* in 1973-74, astronomers studied these moons with Earth-based telescopes. As early as 1951, the geophysicist H. Jeffreys proposed the possibility that Callisto might be partially or totally composed of water in the form of ice. This was suggested by Callisto's very low density, and also its albedo. In a lecture given in 1957 at a meeting of the American Astronomical Society, the astronomer G.P. Kuiper discussed his study of the spectrum of reflected sunlight from the Galilean moons which was based on observations made at the McDonald Observatory. After mentioning a marked difference for Europa and Ganymede, he stated that : "This is most readily explained by assuming that JII and JIII are covered by H2O snow." In the mid-1960s, additional evidence that these moons might be covered with water in some form was found by the Soviet astronomer V.I. Moroz who wrote that : "Europa and Ganymede could very well be covered with ice , if not entirely, at least in large part. Europa shows the deepest ice absorptions and the lowest temperature." This was again based on studying light spectra [A, B]. Then, in the early 1970s, various astronomers were able to provide solid confirmation of the existence of water frost or ice on these moons by analyzing the absorption of the infrared frequencies of sunlight reflected from their surfaces.. This became possible because, at that time, laboratory studies of the spectrum of water ice for those frequencies had just recently been carried out. For example, in their paper Galilean Satellites: Identification of Water Frost (in Science, vol. 178, 1972 ), C.B. Pilcher, S.T. Ridgway, and T.B. McCord report the results of their measurements of infrared reflectivity observed from the solar telescope at Kitt Peak National Observatory. They determined that between 50 and 100% of the surface of Europa, 20 to 65% of the surface of Ganymede, and 5 to 25% of Callisto are covered with water frost . This is one of a number of papers from the same period which attempted to understand details about the surfaces of the Galilean satellites by carefully examining the properties of the light reflected from the Sun and from other sources.
The idea that Europa and other ice-covered bodies in our solar system might possess an ocean of liquid water under a crust of ice was first proposed by John S. Lewis in his paper Satellites of the Outer Planets: Their Physical and Chemical Nature (which appeared in Icarus, vol.15, 1971). It is a theoretical paper which proposes models of the structure of various moons of the outer planets of our Solar System based on making some simplifying assumptions and some hypotheses about the chemical composition of these ice-covered bodies. The available data concerning their composition was quite limited at the time and so Lewis writes: "We must therefore depend to a considerable extent upon our knowledge of the composition of the Sun and the chemical behavior of volatiles [such as H20] at low temperatures in making plausible conjectures regarding the bulk composition of solid material in the outer solar system." Lewis begins the paper with a succinct summary: "Steady-state thermal models for the icy satellites are constructed in which the energy released by radioactive decay [A , B] in the interiors of the satellites is exactly balanced by the net radiative loss from their surfaces. It is shown that the Galilean satellites of Jupiter and the large satellites of Saturn, Uranus, and Neptune very likely have extensively melted interiors and most probably contain a core of hydrous silicates, an extensive mantle of ammonia-rich liquid water, and a relatively thin crust of ices." The heart of the paper is a mathematical analysis of the flow of heat from the core to the surface due to a phenomenon known as convection [A, B, C]. This analysis is based on estimates of the surface temperature of these icy bodies and the amount of heat which would be produced in the core by radioactive elements. The rate of heating in the core is assumed to be that given by the average rate of decay of uranium, thorium, and radioactive potassium. At the end of this paper, Lewis suggests that if an ocean exists in these ice-covered bodies, then there might be a detectable magnetic field to search for: "An extensive electrically conducting mantle forced to convect by an input of heat from below may be conducive to the production of a measurable magnetic field."
In a slightly earlier article entitled Satellites of the Outer Planets: Thermal Models (in Science, vol. 172, 1971), Lewis announced his theory, discussing specifically Callisto: "Steady-state thermal models for the large satellites of the outer planets strongly indicate that their interiors are currently maintained at temperatures well above the ice-ammonia eutectic temperature by the decay of long-lived radioisotopes of potassium, uranium, and thorium. The present-day steady-state thermal structure of a representative satellite, JIV (Callisto) is shown to be characterized by the presence of a thin icy crust over a deep liquid mantle, with a dense core of hydrous silicates and iron oxides." A few years later, in 1974, Lewis gave one of nine Guggenheim Lectures in a series called Man and Cosmos at the Smithsonian Institution. His lecture, entitled The Outer Planets, includes a discussion of the large satellites in our Solar System in which he describes his theory as follows: "A mixture of icy and rocky material, assembled into a body the size of Mercury, heats itself up by the decay of naturally occurring radioactive elements in its interior, causing it to melt. The dense silicates will settle to form a core, which we might think of as being made of mud. That will leave a very thin ice crust floating upon a thick mantle composed of a solution of ammonia in water. This kind of structure has not been observed by spacecraft or by direct observations, but it is a suggestion of what we may someday observe in the course of exploring the satellites of the outer planets."
The papers of Lewis just mentioned and several other theoretical papers that will be discussed here are based on theories about the formation and early history of the Solar System [A, B, C]. Jupiter and its system of orbiting moons may have originated in a way analogous to the formation of our Sun and its orbiting planets. The details of this early history would be responsible for many characteristics of these moons as they are today - their orbits, chemical composition, densities, etc. An article entitled Implications of Jupiter's Early Contraction History for the Composition of the Galilean Satellites by J.B.Pollack and R.T.Reynolds was published in 1974 (in Icarus, vol. 21) , exploring the consequences of theories about the formation of Jupiter and its satellites. The authors write: "Recent calculations of the gravitational contraction history of Jupiter indicate that Jupiter's luminosity was orders of magnitude larger during its early lifetime than it is today. As a result one might speculate that the condensation of icy volatiles to form satellites would be inhibited at close distances to Jupiter and compositional differences among Jupiter's satellites might be generated. We propose that the observed systematic variation of the mean density of the Galilean satellites with distance from Jupiter is a result of the above circumstances." Based on those recent calculations, they argue that the Galilean satellites would have had an abundance of liquid water at least for millions of years in their early history and that "water ice appears to be the only ice likely to condense out in significant proportions, i.e. the Galilean satellites are [presently] mixtures of rocky material and water ice." Their theory also gives an explanation for the fact that the densities of the Galilean satellites decrease with their distance to Jupiter. That is, Io has the highest density and Callisto the lowest.
The International Astronomical Union sponsored a conference at Cornell University in 1974 called Planetary Satellites. A rather substantial volume with the same title was published in 1977, edited by J.A.Burns, which contains 27 papers summarizing the state of knowledge in the mid-1970s concerning the various moons in our Solar System. Most of these papers originated as lectures delivered at that conference. A paper by G.J. Consolmagno and J.S. Lewis, entitled Preliminary Thermal History Models of Icy Satellites, outlines the general ideas and underlying assumptions behind the thermal models which the authors were developing, based on the ideas in the 1971 papers of Lewis. Other papers in the volume deal with the rings of Saturn, the atmosphere of Titan, the surfaces of some of the satellites, theories about the formation of the outer planets and their satellites. Several papers discuss the orbits of satellites, how the orbits evolve over time, and the phenomenon of orbital resonance which turns out to be especially interesting and important for the Galilean satellites.
Another paper from the Planetary Satellites conference, entitled Io's Surface and the Histories of the Galilean Satellites, by F.P. Fanale, T.V.Johnson, and D.L.Matson, discusses in considerable detail the possible nature of Io's surface and how it may have come about. Observations from Earth and from Pioneer 10 and 11 showed clearly that Io is quite different from Europa, Ganymede, and Callisto. The authors argue that Io started with much less water which then was drawn to the surface and may have evaporated rather completely from that body into space leaving the surface covered with the resulting salts: "After considering current data and various compositional hypotheses, we conclude that Io's properties can best be explained if it is postulated that the surface of Io is largely covered by "evaporate" salts produced by defluidization of Io's interior, migration of salt-saturated solutions to Io's surface and subsequent H2O loss to space." "Io's surface seems to represent the end result of a surface dehydration process. On Europa's surface, this dehydration is not complete, and it appears that "clean" H2O ice has been added to it lately at a faster rate than the rate of loss. Ganymede and Callisto (especially Callisto) appear to have very thick ( >100 km) ice crusts overlying huge (>600 km) liquid H2O mantles." The authors obtain these conclusions by developing models for the thermal history of the Galilean satellites by an approach which is somewhat different mathematically than that of Consolmagno and Lewis. Concerning Europa, their model suggests a much thinner ice crust and only the possibility of a liquid water mantle.
Pioneer 10 arrived at the Jupiter system and started sending back valuable data at the end of 1973. That encounter sparked an increased interest in Jupiter and its moons and led to the planning of a volume devoted to the latest research . The volume, which became a 1200-page compendium called JUPITER: Studies of the interior, atmosphere, magnetosphere, and satellites (edited by T. Gehrels), appeared in 1976. One long paper by A.G.W. Cameron and J.B.Pollack discusses the origin of Jupiter and its satellites. Many of the other papers discuss in detail various questions about Jupiter's atmosphere, ionosphere, magnetic field, and radiation belts. More than 200 pages are devoted to Jupiter's moons. There is a paper by Lewis and Consolmagno, entitled Structural and Thermal Models of Icy Galilean Satellites, which gives rather detailed models for Europa, Ganymede, and Callisto. The paper makes various sets of hypotheses about the early history of these satellites and the models are based on computer simulations carried out earlier by Consolmagno. In the conclusion of this paper, the authors suggest on the basis of their models that at present Callisto might have a 1000-km deep liquid water ocean covered by a 200-km thick crust consisting of rock and ice, Ganymede might have a 400-800 km deep liquid water ocean and a 100-km thick crust of ice, and Europa might have a 100-km deep ocean of liquid water below a crust of ice 70-km in thickness.
Here are a few quotes from the Lewis-Consolmagno paper: "Thermal history models are presented for a suite of possible initial structures. Complete melting and differentiation of the ice component of Europa and Ganymede due to internal heat sources are predicted." In the section describing their models for Europa after various periods of time (dating from the origin of the Jupiter system), they write: "After 250 million years, substantial melting has already taken place, resulting in differentiation of the water and silicates. Melting has proceeded almost to the surface, and a crust of pure ice now exists." "After 4.5 billion years, a structure similar to what we expect for the present may exist. A thin crust of ice covers a convecting region of water, which is cooling off the upper layers of the silicate core. Heat production in the core has dropped as well, as the radioactive nuclides decay. However, the center is still effectively isolated from the surface and continues to heat up reaching temperatures of 2800oK." "Internal heat sources seem to be sufficient for Europa and Ganymede to completely melt at some time in their history (at least to within 30 km of the surface.)" "A Europa with 10% water content would have a 70-km ice crust at present, a 100-km water mantle, and a rocky core 1400 km in radius." "Our models predict considerable thermal expansion, and this may produce significant cracks in the crust, leading to upwelling of the less-dense liquid material underneath and eventually to catastrophic overturn of the crustal layers. But the thermal expansion appears to be on a slow enough time scale that plastic flow of the ice should heal such cracks as they develop." "Europa and Ganymede, with thin ice crusts as we predict, would be more easily punctured by an impact: liquid water could then flow from the mantle onto the surface forming a flat, clean plain...."
The two Voyager Missions arrived at the Jupiter system in 1979. During that same year, three important papers were published which changed the theoretical picture considerably. The first paper, entitled On the Internal Structure of the Major Satellites of the Outer Planets by R.T.Reynolds and P.M.Cassen (in Geophysical Research Letters, vol. 6), was written late in 1978. That paper threw some doubt on the theoretical models of the interiors of the Galilean satellites which had previously been proposed. The authors argue that a crust of ice which covers a layer of liquid water and is at least 30-km in thickness would be unstable. That is, even though the liquid water layer is heated from below by radioactive decay in the core, it would nevertheless gradually freeze because of the cold, icy crust above. They write: "Thermal convection in this planetary ice layer is efficient and will solidify an underlying liquid shell in a time that is short compared with the age of the body." Their mathematical analysis is based on studying solid-state convection in the ice crust. If the ice crust is thick enough , then a quantity called the Rayleigh number will exceed a certain critical value and that would imply that the ice crust is not stable. The thermal models predicted by Consolmagno and Lewis would imply that the ice crusts on Ganymede, Callisto, and Europa are thicker than 30 km. The authors then write that: "The considerations of this investigation apply most specifically to Ganymede and Callisto, with probable application to Europa, Titan, and Triton, all of which are expected to contain large fractions of H2O." Thus it would seem that Ganymede, Callisto, and probably Europa should have a frozen crust of ice covering a core heated by radioactive decay, and without a mantle of liquid water in between .
However, as it turns out, there is another possible source of heat - the immense gravitational force of Jupiter. Just a couple of months later, Reynolds and Cassen together with S.J.Peale wrote a paper entitled Melting of Io by Tidal Dissipation which was published in Science, vol. 203, and appeared just a few days before the Voyager 1 flyby of Io on March 5th ,1979. According to Soderblum's survey article in the January, 1980 issue of Scientific American, this paper caused quite a bit of excitement at NASA because of its surprising prediction that there should be widespread volcanic activity on the surface of Io. Within a few weeks, this prediction was confirmed by studying the images of Io* sent back by Voyager 1. The idea is that the gravitational force on Io exerted by Jupiter should vary enough as Io travels around Jupiter to create a strong tidal effect [A, B, C]. The Galilean satellites have orbits which are nearly circular. But not exactly circular! These orbits are elliptical and the discrepancy from being perfectly circular is measured by a number called the eccentricity. The orbit of Io has the largest eccentricity. It only takes 42.5 hours for Io to complete one revolution around Jupiter. This means that Io reaches the point in its orbit which is closest to Jupiter just slightly more than 21 hours after reaching the farthest point. During that interval of time the gravitational force exerted by Jupiter on Io varies by about 17% . The resulting push and pull on the surface of Io creates frictional heat and substantial melting under the surface and, consequently, volcanic activity.
For Europa, there should be a similar tidal effect. It takes slightly more than 85 hours for Europa to complete one orbit. The eccentricity of this orbit is smaller than that of Io, and Jupiter's gravitational force on Europa will vary by about 4% during one revolution, again creating a significant tidal effect over a very short time interval. Cassen, Reynolds, and Peale pursue this idea in an article that was written about one month before the Voyager 2 flyby of Europa on July 9th, 1979. This article was entitled Is There Liquid Water on Europa and appeared in Geophysical Research Letters, vol. 6 in September of that year. The authors write: "It is possible that tidal dissipation in an ice crust on Europa preserved a liquid water layer beneath it, provided that the three-body orbital resonance for Io, Europa, and Ganymede is ancient. The liquid water layer could be a continuing source of the observed surface frost. If Europa's water mantle were ever completely frozen, heating by tidal dissipation would not exceed that produced by radioactive elements, and the mantle would remain frozen."
The phrase "orbital resonance" refers to the fact that the period in which Io completes one orbit around Jupiter is almost exactly half of the period in which Europa completes an orbit, and that period is in turn almost exactly half of the period for Ganymede to complete an orbit [A , B]. It is this rhythm which is responsible for the eccentricity in the orbit of Io and of Europa. The authors argue that if this orbital resonance was present early enough in the history of Europa, then the tidal effect on the ice crust might have generated enough frictional heat to prevent the freezing of a liquid water mantle. They write: "But suppose that Europa's H2O mantle was melted at one time by some other process, perhaps during the satellite's formation. Then heating by tidal dissipation in a growing ice crust might prevent freezing of the entire mantle. The heating rate in the ice crust is greater than in a completely solid body because the unsupported crust is subject to greater deformation, even though the tidal forces are the same. With the current eccentricity, the maximum amplitude of the variable tide on Europa would approach 50 meters for a thin ice crust over water." The mathematical analysis in this paper of the effect of the tidal forces and the effect of thermal convection in the solid crust lead the authors to two possibilities for the present situation on Europa: "Provided that the orbital eccentricity has been near its present value for most of Europa's history, an equilibrium configuration could exist in which the heat generated by tidal dissipation in a thin ice crust (<10 km) is balanced by thermal conduction to the surface. The tidal dissipation would greatly exceed the heat generated by radioactive elements. A deep (~ 90 km) ocean would exist beneath the ice crust." or "Another equilibrium configuration exists in which the entire H2O mantle is frozen, and in which tidal dissipation augments (but probably does not exceed) radioactive heating." "
The authors also consider the possible fracturing of the icy crust. They write: "In the situation in which tidal dissipation is able to maintain a thin, stable crust, one might ask whether or not such a crust would remain intact." Based on a mathematical analysis comparing the tensile strength of the icy crust with the stress which it is subject to by the tidal forces, they conclude that: " Tidal stresses may be great enough to fracture a thin ice crust, thereby permitting water to evaporate and precipitate elsewhere on the satellite." As they explain, such fractures would expose the underlying water to the near vacuum conditions on the surface and would result in vigorous boiling of the water. However, they point out that if a liquid water mantle does become frozen, the crust would be forced to expand, and this would also result in fractures on the surface.
The scientists who studied the images obtained by Voyager 2 published a summary of their findings and interpretations in the November, 1979 issue of Science (in The Galilean Satellites of Jupiter, Voyager 2 Imaging Science, authored by B.A. Smith and 21 other members of the imaging team). Two pages are devoted to Europa. After discussing the hypothetical theoretical models of Europa' s interior which predict a layer of liquid water, the authors suggest that the darker regions on the surface of Europa* may be areas where the rocky core comes rather close to the surface of the ice (within ~ 10 km). If this is so, they argue that the total depth of a liquid water layer should be less than that predicted by the theoretical models ( ~50 km) because otherwise the topography of the core would be unusually great for a body of Europa's dimensions. This leads them to conclude that the density of the core would be low and therefore that it might contain significant amounts of water. The authors also discuss the dark linear markings in the brighter regions of Europa's surface*, suggesting that these might have been caused by expansion of the icy crust due to freezing of an early ocean, creating fractures in the ice which could be filled by fluids from below and which are now visible as dark markings. The widths of these marking as estimated from the Voyager 2 images indicate that the amount of expansion must be about 5 to 15 percent of the surface area. In order to explain this large amount of expansion, they propose that a thin (~50 km) ocean might have been produced over a period of time from water outgassing from the core and that the frozen crust was forced to adjust to the increasing volume of that ocean.
In the January, 1980 issue of National Geographic, there is a marvelous article by Rick Gore entitled What Voyager Saw: Jupiter's Dazzling Realm. It is a long article, filled with beautiful Voyager photos and many quotes from various scientists involved in the Voyager mission about what they expected to see and what they learned from the mission, and it manages to convey the excitement surrounding the new discoveries. Concerning Europa: "Europa, however, was Voyager 2's star. The scientists were predicting that water-rich Europa could be heated by the same kind of tugging as Io - albeit much less so. "We were hoping to see Old Faithful going off," said geologist Hal Masursky. Voyager 2 saw no geysers - but its resolution was only good enough to detect mammoth ones." Commenting about Europa's remarkable flatness and lack of craters , which led scientists to conclude that Europa has a relatively young surface: "An Io-like tidal heating may indeed be keeping the crust of Europa plastic and the ocean beneath either liquid or soft ice. But no one can do more than guess at what mechanisms Europa uses to erase its craters."

In recent years there has been considerable speculation about the possibility of life on, or within, Europa. This has been spurred by the increasing evidence that an ocean might really exist under Europa's icy crust. One of the ideas that has frequently been suggested is that geothermal energy, which is the primary source of energy supporting life in certain deep sea regions here on Earth, might also provide the needed energy source for life at the bottom of an Europan ocean. The first discovery of those deep-sea communities of life on Earth was in 1977 - Robert Ballard's expedition in the Alvin to the rift zone deep under the Pacific near the Galapagos Islands. That discovery was the subject of an article in the October, 1977 issue of National Geographic (Oases of Life in the Cold Abyss, by J. Corliss and Ballard), and a later expedition led to another article in the November, 1979 issue (Return to Oases of the Deep, by Ballard and J. Grassle). Both articles are filled with intriguing photographs showing certain kinds of giant worms and clams, and other exotic creatures that thrive in those regions, seemingly without any dependence on sunlight. (A, B, C).
Those discoveries on Earth, together with the theories of possible oceans on Europa, Ganymede, and Callisto, inspired some individuals to already make the link in the late 1970s. One notable example is the physicist Gerald Feinberg, who came to this idea early in 1979, and realized that a theory that he had developed with the biochemist Robert Shapiro (presented in their book Life Beyond Earth, published in 1980) might explain how life could develop deep in those Galilean oceans. The essential requirement for their theory would be that the internal heat from the rocky core of those bodies reach the ocean in a concentrated form, such as in a volcanic eruption or an upwelling of hot gas, which would create the necessary "deviation from equilibrium."
On June 19th and 20th, 1979, the conference "Life in the Universe" took place at NASA's Ames Research Center. Benton Clark gave a lecture Sulfur: Fountainhead of Life in the Universe at that conference in which he discussed the biochemistry of those deep-sea vent communities discovered on Earth, pointing out that they do depend indirectly on sunlight: Photosynthesis near the surface of the oceans produces the oxygen that those communities require. Clark then explained how sulfur could play the role of oxygen, and that deep-sea volcanic emissions could potentially provide all the necessary ingredients for a self-sustained ecosystem. In the final part of his lecture, Clark raised the possibility that life might exist in undersurface oceans on the icy satellites in our Solar System, including Europa, Ganymede, and Callisto in particular.
In January, 1980, Richard Hoagland published a long article entitled The Europa Enigma in the magazine Star & Sky. It concentrates specifically on Europa, and was inspired by the images of Europa provided by the Voyager mission in July, 1979, and by the theory that tidal heating might maintain an ocean on that body under its crust of ice. At the end of the article, Hoagland also makes the link with the discoveries of ecosystems of ocean bottom life near deep sea vents, and suggests that Europa "has all the ingredients to permit the existence of similar internally nurtured oases of life."
We will discuss the ideas of these individuals in more detail below, and also some even earlier speculations about life in Galilean oceans going back to 1975, before the discoveries made by Ballard's expedition in 1977. The possibility that some of the satellites of Jupiter might have an ocean became somewhat widely known in the 1970s. Isaac Asimov mentions that Ganymede and Callisto might have oceans under a thick crust of ice in his book Extraterrestrial Civilizations, published in 1979. In New Worlds for Old (also published in 1979), Duncan Lunan devotes a good part of his chapter about Jupiter to the Galilean moons. He writes that "Io's surface might have extended salt beds, perhaps deposited by evaporation of water from below ground in the past. If so, Europa and Ganymede might still have sub-surface "oceans." (Both Ganymede and Europa seem to have surface water ice.) " A little later he writes that "On 4 May 1976 Ames Research Centre sent us the most staggering release to date. Ganymede, it seems, may be almost all water - a single droplet larger than Mercury, encased in rock and ice." . In his book cited above, Asimov raised this natural question: can life develop in a "region of eternal darkness, sealed away from the rest of the Universe by an unbroken miles-thick layer of ice? "
Guy Consolmagno, who worked on the theoretical models of oceans on Europa, Ganymede, and Callisto with John Lewis at MIT, included an appendix in his 1975 Master's thesis Thermal history models of icy satellites where he suggested that Europa could have the beginnings of organic chemistry if the rocky core is as rich in carbon as some of the primitive meteorites. He noted that the core would be in intimate contact with the large mantle of water and that geological evolution, like lava flows, might occur, producing a geochemist's delight of possible reactions, easily comparable to the complexity of Earth's salty oceans. He concluded his thesis by writing "... we stop short of postulating life forms in these mantles; we leave such to others more experienced than ourselves in such speculations." In his book Brother Astronomer - Adventures of a Vatican Scientist, published in 2000, Consolmagno gives an account of a conversation that he had with Carl Sagan just before he was to present his work on the models for oceans on the Galilean satellites at a conference about Jupiter in 1975. Consolmagno suggested to Sagan that such oceans might be places to look for life. Sagan responded quite skeptically, saying that "Life needs energy, sunlight. How are you going to get sunlight through a thick crust of ice." In the question & answer session after his presentation, Consolmagno mentioned his idea that there was a possibility of life in the Galilean oceans, immediately adding: "But Dr. Sagan pointed out, there's no energy source for them - no sunlight down there."
Interestingly, in his long article The Solar System Beyond Mars: An Exobiological Survey, which appeared in Space Science Reviews, vol. 111, in 1971, Sagan himself included Europa, Ganymede, and Callisto in a list of bodies in the outer solar system which he believed offered "interesting exobiological opportunities.". Just the presence of water in the form of ice or snow on the surfaces of those bodies led Sagan to make that remark. Arthur C. Clarke made a similar remark about Europa and Ganymede in 1974 in his essay Closing in on Life in Space, writing that they possess "at least one of the preconditions for life: the presence of water " [in the form of frost or ice].
Duncan Lunan's book New Worlds for Old is based on discussions and public lectures sponsored in the mid-1970s by ASTRA-the Association in Scotland to Research into Astronautics. This group was founded in 1953 and is still very active. Lunan's book is densely-packed with perceptive and well-informed speculations about our Solar System. Chapter 9, which is devoted to Jupiter, has a lot to say about the Galilean moons. Speculation about how life might begin to develop on the Galileans is left to the last two pages of that chapter - just a few provocative paragraphs. Here are some quotes: "At ASTRA, Robert Shaw suggested that life might be evolved following the passage of a comet through the Jovian system: now that we know that comets have immense hydrogen haloes, we might expect that when one interacts with the thin atmospheres of the Galileans it generates powerful electric storms, perhaps synthesizing complex molecules as lightning may have done on the primeval earth. " "With the storms, and the successive freezing and thawing of the atmosphere during eclipses, such compounds might find their way into liquid reservoirs below ground - or interact more slowly in the snow. Any life which comes into being has to survive the radiation belts, however ; if sheltered by a crevice, it must have some energy source such as volcanism to take the place of sunlight."
Lunan offers his own speculations based on a hypothesis of A.T. Lawton presented at ASTRA : " . . . in discussion he [Lawton] suggested that there might be a ring of dust surrounding the Solar System, steadily draining into it. In that way the Galilean moons could have been subjected, over a long period, to an infall of interstellar dust [1, 2, 3] rich in heavier elements from supernova explosions. Such surface dust might well give the Galileans complex chemistries. Everything then depends, again, on whether storms transfer such compounds into crevices - to shelter from the radiation, perhaps to trickle down into warmer regions." "Underground water would greatly increase the chances that life might evolve. In a liquid medium, the required chemical interactions are far more likely to occur and primitive organisms have better chances to spread, survive and evolve. There is water ice on Ganymede and Callisto, at least ; their densities are low; is it too speculative to imagine internal heat and subterranean lakes, even seas ? "
After mentioning that Ganymede might be almost entirely water, Lunan writes: "Imagine looking out of a window at that, orbiting it, landing on it . . . . Imagine penetrating the crust, sending the first bathyscaph down . . . . Will the water be clear or cloudy? How far will the lights carry? Could there be life in that unimaginable blackness, clinging to pockets of radioactive heat on the underside of that extraordinary shell ?" Duncan Lunan ends the chapter by writing that "once terrestrial industry is established in Earth orbit, there are . . . probably not more than a hundred years to pass before there are marine biologists on Ganymede."
The book Life Beyond Earth: The Intelligent Earthling's Guide to Life in the Universe is quite different. The authors Gerald Feinberg and Robert Shapiro take a systematic approach, attempting to develop a very broad perspective on how and where life might originate in the Universe. One long chapter is devoted to the general conditions which are essential for life; another chapter looks at a variety of possible chemistries which could serve as a basis for life. Their ideas were presented in a lecture entitled Possible Forms of Life in Environments Very Different from the Earth given by Feinberg at the conference Extraterrestrials: Where Are They? which took place at the University of Maryland in November, 1979. The authors offer the following definition of Life on which their entire discussion is based: "Life is fundamentally the activity of a biosphere. A biosphere is a highly ordered system of matter and energy characterized by complex cycles that maintain or gradually increase the order of the system through an exchange of energy with its environment." For example, they consider the biosphere of Earth to include the totality of all living things together with all nonliving things which enter into their metabolic activity. The authors propose the following three conditions for life to originate and develop: "A flow of free energy. " "A system of matter capable of interacting with the energy and using it to become ordered." "Enough time to build up the complexity that we associate with life." On Earth, these conditions are provided by light and chemical energy, nucleic acids and proteins, and the eons of time of a relatively stable environment.
The last third of the book is devoted to taking the reader on a tour of the Solar System and beyond from the point of view of the general principles that the authors have already fully described. Their discussion of the Galilean satellites is based on the theoretical models proposed by Consolmagno and Lewis in 1976. They state that Europa, Ganymede, and Callisto are fairly similar to each other, and so they just focus the discussion on Ganymede because it is the largest. Here are some quotes from that discussion.
"Beneath the ice crust, which is fifty to one hundred kilometers thick, lies an enormous ocean, five hundred kilometers deep. The situation resembles that of our Arctic ocean, but the Ganymede ocean is vaster. There is twenty-five times as much liquid water under the ice of Ganymede as on all of Earth. Below this ocean is the rocky core, at a temperature that varies from 25o C at the bottom of the ocean to several thousand degrees at the center of Ganymede" "Neither Ganymede's ice surface nor its ocean is pure water. The water contains dissolved impurities of many kinds, just as Earth's oceans do. The precise chemical form of these impurities is unknown, but they may well contain the same elements and simple compounds present in the primitive oceans of our planet. Furthermore, Ganymede's ocean has probably existed in its present form for several billion years. Therefore, this ocean satisfies two of the conditions necessary for life - a suitable material base and enough time for prebiotic and Darwinian evolution to take place."
"The crucial factor which may determine whether life exists in the ocean of Ganymede is whether a suitable energy source has existed to drive the matter away from equilibrium. The water is shielded from the feeble sunlight of Ganymede by the ice crust. It is hard to imagine any useful energy getting through to the ocean from above. However, there is another direction from which energy can reach the ocean - underneath from the hot rocky core. The same radioactive decays that originally melted Ganymede are still producing heat in the core, and this heat works its way out to the ocean in various forms. In order to be of use as an energy source for life, the internal heat must reach the ocean in a concentrated form, such as in a volcanic eruption or an upwelling of hot gas. Otherwise, the heat will just raise the overall temperature at the bottom of the ocean slightly, and will not be available as free energy for life. In our present state of knowledge of the internal workings of Ganymede, we cannot be sure whether rich concentrated energy sources will exist under its ocean. Analogies with Earth would suggest that a significant fraction of the energy would emerge in concentrated form at local hot spots, and at those spots, the deviations from equilibrium that are the beginning of life may occur. (The places on the ocean bottom on Earth where hot springs emerge are rich sites for living creatures. These areas derive their primary energy source from minerals in the hot springs, rather than from the Sun.)" Of course, as the authors indicated , these speculations apply equally well to Europa and Callisto, assuming the validity of the Consolmagno-Lewis models for these bodies. The last parenthetical quote is referring to the discoveries of the Ballard expedition in 1977 of the thriving communities of life at deep ocean vents in the Galapagos Rift zone.
The conference "Life in the Universe" took place at NASA's Ames Research Center in June, 1979. It was a relatively large conference, attended by about 150 people, including scientists from NASA and from the academic world. The lectures covered a wide range of topics, from fundamental exobiological questions to SETI, and were published in 1981 in a volume edited by J.Billingham, also named Life in the Universe. Two of the lectures delivered at that conference mentioned Europa, Ganymede, and Callisto specifically. We have already mentioned Benton C. Clark's lecture entitled Sulfur: Fountainhead of Life in the Universe. In the beginning summary of the written version of his lecture, Clark writes: "Sulfur is ubiquitous in the Universe and essential to all life forms that we know. It supports the chemoautotrophic way of life and the photosynthetic. It may inhabit niches we cannot imagine, and the life zone about a star may therefore be wider than now estimated." "Although it seems most likely that liquid water and organic compounds are essential ingredients for the vast majority of (if not all) biotic systems in the Universe, it will be my theme that sulfur compounds may be of equivalent rank and may well permit the proliferation of life in certain environments not otherwise considered hospitable."
Early in the paper, the author discusses the discoveries made in 1977 with the manned submersible Alvin: "The mission was the geological exploration of ocean-bottom thermal springs on the 2.5-km-deep center of the Galapagos rift zone. Hydrothermal vents were indeed found, and although these are of considerable geochemical and geophysical interest, the most important discovery was the existence of previously unknown species of animals whose communal life is dependent on the primary productivity of sulfur-oxidizing bacteria." Much of the paper describes in some detail the role of sulfur in the chemistry of the Universe and our Solar System, its role in planetary evolution and especially in the chemistry of life. Later , in a section entitled "Galapagos Discovery", he writes: "The finding of small, isolated, and more or less complete ecosystems at the mouth of active hydrothermal submarines vents is important since such systems do not depend on photosynthesis for their primary productivity." He points out that those deep sea vent communities do require dissolved oxygen in the seawater, which is generated mainly by photosynthesis near the surface. Then he writes: "It is interesting to speculate, however, that submarine volcanic emissions could provide all the necessary ingredients for a self-sustained ecosystem." He suggests some alternative chemistries which such volcanic emissions could support, including one specific set of linked, sulfur-based, energy-yielding chemical reactions which could be a basis for a biological system.
The last section is entitled "Other Worlds." Here are some quotes: "The assumption that only Earth-like environments qualify as CHZs [continuously habitable zones] is not a secure one. We have been biased by the idea that photosynthesis is of such fundamental significance that advanced biotic systems can persist only in environments coupled to illumination. However, the existence of niches only very indirectly coupled with the solar photon flux, such as the Galapagos vent communities, other benthic and marine mud ecosystems, and salt-marsh environments, emphasizes that the most fundamental requirement is energy flow to provide recycling of, or a fresh supply of, chemical potential energy." "The basic requirements of life may simply be (1) a flux of energy, (2) a stable temperature regime compatible with the biochemistry of the organisms, (3) a liquid milieu, and, of course, (4) an initial supply of building-block elements, such as C, H, N, O, P, S, and transition metals. These elements need not, in principle, be replenished since they can be recycled. Under these conditions, certain non-Earth-like environments may not only be conducive to life, but may be available in far greater abundance in the Universe."
"Consider H2O-rich bodies. In our Solar System, this includes not only Earth, but quite possibly Mars and Triton, and certainly Ganymede, Callisto, and Europa. Liquid water does not exist at the surface of any of these bodies, except Earth, but we should not discount the existence of "buried" liquid water reservoirs. " Clark briefly summarizes the possible sources of such undersurface liquid water reservoirs, including tidal effects such as those responsible for volcanic action on Io. "Regardless of the manner in which they are formed, there is good reason to consider buried liquid water reservoirs as possible life-supporting environments. The probable availability of dissolved salts, including sulfur compounds, and the existence of energy flow, in the form of planetary heat flux, satisfy the life-supporting requirements listed above. Coupling to stellar or planetary luminosity may be completely unnecessary."
"Sulfur is ubiquitous and probably plays several important roles in any exobiological organization. Although it can participate in photosynthesis, it also permits the chemoautotrophic way of life. Habitable zones include not only the surface ocean environment, but also the much more probable subsurface oceanic regions. Earth-like environments as abodes for life may be the exception rather than the rule. Occupation of the much more abundant buried zones is possible, and these should ultimately become an object of exploration. Whether such environments can support life long enough and at a sufficient level of activity to permit the evolution of highly encephalized forms (intelligent life) is conjectural. " Clark concludes his paper by mentioning the research fields that would have a bearing on the speculations he has presented, including especially the study of large planetary satellites such as the Galilean satellites of Jupiter, their thermal history and the question of how long subsurface liquid water might have existed on such satellites. Another question that he emphasizes is whether totally chemoautotrophic ecosystem are likely to survive on a large enough scale and for long enough time periods to permit higher evolution.
Richard C. Hoagland's article The Europa Enigma can be found in its entirety on the Enterprise Mission website . It was widely publicized at the time by Terence Dickinson, the editor of the magazine Star & Sky in which the article appeared. He issued a news release which prompted reports about Hoagland's ideas in numerous newspapers. This article inspired Arthur C. Clarke to make Europa and the possibility that life might exist there one of the themes for his novel 2010: Odyssey 2. Hoagland presents his own theory of how complex organic chemistry, which could be the precursor of life, might have come to exist in a possible ocean on Europa. His starting point is the often-stated analogy between Jupiter and its system of orbiting moons and a sun with its system of orbiting planets. Recounting his thoughts as he watched the images from Voyager on the TV screens at JPL, he writes: "But that night, as we swept through the Jovian system and Voyager returned image after image of vastly different worlds - each Jovian satellite more stunning, each more intriguing than the last, each a place which would be a major planet if it orbited the sun - it was then that the dry, academic rhetoric about "miniature planetary systems" suddenly leaped off the screens and became a set of mind-expanding possibilities." He then continues: "At one time, so the theorists tell us, that image was far more accurate. The newly-forming Jupiter, accreting out of the primeval solar nebula of swirling dust, hydrogen, helium, and traces of other elements, resembled in every essential respect a newly-forming star. It glowed - with a fierce ruby light - radiating as much energy as a traditional main sequence red dwarf star, about one ten-thousandth of the current sun. "" The critical difference was, of course, that the sun shines by nuclear energy resources and Jupiter was drawing on far more limited reserves, the transformation of the gravitational energy of its collapse into waste heat." "But, in between Jupiter's moment of "ignition" and its "fading" there must have been a window, one brief slice of time when Europa basked in energy as rich as any streaming out across the orbit of Earth . . . or Mars." "It was then that Europa must have had real oceans and cloud-puffed skies with gentle rains or slashing hurricanes to turn those sparkling seas to froth before a storm." "And yet Europa's fate was sealed. It died while Earth itself was still cooling toward the moment when its first oceans could be born. Jupiter continued to evolve, growing small and dim." "In a geological instant the waters that splashed across this oh-so-young Europa froze and a vast satellite-wide ocean was suddenly transformed into a shimmering expanse of ice, reflecting for eternity the faded "sun" of its brief fling at life. Jupiter is now held motion-less above one shining hemisphere by Europa's now tide-locked synchronized rotation." Hoagland then discusses at length one way in which organic molecules might come to be synthesized in the early atmosphere of Europa, which he suggests might have been similar to that of Earth in its early history. He argues that this early atmosphere would have been very heavily ionized and that it would be lost into space over a period of millions of years, creating a ring along its orbit of heavy ions. He then writes that, as a result, interaction with the primeval magnetic field of Jupiter would create intense electric currents between the poles of Europa and the Jovian photosphere. This would have led to "heating of the atmosphere above the poles", "massive superbolts of lightning, even in clear air." "And one more thing: organic synthesis reactions between the major and minor constituents within this atmosphere!" He also mentions several other sources of organic synthesis. "The result should have been a veritable rain of molecules falling from the skies above this youthful planet, everything from alcohols to penultimate amino acids." In this way, before a frozen crust covers the surface of Europa, an ocean rich in organic compounds could be produced.
After summarizing the ideas of Cassen, Peale, and Reynolds concerning the possibility that tidal forces exerted on Europa might maintain a sea of liquid water under its icy crust, Hoagland continues: "If true, the continued existence of the solar system's deepest planetary ocean - across 4.5 billion years - presents us with a staggering set of possibilities, including the independent evolution beyond those pre-organic chemicals and acids into the object of our centuries-long quest: the solar system's second world of life." He then discusses the extensive, dark cracks covering Europa's surface, suggesting that the presence of these markings indicates that the ice crust is thin and that their darkness might be due to "radiation polymerized organic molecules brought up from far beneath, staining the surface crust for miles beyond the actual fracture of the ice." "Even if only relatively simple molecules, the sudden exposure to raw solar ultraviolet and the high energy radiation background of the surface would inevitably cross-polymerize these compounds into different combinations, quite likely producing brownish stains along the fissure identical in chemistry to those produced in all those laboratory flasks!" Proposing a description of Europa as a "pressure-cooker planet," Hoagland explains: "Europa's immense good fortune was that Jupiter did die leaving it with a perfect atmospheric seal against the loss of all its volatiles. With the establishment of the miles-thick icebound crust, volcanic activity from the ocean floor would have continued for several million, if not billions of years, totally uncaring that the products - water vapor, carbon dioxide, ammonia, nitrogen, surphur, etc. - were now trapped beneath a lid, represented by the crust fifty miles above." "The chemical and organic evolution of those organic molecules produced during Europa's first few million years could have continued under that canopy of ice, augmented by a variety of energy resources, and aided by that one commodity every exobiologist agrees is quite essential: Time - about 4.5 billion years of it." "And the most intriguing clue that this process at this moment is occurring, is those peculiar surface markings covering Europa like no other planet in the solar system."
"Finally, what if evolution on Europa did continue, past microbes living in their anaerobic ocean, past organisms using merely energy of fermentation? Suppose the combination of several billion years and the unique environment fostered - forced - the evolution of much more complicated organisms? Could there, in fact, be the equivalent of plesiosaurs swimming in the forever dark beneath Europa's blinding landscape: the evolutionary cousins of the great blue whale; the intellectual equivalent of dolphins, or ourselves, locked in that icy prison, forever trapped in orbit around their almost star, who have never seen the real stars and have no way of knowing anything beyond their deep, dark, liquid water?" Near the end of his article, Hoagland discusses the discoveries of ecosystems of ocean bottom life near deep sea vents, which are based on chemical synthesis rather than sunlight, and the relevance of these discoveries for Europa: "Europa's ocean, according to the line of reasoning in this article, potentially has all the ingredients to permit the existence of similar internally nurtured oases of life."
With the wealth of discoveries and data obtained by the two Voyager missions of 1979, a conference called "The Satellites of Jupiter" was soon organized and took place in May, 1980, sponsored by the Institute of Astronomy of the University of Hawaii and supported by NASA, the International Astronomical Union, and several other scientific organizations. There is an excellent article in the proceedings of this conference by Cassen, Peale, and Reynolds, entitled Structure and Thermal Evolution of the Galilean Satellites, which discusses in detail the then current ideas about the internal structure of the Galilean moons, and gives a rather thorough account of the history of those ideas. Concerning the possibility of the existence of oceans on Ganymede and Callisto, the authors write: "The water mantles of both satellites are probably completely solid, unless they are significantly contaminated by dissolved salts or ammonia. The major uncertainties in thermal models of these bodies are due to uncertainties in the creep properties of ice and the degree of contamination of the water mantles. " Concerning Europa: "Tidal heating undoubtedly has contributed significantly to Europa's thermal history, but whether it is enough to maintain liquid water depends on the viscosity of the ice, the history of the orbital resonances, and the quantities of impurities in the ice." But the authors are clearly somewhat less optimistic about the possibility that Europa might still have a liquid water mantle than in their earlier paper of 1979 (Is there liquid water on Europa?). A small but crucial mathematical error in that paper weakens considerably the argument presented there. The authors discuss this error and its implications in another paper in the November, 1980 issue of Geophysical Research Letters, entitled Tidal dissipation in Europa: A correction.
However, in a brief paper entitled Liquid water and active resurfacing on Europa , which appeared in the January, 1983 issue of Nature, Cassen, Peale, Reynolds, together with S.W. Squyres present some additional, new arguments which then strengthened the case for the existence of a liquid water ocean on Europa. They first reexamine the principal heat sources that could effect the surface temperature, namely the heat produced by radioactive decay in the core, by tidal forces acting on the icy crust, and by tidal forces acting on the core itself. This last source was not considered in the earlier paper of 1979 by the first three authors. Under certain hypotheses, the authors argue that their calculations are at least consistent with an H2O-layer (liquid water and ice) many tens of kilometers thick having an icy crust about 16 kms thick. But they point out that the calculations are very sensitive to even slight changes in the hypotheses made, and then discuss some observational evidence. Based on the paucity of craters on Europa, the authors arrive at an estimate of the viscosity of the surface ice. They then argue that this viscosity requires some kind of insulating blanket, such as an extensive layer of frost on the surface. The authors then write: "Fracturing of a thin ice crust over liquid water could provide such a blanket. Water exposed by fracturing would not flood the surface , due to the buoyancy of the crust, but would boil, producing vapour that would condense as frost over a large area. Frosts typically have very low densities and thermal conductivities, and could provide the insulation required. An insulating layer could also result in an average crustal thickness much less than the mean value of ~16 km calculated for conduction in solid ice alone." The authors conclude by presenting observational evidence in favor of the presence of an extensive frost layer.
One of the crucial requirements for the existence of life in an undersurface ocean on Europa would be adequate sources of energy. Precisely this question is studied in the paper On the Habitability of Europa by R.T. Reynolds, S.W. Squyres, D.S. Colburn, and C.P. McKay, which appeared in Icarus , vol. 56, in 1983. In their introduction, the authors write: "Given the indications of a substantial liquid water ocean on Europa, and the affinity of life forms on Earth for an aqueous environment, it is perhaps appropiate to investigate the possibility of Europa's habitability. In this paper we consider the type of environment that might exist in an ocean on Europa and the suitability of that environment as an abode for life. The description of a hypothetical ecosystem on Europa would entail a knowledge of the environment far beyond anything currently available. It is possible, however, to address the much more limited question of the availability of basic requirements for living systems. These include (1) a proper physical environment (appropiate temperature, pressure, etc.), (2) long-term stability of that environment, (3) necessary biogenic elements, and (4) adequate energy sources." The authors then point out that the presence of a liquid water ocean would imply that conditions (1) and (2) are satisfied, and argue that quite reasonable assumptions about the formation of Europa imply that (3) is satisfied too. For these reasons, the paper concentrates specifically on the question of whether biologically useful energy sources should exist on Europa, examining in turn thermal energy, solar energy, and electrical energy.
Thermal Energy. An ocean on Europa would be heated as a result of radioactive decay in the core, tidal forces on both the core and the icy crust, and the release of stored energy from an earlier period of higher heating. The authors argue that this energy would probably not be useful for supporting life. Although there would be temperature differentials in such an ocean (e.g. an increase in temperature with depth), life forms which depend on such temperature gradients as an energy source would probably have to be kilometers in length. The authors then discuss the possibility of concentrated sources of heat existing on Europa, analogous to the deep sea vents and hot springs that were discoverered in the late 1970s here on Earth. The calculation of the amount of heat energy produced in Europa's silicate core by radioactive decay and tidal forces does not by itself imply that ocean-bottom volcanic activity is very probable. Nevertheless, combined with other factors, the authors write that such activity is at least a possibility and that current data and modeling techniques are just not good enough to assess the chance that such concentrated energy sources should exist.
Solar Energy. "We assume that solar energy will only reach the liquid water where the ice crust has been recently fractured. An important quantity is therefore the annual amount of Europa's surface area over which liquid water is exposed to sunlight by crustal fracturing." Such fracturing may produce a layer of frost on the surface. Also, the exposed liquid water will begin to freeze rather quickly. By making some reasonable estimates of the thickness and density of the frost layer, the authors arrive at an estimate of 5 square kilometers for the total area of Europa's surface where the underlying liquid water would be exposed to sunlight during one year. Then they can estimate the amount of solar energy this exposure can provide to an undersurface ocean on Europa, arriving at a figure of 2x1022 ergs per year.
Electrical Energy. "Another possible source of biologically useful energy could result from the motion of Europa through Jupiter's magnetosphere." The authors argue that the induced electrical current will tend to flow through the liquid water ocean (if it exists) from one pole of Europa to the other and the magnitude of this current depends on the conductivity of the ice crust at the poles. They conclude that this source of energy would only be significant when the underlying liquid water ocean is actually exposed at the poles. The authors raise the possibility that such an electrical current could create enough heat to permanently maintain areas of exposed water at the poles, which would also allow significantly more solar energy to reach the liquid water.
There is a considerable literature still to come after 1983. One excellent guide to more recent articles and links about Europa, the possibilities that this moon of Jupiter offers, and all the related issues is the essay by Charles Tritt: Possibility of Life on Europa. For some references and links concerning the situation as it seems at the present time, the reader should look at the Commentary.