Europa: The Ocean Moon
By Richard Greenberg
Springer-Praxis, 380 pages, $89.95
by Michael Benson
Unlike the luminous sphere visible at the moment from about half of the Earth’s surface, the moons of other planets have had few opportunities to infiltrate human history. Largely unsuspected, entirely invisible, they were incapable of raising terrestrial tides, inspiring poetry, or triggering lunacy of any sort. This situation only started to change on the night of January 7th, 1610, when a certain Pisan-origin math professor took a homemade spyglass out into the Padua night and pointed it towards Jupiter. Shimmering but distinct through a thin layer of winter air and over 500 million miles of deep space, the planet revealed itself for the first time to human eyes as a sphere – a world in its own right. Galileo Galilei also noted three points of light strung out in a line along Jupiter’s equator. Their position intrigued him, but he had no reason to believe they were anything other than fortuitously positioned stars. By the next night, however, they’d not only changed their formation in relation to each other but, contrary to expectation, had tagged along with Jupiter as it moved through the sky in a westerly direction. A few nights later, a fourth bright star appeared. It too stayed in formation with the planet.
By January 15th Galileo had famously deduced that planetary bodies were revolving around Jupiter. He attempted to name them the “Medicean Stars,” after his patron Cosimo de Medici, but the name didn’t stick. They’re now the Galilean Satellites, with their individual names – Io, Europa, Ganymede and Callisto, in order of their distance from the planet – the result of a suggestion given to another early observer of Jupiter, German astronomer Simon Marius, by his countryman Johannes Kepler. Kepler thought they should be named after mythical Jupiter’s concubines. He also coined the term “satellites” to describe them.
Galileo’s discovery was of a particularly resounding cosmological significance because it provided persuasive evidence that Copernicus, and many centuries previously Aristarchus of Samos, was right: Given the evident dynamics of the Jovian system, it was now much more likely that the planets revolve around the sun, not the Earth, with only our own moon left to orbit us. When Galileo published his “Message from the Stars”[i] in Venice only two months later, terrestrial tides (or at least, academic and theological storms) were for the first time raised by moons much too far away to do so with their own gravity. Unnoticed among its other repercussions was the not uninteresting fact that the book became the first runaway international science best seller.
Although other similar objects were soon discovered in orbit of other planets, among extraterrestrial satellites it was the Galileans that held the most sway over human affairs for the next four hundred years. Apart from their revolutionary cosmological implications they were immediately seized upon for wholly practical reasons: their regular eclipses by Jupiter provided a way to create an absolute time standard. When compared with the local time at a given place on Earth, the Jovian clockwork could allow the measurement of the longitude of that position – something previously virtually impossible. While the instability of ships at sea made close observation of Jupiter’s moons impractical for maritime navigation, the method worked well enough on land, and Galileo’s discovery soon helped revolutionize cartography.[ii]
And their influence didn’t stop there. Only 60 years after their discovery those four metronomic glints provided the first proof that the speed of light, contrary to previous supposition, was both finite and measurable. While conducting a campaign of observations of Jupiter’s moons in 1671, Dutch astronomer Ole Rømer noticed puzzling discrepancies in the timings of their eclipses. At the Paris Observatory, Italian astronomer Giovanni Domenico Cassini had earlier noted the same phenomenon and tentatively attributed it to light having a finite speed. But he didn’t pursue the hypothesis, and when Rømer joined him as his assistant a couple years later, he added his own observations to Cassini’s – and saw that the times between eclipses shortened when the Earth was on the same side of the sun as Jupiter, and lengthened as the two planets drew further away from each other. On November 9, 1676, he accurately predicted a ten-minute delay in the eclipse of Jupiter’s innermost moon, Io. It was the first measurement of a universal quantity ever made.[iii]
Having helped unseat humanity from the center of all creation, enabled the production of unprecedentedly accurate terrestrial maps, and illustrated the fundamental speed limit of the cosmos, Jupiter’s moons essentially desisted from playing more of a role in our evolving understanding of the universe until about three hundred years after Rømer, when the advent of spaceflight created an explosion of information no less revolutionary than that triggered when Galileo first tilted his revolutionary vision machine at the sky. Latter-day iterations of those lenses could now be vaulted through space to encounter objects once only discernable as points of light wavering in our planet’s restless atmosphere.
Fairly crude Earth-based photometric and spectroscopic observations of Jupiter’s four Galilean satellites conducted in the 1960’s and 70’s had indicated that the outer three were surfaced by water ice, with the innermost apparently covered by an unknown reddish-orange material. But Io, Europa, Ganymede and Callisto only began to become distinct places to us in 1979, when the twin Voyager missions flew past Jupiter and sent back the first high-resolution pictures of its moons.[iv] The Voyagers revealed the Jovian archipelago to be far more complex, dynamic and variegated than previously thought likely.
Which is not to say that absolutely no prior suspicions regarding Jupiter’s satellites had found their way into print, as Richard Greenberg, a professor at the University of Arizona’s Lunar and Planetary Sciences Laboratory—one of the nation’s leading outposts of those contemplating things extraterrestrial—recounts in his magisterial new book Europa: The Ocean Moon. In a legendarily prescient paper published in Science magazine just before Voyager-1 encountered Jupiter in 1979, Greenberg recounts, University of California physics professor Stanton Peale argued that deviations in the orbits of the inner three Jovian moons would inevitably result in tidally-generated heat within them, with most of it concentrated in Io and Europa, the inner two. Io in particular, Peale argued, would most likely prove to be a volcanic world with a largely molten interior. Less than a week later, Voyager’s photographs revealed multiple towering volcanic plumes extending up to three hundred kilometers from Io’s yellow-orange surface. The first active geological processes ever observed on another world, they provided one of the most spectacular – and certainly the most spectacularly well-timed – proof in the history of planetary science.
The two peripatetic Voyagers also photographed Greenberg’s subject, Europa, though from a greater distance than Io. Their pictures were good enough to reveal that Jupiter’s second large satellite, while also outstandingly odd, couldn’t have been more different from its volcanic sister moon. Europa possesses a far more subdued, monochromatic face, an icy crust both weirdly fissured and highly reflective, with very little surface relief evident. Despite its global web of tangled lines, in fact, the moon appeared to be virtually cue ball smooth in the Voyager photographs. On closer inspection, however, the pictures revealed a significant similarity with Io: very few asteroidal or cometary impact craters were visible. This indicated that Europa’s surface was quite young in geological terms – evidence that the tidal energies predicted by Stanton Peale might be having an effect here as well. And given the example of nearby Io, sub-surface volcanism couldn’t be discounted on Europa, either – perhaps hidden by a liquid ocean under what was understood to be water ice. Or so, anyway, did the more adventurous planetary scientists speculate.
Apart from the apparent youth and smoothness of the surface, however, hard evidence for such an ocean was lacking, and there was no way to take a closer look, either: both Voyagers whipped onwards towards Saturn on their gravity-assisted tour of the outer planets. Still, their Europa images arrived at about the same time that the “black smoker” ecosystems were discovered in the deep ocean floors of Earth. These teeming colonies of organisms are reliant on the hydrothermal energy pouring from submarine volcanic vents, and seemingly independent of the photosynthesis-based biosphere of the surface.[v] Europa thus quickly became the object of a debate regarding the possibility that an ocean might exist there, and that life could have arisen there.
Still, this was a highly theoretical debate, because it was tempered by some incontrovertible facts. Jupiter and its satellites are so far from the Sun’s fires that Europa’s surface temperature is exceedingly cold – an estimated –260 degrees Fahrenheit. Absent substantial indigenous heat, which (apart from the circumstantial evidence provided by its volcanic sister moon Io) couldn’t be proved either way with Voyager’s data, this should have been more than cold enough to freeze the putative Europan ocean down to its bedrock eons before humans learned how to launch cameras unto heaven. Accordingly, the possibility of liquid water there was dismissed by a plurality of planetary scientists.
This estimation changed almost at a stroke when a mission specifically dedicated to the study of Jupiter and its moons arrived there in December of 1995. NASA’s somewhat beleaguered, but functional Galileo Orbiter, which had suffered a serious in-flight technical malfunction that had nearly crippled its mission, was the first spacecraft to orbit one of the outer planets. Although its umbrella-shaped high-gain antenna had failed to unfold during its six-year flight, rendering the spacecraft incapable of high-volume communications with Earth, NASA engineers had devised an ingeniously ameliorative series of fixes. These enabled a much-reduced but still significant quantity of photographs and other data to be transmitted.[vi]
And it was in the nature of an orbital as opposed to a planetary fly-by mission that interesting features spotted during one pass of a Jovian moon could be returned to for closer inspection later – a major improvement on the hit-and-run Voyagers. Although its data-flow was a trickle compared to what might have been, Galileo’s great virtue was its capacity to return, repeatedly, to points of interest over the course of years.
This was fortunate, Greenberg recounts, because early Galileo passes of Europa in late 1996 and early 1997 revealed a sight unprecedented in the short history of space exploration. Regions of the surface were patched by regions of immobilized floes – places where the ice had apparently melted through and then been refrozen. Several of these regions were filled with tilted, rotated, or otherwise displaced pieces of crust –icebergs that had evidently rafted out of place due to some thermal process, and then been locked back into place as the crust reformed. A giddy sense of excitement began percolating through the planetary science community: Galileo’s Europa photographs immediately revived, in sensational fashion, the prospect that a liquid ocean could pullulate under the moon’s surface ice. They also revived the possibility that life might have evolved there.
All known life requires liquid water, energy sources, and organic materials. Given the estimated age of the moon’s surface, the melt-throughs photographed by Galileo had evidently occurred at some point in the geologically recent past. This was extremely significant. Eons of cometary impacts, not to mention a steady accumulation of second-hand smoke from sister moon Io’s volcanic plumes, were known to have deposited organic materials and chemical oxidants across Europa’s surface. But an interchange between its surface and ocean was thought to be required if a strong case for indigenous life was to be built. Galileo’s initial images, then, provided the necessary but not yet sufficient evidence for such an exchange – for all three of life’s requirements, in fact. While this didn’t yet prove anything, it certainly re-started the debate in a sensational fashion. Overnight Europa became “the sexiest planet in the solar system,” as Greenberg puts it.[vii]
Robot Galileo’s human namesake famously ran into a bit of trouble with the Inquisition when he insisted that all the evidence confirmed that the Earth, contrary to long-held belief, moved. In January of 1997 – 387 years to the month since Jupiter’s moons were discovered – the same Pope who’d finally acknowledged that the astronomer was right granted an audience to a group of visiting scientists who were running NASA’s eponymous Jupiter mission. Galileo Project Director Bill O’Neil and Project Scientist Torrence Johnson presented John Paul II with an album containing the spacecraft’s Europa photographs and explained their implications.[viii] Studying the moon’s tilted, displaced, refrozen icebergs for a long minute, the pontiff finally looked up at his visitors. “Wow,” he said.
Wow, indeed. So where do we stand now, one year shy of four centuries since moons were first spotted in orbit of wrathful Jupiter? The prior role of the Galilean satellites in modifying our understanding of the universe might seem an extraordinarily hard act to follow. One of the few conceivable ways they could do so might be if the discovery of life among them provided a fittingly grand finale to the story of humanity’s investigations of them. Provocatively enough, fascinatingly enough, such a discovery isn’t just not impossible: it’s possible – even probable. Or such is the careful impression left by Europa: The Ocean Moon. And Richard Greenberg’s thesis certainly isn’t unique to him; it has gained wide credence in the planetary sciences community since Galileo’s first pictures of Europa created a sensation.
Greenberg’s book is something that has only been a workable proposition during the last decade or so: a sizable tome, packed with text and many images, entirely devoted to a single satellite of a distant planet. It opens with a vivid evocation of hypothesized jellyfish and plant life floating within Europan tidal ecosystems, and proceeds to build a compelling case that all the ingredients and conditions exist to sustain “a long-lived global biosphere on Europa,” as Greenberg writes. If there’s a single book that should be required reading among those planning NASA’s future robotic deep space missions, it’s this one.
In 1976, a young scientist with a calling card bearing the rarified job description “celestial mechanician” was chosen to be a member of the Galileo imaging team, the group in charge of pointing a spacecraft’s camera and interpreting its results. Greenberg was selected on the strength of an application that argued that what would be seen among the planet’s moons, when they were finally observed up close, could profitably be studied in light of the gravitational, or “tidal,” dynamics there.[ix] Jupiter, by far the largest planet, is massive enough to cause even the sun to wobble slightly from its gravitational pull. The forces affecting its moons are therefore considerable.
As it turned out, three of the four large moons that help define the Jovian system were so decisively shaped by gravitational energies that they couldn’t have been better suited for study by Greenberg’s vocation. Celestial mechanics, which is the application of physics to the study of the motions and properties of astronomical objects, gave him a distinct advantage over the geologists that dominated the Galileo imaging team. As a truly alien world Europa, as Greenberg makes clear in Ocean Moon, possesses a surface that can’t readily be explained by recourse to terrestrial analogies, which geologists tend to rely on. In a process almost entirely unlike the Earth’s largely internally generated tectonic and thermal processes, the forces driving Europa seem to come from outside.[x]
While it had been known for centuries that the inner three Galilean satellites orbit in a mathematically perfect resonance – for every rotation around Jupiter of the outermost moon, Ganymede, Europa goes around twice and Io does so four times – what hadn’t been sufficiently appreciated was that the resulting regular repetition in their alignments enforces irregular, elliptical orbits for all three. Well before his active involvement the Galileo program Greenberg had worked out the true deviations in their orbits due to this “resonance.” But he hadn’t followed the process through to its logical conclusion. In Ocean Moon he reports receiving a call from Stanton Peale, the man who did. While still researching his exquisitely timed paper predicting volcanism on Io, Peale called Greenberg seeking confirmation of the exact orbital eccentricity, or deviation from a true circle, of the Galilean satellites. Greenberg had the goods on that, and readily shared them with his colleague:
“Do you know what this means?” [Peale] asked. “No, what?” I cluelessly responded. “It means that there must be an incredibly high rate of tidal heating in Io and probably in Europa as well.” I became the first of many celestial mechanicians to slap their foreheads: Why didn’t I think of that!
As Greenberg puts it, everything interesting about Europa follows from the fact that its orbit isn’t perfectly circular. If there had been only one large moon orbiting Jupiter, it would travel in a nearly perfect circle and its surface would have been tugged into a slightly oblong shape millions of years ago due to the effects of the planet’s gravity. The tidal stresses that had distorted it would then have effectively relaxed as it froze solidly into that shape. But as Galileo was the first to realize, there are four substantial moons, ranging in size from about the same as the Earth’s moon (Io and Europa) to just over and just under that of the planet Mercury (Ganymede and Callisto). Their gravity, and their ever-shifting yet repetitive alignments as they orbit, continuously stir things up within the Jovian system.
In effect, their ceaseless gravitational interplay keeps the inner three stressed. As tidal forces ricochet back and forth, the inner three moons rock from side to side as they orbit, ensuring that neither their crusts nor their interiors ever freeze into a fixed shape. The result, on Europa, is the continuous flexing thought to creak through its global shell, cracking and displacing its surface and creating a friction-generated heat allowing for liquid water underneath. (The result on neighboring Io, the nearest to Jupiter, is clearly a massive global sub-surface reservoir of red-hot magma, capped by a lurid sulfur surface: the most volcanic object in known space.)
Much of this of course took quite some time to work out, and Europa: The Ocean Moon chronicles a remarkable set of inter-linked, collaborative feats of deduction regarding elements of the above picture by Greenberg’s small interdisciplinary group of graduate students in Tucson. At its peak the team included accomplished geologist and remote-imaging specialist Paul Geissler,[xi] physicist and astronomer Gregg Hoppa, and structural geologist Randy Tufts. (Tufts died in 2002, only a few years after achieving a remarkable insight into the nature of Jupiter’s second moon.)
As Galileo’s images came in, the first order of business of Greenberg and his group was to try to establish, with as little wiggle-room as possible, that Europa in fact possesses a liquid ocean. Apart from the icebergs, could its surface contain what were in effect coded messages confirming the presence of sub-surface water? Their second, related task was to determine how thick its ice might be – an issue crucial to the likelihood of life there. Ironically, they found the relatively small number of pictures from the semi-crippled Galileo Orbiter to be a blessing: the low rate of data flowing from the spacecraft meant that even their small group could scrutinize each image at great length. Some of their insights ended up possessing a revelatory quality that will certainly be savored by planetary scientists for a long while.
One such finding, Randy Tufts and Gregg Hoppa’s decryption of the moon’s mysteriously arcuate fault lines, was the first substantial empirical confirmation of a sub-surface ocean. These curving “cycloidal” fissures had long counted among the moon’s most puzzling features. Even the more distant Voyager images of Europa in 1979 had revealed long, linked chains of curvilinear cracks, each joined to the next by a kind of cusp. And Galileo’s higher-resolution images contained many more examples, revealing that many of Europa’s faults that aren’t linked into chains also curve. But why?
The answer was the product of research that could only have been fruitful if conducted under the light of celestial mechanics. One way to start unraveling Europa’s mysteries, Greenberg had reasoned, was to plot the evolution in both direction and power of Jupiter’s gravity as it plays across the moon’s surface during each of its 85-hour orbits of the giant planet. Accordingly he assigned Gregg Hoppa to calculate the strength, duration and direction of the stresses effecting Europa during each of its rotations around its parent world. By the spring of 1998, Hoppa had produced a series of tidal stress charts, which he posted on the walls of the room he shared with geologist Tufts. Their well spaced, gradually shifting lines looked a lot like iron filings that have been aligned due to the passage of an invisible magnet.
A balding, soft-spoken man then in his early 50’s, Tufts told me that he’d been taking a break one night from laboring over his doctoral dissertation during the summer of 1998 when it occurred to him that the curvature of Europa’s faults might be the result of the shift, in both direction and power, of Jupiter’s gravity during each of the moon’s revolutions around the planet.[xii] It followed that this directional shift would recur with each orbit. With growing excitement he considered the possibility that Europa’s cycloidal chains, so repetitive in space, might have resulted from repetitions in time – from the metronomically repetitive gravitational forces that unfold during each orbit. Studying Hoppa’s print-outs and sketching curvilinear lines in his notebook, Tufts saw that whenever he followed Jupiter’s ever-shifting gravitational influence with his pencil, he I fact ended up with looping, linked cracks – fissures in the ice that appeared to propagate in curving, stop-and-go chains, exactly as they do in Galileo photographs. It was a classic “Eureka” moment. Later, Hoppa punched the varying amplitudes and timings of his stress calculations into his computer and successfully got it to generate the same curving lines.[xiii] Europa’s arcuates mystery had been solved.
In fact it was only later, almost as a kind of after-thought, that the team realized that the process they’d discovered required the presence of a global sub-surface ocean many tens of kilometers deep. Without that little detail, the liquid water presumed to be exerting pressure from below couldn’t have had the heft to produce the cracks. Tufts and Hoppa had divined the first truly substantial empirical proof of Europa’s ocean.[xiv] It was front page news across the United States: Jupiter’s second moon had vaulted to the head of the short list of extraterrestrial objects thought to possess the potential of hosting indigenous life. (The other leading contender is Mars, though lately Saturn’s icy moon Enceladus, which bears a distinct resemblance to Europa and also possesses sub-surface liquid water, has also entered the running.)
Having come about as close to proving a Europan ocean as possible without actually drilling through the ice, Greenberg and his group proceeded to pursue multiple threads of inquiry, almost all of which pointed to a crust only a few kilometers thick – thin enough to allow for various forms of contact between the ocean and surface. Much of Europa: The Ocean Moon is devoted to documenting this highly elaborated effort, which encompassed Europa’s faulting, its iceberg-spackled melt-through regions, and the moon’s relatively few impact craters, all of which tend towards a flattened appearance when above a certain size. That flattening provides still more evidence of a relatively thin crust, as the asteroids that made these craters are presumed to have punched straight through to liquid water, which then filled the hole and froze. To judge from Galileo’s photographs, the results look more like what happens when a bullet bounces off bulletproof glass, producing a network of cracks, than a dish-shaped crater of the lunar variety.
The book’s many photographs include a number of fascinating, colorful “puzzle-piece” forensic reconstructions of Europan crustal features that have moved out of alignment due to the same gravitational forces that created the cycloids.[xv] Some of them bear an uncanny resemblance to the Suprematist canvases of the Soviet avant-garde of the 1930’s.
It was in their finding that Europa must possess a relatively thin (and thus permeable) crust that Greenberg’s group presented their most direct challenge to what was by then an already well-entrenched orthodoxy in Europa studies. Established by the geologist that dominated Galileo’s imaging team, this understanding of Europa held that the moon’s ice shell is many tens of kilometers thick. If true, that would mean that the moon’s sub-surface ocean was effectively isolated from the organic chemicals on its surface, making it less likely to contain life. The dispute between the thin-ice findings of Greenberg and his group and the largely geologist-driven thick ice understanding of Europa is where politics enters the picture.
One would think that the planetary sciences community would eagerly embrace any evidence supporting the thesis that Europa might be thin-skinned enough to possess conditions potentially suitable for life. But as Greenberg makes clear, this isn’t necessarily so. Europa: The Ocean Moon is interesting for a number of reasons; not least of them its withering critique of the ways in which academic politics undermined objective scientific research within the Galileo program (allegedly, of course, but much evidence if provided). Greenberg is very blunt. The imaging team that he belonged to for 26 years “included some of the more politically skilful, aggressive, and powerful members of the scientific community.” “Heavy-hitting geologists” who’d gained exclusive rights to conduct initial interpretations of the incoming data and who relied heavily on “inexperienced students” for this all-important task dominated the planning of the spacecraft’s observations. Snap judgments about the nature of Europa were fuelled by the need to provide the media with an instant analysis of early photographs. These necessarily provisional findings ended up codified into positions that were than defended at all costs, because scientific and institutional reputations had already been staked. The result was that questionable conclusions became canonized, leading to the acceptance in most scientific papers (sometimes even a coerced acceptance, or so Greenberg implies) that Europa’s ice must be 20 kilometers thick or more.
If Greenberg appears willing to abide errors in interpretation, and even admits to making a few himself, he clearly can’t stomach what he regards as a willful insistence on sticking to increasingly untenable positions for reasons of individual and institutional pride. He sketches a disturbing picture in which much pressure was put on researchers to produce findings in conformity with the prevailing (i.e., thick-ice) thesis. Younger, untenured researchers, Greenberg charges with palpable anger, actually risked their careers if they didn’t do so. This was no honest disagreement between scientists, he indicates, but rather a kind of conspiracy led by “apparatchiks” within a “nomenklatura” that includes a sizable slice of the US planetary science elite. In places Greenberg fairly seethes over the “political hustlers and enforcers” who have committed “inexcusable errors in research methods and results.” He’s not shy about naming names, either.
Clearly no non-scientist can adjudicate this apparently quite poisonous dispute. I will say that Greenberg’s quoting of the original Galileo in condemnation of the “malignity, envy and ignorance” of those who would “force the course of nature to conform to their dreams” strikes me as a shade ill advised. The clear implication that he and his team stand in Galileo’s shoes is unworthy – not because it’s untrue (it certainly isn’t) but because it’s unnecessary. Europa: The Ocean Moon makes a more than sufficient argument in support of its case that Jupiter’s second satellite possesses conditions potentially suitable for life without needing to equate its author’s thick-ice, isolated-ocean opponents with the leaders of the Inquisition.
Because finally Europa: The Ocean Moon, like the spacecraft providing most of its images, is evidence that a flawed system worked more or less correctly – at least in the end. Greenberg’s early work led to his inclusion on the Galileo imaging team. He was able to bring together an evolving brain trust of brilliant grad students at one of the country’s leading planetary sciences facilities, where his tenure shielded him, allowing him to publish findings in contradiction of more orthodox views. The density and complexity of the book’s argumentation, the sheer quality of its evidently well-funded research, the originality and even conceptual beauty of its findings, all are evidence of an organizational structure for deep-space research which may be inefficient, and riven in places by factionalism and careerism, but which was ultimately effective.
And it has to be asked; didn’t all the institutional weight behind the thick-ice view of Europa in the end force Greenberg to hone his argument? Would his book be this good without it? After all, only a few years have passed since robot Galileo’s last encounter with Jupiter’s orbiting oceanic satellite, and yet on the evidence of Europa: The Ocean Moon, the work of Greenberg’s small squad of interplanetary code-crackers won’t merely endure – it will prevail. Citing budget constraints, NASA has cancelled two dedicated missions to Europa just in the last six years. It should make actually launching one an overriding priority: there can be no more important destination in all of planetary science.
[i] Historian William Burrows points out that the more common translation of Siderreus nuncius, “Starry Messengers,” is less reflective of Galileo’s meaning. This New Ocean (Random House, 1998) footnote, pg. 16
[ii] Dava Sobel, Longitude (Penguin Books, 1995) pg. 27
[iv] Two prior fly-by missions did not carry sufficiently advanced cameras for much knowledge of the moons to be obtained.
[v] In fact it has since been established that they make limited use of nutrients that filter down from the surface.
[vi] For a detailed look at the Galileo mission, which ended in 2003, see my article “What Galileo Saw,” The New Yorker, 9/1/2003; it’s also in Best American Science Writing 2005 (HarperCollins).
[vii] Sex appeal aside, many planetary scientists end up referring to their object of fascination as a planet, even if it’s actually orbiting one.
[viii] O’Neil interview with the author, Paris, March 2000.
[ix] In celestial mechanics the word “tide” is used to denote the effects of gravity on anything, not necessarily a liquid.
[x] Clearly the tides raise in both the Earth’s oceans and to a lesser but measurable extent on its crust are an exception to this statement.
[xi] Full disclosure: I worked extensively with Paul Geissler to produce many of the color images in my book “Beyond: Visions of the Interplanetary Probes” (Harry N. Abrams, 2003)
[xii] Interview with the author, Padua, October 1999
[xiii] For compelling images and animations of this, see http://pirlwww.lpl.arizona.edu/~hoppa/science.html
[xiv] Their findings were later corroborated by Galileo’s magnetometer, which discerned a flux in Europa’s magnetic field consistent with a global layer of conductive sub-surface salty water.
[xv] As Greenberg puts it, Hoppa’s gravitational stress charts “eventually became the basis for explaining most of the major tectonic patterns on Europa.”