Exploration History of Europa

8/12/2019 Exploration History of Europa

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EuropaEdited by

Robert T. PappalardoWilliam B. McKihnonKrishan K. Khurana

With the assistance ofRene Dotson

Wth 85 collaborating authors

THE UNIVERSITY OF ARIZONA PRESSTucsonin collaboration with

LUNAR AND PLANETARY NSTITUTEHouston


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ContentsList of Contributing Authors ................... xiAcknowledgment of Reviewers ................ xvForewordT Johnson ..........xvii

PART I: HISTORY, ORIGIN, AND DYNAMICSThe Exploration History of Europac. Alexander R. carlson, G. consolmagno, R. Greeley, and D. Monson ...........3Formation of Jupiter and Conditions forAccretion of the Galilean SatellitesP. R. Estrada, I. Mosquera, J. J. Lissauerr G. D'Angelo, and D. P. Cruikshank ,.................27Origin of Europa and the Galilean SatellitesR. M. Canup andW. R. Ward.... .....59Tides and Tidal Heating on EuropaC. Sotin, G. Tobie, J. Wahr andW. B. McKinnon............. .................. g5Rotational Dynamics of EuropaB. G. Bills, E Nimmo, O. Karatekin, T Van Hoolst, N. Rambaux, B. Levrard,and J. Inskar...... ........119

PART II: GEOLOGY AND SURFACEGeologic Stratigraphy and Evolution of Europa's SurfaceT Doggett, R. Greeley, P. Figueredo, and K. Tanaka .....137Europa's Crater Distributions and Surface AgesE. B. Bierhaus, K. Zahnle, and C. R. Chapman........,...... ................ 16lEuropa's Impact Craters: Probes of the Icy ShellP M. Schenk and E. P. Turtle ...... 1glTectonics of EuropaS. A. Kattenhorn and T. Hurford.... ................ 199Morphology and Evolution of Europa's Ridges and BandsL. M. Prockter and G. W. Patterson.......... .....237

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Chaotic Terrain on EuropaG. CoIIns and E Nimmo .............25gEuropa's Surface CompositionR. W. Carlson, W. M. Calvin, J. B. Dalton, G. B. Hansen, R. L. Hudson,R. E. Johnson, T. B. McCord, and M. H. Moore.................... ....,......2g3Surface Properties, Regolith, and Landscape DegradationJ. M. Moore, G. Black, B. Buratti, c. B. phiilps, J. spencer, and R. sullivan ....................32g

PART III: INTERIOR, ICY SHELL, AND OCEANInterior of EuropaG. Schubert, F. SohI, and H. Hussmann. .......353Thermal Evolution of Europa's Silicate InteriorW. B. Moore and H. Hussmann ...369Geodynamics of Europa's Icy ShellF. Nimmo and M. Manga .............3g1Heat Transfer in Europa's Icy ShellA. C. Barr and A. P. 5howman............... .......405on the chemical composition of Europa's Icy sheil, ocean, and underlying RocksM. Yu. Zolotov and J. S. Kargel ...43IOceanography of an Ice-Covered MoonS. Vance and J. Goodman ............45g

PART IV: EXTERNAL ENVIRONMENTObservations of Europa's Tenuous AtmosphereM. A. McGrath, C. J. Hansen, and A. R. Hendrix ..........4g5composition and Detection of Europa's sputter-induced AtmosphereR. E. Johnson, M. H. Burge4, T A. Cassidy, E Leblanc, M. Marconi,andW. H. Snryth .........507Europa's Radiation Environment and Its Effects on the Surfacec. Paranicas, J. F. coope4 H. B. Garrett, R. E. Johnson, and s. J. sturner .....52gEuropa's Interaction with the Jovian MagnetosphereM. G. Kivelson, K. K. Khurana, and M. Volwerk..........:...... ............545Electromagnetic Induction from Europa's ocean and the Deep InteriorK. K. Khurana, M. G. Kivelson, K. p. Hand, and C. T RusseII.. .....571


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PART V: ASTROBIOLOGY AND PERSPECTMSAstrobiology and the Potential for Life on EuropaK. P. Hand, C. E Chyba, J. C. Prscu, R. W. Carlson, and K. H. Nealson .......599Radar Sounding of Europa's Subsurface Properties and hocesses: The View from EarthD. D. Blanlecnship, D. A. Young, W. B. Moore, and J. C. Moore .....631Future Exploration of EuropaR. Greeley, R. T Pappalardo, L. M. Prockter A, R. Hendrh, and R. E. Lnck.....................655Europa: Perspectives on an Ocean WorldW. B. McKinnon, R. T Pappalardo, and K. K. Khurana.........,... .....697Appendx: Earopa Galleo and Voyager Image Mosaic Maps ....,... ........... 711Color Section following page 586

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The Exploration History of EuropaClaudia Alexander and Robert CarlsonNASA Jet Propulsion Laboratory/Clrnia Insttute of Teclnology

Guy ConsolmagnoSpecolaVatcanaRonald GreeleyAr zo na S tate U nv ers itlDavid MorrisonNASA Ames Research Center

Almost 400 years after Galileo's momentous discovery of Europa (January 1610) with aone-inch magnifier, the New Horizons spacecraft took images of Europa (February 2007) witha high-quality CCD imager. In the intervening years, theoretical work coupled with improve-ments in technology have led scientists from one paradigm to another. In Galileo's era, themovements of Europa, logged by hand, were part of a paradigm shift regarding Earth as thecenter of the solar system. Another paradigm shift was the importance of sputtering, a processthat may be critical to Europa as an object of biological interest, and that was treated like sci-ence fiction when the physics were first proposed in the 1970s. In the late twentieth centurythe Galileo spacecraft collected evidence that suggested Europa to have a subsurface ocean.The evolution of thought coupled with the improvements in technology are presented in thischapter.

1. INTRODUCTIONEuropa, the smallest of the Galilean satellites, was theleast-well-observed target during the two Voyager flybys ofthe 1970s. Post-Voyager, this small ice-covered world cata-pulted into prominence as a place in the solar system withextensive water oceans that may be habitable. The begin-ning of this Europa revival can be traced to the Nature paperby Squyres et al. (1983) suggesting that the linear fracture-like markings and absence of craters revealed by Voyagerwere evidence for liquid water and active resurfacing. Con-sequently, Europa emerged as a prime target for explora-tion by the Galileo mission of the 1990s, and the primaryfocus of the first of Galileo's two extended missions. To-day Europa is a candidate for a dedicated orbiter mission,with eventual landings on its surface. Many astrobiologists

consider Europa, among leading candidates, to be a moreinteresting potential abode for life even than Mars, sincethe discovery of life in its oceans would constitute com-pelling evidence for an independent origin of life, a sec-ond genesis within our solar system, whereas dependingupon the context, martian life might have significant tiesto "seeding" from Earth.Until the advent of the "space age" circa 1950, the onlyinformation we had about Europa was its mass, diameter,surface brightness in various colors, and orbital commen-surability with the other large satellites of Jupiter. None-

theless, these were sufficient to allow astronomers to anive,eventually, at a global picture of Europa not far removedfrom the perspective afforded by contemporary spacecraft.This chapter traces the history of Europa studies over fourcenturies, from Galileo (the man) to Galileo (the mission).2. FROM DISCOVERY TO THE BIRTH OFMODERN PLANETARY SCIENCEFrom its discovery in 1610 through V/orld War II, Europalanguished in the backwaters of astronomy. While discov-ery of Jupiter's satellite system played a major part in es-tablishing the Copemican cosmology and founding the newscience of planetary dynamics, when reasonable data on sizeand mass were available, the astronomers of subsequentcenturies did not bother to speculate about the physical or

chemical properties of the satellites of Jupiter. Even whenthe tools of astrophysics became available in the early twen-tieth century, planetary scientists seem to have been amongthe last to embrace new techniques.2,1. Discovery

Europa was one ofthe threejovian satellites discoveredin the opening weeks of the era of telescopic astronomy.In January 1610, Galileo Galilei used the "occiale" to ob-serve the heavens - a telescopic invention that was de-


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^' 4' u h^?1^l.t',i;' '" oi,7, t-t-o,r,'f-.^,t',1 ,,t

signed to serve maritime and land enterprises' His tool wasprmitive by today's standards - about one inch of aper-iure, obtaining a magnification of about 20 times that ofthe naked eye. Nevertheless, over the nights of January 7-15, 1610, Galileo conducted observations that would con-tribute to changing the paradigm about the solar systemforever.Figure I shows a representation of the notes Galileo tookas he made these historic observations' Galileo noted that"three little stars, small but very bright, were near theplanet." On his second night of observing, Galileo noteditrat the three little stars werc "all west of Jupiter, and nearertogether than on the previous night." Subsequent observa-tions confirmecl that the satellites (three little stars) wentaround Jupiter in the prograde fashion. This was the firstexample of one heavenly body orbiting another, therebyproviing an argument for the controversial heliocentriciheory oflCopemicus. The first findings from Galileo's studyof the Jupiter system were published just 60 days after theobservations were made, in March 1610 in the short mono-graph entitled Sielerius Nuncius. Bavarian astronomer SimonMarius also made observations of the moons in thistimeframe, without publishing his results. Although Galileoproposed calling them the Medician satellites, in honor ofitlr Vl"i"i patrons, over time, the term "Galilean satellites"

thi; stav {v+r+her to th ast lcallisto]awd the w estYw sta r [G anymecie]aqpeared LaYger thawlh atheY star"'I did wotqU attewt.owtothqrtt lu'.i'>, *, rr' *.* dta''oes befuteewth"v" awdiwpLtcv'' ' (: ' :' - \ '' - r l^^., l$ l, -le t t e +l^nF l. ,,et),r^i,';i ^ t,),,',' o"" ir,::;r3^::::.^t ti"*e thaL thegle^f t ^p "n T '-'- ",,. .. wye {Lxed stars..


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that does not exist with the other satellites. As noted byGreenberg (2005), "everything interesting about Europafollows from the fact that the eccentricity [of its orbit] isnot zero."2,2. Mass

The masses of Europa and the other Galilean satellites, asa fraction of Jupiter's mass, were first deduced by Laplace(1805) in his treatise Celestial Mechanics, using an elabo-rate theory of the mutual perturbations of those moons. Hispublished result for Io was low by a factor of 4, but themass of Europa, 2.3 x l0-s of the mass of Jupiter, waswithin l07o of the modern value. (He overestimated themass of Ganymede by l3Vo, and underestimated Callistoby 257o.) At the end of that chapter, he pointed out that themasses of Io and Europa were "almost equal [to that of theEarth's Moon]."Laplace's calculations depended on careful choice ofeclipse timing data, which he himself noted were problem-atical at that time. These data were improved with obser-vations of the moons by Baron Marie-Charles Damoiseauand others during the first half of the nineteenth century(see Sampson, l92l). Nonetheless, reference papers andpopular textbooks of the period continued to use Laplace'smass values.Mass determinations including more modern values werereviewed by Brouwer and Clemence (1961), Kovalevs(1970), and Duncombe et al. (1974). The prespacecraft aver-age given by Morrison and Cruikshank (1974) is 480 t l0in units of l02o kg.2,3. Diameter

An accurate diameter of Europa was far more difficultto obtain than its mass in the prespacecraft era. Throughthe years, techniques for determining this diameter haveincluded timing satellite eclipses and occultations; the filarmicrometer (a measuring microscope using movable threadsto measure the size of Europa's disk as seen in a telescope'seyepiece); a double-image micrometer used by Dollfus atPic du Midi; a diskmeter used by Kuiper at the Mt. Wilsonand Palomar telescopes (including the Hale 5 m); and op-tical interferometry employed by Michelson, Hamy, Danjonand others (Dollfus, 1975).Secchi (1859) provided an early result, listing the diam-eter of Europa of 3330 km, 6Vo too large. He also listedmasses of the satellites in terms of an Earth mass, basedon the lnplace (1805) work. Combined with the Europamass estimate, a calculation can produce a density of 2.5 gl" -z (207o low). Secchi himself never calculated this den-sity nor discussed the implications of these numbers.Astronomy books popular in their day, by Herschel(1861) and Newcom (1878), presented similar diametersand masses for the Galilean moons, but they neither cal-culated densities nor attempted interpretation of composi-

Alexander et al.: Exploration History of Europation from mass values. Consideration of Europa as a place,a planet-sized body with a geological history, was appar-ently not grounds of contention for nineteenth-century as-tronomers.2.4. Brightness, Albedo, and Shape

In the field of photometry, otherwise known as the quan-titative measurement of light, the most rudimentary obser-vation is the integrated brightness as a function of time. Thisintegrated brightness is, for a rotating body, a periodic func-tion, where the amplitude - the change in the intensity ofcollected light over a rotation period - can be used to in-fer not only the rotation period of a body but its shape. Inthe nineteenth century, the perceived variability of thebrightness and shape of these objects, as reported by anumber of reputable observers, constituted the primarycontroversy related to the nature of the Galilean moons.Secchi (1859), for example, said of Ganymede that "onecan see various spots and a notable flattening, from whichI conclude that it spins with a very short period. It seemsat some times to be quite flattened, and at other timesround . . ." (translation by G.J.C.). Secchi suggested that thiscould be explained if it precessed very rapidly, wobblinglike a child's top. Likewise, Newcomb (1878) reported that"the light of these satellites varies to an extent which it isdifficult to account fo except by supposing very violentchanges constantly going on their surfaces."Pickering et al. (1819) produced the first accurate rela-tive photometry of these moons. Their photometer useda series of prisms to view simultaneously the images ofthe satellite (or star) and a calibration object, whose bright-ness could be dimmed to match the objects' brightness bycrossed Nichols or a variable aperture. Their conclusion wasstraightforward: "It has been thought by many astronomersthat the light of the satellites of Jupiter was viable. Thisview is not sustained by the present measurements . . ." Theinherent difficulty of visually estimating the brightness ofthese moons so close to the overwhelming light of Jupiteris the reason suggested by Pickering et al. for the misleadingreports of their contemporary nineteenth-century observers.These authors also produced a table of relative albedos;the actual values depended on which set of diameters theyused, but a typical set of values gave the ratio of the albe-dos relative to Io for Europa, Ganymede, and Callisto as1.21,0.625, and 0.359. They did not notice or comment onthe curious fact that the more dense moons, Io and Europa,are significantly brighter than the less-dense moons ofGanymede and Callisto.Although this work disproved the reported brightnesschanges, the idea that the moons. were highly elliptical wasstill widely accepted. Thirty years later, Pickering (1908)reported on further observations, including measurementsof the moons' diameters made with a filar micrometer. Hefound a signifrcant difference between the polar and equa-torial axes of all the satellites, and he discussed at length


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Europa

the question of "elliptical shape" of the moons reported byothers before him. Searching the literature, he noted thatfrom 1850 to 1895 Io was observed to be "elliptical" by13 observers, and Callisto by l0 observers. (By contrast,Europa was reported to be "elliptical" in shape only by twoobservers, the fewest of the four satellites.) From today'sperspective, we suspect that apparent differences betweenpolar and equatorial diameter were due to a brighter polarup on Ganymede and a darker cap on Io, whereas the diskof Europa is more uniform'Pioneering photoelectric photometry was canied out byStebbins (1927) and Stebbins and Jacobsen (1928); thesedata were subsequently converted to the UBV photometricsystem and augmented by Harris (1961). The Stebbinsobservations of 1927 demonstrated that all four Galileansatellites were in synchronous rotation, keeping the sameface toward Jupiter, and therefore tidally locked. The larg-est lightcurve amplitude dichotomy was found for Europa,particularly in the blue and ultraviolet, suggesting an asym-metry between leading and trailing hemispheres ' Burns(1968) was the frrst to suggest that asymmetric magneto-spheric fluxes and radiation effects could lead to the hemi-spherical differences in color observed by Hanis. Muchmore extensive photometric studies were carried out by inthe late 1960s and early 1970s by Johnson (1971)' Johnsonand McCod (1971), Morrison et al. (1914), and later byothers (see section 3.2. and chapter by Carlson et al')'2.5. Density and Bulk ComPosition

One of the most striking aspects of the Galilean satel-lites to modern planetary scientists is the dichotomy be-tween the two inner satellites, with their lunadike densities, and the outer two, which have densities too low for arocky composition. Today we recognize that density is ameasure of bulk composition, and thus we see this di-chotomy as a fundamental challenge to theories of the ori-gin and evolution of these objects, e.g', that all four satel-lites evolved from the same gaseous cloud of volatiles (thecomposition of which includes water vapor and carbon di-oxide, volatiles that form ice at low temperatures; see sec-tion 3.6).One of the stranger bits of history comes from Cham'bers (1861, p. 61), who was perhaps the first to calculatedensities for the satellites. Unfortunately, his work wasflawed by a mathematical error; although he presented,reasonably, Europa's diameter as 2l9l miles (3500 km'l27o large) and used Laplace's mass, his tabulated densi-ties arc for some reason consistently low by a factor of l1'5;thus every moon is given a density significantly below thedensity of water. For Europa, Chambers reported a densityof 0. l7l g cm-:. He did not comment on the signif,rcanceof these remarkably low densities.Pickering (1908) was the first to consider density andalbedo as clues to composition. He used a Europa diam-eter of 3540 km and calculated a density of 1.91 g cm-'Pickering also commented: "It thus appears that ' ' ' the

three inner satellites are therefore composed of some lightcolored material . . . It will be noted that the densitiesgiven. . . are extremely small for solid bodies. . ' lS]uchgur"t almost necessarily imply that these bodies are ei-ther enveloped in clouds supported in suitable atmospheres,or that they are composed of dense swarms of meteorites'Their density and brightness are what we should expect ifthey were composed of loose heaps of white sand ' ' ' Asidefrom the impossibility that such small light bodies shouldpossess dense cloud-laden atmospheres, their varying albe-os preclude the former supposition." Although Pickering'ssuggestion that Europa was a swarm of white sand strikesus as unlikely, he does get credit for being the first personto assemble the essential data and attempt to interpret it interms of the possible composition and structure of thesemoons, concluding that they are low-density bodies withbright surfaces.The first published suggestion that these moons couldbe made of ice came in an almost off-handed way by Jef'freys (1923). In a paper arguing that the gas giant planetswere colder than previously suggested, and made substan-tially of ices, he comments that "the necessity of suppos-ing that the outer planets are composed of something lighterthan terrestrial rocks receives additional support when weconsider their satellites . . . The densities of [Io and Europa]are comparable with those of tenestrial rocks, but those for[Ganymede and Callisto] are too small, and are again com-parable with the densitY of ice."This suggestion of an icy composition for these moonswas influential. In a widely used astronomy textbook, Rs-sell et al. (1945) wrote that of the Galilean satellites, Io andEuropa "are therefore not far from the same size as ourmoon [with densities ofl2'7,2.9lgcm-t].It is thereforeprobable that fthey] are masses of rock, like our own satel-lite. Jeffreys suggests that the third and fourth may be com-posed partly of ice or solid carbon dioxide."

3. FROM THE BIRTH OF MODERNPLANETARY SCIENCE THROUGHTHE PIONEER MISSIONSThe situation at mid-century was summarized by CeciliaPayne-Gapschkin in her 1956 college textbook: "The den-sity of Io and Europa is not very different from that of ourmoon, and they are probably roc bodies. Callisto is verylikely a chunk of ice' Perhaps Ganymede is partly rock,partly ice. All four satellites . . . can hardly have atmos-ph"r"t; surface gravity and velocity ofescape are too low"(Payne-Gapschkin, 1956). No one had yet noted the con-tradiction between lunar-type density and ice-like albedosof Io and Europa - the key to recognizing their uniqueproperties, although the explanations were entirely differ-ent for these two small worlds'In the 1950s Gerard P. Kuiper (University of Chicagoand McDonald Observatory) and Fred L. Whipple (HarvardCollege and Smithsonian Astrophysical Observatory) initi-ated the renaissance of planetary astronomy and its trans-


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formation into the much broader modern field of planetaryscience. They began the trend toward thinking of membersof the solar system as individual worlds that could be stud-ied by the guantitative tools of astrophysics. With the cre-ation of NASA in 1958, the potential for a radical jump inour knowledge of the solar system occurred, and both Kui-per and Whipple were among the first to propose spacescience missions to the Moon and planets.A summary of what was known about solar system ob-jects shortly past mid-century was published in the Univer-sity of Chicago volume Planes and Satellites (Kuiper andMiddlehurst, 1961). Fifteen years later, a comprehensivereview of prespacecraft knowledge of all the satellites (52then known) was published in Space Science Reviews byMorrison and Cruikshank (1974). Along with our own rec-ollections of this period, much of the information in thissection is derived from this 1974 discussion. We have alsoused the popular history on the Pioneer missions: PioneerOdyssey (Fimmel et al., 1917).3.1. Early Ideas on CompositionIn 1963, Whipple wrote: "It is thought-provoking. . .that the two inner Galilean satellites should be so much likethe Moon while the two outer ones should contain such alarge proportion of lighter materials. These facts must un-doubtedly constitute an important clue to the formation, notonly of the satellites, but of Jupiter itself. All four of theGalilean satellites differ in one respect from the Moon: theyare all much better reflectors of light . . . Their surfaces,therefore, must be quite different in character from that ofthe Moon, at least partially covered with some HrO ice andalso other frozen gases."

Discussions of surface composition date to Kuiper's sug-gestions that low-density objects condensed at low tempera-tures from the volatiles in the solar nebula, predominantlyHrO. While Kuiper was exploring the Galilean moons froman astronomical perspective, Harold C. Urey was fittingthem into his groundbreaking picture of the solar systemas a chemical system. In his 1952 book The Planets, hespeculated that the outer solar system was filled with waterice. Noting that Ganymede and Callisto had densities similarto water ice, he wrote "[T]he interiors of objects of similarmass [to the Moon] regardless of their original temperaturesmust have risen above the melting point of ice in their in-teriors, and hence the water of the Jovian moons must allbe at or near their surfaces. In fact, water flows instead ofterrestrial lava flows may occur from time to time" (Urey,1952, pp. t6l-162).Cameron (1973) proposed from more detailed modelingthat the composition in the solar nebula changed fromhigher-density, rocky material near the Sun to ice-richmaterial beyond Jupiter. Urey's student John Lewis pro-vided quantitative models of the chemical equilibrium con-densation of such a nebula in a series of paper in the early1970s (Lewis, 197|a,b, 1972, 1973), a subject we will re-turn to when we discuss thermal evolution in sections 3.6

Alexander et al.: Exploration History of Europaand 5.8. All these solar nebula chemical models were con-sistent with a composition of the satellites of Jupiter ofroughly half rock and half ice - a description that matchedCatlisto and Ganymede but not Io and Europa.The effort to actually determine which ices were presentbegan with low-resolution infrared spectral observations. Inone of the first applications of new lead-sulfide infrareddetectors to planetary astronomy, Kuiper (1957) publishedan abstract describing the spectra ofthe Galilean satellites.He noted that the spectra of JII and JIII, Europa and Gany-mede, were reduced in relative intensity beyond 1.5 rmcompared to JI and JIV. His interpretation was that "HrOsnow" covered the surfaces of Europa and Ganymede, butnot Io or Callisto. He also noted that the presence of H2Owas consistent with Europa's high visual albedo, whileGanymede's darker surface could be contaminated by sili-cate dust. Moroz (1965) was the first to publish spectra ofthe Galilean satellites, from which he found that Europa andGanymede's spectra resembled those of the martian polarcap and the rings of Saturn. He therefore assumed that thesurfaces of Europa and Ganymede were largely covered byH2O ice.3.2. Infrared Spectra and Thermal Properties

Beginning in the mid-1960s, NASA began to support theconstruction and operation of a new generation of opticaltelescopes to be used for planetary studies. These includedseveral instruments on Mauna Kea, Hawaii, which wasbeing recognized as one of the best sites in the world topursue the new field of infrared astronomy. In the early1970s groundbased studies using these and other telescopesyielded direct measurements of the temperatures of theGalilean satellites, infrared spectra that were diagnostic ofsurface composition, and improved multicolor photomet-ric lightcurves.High-precision multicolor spectrophotometry in the vis-ible and near infrared (from 0.3 to 1.1 pm) were obtainedfor the Galilean satellites by Johnson (1971), Johnson andMcCord (1971), and Wamstecker (1972). Longer-wave-length infrared detectors and interferometric spectrometersmade possible the extension of these reflectivity curves toa wavelength of 4 .rm by Pilcher et al. (1972) and Fnk etal. (1913). From an analysis of the depths of the water iceabsorption bands, Plcher et al. (1972) concluded that be-tween 507o and 1007o of Europa's surface was comprisedof HrO ice. Pilcher et al. (1972) further concluded that boththe albedos and the infrared band depths of Enropa,Ganymede, and Callisto could be understood in terms ofvaiations in only one parameter, the fraction of exposednon-ice surface material. The variations with rotation invisible and UV reflectance, however, and in particular thestrong minimum in Europa's brightness aligned along itstrailing side (Morrison et al., 1974), suggested some exo-genic effects, perhaps involving interaction with thejovianmagnetosphere. For a comprehensive review of these dataand their interpretations, see Johnson and Pilcher (1917).


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Duropa

Thermal radiation from the Galilean satellites was firstmeasured in the 8-14-pm atmospheric window by Murray(1975) and Low (1965). They found the lowest brightnesstemperature for Europa (120 K), consistent with its veryhigh albedo. Morrison and Cruikshank (1973) extended theinfrared data to 20 rm and also detected a temperature vari-ation with orbital phase in which the leading side of Europawas warmer, consistent with its lower visual albedo.Observations of the thermal response of the surface tothe relatively rapid changes in isolation during satelliteeclipses can be used to determine the thermophysical prop-erties of the satellites (Morrison et al, 1972; Morrison andCruikshank, 1973). Europa was the least-well observed bythis technique, but its thermal inertia was similar to that ofGanymede (about 2-3 g K s-l), indicating a low-conduc-tivity, porous surface material. These eclipse observationsalso gave the first hints of Io's volcanic activity, althoughthey were not correctly interpreted at the time.3.3. Magnetospheres

Along with photometric observations of the jovian sys-tem, in the mid-twentieth century groundwork was laid forunderstanding of the plasma environment that influences theGalilean satellites. Hannes Alfvn, the Swedish Nobel lau-reate for his work in plasma physics, first spoke in 1937about the pervasiveness of electric currents in the plasmaof interplanetary space: "Space is filled with a network ofcurrents that transfer energy and momentum over very largedistances." In 1939, Alfvn proposed a theory of magneticstorms to explain aurorae and other aspects of plasma dy-namic phenomena in near-Earth space, including the exist-ence of hydromagnetic waves (AUvn, 1942), that wouldplay so prominent a role decades later in interpreting themagnetic signal at Europa.For the first forays into the Jupiter system, concernsabout the radiation environment were paramount. In agroundbreaking discovery, Burke and Franklin (1955) hadmeasured decametric radio signal bursts from Jupiter at22.2MH2. Soon an incredible spectrum of radio signals wasbeing recorded from Jupite covering 24 octaves. Commen-surate with the discovery of Earth's Van Allen belts, Jupiterwas suddenly recognized as notjust a gas giant sumoundedby the vacuum of space, but a planet with a substantial mag-netic field containing extensive energetic plasma, capableof sustaining radio emission, emanating from its own VanAllen belts. The radio signals suggested radiation similarto that emitted by pulsars - namely synchrotron radiationwhere electrons are trapped in so strong a magnetic fieldthat they move at relativistic speeds and emit powerful ra-dio signals (Drake and Hvatum, 1959; Roberts and Stanley,1959). The signals suggested that the inner magnetospherealso contained cyclotron radiation, a type of radiation emit-ted by electrons that arc not moving at relativistic speeds,but with high energy nonetheless. Another fundamentaldiscovery came in 1964 when some of the jovian emissionwas attributed to a complicated electrodynarnic coupling

between Jupiter and its moon Io (Biggs, 1964). A discus-sion of the discoveries in the jovian magnetosphere fromthis period and their relation to the interpretation of phe-nomena related to the electrodynamics of Io (precursors tounderstanding the same phenomena near Europa) may befound in Physics of the Jovian Magnetosphere (Dessle41983). A growing realization of the magnitude and efficacyof the jovian magnetosphere provided much of the motiva-tion for the Pioneer missions to the outer solar system.3.4. The Pioneer Missions

Following the successful flight of Mariner 2 to Venus in1962, NASA planners began to consider ambitious missionsto the outer solar system. The key to accessing these dis-tant planets was gravity assists based on close flybys ofJupiter. The positions ofthe outer planets in the early 1980swould offer an exceptional opportunity for a "grand tour"with flights to Jupiter, Saturn, Uranus, Neptune, and evenPluto. Such a fortuitous alignment would not repeat formore than a century. However, the grand tour could not beachieved without flying spacecraft deep into the jovianmagnetosphere in order to achieve the desired gravitational"slingshot" effect. Thus an argument was made for a pre-cursor mission, designed to investigate two potentially le-thal problems: concentrations of dust in the asteroid belt,and damage to electronics from high-energy particles in thejovian magnetosphere.This precursor, called Pioneer and assigned to NASAAmes Research Center, received Congressional approval in1969. Its primary objectives were to (1) explore the inter-planetary medium beyond the orbit of Mars; (2) investigatethe nature of the asteroid belt, assessing possible hazardsto missions to the outer planets; and (3) explore the envi-ronment of Jupiter, including its inner magnetosphere. Thechoice of a spinning spacecraft carrying two magnetom-eters, a plasma analyzet (for the solar wind), a chatged-particle detector, and an ion detector reflected this emphasis.The Pioneers also carried ihree rudimentary remote-sens-ing instruments: an imaging photopolarimeter, ultravio-let photomete and infrared radiometer, some of whichstruggled to perform in the hard radiation environment.Pioneer l0 was launched on February 27,1972, followedby Pioneer 11 a year later. These were the first human arti-facts to leave Earth with sufficient energy to escape the solarsystem entirely. Both spacecraft transited the asteroid beltwithout incident. The Pioneer 10 Jupiter encounter, whichlasted several weeks, took place in December 1973. Pio-neer 10 obtained the first spacecraft view of Europa, con-structed from the photopolarimeter data, shown in Fig.2.Although it was very low resolution, on the order of 200 kmpixel-I, the image did show a heterogeneous surface. Basedon these results, Pioneer I I was targeted for an even closerpass by Jupiter a year later, 40,000 km above the joviancloud tops.As shown in Fig, 2, the photometry obtained on the satel-lites was crude and rudimentary. Careful tracking of the


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Fig. 2. The first spacecraft image of Europa, obtained in Decem-ber 1973 by Pioneer 10.spacecraft did provide improved masses for the satellites,yielding a refined density for Io of 3.53 g cm-3 and for Eu-ropa of 299 g cm-3, suggesting that these two inner satellitesreally did not have the same compositions and structure.3.5. Formation of the Galilean Satellites

The post-Pioneer perspective on the Galilean satellites'"vas presented in the first of what would be four compre-hensive multiauthor books published by the University ofArizona Press. Planetary Satellites, edited by Joseph Burnswith 33 collaborating authors and 598 pages (Burns, 1977),was based on a conference held at Cornell University in1974; the last section of Jupite4 edited by Tom Gehrels(Gehrels,1976), was based on a conference held at the Uni-versity of Arizona in 1975. These books documented indetail the subjects lightly summarized in this section. Auseful popular history is Voyage to Jupiter (Morrison andSanz, 1980).In some ways the jovian system can be considered aminiature solar system. The regularity of the Galilean sat-ellite orbits suggests that they were formed within a cir-cumjovian nebula analogous to the solar nebula that gaverise to the planetary system, including a decrease in den-sity with incrcasing distance from the primary. Thus themost fundamental properties of the satellites are closelylinked to the formation of Jupiter itself.Theory suggested that Jupiter's formation was charac-terized by three main stages (Pollack and Fanale, 1982):(l) early, slow contraction, in quasihydrodynamical equi-librium; (2) rapid hydrodynamical collapse, triggered by thedissociation of hydrogen; and later (3) slow contractionleading to gradual cooling (e.g., Bodenheimea 1974).The

Alexander et al.: Exploration HistotT of Europacircumjovian nebula probably developed during stage2when the collapsing proto-Jupiter shrank inside the currentorbits of the Galilean satellites. Cameron and Pollack (1976)proposed that accretionary processes occuning in the diskled to formation of the satellites, by processes analogousto those that took place in the solar nebula.Constraints on the temperature conditions in the cir-cumjovian nebula were provided by the trend in bulk com-position of the satellites, as first modeled by J. Pollack ofNASA Ames and his colleagues (Pollack and Reynolds,1974; Cameron and Pollack, 1976; Pollack et al., 1976).The solar nebula, cooling with distance from the Sun, wasmodeled to have azone cool enough that volatiles such aswater vapor would finally condense. The transition from thezone where roc material predominated, to that where vol-atiles would readily condense, was designated the "snowline." By analogy, the density dichotomy among the sat-ellites suggested that there was a snow line in the joviannebula between Ganymede and Europa. Presumably thehigh luminosity of the proto-Jupiter inhibited the conden-sation of H2O inside the orbit of Ganymede.In this emerging view, Europa's history bore some simi-larities to that of Earth, with its bulk composition dictatedby the moderately high temperatures in the part of thenebula where it formed, followed by the acquisition of asurface veneer of H2O ice from impacting comets or othervolatile-rich planetesimals. Observations, howeve gave nohint of the thickness of Europa's veneer of ice.3.. Interiors and Thermal Evolution

Thermal modeling had been initially driven by theoreti-cal concepts, not new data. John Lewis (now at the Massa-chusetts Institute of Technology) continued Urey's line ofresearch on the cosmochemical nature of the extended so-lar system with a series of papers (Lewis, 1969, 1972) thatoutlined the various types of ice that should be in stableequilibrium with a cold solar nebula of cosmic abundances.Lewis (l97Ia,b) presented a simple heat-balance calcula-tion showing that if the radiogenic heat from the roc frac-tion of an ice-rock moon in cosmic proportions were insteady state with the outflow of heat via conduction, thentemperatures would rise well above the melting point of icein bodies as small as 1000 km radius. He further recognizedthe role of high-pressure ice forms in the evolution of thesebodies, the importance (and difculty) of modeling convec-tive heat transport, and the likely alteration of the surfacesdue to evaporation of the ices, the infall of meteoritic ma-terial, and the formation of minor species from "photolyti-cally labile material."Lewis in turn directed a student who developed a moredetailed model (Consolmagno, 1975) for the thermal evo-lution of icy moons. This model took into account the de-caying levels of radionuclide abundances, phase changesbetween different forms of ice (as well as the melting ofthat ice), and the heat released during the formation of arocky core once the ice melted. Convection was assumed


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10 Europafor the molten regions of the body but, it was argued, wasless likely to be important for the ice (o at least, difficultto model) given the uncertain and non-Newtonian proper-ties of dirty water ice with multiple high-pressure phases.Assuming Europa was made of 907o roc material withthe density of an ordinary chondrite and l}Vo HrO, themodel (Consolmagno, 1975) predicted that on Europa "athin crust of ice covers a convecting region of water, whichis cooling off the upper layers of the silicate core."In an appendix, Consolmagno speculated on the impor-tance of the chemical evolution of the rocky core with theliquid water mantle. He closed this argument by writing,"Given the temperatures of the interiors, and especially ofthe silicate layers through which liquid will be percolating,the possibility exists of simple organic chemistry takingplace involving either methane from the ice or carbon inthe silicate phase. However, we stop short of postulating lifeforms in these mantles." Similar arguments on the meltingof the icy Galilean moons (although without the specula-tion about life) were presented in Consolmagno and Lewis(1976) and Fanale et al. (1977), who agreed that Europawould likely differentiate within the first 500 m.y., with aliquid water layer (ocean) underlying a water ice crust, bothtogether forming a layer on the order of 100 km depth overthe roc core.As all these authors acknowledged, thermal modelingrequired more knowledge than was actually available at thetime for the initial composition, radioactive heat sources,temperature of formation, and the thermal and physical be-havior of unconstrained mixtures of ices and rocky mate-rial, and furthermorc required speculation about the totalrmount of anhydrous vs. hydrous silicates in the initial infallof material to form the satellite. Uncertainty as to how muchaccretional energy was retained has always made it difh-cult to assess how effective this heat source was at earlydifferentiation. Lack of data never prevented theoreticiansfrom speculating, howeverIt was Io that first drew public attention to the thermalevolution of the Galilean satellites. In a paper dramaticallypublished in Science a few days before the Voyager 1 en-counter, Stan Peale of the University of California, SantaBarbara and Pat Cassen and Ray Reynolds of NASA AmesResearch Center calculated the expected heating of Io fromtidal stlesses (maintained by the Laplace resonance) andpredicted currently active volcanism (Peale et al., 1979).Their prediction was almost immediately confirmed by Voy-ager images of Io, first of volcanic flows on the surface andthen by spectacular plumes of gas rising from ongoing erup-tions identified a few days after the flyby. We will returnto the analysis of the interior and implications of tidal heat-ing for Europa in section 5.8.

4, THE VOYAGER ERAThe Voyager "grand tour" followed closely upon Pio-neers 10 and ll. The years around the Voyager missionssaw a revolutionary change in our understanding of theGalilean satellites. In many respects, these small satelites

were the stars of the two Voyager Jupiter encounters, emerg-ing as full-fledged worlds with unique geological histories.Europa was seen as a rocky world with a young surfacelayer of plastic ice and enigmatic surface features that hintedat a dynamic thermal history. And after the dramatic dis-covery of active volcanism on Io, these two satellites wouldnever again be considered as "twins."4.1. The Voyager Missions

The back-to-back Voyager missions represented a scaleddown but still highly capable implementation of the grandtour concept. Built and operated by the Jet Propulsion Labo-ratory (JPL) of the California Institute of Technology, theVoyagers were large three-axis-stabilized spacecraft with afully articulated scan platform, permitting detailed study ofthe planet and satellites. Voyager I was launched in Sep-tember 7977 on a path to Jupiter and Satum and Titan; Voy-ager 2 undertook the l3-year trip to Jupiter, Saturn, Ura-nus, and Neptune. Both eventually followed the Pioneers inleaving the solar system entirely.Voyager imaging data determined the diameters (andhence densities) of the Galilean satellites to yield whatare essentially the modern values (for Europa: diameter3100 km; density 3.0 g cm-3), and their surface featureswere extensively mapped. Europa was the least well imagedof the Galilean satellites, an unavoidable result of its orbitalposition during the two flybys in March and July 1979.Thebest data were obtained by Voyager 2 on July 8, 1979, at aclosest range of 206,000 km, when a fraction of one hemi-sphere was mapped at a resolution of about 2 km pixel-t.Its bright (high-albedo) surface was found to be remark-ably smooth and almost free of craters. The surface con-sisted primarily of uniformly bright tenain crossed by longlinear markings and a few very low ridges (elevation up toa few hundred meters). Voyager scientists characterized itas looking like "a white billiard ball faintly crossed by linesapplied with a pen" or looking "cracked like a broken egg-shell" (Morrison and Sanz, 1980).The absence ofcraters or surface relief(Fig. 3) suggestedto Voyager scientists either recent resurfacing or a "soft"icy surface, while the long "cracks" seemed to imply a brit-tle crust subject to tectonic stresses. While the ice crust wasestimated as roughly 100 km thick, there was little consid-eration to the possibility that much of this material mightactually be liquid today. However, the suggestion was madethat there might be episodic heating of Europa, perhapsanalogous to the tidal energy source currently so activeon Io. Perhaps the most enigmatic features, visible onlynear the terminator on the best images, were cycloid ridgesystems with wavelength on the order of 100 km and totallength of more than 1000 km.The Voyagel encounters were an unforgettable experi-ence for the participants, who included nearly 100 mem-bers of the press, about half of whom encamped at JPL fora month in both March and July 1979, as well as the thenGovernor of Califolnia, Jerry Brown. In those days NASAinitiated an open cornmunications policy with daily press


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Fig. 3. Among the firsr close-up images of the linear crack-likefeatures of Europa's surface obtained from Voyager 2, July 9,1979, at a range of 246,000 kilometers.conferences, informal science seminars, and frequent per-sonal exchanges involving mission staff and scientists. Somemembers of the press, such as Henry Cooper (who was thenwriting forThe NewYorker), Kelley Beatty of S andTete-scope, and Jonathan Eberhart of. Science News, were givensome access to the science work areas where the data werebeing analyzed and discussed. The spacecraft raw images(coming in at a rate of one every 90 seconds) were alsodisplayed "live" on the JPL internal TV net, so that every-one present could participate in the shared excitement ofdiscovery. Popular books written in this timeframe includeThe New Solar System (Beatty et at., l98B).The circumstances in which the imaging resolution andother data rapidly improved as the spacecraft approachedits targets allowed a unique compression of the scientihcmethod, canied out on a public stage. Data such as satel-lite images that were received one day were interpreted byeach science team to a level appropriate for the press con-ference the following day. But everyone knew that by thetime these results were presented, together perhaps withideas about their interpretation and significance, new andbetter data were already in the pipeline. In the final daysbefore each encounte the imaging resolution doubled fromone day to the next. Within a week, hypotheses could beformed, tested, and rejected or modified several times. Thiswas a heady experience never equaled in any other spacescience mission.Carl Sagan had famously written that only one genera-tion was privileged to witness the transition from planets(and satellites) as mere points of light in the night s toreal worlds, each with its own unique history. For theGalilean satellites, that transition took place within just thefive months that encompassed the Voyager encounters.

Alexander et al.: Exploration History of Europa I I4,2. Non-Ice Material and processes

In the years immediately preceding Voyager's Jupiterflybys, groundbased observers discovered the neutral torusand plasma torus associated with Io, the first being discov-ered by R. A. Brown's observation of Io's sodium cloud andthe second by Kupo et al.'s 1976 discovery of ionized sul_fur in Jupiter's magnetosphere (see review by Thomas etal., 2004). Voyager in situ and ultraviolet measurementscharacterized the density and spatial extent of this iogenicplasma and demonstrated the intimate coupling between thesatellites and the jovian magnetosphere. These observationsprompted Eviatar et al. (1981) to suggest that ion implan_tation was occurring on Europa's trailing side; that Io waspainting that hemisphere with sulfur. Concurrent with thegroundbased torus observations, the International UltravioletExplorer (IUE) was being used to characterize the ultravio-let reflection spectra of Jupiter's satellites. Lane et al. (1 93 1)examined trailing-leading side ratio spectra, finding a broadabsorption feature centered at 280 nm that they identifiedwith SO2 and suggested was formed by ion implanration.Voyager images enabled mapping of Europa's surface,in optical pass bands from the ultraviolet to orange regions,and then study of the distribution of chromophores. Sulfurhad earlier been suggested (cf. Johnson and McCord, l97l;Wamsteke1 1972), and analyses by Johnson et at. (19g3),McEwen (1986), and Nelson et al. (1986) found distribu-tions consistent with implantation. The sulfur chemistry wasnot completely understood, and the association of darkmaterial with geological features was (and remains) unex_plained. Possibiliries include enrichment by geological pro_cesses or emplacement of material from below, or both (seechapter by Carlson et al.).In addition to observational work, two separate com_munities began laboratory work that would bear fruit forplanetary exploration in the decades to come. Experimen-tal work showed that ion bombadment of minerals and saltsdarkened surfaces and caused spectral shifts (Hapke,200l;Nash and Fanale, 1977).Ion bombardment also ejects mol-ecules from the surface by a process known as sputtering.Brown et al. (1978) and Lanzerotti et al. (1978) showed thation bombardment of low-temperature ices breaks chemicalbonds, creating new species, and efficiently sputters theparent and daughter molecules. The production efficiencyfor producing O, was found to be strongly temperature de-

pendent, but at temperatures relevant to the surface of Eu_ropa. HrO, H2, nd O, proved to be important componentsof the ejecta. On the basis of this work, Johnson et al.(1982) predicted the presence of an O, sputrer-producedatmosphere on Europa (see section 4.4).4,3, First Thoughts on Habitability

Consolmagno (1975), Consolmagno and Lewis (1976,197"7, 1978), Reynolds et al. (1983). and others, perhapsmost famously Arthur C. Clarke (inspired by conversationswith some of these investigators) in his 1982 novel 2010,opened questions about the habitability of Europa. Two sce_


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t2 Europanarios presented themselves - life on' and life under, thesurface. Surface life, subjected to the radiation environmentand near-vacuum conditions' seemed highly unlikely' WhenConsolmagno first informally suggested life in a subcrustalocean, he was reminded by Carl Sagan (personal commu-nication) that life on Earth depended on sunlight for itsenergy, a source not available in the dark oceans'An important change in the understanding of the en-ergetics of life occuned when tenestrial life forms werediicovered associated with black smokers deep in Earth'soceans (Corliss et al., 1979; Jannasch and Wirsen, 1979;Corliss et at., l98l; Sullivan and Palmisano, 1981)' Thediscovery dramatically demonstrated that chemotropic lifecould thrive on a geochemical source of reduced com-pounds, although it was later pointed out that such processesiook place in the presence ofan ocean saturated in molecu-lar oxygen produced by photosynthesis. For Europa, com-parable oxygen- and oxidant-producing reactions may benduced by radiolysis at the surface that, with subduction,could form a radiation-driven ecosystem (Chyba, 2000;Chyba and Hand,200l; chapter by Hand et al')'in those early days, speculation about the habitability ofa proposed europan ocean included assumptions such asinien abundances of nutrients by comparison with theMurchison meteorite, assuming Europa to be of carbona-ceous chondritic origin (Oro et aI', 1992; Reynolds et al',1983). The metabolic pathways of chemotropic microorgan-isms were only moderately well understood' Radiation andinsolation were inferred based upon (incomplete) samplingand characterization of the radiation environment near theL-shells crossed by Europa (Eviatar et al., l98I)' Quanti-tative discussion of biosignatures would have to wait untilhigher-resolution data on the geochemistry of Europa couldbe obtained.4.4. Post-Voyager Perspectives on ThermalHistor the Presence of an Atmosphere'and Europan ElectrodYnamics

The predictionby Peale et al. (1979) that tidal heatingforces would result in a volcanic Io, and the subsequent dis-covery by Voyager of the geysers Pele and Loki in full erup-tion, electrified the planetary science community' Europa'syouthful surface and global tectonic activity were hypoth-sized to originate, like those of its neighbor Io, from tidalinteractions with its innermost and outermost orbital neigh-bors. Reynolds and Cassen (1979) argued that the ice layerwould be unstable to convection and that thermal convectionwould freeze a liquid layer in a time short compared to thelifetime of the satellite (C. Alexander at the time was thestudent who ran many of Reynolds'convection models)' Butthe calculations were critically sensitive to material param-eters and assumed initial conditions, because the heat pro-duction and subsolidus heat removal processes were of com-parable magnitude. For example, the Squyres et aI' (1983)solution (next discussion) was based upon an assumptionthat hydrated silicates do not lose appreciable strength until

dehydration occurs and liquid water is released - an as-sumption that impacts the calculated ability of the body toflex and respond to tidal forces.Squyres et at. (1983) introduced the paradigm of a rela-tively thick ice crust, potentially overlying a liquid waterlayei that remained to be proved. The paradigm included

the notion that tidal heating could maintain a liquid oceanlayer, but heating rates would be slow enough to maintaina frozen crust tens of kilometers thick, and that convec-tion would be the dominant mechanism for heat transportfrom the ocean to the surface. Alternatively, Helfenstein andParmentier (l 985) suggested that nonsynchronous rotation(an ocean or "soft ice" layer that effectively decoupledthe surface from the interior) might provide a fracturingmechanism to explain the geologic features revealed byVoyager.Early speculation about a tenuous atmosphere on Europa,generated by sublimation, just as it is on a comet, was ef-fectively damped by the measurements of the low surfacetemperature of Europa. Even when suggestions were madeconeming migrating ice on Ganymede, Europa seemed toocold for such processes to take place. Alternatively, Johnsonet al. (L982) predicted a bound O, atmosphere for Europa,with a column density of -2-3 x 10le m-2, generated bysputtered water ice.After the discovery of electrodynamic signals associatedwith Jupiter's interaction with Io (Bigg, 1964), Neubauer(1980), in a seminal paper, studied the motion of a largeconducting body through space, and determined that energywould be radiated away in the form of Alfvn waves' Rey-nolds et at. (1983) were the first to discuss an electrical Eu-ropa, although they assumed charge separation due to a J xB lorce (where J is the current and B the magnetic field)that would generate an electric field and associated currentsclosing through Jupiter's ionosphere. The Galileo missionwould exploit the interactions between Europa and the mag-netosphere to provide the most compelling evidence forliquid water in its interior, looking for the evidence for thepresence of a conducting layer in these Alfvn waves andother electrodynamic phenomena first discussed at this time'In the years after Voyager, the final two of the four com-prehensive volumes (see section 3'5), published by the Uni-versity of Arizona Press and featuring the Galilean satellites,were released . Satellites of Jupiter, edited by David Moni-son with 47 collaborating authors and972 pages (Morrison,1982), summarized the state of knowledge after the flybys'A still more mature perspective was reflected in Satellites,edited by Joseph Burns and Mildred Shapley Matthews,with 45 collaborating authors and l02l pages (Burns andMatthews, 1986).5. THE GALILEO SAGA

For planetary scientists, the decade after Voyager wascharacterized by budget threats (in 1981 the Offrce of Man-agement and Budget argued for terminating all planetaryeiploration and redirecting JPL into defense work) and the


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loss of the shuttle Challenger in January 1986, an event thathalted all deep space launches, including that of the HubbleSpace Telescope (HST), and forced a major redesign of theGatileo mission to Jupiter. After the rapid-fire successes ofthe Mariner and Pioneer missions, crowned by the triumphsof Viking (to Mars) and Voyager in the late 1970s, this wasa severe disappointment. Many young scientists who hadentered planetary studies thinking that there would be atleast one new mission per year faced a new and unpleas-ant reality. However, when the next generation of planetarymissions such as Galileo and Magellan did fly, they pro-duced data of unprecedented quality and quantity. Reviewsof the Galileo mission as a whole can be found in John-son et aI. (1992), Barbieri et al. (1997), Harland (2000),Fischer (2001), Cruikshank and Nelson (2007),and Meltzer(2007). Technical details of the science investigations arecomprehensively described in the book The Galileo Mission(Russell, 1992).5.1. The Galleo Mission

The mission that would come to be known as Galileowas under discussion long before Voyager was launched.At NASA Ames Research Center, a science working groupstudied a Jupiter Probe mission that combined a spinningspacecraft (with heritage from Pioneers l0 and 11) withan entry probe. This team focused on investigating jovianatmospheric composition and the magnetosphere. A paral-lel study at JPL investigated the potential of a three-axis-stabilized spacecraft like Voyager that could orbit Jupiterand undertake detailed study of thejovian satellites and thedynamics of the planet's atmosphere. Both teams argued fora long-lived mission that could make many orbits of Jupi-ter, mapping out the magnetosphere and providing multi-ple close flybys of the satellites. These two concepts werer'rerged in a study led by James van Allen that recom-mended to NASA a multidisciplinary mission called Jupi-ter Orbiter Probe (JOP). While there was obvious value inpursuing so many objectives with a single mission, thesechoices also planted seeds of conflict that would emergewhen diffrcult choices had to be made in planning trajec-tories and prioritizing investigations. Given Galileo's limi-tation on computing power and onboard memory (whichused tape recorders to store data), this turned out to be avery challenging mission from an operational perspective.The 16 instruments canied by the Galileo orbiter in-cluded a magnetometer mounted on a boom to minimizeinterference with the spacecraft; a plasma instrument fordetecting charged particles (PLS); a Plasma wave detector(PWS) to study waves generated by the particles; an ener-getic particle detector (EPD) for measurements of plasmaat the very-high-energy end of the spectrum; and a detec-tor of cosmic and jovian dust. It also canied the Heavy IonCounter (HIC), an engineering experiment added to assessthe potentially hazardous charged particle environmentssunounding the spacecraft. The remote sensing instrumentsincluded the camera system [SSI, the first charge-coupled

Alexander et al.: Exploration History of Europa 13device (CCD) on a deep-space projectl, designed to obtainimages of Jupiter's satellites at resolutions from 20 to 1000times better than Voyager's bes the Near Infrared MappingSpectrometer (NIMS) to make multispectral images (imagecubes) for chemical analysis, the first imaging spectrom-eter ever flown; an Ultraviolet Spectrometer (UVS) to studygases; and a Photo-Polarimeter Radiometer (PPR) to meas-ure radiant and reflected energy. The payload also includedan extreme ultraviolet detector (EUV) associated with theUV spectrometer on the scan platform.The JOP mission was ready to be proposed to Congressas a "new start" even before the September 1977 Yoyager Ilaunch. In March 1977 funds were approved following anunprecedented roll-call vote in the Senate that dealt withthis specific mission alone. The official start of the project,now named Galileo, was in October 1977, with a plannedlaunch in January 1982 using the space shuttle.The launch of Galileo was repeatedly delayed, prima-rily by slips in the space shuttle schedule and changingconfiguration of the associated upper stage launch vehiclesthat were required to accelerate the spacecraft from low-Earth orbit and on to Jupiter. Following the Challenger acci-dent in 1986 and subsequent decisions not to use the high-energy Centaur upper stage with the shuttle, the mission wasreconfigured for launch in October 1989. Because the avail-able solid-fuel IUA upper stage was less powerful than aCentaur, the new trajectory included flybys of Earth (twice)and Venus. Concerns about heating close to the Sun led themission plnners to delay deployment of the main 4.8-mhigh-gain antenna until after the Venus flyby. It then failedto open fully, eliminating all high-gain transmissions fromthe spacecraft. As a consequence, less thanl%o ofthe origi-nally planned data could be returned from the mission.Heroic changes in software and adoption of sophisticateddata compression schemes bought back about a factor of10, but throughout its lifetime Galileo was "data starved,"requiring very careful planning to ensure maximum sciencereturn (O'Neil, 199'7).5,2. Satellite Tours

Galileo was in the Jupiter system from December 1995through September 2003, almost a full Jupiter year. The"tour" for the Galileo orbiter was designed to enable de-tailed observations of the satellites and Jupiter. Jupiter or-bit insertion (JOI) was achieved with a close flyby of Io,with subsequent orbits anchored on satellites with the dualpurpose of studying each satellite and using the satellitegravity to modify the orbit for the next loop. Each orbit wasdesignated by the flyby satellite. Thus, orbit "Gl" was thefirst orbit in the mission and Ganymede was the target forthe close flyby; "C3" was the third orbit with a flyby ofCallisto; etc. Substantial data could also be collected for theother satellites from a greater distance; for example, the firstimages of Europa were taken on orbit Gl. Even thoughthese far-encounter views were only equivalent to the Voy-ager resolution (typically l-2 km pixel-r), they were use-


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TABLE l. Europa data takes dving Galileo's prime and extended missions'Orbit Mission Date Altitude (km) Hemisphere ofClosest Approach Local TimeG1 Prime

PrimePrimePrimePrimePrimePrimePrimePrimeGEMGEMCEMGEMGEM

Iune 2'1, 1996Sept. 6, 1996Nov.4, 1996Dec. 19, 1996Feb.20, 1997April 5, 1997Iune 25, 1997Sept. 17, 1997Nov.6, 1997Dec. 16, 1997Feb. 10, 1998March 29, 1998May 31, 1998July 21, 1998Sept.26, 1998Nov.22, 1998Feb. I, 1999Nov.26, 1999Jan. 3, 2000May 20, 2000Jan.17,2002

Between trailingand antijovianTrailing (Upstream)AntijovianLeading (Wake region)Trailing (Upstream rcgion)Between antijovianand leadingBetween leadingand subjovianTrailing (Upstream)Leading (Downstream)Trailing (Upstream)(Gravity only)Trailing (Upstream)Leading (Downstream)(No data obtained becauseof spacecraft safing)Leading (Downstream)(No data obtained becauseof spacecraft safing)Trailing (Upstream)SubjovianTrailing (Upstream)AntijovianSubjovian

16.7 (night sector)12.9 (nigh

I 1.0 (day)14.7 (day)14.4 (day)l0.l (day)9.9 (day)

9.8 (night)2.9 (night)

156,000673,00041,000692586 km24,600

G2C3E4E6G7C9c10EllEt2El3Et4El5El6Et7E18El9t25826G28t33

GEMGEMGMMGMMGMM

621,00020432013562164425t5r 8343s82227114398860351593,321r,002,152

GEMGEM

Excellent images were taken on encounters where Europa was not the prime target of Gl, G2,C3, G7, C9, ClO,125, G28, and I33, some with resolutions as good as 420 m pixel-t, and often in color. GEM = Galileo EuropaMission; GMM = GaIiIeo Millennium Mission. Local time is relative to Jupiter; neither remote sensing nor freldsand particles (F&P) data was collected on El, E16, and E19 for anomaly and telemetry limitation reasons. Dataextracted from Kurth et al. (2001).

ful because of the high quality of the Galileo CCD cameraand the opportunity to view the targets at different phaseangles and rotational phases,For the first two years, the Gqlileo orbiter made closeflybys only of Callisto and Ganymede, using these satel-lites to shlink and circularize the orbit. The strategy for im-aging Europa was to acquirc enough information during thistime to be able to plan specific close passes as the space-craft orbit permitted. The team implementing this imagingstrategy was a talget planning and sequence design teamled by Ron Greeley and associates at Arizona State Uni-versity, during the prime mission, and co-organized withJames Head, at Brown University, for the extended mission.All such remote sensing sequences were sevelely impacted,of course, by the failure of the high-gain antennae to de-ploy. This was particularly problematic for imaging andother high-data-rate instruments. Each picture was a pre-cious cornmodity, resulting in considerable debate withinthe team and the Galileo Project Science Group over theallocation of data downlink resources.For Europa, medirm-resolution (few hundred meters perpixel) images provided regional views, and high-resolution

(tens of meters per pixel) images provided samples of keytemains and features, set within the context of the regionalview. For morphology, low-Sun-angle illumination wasdesirable to enhance tenain characteristics, while color andphotometry objectives were best met with high-Sun views.In all, the Gclileo spacecraft encountered Europa 12times (see Table 1). The first three encounters took placeduring the prime mission, and the bulk of the remainingencounters occurred during the Galileo Europa Mission(GEM). The final encounter was part of the Galileo Mil-lennium Mission (GMM), an extension to allow the Jupitersystem to be studied by two spacecraft at once: GaLileo andCassini, passing by in the year 2000 on its way to Saturn.Ironically, just as the Galileo spacecraft was aniving atJupiter, policy makers were gathering to decide how bestto destroy the spacecraft to protect Europa. Even before the1989 launch, rhe Galileo project plan was revised to includethe following: "In addition, the Project will supply dataobtained bearing on the biological interest of the Joviansatellites to the [NASA] Planetary Protection Officer in atimely rnanner. This information will be provided by letterbefore the end of mission and while the spacecraft is cou-


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Alexander et al.: Exploration Hstory of Europa l5

Fig. 4. (a) The trailing hemisphere of Europa - so designated because the hemisphere facing the camera happens to be the trailingside of the moon's orbital motion around Jupiter and also subtends the stream of magnetospheric plasma, that flows faster than theorbital motion of Europa. The impact crater Pwyll is the central point on the lower right of the image from which the white rays aresplayed. te longitude of the center of this hemisphere is approximately 290oW. (b) This image shows Pwyll rotated to the west (left)about 70o from the previous image such that the trailing hemisphere is oriented to the left and the leading hemisphere to the right; theConamara Chaos region is now shown on the left; the Cadmus and Minos reqions are in the upper right; the "wedges" region is justbelow the equator and west of the antijovian (center) point. The longitude of the center of this hemisphere is 220oW.trollable. If the Planetary Protection Officer finds that asatellite should be protected further than Category I[ re-quirements call for, the Project will negotiate options thatwill preclude an impact of that satellite by the Orbiter:' InApril1999, after the final GEM Europa encounte a Na-tional Research Council board was convened to make rec-ommendations on how to best prevent forward contamina-tion of Europa (National Research Council,2000), and theproject began considering the logistics of the final disposi-tion of the spacecraft.5.3. Geology Unique to Europa

The hypothesis of a europan ocean \ryas considerablystrengthened in the extended GEM. Figure 4 shows thehemisphere(s) in which most of the features relevant to theefforts to deduce the presence of an ocean are to be found.With the first close flyby of Europa on E6, images wereobtained with resolution up to 2l mlpixel. These repre-sented a huge increment in detail, but because of the con-strained data budget they could not always be placed in theirlow-resolution context. This problem of high-resolution"postage stamp" images without full context was a chal-lenge throughout the mission. These high-resolution imagesshowed the "triple-band ridges" (Fig.5) and "chaotic ter-rains" (Fig. 6) in detail, both of which indicated the highprobability of sub-ice activity at the time that the features

formed. Chaotic terrain was particularly intiguing, becauseit seemed to indicate places where the crust has been ex-tensively disrupted by internal processes. Reconstructionsof the geometry of the chaos terrain showed that the vari-ous pieces could be fit back together like ajigsaw puzzle.Even before the high-resolution images were obtained,the concept of a young, active sub-ice ocean was begin-ning to emerge, as reported by Greeley at the conference"The Three Galileos: The Man, The Spacecraft, and theTelescope" (Barbieri et al., 1997). Held January 1997 inPadova, Italy, the sessions convened in the same universityvenue as Galileo's lectures, and the conference culminatedwith a visit to Pope John Paul II in Rome, ananged by GuyConsolmagno and the Director of the Vatican ObservatorGeorge Coyne.By the end of 1997, the Galileo science team was inconsensus that Europa is a fascinating object that meritedmore extensive exploration, leading to the GEM extensionof operations through repeated flybys of this moon. Sum-maries of the geology of Europa as it was understood atthe time can be found in Carr et al. (1998) and Greeley etal. (1998). A series of papers from the imaging team, initi-ated in 1997 and published in 1998, addressed detailedaspects of the "notion of the ocean" within Europa, includ-ing a paper by Sullivan et al. (1998) on "wedges"; an analy-.sis by Pappalardo et al. (1998) of solid-state convection;and a paper by Gesler et al. (1998) on evidence for non-


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t6

Fig.5. Ridges and triple bands. Original image taken during the Gl encounter, subsequent high-resolution images taken on E6.

Fig.6. The Conamara Chaos region. The lower-resolution image was taken on E6, the high-resolution image on El2'

synchronous rotation. This was followed by apapetby Pap'palardo et al. (1999) on the evidence for an europan ocean'The mounting evidence for nonsynchronous rotation wasbased in part on images that show strike-slip faults (hori-zontal movement long fractures in the crust). More than100 such faults were identified by Greenberg's team [seeGreenberg et al. (2002) for a complete list of citationsl, whonoted that left-lateral offsets occur preferentially in thenorthern hemisphere, while right-lateral offsets dominate inthe southern hemisphere. These features correlated wellwith the stress field - a map of the extensional and com-pressional forces that drive surface deformation. The crackpattern on the surface was found to be consistent with theinduced stress field found by models of nonsynchronousrotation and the presence of a sub-ice ocean. In addition,cycloidal ridges, first seen by Voyager, was evidence thatdiurnal tides were important drivers for surface deformation(Hoppa et al., 1999; Greenberg et a1.,2002).However, the evidence of an ocean based on surface fea-tures simply refered to the time when the features formed,and did not necessarily suggest that an ocean exists today.

This consideration required an assessment of the age of theeuropan surface, coupled with data from the Galileo mag'netometer,5.4. Age of the Europan Surface

Very few impact craters are seen on Europa's surface.The frequency of impact craters superposed on planetarysurfaces provides an estimate of surface ages, based onmodels of the flux of impacting objects. The impactingpopulation for the jovian satellites is presumably very dif-ferent from that of the inner solar system, which is domi-nated by asteroidal sources. The impact flux in the innersolar system has been calibrated using crater counts on thelunar maria, for which chronological dates are known (e.g.,Hartman,lgSl). It is presumed that the primary impactingpopulation in thejovian system is heliocentric (not planeto-centric) and consists of comets, i.e., of debris from theKuiper belt and Oort cloud. To date a relatively young sur-face like that of Europa, one needs to know the flux of small(kilometer-scale) comets. Observations from Earth indicate


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that very few kilometer-scale comets penetrate the innersolar system, so estimating the flux at Europa involves achain of assumptions. One must also take into account theconsiderable focusing of incoming projectiles by the joviangravity, and this in turn depends on the dynamics of thispopulation (Moore et 1., 1998).Flux uncertainties produced initial estimates for the ageof Europa's surface ranging from more than 2 b.y. to lessthan 10 m.y. With refinements in models for the impactingflux, most planetologists now regard the surface to be 30-70 m.y. old (hhnle et al., 2003), a geological blink of theeye in comparison to most planetary surfaces (Carr 1999).Consequently, it seems unlikely that an ocean sufficientlyextensive to account for the surface features would havefrozen in such a short period of time geologically. TheGalileo imaging team [see Pappalardo et al. (1999) andGreeley et al. (2004) for citations related to this growingconsensusl therefore concluded that an ocean is probablypresent today; this conclusion was further supported by themagnetometer data discussed below. Questions remain asto the thickness of the ice shell over the ocean, which isone of the key issues to be addressed by a future mission(see section 6).5.5. Inference of an Induced Magnetic Field

The most powerful arguments for a current liquid waterocean on Europa were derived from measurements of inter-actions with the jovian magnetospherc. Kargel and Consol-magno (1996) suggested that evidence of an ocean rich indissolved salts and thus electrically conducting might bevisible in the magnetic eld around Europa. Saltwater, al-though a poor conductor compared to metals like cop-per wire, is much more conductive than some natural non-metallic solids expected at Europa, such as water ice, andsalty hydrated silicate-ice polyphase aggregates. On a plan-etary body, when found physically near the surface and notdeeply buried, curents engendered in an idealized saltybrine, with the conductivity of terrestrial seawater, shouldbe capable of producing a magnetic signature detectableabove the surface (Zimmer et al., 2000), if that liquid islocated physically near the surface and not buried toodeeply, with a signature that is slightly different in charac-ter than that produced by a magnetic core of iron and nickel,like that of Earth.A dipolar magnetic perturbation was observed in themagnetic field data from Galileo's initial pass by EuropaonF4 (Kivelson et al., 1997,2000). The magnetic field de-pression expected in the presence of the ambient jovianmagnetic field of 450 nT was 100 nT. The actual measureddepression was about 50 nT. TVith additional passes, thosesampled during GEM, it became clear that the magneticmoment of Europa changed sign in phase with the chang-ing sign of the ambient jovian magnetic field's radial com-ponent. Such a change would be expected in the presenceof Alfvn wings, the plasma structure first predicted byAlfvn in the 1930s for a conducting object moving througha magnetic field. As modeled by Neubauer (1998), Alfvnwings would create perturbations in the radial direction. The

Alexander et al.: Exploration History of Europa l7observed deflection provided compelling evidence of thepresence of a global-scale conducting shell located within100 km of the surface lsee Kivelson et al. (2004) and thechapter by Khurana et al. for an extended discussion].However, although the symmetry of the Alfvn wings,with offsets in the radial direction toward and away fromJupiter, appear generally consistent with asymmetries foran induced magnetic response (Neubaue7 1998) due to thepresence of a highly conductive material (liquid, salty wa-ter) beneath the crust, it has been shown (Saar et al., 1998)that the signature can also be reproduced solely using elec-tric currents in the thin exosphere. In 1998, the signatureof the footprint of an Alfvn/Birkeland-style current wasdiscovered in the aurora of Jupiter (Clarke et a1.,2002),demonstrating that there is an electrodynamic response ofEuropa in the jovian magnetosphere that mimics that of Io,where Io is capable of producing a palpable auroral signa-ture at Jupiter with atmospheric currents alone (Clarke etal., 2004). It should also be noted that a salty ocean withelectrical conductivity strong enough to support currentrequires that the mole fraction of the active brine componentbe suff,rciently high and it is not clear, with current work,that the required high fractions exist in nature (McCarthyet a1.,2006; Grimm and Stillman, 2008). For morc on thesalinity of a potential europan ocean, see section 6.5.6. Europa's Sputtered Atmosphere and Exosphere

The interaction of magnetospheric plasma with the sur-face results in the creation of an atmosphere. Observationsof an atmosphere at both Ganymede and Europa were con-firmed independently, just prior to Galileo orbit insertionby groundbased observers using the Goddard High-Resolu-tion Spectrograph of the Hubble Space Telescope. Molecu-lar oxygen was observed in the atmospheres of Europa andGanymede (Hall et al., 1995,1998) and separately in thenear-surface of Ganymede (Spencer et al., 1995) and laterEuropa (Sp e nc e r and Calv in, 2002). The I 356- and I 304-emission lines suggested that the emission resulted from theelectron impact dissociation of Or, a process that leaves oneoxygen atom in an excited state (see McGrath et a1.,2004;chapter by McGrath et al.).With Galileo, the principal sputtering agents were shownto be S and O at energies of hundreds of keV (Paranicaset al., 2001, 2002), as predicted pre-Galleo by, e.g.,Johnson (1990). Ip (1996), in modeling the process, in-cluded a secondary source of sputtering, as if a207o frac-tion of exospheric ions were recycled to Europa's surface.His resulting column density was higher than those of theJohnson group, as well as values derived from Hubble ob-sewations, but in subsequent models Johnson chose to in-clude the "back-sputtering" element as well. Thble 2 showsa comparison of estimates of the column density of Europa'sneutral atmosphere, and curent derived from the modeledelectron distribution and calculated Pedersen and Hall con-ductivities.Although most of the material in the atmosphere rcmainsbound to Europa, a portion is ejected with sufficient energyto escape, forming a neutral cloud in Europa's orbital path.


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18 EuropaTABLE 2. Evolution of derived atmosphericcolumn densities for EuroPa.

Source Column density (m-2) Alfvn Cunent (A)Johnson (1990)Hall et al. (1995)Ip (t996)Hall et aL (1998)Saur e, al. (1998)

to wavelength regions inaccessible to groundbased tele-scopes. An example is the water band at 3.1 rm that is sostrong that H2O ice behaves like a metal and produces aspecular refleition feature. The shape of this reflectancep"uk n"ut 3.1 m is diagnostic ofthe lattice order in the topmicrometers of the surface that can vary from crystallineto amorphous. The presence of amorphous ice on Europa'ssurface was found by Hansen and McCord (2004)' Amor-phous ice can be crystallized by heating, while the disrup-iion caused by particle radiation can amorphize crystallineice. Model comparisons show that the surface ice is pre-dominantly amorphous on Europa, although below a depthof


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Predictions ofradiolytic products for icy satellite obser-vations were given by Johnson and Quickenden (1997) andincluded molecular oxygen and hydrogen peroxide (HzOz)as expected radiolytic products. Early in the Galileo mis-sion, infrared spectra ofEuropa were obtained close to thesatellite, but long-wave measurements were severely com-promised by radiation-induced noise. By obtaining Europaspectra from great distances, at nearly the orbit of Gany-mede, an infrared feature of HrO, was identified and cor-roborated by ultraviolet spectra (Carlson et al., 1999a).Since hydrogen peroxide is rapidly dissociated by near-UVsolar radiation, a large production rate vvas implied, leadingto the conclusion that radiolysis is a major factor in deter-mining Europa's surface chemistry (see chapter by Carlsonet al.). The fluxes of ionizing radiation at the Galilean satel-lites were compiled by Cooper et al. (2001) and are updatedin the chapter by Paranicas et al.Gardening by micrometeoroid impact competes with ra-diolysis and sputtering, and buries the radiolytic productsquite rapidly, forming a fluffy regolith (Buratt, 1995) withlarge effective areas that can react with and trap O, andother atmospheric species. Since the regolith depth is largecompared to the penetration depth of most of the impactingparticles, the lower layers are shielded and the regolith canstore and protect oxidants for eventual delivery to Europa'socean by crustal processes (Prockter and Pappalardo,2000).Radiolysis, while forming interesting chemical products,unfortunately also hides the chemical history and ultimatesource ofmaterial. The origin ofhydrated species, observedby NIMS and earlier by Pollack et al. (1978) and Clark andMcCord (1980), is an important case. One suggestion forthese hydrated species is hydrated salts, upwelled and em-placed on the surface from the ocean below (McCord et al.,1998b). An alternative suggestion is hydrated sulfuric acid(Carlson et al., 1999b), which is the stable end-product ofthe radiolysis of sulfurous material in H2O ice, but here thesource of sulfur is hidden - it could be endogenic salts,exogenic sulfur ions, or any other form of sulfur. The hy-drate is associated with disrupted surface arcas in a mannerthat suggests an endogenic source for the sulfur (McCordet al., 1999), although surface heating processes (diapersand shear heating) can produce lag deposits that may ex-plain the geological associations without invoking an en-dogenic source (Fagents et al., 2000; Fagents,2003). Thenature of Europa's hydrated species is an important but as-yet-unresolved question. Europa's sodium and potassiumextended atmosphere may be key to resolving the debate,but Europa's plasma interactions need better understandingto accurately determine the iogenic Na and K fluxes ontoEuropa.5.8. Interior and Tidal Heating

Radiogenic heating introduces conduction and convec-tion, processes that deform the interior of a body. The over-all shape of the body (the ice shell) will also deform (overlonger timescales) as a result of lotation about its axis, thedistribution of mass within, and tides. The degree to which

Alexander et al.: Exploration History of Europa 19any of these processes dominates depends upon certainrheological properties of ice such as viscosity, melting point,degree of impurity, dislocation creep, etc., collectivelyknown as the lagging response of Europa's material. In thecase of tides, the minimum kinetic state would be that of acircular orbit, all spins aligned, and rotation of the moonsynchronized with its orbital motion. The pace at which thisfinal state is achieved depends critically on the lagging re-sponse of the material.The orbital commensurability, first noticed by Galileo,in which Io orbits Jupiter four times in the time it takesGanymede to go around once, contributes to a very impor-tant phenomenon: Conjunction (the configuration where Io,for example, would over take Europa, and all three line upwith Jupiter) always occurs in accord with the major axesof the (elliptical) orbits, but the conjunction of Io with Eu-ropa always occurs on the opposite side ofJupiter from thatof Europa with Ganymede. Because of this resonance, themoons have a forced component of orbital eccentricity; theorbits are not merely elliptical, but in fact the repeated gravi-tational pull in the same geometry results in a small butnonzero tug on the body that not only prevents the orbitfrom circularizing, but prevents the material of the bodyfrom dissipating heat in such a way that the minimum ki-netic energy configuration can be achieved. Thus the reso-nances maintain the orbital eccentricities of the Galileansatellites, leading to an enhanced component of tidal dissi-pation, particularly for the innermost moons.The thermal evolution of Europa, especially as rclatedto tidal heating, was expected to directly influence the char-acteristics of the ice shell and the putative ocean, whichwould in turn affect the amount of heating produced bythose tides. A leading model by Yoder (1979) and Yoder andPeale (1981) suggested that the orbital interactions amongIo, Europa, and Ganymede evolved through time. For exam-ple, all thee satellites might have been temporarily capturedinto low-order Laplace-like lesonances, and then evolvedinto their present state with increasing rates of heatingthrough time. Alternativ ely, G re e nb e rg (1982, I 987) arguedthat the Laplace resonance is primordial and that heatingrates were even higher in Europa's early history than theyare today. Post-Galileo models, such as those of Canup andWard(2002) and Peale and Lee (2002), provide insight intoboth possibilities, but the issues of timing and rates remainlargely unconstrained. However, refinements in values fordensity and moment of inertia (Anderson et al., 1998) haveenabled updated models of the interior structure and dif-ferentiation (McKinnon, 1996; Schubert et a1.,2004; chapterby Schubert et al.), as well as the infemed thermal evolu-tion. Although a great many unceltainties remain, theseresults generally support the notion that a partial meltingof the rocky interior has occuued, which could drive basal-tic volcanism at the rock-ocean interface, even today.Unfortunately, only the very latest geological record onEuropa is preserved on the surface, and there are no sur-face clues to the properties of the subsurface beyond about40 to 90 m.y. ago. It is possible that the tidal heating ratesvary through time, even for the modern epoch. Geological


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20 Europamapping of the visible part of Europa's history (Greeley etol.,' IOOO,2004; Thomas et al., 2007) is based on the iden-tification of specific features and placement of their forma-tion in a relative time sequence (see chapter by Doggett etal.). Initial results suggest that the earliest events involvedfoimation of extensive ridge systems, driven by global-scaletectonic deformation. Formation of chaos terrain, lowdomes, and smaller features came later and is thought toreflect local upwelling of warmer ice and/or water from theocean, but not the global-scale processes indicated by theerlier extensive ridge formation. Regardless of the debatewithin the community on the details of specifc feature for-mation and timing, there is general agreement that most ofthe surface features represent processes within and belowthe ice shell, and that the ice shell is likely to have beenrelatively thin at the time of the formation of the features'The nature of the ice represented by the young surface fea-tures today are expected to be representative of conditionsover a much longer period of time, although changes in icethickness on a dramatic scale as expected for freezing ormelting on long timescales have been modeled (Mitri andShowman,2005).Many of the issues regarding interior structure and evo-lution, and the possible links to the surface, require newgeophysical measurements and observations of the surface'For example, measuring the amplitudes of the diurnal tidesand the gravity field would constrain models of the oceandepth an thickness of the ice shell' If the tidal amplitude islarge (i.e., 30 m), then a thin shell (less than tens of kilome-t"rr) it present toda but if the amplitude is -1 m, then thewater is probably completely frozen (Moore and Schubert'2000). Piesently we have less than only about l07o of thesurface of Europa imaged in sufficient resolution to assessthe existence of ancient surface features; expioration of thesolar system has shown that many bodies have terrain di-chotomies, such as the ancient cratered highlands dominat-ing the lunar farside vs. the younger mare regions on thenearside. Until Europa is mapped globally under uniformconditions of illumination and resolution' we will not knowthe full extent of the "visible" h

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