Io after Galileo - pdfs.semanticscholar.org · his telescope, which he named the Medici stars after his sponsor. After a few observations, After a few observations, however, Galileo

INSTITUTE OF PHYSICS PUBLISHING REPORTS ON PROGRESS IN PHYSICS

Rep. Prog. Phys. 68 (2005) 303–340 doi:10.1088/0034-4885/68/2/R02

Io after Galileo

Rosaly M C Lopes1 and David A Williams2

1 NASA Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA2 Department of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404,USA

Received 18 August 2004, in final form 22 October 2004Published 5 January 2005Online at stacks.iop.org/RoPP/68/303

Abstract

Io, the volcanically active innermost large moon of Jupiter, was a target of intense studyduring the recently completed NASA Galileo mission to Jupiter (1989–2003). Galileo’s suiteof instruments obtained unprecedented observations of Io, including high spatial resolutionimaging in the visible and infrared. This paper reviews the insights gained about Io’s surface,atmosphere and space environment during the Galileo mission. Io is thought to have a largeFe–FeS core, whose radius is slightly less than half the radius of Io and whose mass is 20%of the moon. The lack of an intrinsic magnetic field implies that the core is either completelysolid or completely liquid. The mantle of Io appears to undergo a high degree of partial melting(20–50% molten) that produces ultramafic lavas dominated by Mg-rich orthopyroxene in anapparent ‘mushy magma ocean’, suggesting an undifferentiated mantle. The crust of Io isthought to be rigid, 20–30 km thick, cold away from volcanic heat sources and composed ofmafic to ultramafic silicates. Tidal flexing due to Io’s orbital resonance produces ∼100 mtides at the surface, generating heat that powers Io’s volcanism. Silicate volcanism appears tobe dominant at most hot spots, although secondary sulfur volcanism may be important in someareas. The key discoveries of the Galileo era at Io were: (1) the detection of high-temperaturevolcanism (ultramafic, superheated mafic or ‘ceramic’); (2) the detection of both S2 and SO2 gasin Ionian plumes; (3) the distinction between eruption styles, including between Pelean plumes(originating from central vents) and Promethean plumes (originating from silicate lava flowfronts); (4) the relationship between mountains and paterae, which indicates that many pateraeare formed as magma preferentially ascends along tectonic faults associated with mountainbuilding; (5) the lack of detection of an intrinsic magnetic field; (6) a new estimate of globalheat flow; and (7) increased understanding of the relationship between Io, its plasma torusand Jupiter’s magnetic field. There is an apparent paradox between Io’s potentially ultramaficvolcanism (suggestive of a primitive, undifferentiated mantle) and the widespread intensity ofthe volcanism on Io (which should have produced a volume of lava ∼140 times the volumeof Io over the last 4.5 Ga, resulting in more silicic materials). The resolution of this paradoxrequires either an Io that only recently (geologically) entered its tidal resonance and becamevolcanically active or a response of Io’s lithosphere–mantle to tidal heating that has in someway prevented extreme differentiation. Understanding this problem is one of many important

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304 R M C Lopes and D A Williams

issues about Io that remain unresolved. We conclude this paper with a discussion of the typesof future observations, from the ground and from space, that will be needed to address theseissues.

(Some figures in this article are in colour only in the electronic version)

Io after Galileo 305

Contents

Page1. Introduction 3062. Early observations 3063. Voyager-era discoveries 3084. Galileo at Io 311

4.1. The Galileo mission 3114.2. Geology and geophysics of Io 3144.3. Io in the Jupiter environment 3284.4. Monitoring Io from Earth: HST and adaptive optics 330

5. Outstanding questions and future missions 331Acknowledgments 334References 334


1. Introduction

Io is the innermost of the four large satellites of the planet Jupiter. It was discovered bythe Italian scientist and astronomer Galileo Galilei on 8 January 1610, and was named afterthe daughter of Inachus, who was one of the ancient Roman god Jupiter’s illicit lovers. Io’sunusual spectroscopic characteristics were recognized in the 1970s, prior to the NASA Voyagerspacecraft flybys in 1979. Also just prior to the Voyager flybys, Io’s 4 : 2 : 1 orbital resonancewith Europa and Ganymede was predicted to induce severe tidal heating and subsequent activevolcanism on Io (Peale et al 1979). The Voyager spacecraft confirmed the prediction that Iowould be found to be not only volcanically active, but also the most volcanically active bodyin the solar system. Plumes of gas and dust rise hundreds of kilometres above the surface ofmulticoloured volcanoes. Dark and bright lava flows extend for hundreds of kilometres acrossthe surface and enigmatic mountains, barely visible except in low sun angle shadows, pepperthe surface. The initial studies of Io from Voyager data have given way to wide-ranging newstudies using an incomparable data set collected by NASA’s Galileo orbiter over 8 years in theJovian system (1996–2003). The purpose of this paper is to review our understanding of Ioafter the Galileo mission: what we knew before, what we have learned thus far and what westill need to learn. We will also discuss the future of Io exploration and discuss what missionsmay provide information on Io in the future.

2. Early observations

The earliest observations of Io and the other Galilean satellites have had special significancein the history of astronomy. In 1610, Galileo Galilei discovered the four large moons usinghis telescope, which he named the Medici stars after his sponsor. After a few observations,however, Galileo concluded that the four objects were not stars but satellites in orbit aroundJupiter. Galileo’s studies of the motion of the newly discovered satellites had a profound effecton human history, becoming the cornerstone of evidence that confirmed the Copernican theoryof the universe.

Continued observations of the Galilean satellites, as the four moons became known, hadfurther important implications for physics. By 1675, Danish astronomer Olaus Romer hadnoted that the times of the eclipses and occultations of the four moons by Jupiter showeda phase shift with a periodicity of about 6.5 months. He concluded that when Jupiter is atopposition (when Jupiter and Earth are closest, on the same side as the sun) light from the Joviansystem must travel a distance of approximately 4 astronomical units to reach Earth. However,when Jupiter is at conjunction (when Earth and Jupiter are farthest apart, on opposite sides ofthe sun), light travelled about 6 astronomical units on its journey to Earth. Romer concludedthat this phase shift in the arrival time of Jovicentric events meant that light has a finite velocity,and he used the motions of the Galilean satellites to determine the speed of light. In 1805,Laplace demonstrated that the Galilean satellites have a special orbital configuration (known asthe Laplace resonance) which suggested a special dynamical relationship between Io, Europaand Ganymede. Further work on the orbital resonances was done by Sampson (1921) but itwas not until the 20th century (Peale et al 1979) that the role of the orbital resonance in Io’svolcanism was discovered. In addition to the 4 : 2 : 1 orbital resonance between Io, Europaand Ganymede, Io undergoes synchronous rotation, in which the rotation rate on its axis isequal to its revolution rate about Jupiter (like the Moon’s about the Earth). This dynamichas the effect that (from the point of view of Jupiter) only one side of Io is visible, which iscalled the subjovian hemisphere. The opposite side of Io that cannot be seen from Jupiter iscalled the antijovian hemisphere.


Table 1. Io’s basic orbital and physical properties.

Mean radius 1821.6±0.5 kmBulk density 3528±3 kg m−3

Orbital period 1.769 daysOrbital eccentricity 0.0041Orbital inclination 0.037Orbital distance, a 421 800 kmRotational period synchronous with orbitMaximum moment of inertia 0.3769±0.0004Potential Love number, k2 1.292±0.003Mass (8.9320 ± 0.0013) × 1022 kgSurface gravity 1.80 m s−2

Global average heat flow >2.5 W m−2

Radius of core 656 km (if pure iron)947 km (iron and iron sulfide mixture)

Surface equatorial magnetic field strength <50 nTGeometric albedo 0.62Local topographic relief up to ∼17 kmActive volcanic centres at least 166Typical surface temperature 85 K (night) to 140 K (day)Atmospheric pressure <10−9 bar, higher at locations of plumes

Sources: Lopes-Gautier 1999, Anderson et al 2001, Kivelson et al 2002, Geissler et al 2003, Lopeset al 2004, McEwen et al 2004.

Prior to observations from spacecraft, knowledge about Io’s surface was inferred frombrightness variations in reflected sunlight as Io moved around Jupiter. These telescopic observa-tions showed that Io’s brightness varied according to its position in its orbit, suggesting that themoon always keeps one face towards Jupiter. In 1927, photometric and colourimetric observa-tions (Stebbins 1927, Stebbins and Jacobsen 1928) confirmed that Io has a pronounced variationin brightness with orbital phase angle. During the next few decades, gradual improvement ofphotometry and colour data (see summary by Harris (1961)) showed that Io is the reddest objectin the solar system and has a marked colour variation with orbital phase angle. Furthermore,these early observations showed that Io differed from the other Galilean satellites (and mostother satellites in the outer solar system) because of the absence of water bands in its spectra.

The peculiar nature of Io’s surface became more evident in the early 1960s, when Binderand Cruikshank (1964) reported an anomalous brightening of Io’s surface as it emergedfrom eclipse. This first report of ‘post-eclipse brightening’ and the suggestion of a possibleatmosphere spurred more telescopic observations, but while the presence of an atmosphere wasconfirmed (e.g. Taylor et al (1971)), post-eclipse brightening has remained controversial (e.g.Franz and Mills (1971)) and has not been confirmed to this day, though several explanationswere put forward (Sinton 1973, Frey 1975, Nelson and Hapke 1978).

The first evidence of an electromagnetic link between Io and the Jovian magnetosphere wasput forward by Bigg (1964), who found that bursts of decametric radio emission by Jupiter wereapparently controlled by Io’s orbital position. Models of electrodynamic interaction betweenJupiter and Io (Piddington and Drake 1968, Goldreich and Lynden-Bell 1969) addressed thecoupling mechanism between Io and Jupiter’s inner magnetosphere.

The first clues to Io’s bulk composition came from measurements of Io’s mean radiususing a stellar occultation (O’Leary and Van Flandern 1972, Davies 1982) and mass derivedfrom the Pioneer flyby in 1973. The bulk composition of 3.54 g cm−3 indicated silicateswere dominant on Io, but the surface’s high albedo and cold temperatures indicated frosts(see table 1). Telescopic observations using improved spectral reflectance techniques were


used to attempt to determine Io’s surface composition. Johnson and McCord (1971) suggestedpolysulfides as a possible colouring agent for the surface. The idea of sulfur on Io was stronglysupported by laboratory experiments by Wamsteker (1973) which showed that sulfur and itscompounds matched the strong UV absorption and reflectance spectrum of Io. The discoveryof a strong absorption band near 4 µm (Cruikshank et al 1977, 1978, Fink et al 1978, Pollacket al 1978) could not be explained by sulfur. It was later found to be due to sulfur dioxide(Smythe et al 1979) which is now known to be the dominant compound covering Io’s surface.

A major discovery during the 1970s was of the Io sodium cloud (Brown 1973, Traftonet al 1974) and later of a potassium cloud (Trafton 1975). The flyby of the Jupiter systemby Pioneer 10 in 1973 revealed that Io has an ionosphere and thin atmosphere (Kliore et al1974). Pioneer measurements also showed a cloud of neutrals along Io’s orbital path (Carlsonand Judge 1974). Ground-based measurements in the mid-1970s discovered ionized sulfuremission in the inner Jovian magnetosphere but on the opposite side of Jupiter from theposition of Io at the time (Kupo et al 1976). Subsequent studies revealed this to be a plasmatorus (Broadfoot et al 1979, Sandel et al 1979).

The first indications of volcanic activity were given by infrared (IR) photometry andradiometry that showed higher brightness temperatures at 10 µm than at 20 µm (Hansen 1972,Morrison et al 1972, Sinton 1973), but the paradigm at the time was that Io was a cold and deadworld, and the observations remained a puzzle. However, shortly before Voyager 1 arrived atthe Jupiter system in March 1979, several papers presaged the discovery of active volcanism.Nelson and Hapke (1978) reported a spectral edge at 0.33 µm and proposed that sulfur wasthe major contributor to this spectral feature. They suggested that the presence of allotropesof sulfur explained this and several other spectral features, and that these allotropes could beproduced by melting of yellow sulfur and subsequent quenching, possibly in the vicinity of avolcanic fumarole or hot spring. More surprising were Witteborn et al’s (1979) observationsof an intense temporary brightening of Io in the 2–5 µm range, which they explained as anemission feature caused by part of Io having a surface brightness temperature of about 600 K.A few days before the Voyager encounter with Io, the theoretical work of Peale et al (1979)was published. The authors addressed the dissipation of tidal energy in Io and predictedthat ‘widespread and recurrent volcanism’ might occur. The prediction was spectacularlyconfirmed during the Voyager encounter.

3. Voyager-era discoveries

The Voyager 1 flyby of March 1979 and subsequent Voyager 2 and ground-based observationssubstantially changed our understanding of Io. Io was a surprise from the first images (figure 1),which showed an intensely coloured surface, indicating the presence of sulfur compounds,and the absence of impact craters, indicating a young surface (Smith et al 1979). Peale et al’s(1979) prediction of active volcanism was dramatically confirmed when navigation images,deliberately overexposed, showed a 200 km high plume (later known as Pele) on Io’s limb(Morabito et al 1979). When images revealed other plumes and the infrared interferometerspectrometer (IRIS) instrument detected SO2 gas from the Loki plume and surface hot spots(Pearl et al 1979) there was no doubt that rampant volcanism was present on Io.

The major debate ignited by the Voyager data concerned the composition of the surface andnature of volcanism—sulfur or silicates? Arguments favouring a composition predominantlyof sulfur were based on spectral data such as those from Wamsteker et al (1974) that indicatedsulfur on the surface and from Fanale et al (1979) and Smythe et al (1979) that showed SO2

frost to be present on the surface. The distribution of SO2 on the surface was mapped byHowell et al (1984) using the infrared band at 4.1 µm, and by Nelson et al (1980) using the


Figure 1. Voyager 1 image mosaic of the eruption of Pele on Jupiter’s moon Io. This image showsmost of the different types of materials found on Io, including paterae (volcanic vents), flucti (lavaflow fields), mountains and diffuse pyroclastic deposits. The Pele volcanic plume in this mosaicrises ∼300 km above the surface in an umbrella-like shape. The red, sulfur-rich plume falloutcovers an area the size of Alaska. The vent is a dark spot just north of the triangular-shaped plateau(right centre). To the left, the surface is covered by colourful lava flows rich in sulfur. Credit:NASA Photojournal PIA00323.

UV band (0.28 µm). Apart from the detection of SO2 over the Loki plume (Pearl et al 1979),rotational lines from an SO2 atmosphere were detected in microwave observations by Lellouchet al (1992). Ionized sulfur was detected in the Io torus (Broadfoot et al 1979) and the presenceof various sulfur compounds such as H2S (Nash and Howell 1989, Salama et al 1990), Na2Sand Na2O2 was suggested. Tentative identifications of SO3, Na2SO3 and NaHSO4 were madeby Khanna et al (1995). Evidence for a predominantly sulfur composition was also suggestedby the surface colours observed by Voyager which suggested different allotropes of sulfur(Sagan 1979, Pieri et al 1984). However, the validity of colour arguments was questioned(Gradie and Moses 1983, Young 1984, Nash 1987).

A predominantly silicate composition was favoured by knowledge of Io’s density(∼3500 kg m−3), which is similar to that of Earth’s moon, and by the topography of someof Io’s surface features (Clow and Carr 1980, Moore et al 1986). The two compositions(sulfur and silicates) can in principle be distinguished by their eruption temperatures, sincesulfur melts at lower temperatures than basaltic silicates. Thus molten sulfur pools and flowswere expected to be in the range 400–600 K and basalts over 1000 K. However, coolingsilicates show temperatures in the same range as sulfur; therefore, the temperatures of hotspots detected by IRIS (Pearl et al 1979), which were below 650 K, could be interpreted aseither composition. The first strong evidence for silicate volcanism came from ground-basedobservations that showed eruption temperatures over 800 K, definitely too high for sulfur(Johnson et al 1988, Veeder et al 1994). One of the main objectives for the Galileo remotesensing observations of Io was to determine the composition of Io’s surface and the nature ofvolcanism.


Other questions for Galileo included the distribution of SO2 on the surface and itsrelationship to plumes and to the origin of Io’s atmosphere. Prior to Galileo, models foratmospheric SO2 origin could be classified as one of three types: buffered, volcanic andsputtered (Lellouch et al 1992). Results from millimetre-wave observations (which allowedthe first ground-based detection of Io’s atmosphere) favour the volcanic model (Lellouch et al1992) but stressed that some aspects of all three types might be important.

The frequency and rate of Io’s eruptions was poorly constrained before Galileo, as Voyagerhad only two flybys, and ground-based observations were infrequent. However, ground-basedobservers made great strides in detecting eruptions on Io, particularly those from Loki, Io’smost powerful volcano (e.g. Howell and Sinton (1989), Spencer et al (1992)). Outbursts,defined as events that at least double the total 5 µm flux from Io (Howell et al 2001) seemedto occur several times a year. Spencer and Schneider (1996) estimated the probability of anoutburst at 12 ± 7 events per year if the events lasted for 1 day.

Another major question for Galileo concerned limits to Io’s heat flow. Hot spots playa major role in radiating Io’s internal heat to space. Voyager IRIS instrument observedthermal emission from hot spots, but had poor spatial and temporal coverage. Disk-integratedobservations from Earth can have good time and longitudinal coverage, but prior to observingtechniques involving adaptive optics (e.g. Marchis et al (2000)), it was not possible todetermine latitude of hot spots observed and, therefore, reliable correction for foreshorteningwas not possible. Additionally, it was difficult to separate the volcanic emission from thepassive thermal emission due to reradiated absorbed sunlight (Veeder et al 1994, Spencer andSchneider 1996). Estimates of heat flow prior to Galileo ranged from about 1.5 W m−2 (e.g.Morrison and Telesco (1980), Johnson et al (1984), McEwen et al (1985)) to at least 2.5 W m−2

(Veeder et al 1994).Other Voyager observations that spurred significant work include the observations of active

plumes (e.g. Strom and Schneider (1982)), mapping the geomorphology (e.g. Schaber (1982),McEwen et al (1989), Crown et al (1992)) and constraints on the resurfacing rate. Io’s totallack of impact craters and abundance of volcanic centres implied that volcanic products coverthe surface at sufficiently high rates to obliterate all craters. Johnson and Soderblom (1982)estimated a rate of 0.1 cm year−1, assuming a cratering flux compared with that of our ownmoon. Resurfacing materials could be extensive lava or sulfur flows (Blaney et al 1995) orplume deposits (e.g. McEwen et al (1989)).

After both Voyager spacecraft left the Jupiter system, observations of Io continued to bemade from Earth using infrared instruments on ground-based telescopes (e.g. Veeder et al(1994), Spencer et al (1992, 1997a)), which detected previously unknown hot spots andtemporal variations in their activity. Loki, in particular, was found to have brightening andfading cycles (see summary by Rathbun et al (2003)). Observations at millimetre wavelengthsusing the 30 m Institut de Radio-Astronomie Millimetrique (IRAM) detected rotational linesfrom SO2 (Lellouch et al 1992). Observations made using the International UltravioletExplorer satellite (Nelson et al 1980) in the mid-UV (0.22–0.33 µm) suggested a longitudinalvariation in the distribution of SO2 on Io’s surface, with SO2 being more abundant on theleading hemisphere (longitudes 72–137˚W), a suggestion later reinforced by ground-based IRobservations (Howell et al 1984).

Observations using the Hubble Space Telescope (HST) revealed a major surface changein the area of Ra Patera in 1994, established a connection between red deposits and plumeactivity (Spencer et al 1997b) and detected the Pele plume (Spencer et al 1997c) and severalpreviously unknown hot spots (Goguen et al 1998). The integration of these results with thosefrom Galileo, which began observing Io in 1996, brought about new major insights into Io’svolcanic activity.


Figure 2. Schematic diagram of the Galileo spacecraft, showing operation components and scienceinstruments.

4. Galileo at Io

4.1. The Galileo mission

The Galileo mission to Jupiter (Johnson et al 1992) was conceived by the US NationalAeronautics and Space Administration (NASA) in the mid-1970s as a Jupiter orbiter to followon to the Jupiter flybys that were to be conducted by the two Voyager spacecraft in 1979.The Galileo spacecraft would consist of two primary components: a Jupiter orbiter, and aJupiter atmospheric probe. The orbiter would utilize a unique, dual three-axis-stabilized/spin-stabilized operations and control platform (figure 2), in which fields and particles instrumentswere attached to the spun section (to sample the magnetic and charged particle environment ina region surrounding the spacecraft as it traversed Jupiter’s intense magnetosphere), whereasthe remote sensing instruments were attached to a despun section to obtain precise imagingof Jupiter’s atmospheric phenomena and satellite surface features. The Galileo orbiter wouldcontain the latest designs in remote sensing instruments, including the first digital charged-couple device (CCD) camera and the first imaging spectrometer flown on a planetary mission.The atmospheric probe, the first to sample the atmosphere of a gas giant outer planet, wasdesigned to parachute into Jupiter’s atmosphere and measure temperature, pressure, density,humidity and chemical composition as it sank through hundreds of kilometres of atmosphere.Signals were to be relayed from the probe to the orbiter and stored on an onboard tape recorderfor later transmission to Earth.

The US Congress approved funding for the Galileo mission in 1977, and development soonbegan. The spacecraft was originally designed to be launched from a US space shuttle orbiter in1982 and utilize a direct trajectory to arrive at Jupiter in 1985. However, due to developmental


Figure 3. Trajectory and key mission events of the Galileo mission to Jupiter. When the liquid-filled booster was cancelled for safety reasons after the Challenger accident, use of a solid-fuelledbooster required a series of gravity-assist flybys for Galileo to reach Jupiter. The Venus–Earth–Earth gravity assist (VEEGA) trajectory was developed to meet this requirement.

problems with the US shuttle programme, the Galileo spacecraft was redesigned three times(for launch first on the shuttle, then on multiple unmanned rockets and eventually back to theshuttle) between 1979 and 1985. A planned launch in 1986 was delayed after the Challengeraccident, after which the direct-to-Jupiter trajectory was lost due to safety issues with theplanned liquid-fuelled booster stage. An alternate trajectory was developed that utilized asafer solid-fuelled booster and a series of gravity assists by Venus and the Earth (twice) thatwould enable Galileo to reach Jupiter, but only after a 5 year cruise through the inner solarsystem (figure 3).

After a tortuous, 12 year development period, the Galileo spacecraft was finally launchedon 18 October 1989 by the crew of STS-34 aboard the space shuttle Atlantis. During its 5 yearcruise phase (figure 3), Galileo became the first spacecraft to conduct infrared imaging ofVenus’ lower cloud deck (figure 4) (Belton et al 1991, Carlson et al 1991, 1993), to obtainmultispectral imaging of the lunar far side and north polar regions (confirming the existence ofthe far side South Pole–Aitken basin and that the north polar light plains are impact ejecta rather


Figure 4. Key milestones of the Galileo mission during its cruise phase to Jupiter. Clockwisefrom upper left: the first infrared images of the cloud deck of Venus, from NIMS (February 1990);the first (false colour) multispectral images of the eastern lunar far side, and confirmation of theSouth Pole–Aitken impact basin (from SSI, December 1990); (centre) the first high resolutionspacecraft images of an asteroid, 951-Gaspra (October 1991); the first (false colour) multispectralimages of the lunar north polar region (December 1992); the first discovery of an asteroidal moon,Dactyl orbiting 243-Ida (August 1993); and the first direct observation of a cometary impact witha planetary body (a fragment of comet Shoemaker-Levy-9 striking Jupiter, July 1994). Credit:images courtesy NASA.

than volcanic material: Belton et al (1992a, 1994)), to image an asteroid at high resolution(951-Gaspra on 29 October 1991: Belton et al (1992b)), to discover a moon of an asteroid(Dactyl orbiting 243-Ida on 28 August 1993: Belton and Carlson (1993), Belton et al (1996))and to directly observe and image an impact of one planetary body with another (CometShoemaker-Levy/9 with Jupiter during July 1994: Carlson et al (1995, 1997), Chapman et al(1995), Hord et al (1995), Martin et al (1995)).

The Galileo spacecraft arrived at Jupiter on 7 December 1995, and successfully deployedthe atmospheric probe (for a summary of probe results, see Young et al (1996)). Galileowas designed to orbit the planet Jupiter for a 2 year nominal mission (1996–97) and collectdata primarily on the planet’s atmosphere, and secondarily on its moons, rings and magneticfield. However, the failure of Galileo’s high-gain antenna to deploy (discovered during thecruise phase) drastically reduced the quantity of images and data that could be returned toEarth. Thanks to heroic work by engineers at NASA’s Jet Propulsion Laboratory (where thespacecraft was operated), the spacecraft’s software was reconfigured to utilize the low-gainantenna which along with newly installed data compression algorithms maximized the amountof data that could be returned. However, primary targeting was changed from the planet Jupiter


itself to the Galilean satellites, which could be more thoroughly studied using the limitedspacecraft resources. Because of excellent fuel margins and nominal spacecraft operations,NASA authorized two mission extensions to gather additional data on the Galilean satellites:the Galileo Europa mission (GEM, 1998–99) and the Galileo millennium mission (GMM,2000–02). These extensions are important for Io, as all the high resolution data obtainedby Galileo were collected during these mission extensions. Data collection by the Galileospacecraft continued up to the end of the mission, when Galileo was commanded to enterJupiter’s atmosphere and vaporize itself (this was to remove any chance of Galileo collidingwith Europa at some point in the future when fuel ran out and control was lost). Thus, after26 years of project activity at NASA, almost 14 years in space, about 8 years in the Joviansystem and 35 orbits of Jupiter completed, the Galileo mission to Jupiter came to a close on21 September 2003.

The non-Io-related discoveries made by Galileo at Jupiter are varied and numerous, andthere is insufficient space here to mention more than a few. Perhaps the most well knowndiscovery is that of a ∼100 km thick liquid water ocean underneath the frozen crust of the moonEuropa (e.g. Carr et al (1998), Greeley et al (1998), Kivelson et al (2000)). This discovery in thelate 1990s (along with the 1996 discovery of purported evidence of past microbial life in Martianmeteorite ALH84001) in these authors’ opinion strongly contributed to the growing interestin the nascent field of astrobiology, which led to the development of the NASA AstrobiologyInstitute in the late 1990s. It also served as the primary motivation for proposing the EuropaOrbiter mission, which has now evolved (thanks to the 2002 Planetary Decadal Survey) into theJupiter icy moons orbiter (JIMO), a multi-billion dollar project planned for launch to Jupiter inthe next decade (see section 5). Some of the other major discoveries made in the Jovian systemby Galileo include the following: discovery of enormous downdrafts associated with Jupiter’shot spots and widespread inhomogeneity in the distribution of water in Jupiter’s atmosphere(Young et al 1996); discovery of an active dynamo inside Ganymede (Kivelson et al 2002);discovery of sub-surface oceans at Callisto and Ganymede (figure 5, see also Khurana et al(1998), Kivelson et al (2002)); discovery of a tenuous carbon dioxide atmosphere on Callisto(Carlson 1999); discovery that Jupiter’s rings are geologically young and are composed ofimpact debris from Jupiter’s small inner satellites (Burns et al 1999); discovery of ammoniaice clouds in Jupiter (Baines et al 2002); and discovery (from observations in moonlight)of the close relationship of Jupiter’s lightning to deep convective features in Jupiter’s clouds(Ingersoll et al 2000, Roos-Serote et al 2000). The remainder of this paper will focus onthe new understanding developed of the volcanic moon Io, mostly from data gathered duringthe Galileo mission. For a good review of the new insights gained regarding Jupiter, itsatmosphere, magnetic field and the Galilean satellites, please see the new book by Bagenalet al (2004).

4.2. Geology and geophysics of Io

Results from the Galileo mission substantially changed our understanding of Io’s geologyand geophysics. Remote sensing instruments (visible and infrared) were used to study thesurface features and volcanic activity, while the tracking of the spacecraft itself provided newconstraints on the interior. Gravity measurements from tracking (Anderson et al 2001, Bagenalet al 2004, chapter 13) indicate that Io has a large iron/iron sulfide core, comprising about20% of the satellite’s mass. These measurements confirmed the prediction of Consolmagno(1981) and the hydrostatic shape measurements of Gaskell et al (1988) using Voyager data.Galileo’s close flybys of Io failed to reveal an intrinsic magnetic field, suggesting that littlecore convection is taking place (Kivelson et al 2004, Schubert et al 2004).


Figure 5. Artists’ concept diagrams showing cutouts of the four Galilean satellites of Jupiter,based on data obtained through the Galileo mission. Io is thought to have a mushy magma ocean(Keszthelyi et al 1999, 2004), whereas Europa, Ganymede and Callisto all are thought to containliquid water oceans or layers in their interiors (see, e.g. Kivelson et al (2004)). Image courtesy ofJPL/NASA.

Remote sensing observations from Galileo revealed the surface in unprecedented detail.Most of the new insights gained during the Galileo mission come from analysis of surficialand atmospheric phenomena. The following sub-sections list the major discoveries at Io.In these sections we will refer primarily to Galileo’s three remote sensing instruments,the solid-state imaging camera (SSI), the near-infrared mapping spectrometer (NIMS) andthe photopolarimeter–radiometer (PPR).

4.2.1. Surficial features. The surface of Io contains three primary types of features(figure 6): (1) broad, flat, layered plains, which may or may not be covered with visible,diffuse pyroclastic volcanic deposits; (2) volcanic structures including paterae (caldera-likedepressions), flucti (lava flow fields) and tholi (shield volcanoes and other positive-reliefstructures); and (3) mountains of volcano–tectonic origin. The complementary imagingcoverage of Galileo and Voyager has allowed these features to be mapped in a global scale,thus giving us a window into not only local but also global processes.

Between the paterae, mountains and other major surficial features, Io’s surface appearssmooth except for scarps that cut across the plains. Some scarps are linear and occur in parallelgroups, which suggest a tectonic origin. Other scarps, however, are irregular and appear to beerosional, sometimes forming a series of mesas or large plateaus. The presence of erosional


Figure 6. Galileo solid state imager mosaics of three regions of Io’s antijovian hemisphere. Allmosaics use relatively low-resolution (1.4 km per pixel) colour images from June 1999 merged withhigher resolution greyscale mosaics obtained later in the mission. Together, these three regionsaccentuate the different types of geologic materials and terrains on Io. North is up except wherean arrow indicates otherwise. Top: the Chaac–Camaxtli region (Williams et al 2002), imagedin February 2000 at 186 m per pixel. This region contains paterae in various shapes, sizes andcolours, indicative of varying volcanic and tectonic influences on their formation. Middle: theCulann–Tohil region (Williams et al 2004), imaged in October 2001 at 330 m per pixel. Thisregion contains paterae, flucti (lava flow fields) and two types of mountains: tectonically-derived(Tohil Mons) and volcanically-derived (Tsui Goab Tholus). Bottom: the Zamama–Thor region(Williams et al in revision), imaged in October 2001 at 360 m per pixel. This region is dominatedby two eruptive centres, Zamama and Thor, which have produced various eruptive styles. Thorwas the site of the highest plume event ever recorded on Io, reaching more than 500 km above thesurface in August 2001. All three regions contain areas of coloured diffuse deposits, indicative ofexplosive pyroclastic material of different compositions. Credit: Chaac–Camaxtli colour mosaicprocessed by Alfred McEwen, University of Arizona; Culann–Tohil and Zamama–Thor colourmosaics processed by Moses Milazzo, University of Arizona.


features on Io is somewhat puzzling because of the lack of a significant atmosphere or flow ofliquid water. McCauley et al (1979) suggested that the eroding agent is the explosive escapeof sulfur dioxide from a subterranean SO2 ‘aquifer’. The idea of subterranean SO2 reservoirsgained some support after the analysis of Galileo data of the movement of the Prometheusplume (Kieffer et al (2000), see below).

Volcanic features dominate Io’s surface, and Io’s volcanoes cover a wide range ofsizes and present varying characteristics such as power output, persistency of activity andassociation with plumes. Interestingly, most of Io’s volcanoes manifest themselves as caldera-like depressions, referred to as paterae (see below). In contrast, unlike terrestrial constructs,Io’s volcanoes rarely build large topographic structures such as shields or stratovolcano-likemountains. There are only a few structures, called tholi (singular tholus, meaning ‘dome’),scattered across Io. Two of these (Apis and Inachus Tholi) are located on Io’s Jupiter-facing (subjovian) hemisphere, and resemble Venusian ‘pancake domes’ in shape. Thesefeatures were suggested to be evidence of possible shield-building basaltic volcanism (Schaber1980). On Io’s anti-Jupiter-facing (antijovian) hemisphere, several small tholi were identifiedin relatively high resolution Galileo image mosaics (figure 6); however, these features aremore similar in morphology to terrestrial shields found in locations such as Iceland andHawaii.

Io’s surface shows some remarkably large lava flows (fluctii, singular fluctus, meaning‘flow field’). The lava flow field from Amirani is ∼300 km long, the largest active flow fieldknown in the solar system. Io’s large lava flows are possibly analogues of the continentalflood basalt lavas on Earth, such as the Columbia River basalts in the USA. These ancientterrestrial flows were never directly observed, but are suspected of producing major climaticeffects (Self et al 1997). Repeated imaging of Amirani during the Galileo flybys allowederuption rates to be estimated (50–500 m3 s−1, Keszthelyi et al (2001)). These are consideredmoderate effusion rates, and the ability of lava to travel large distances at moderate effusionrates suggests not only that the lavas had a low viscosity, but also that they were emplaced asinsulated flows, so that the cooled crust would insulate the hot material underneath. Thermalprofiles along the Prometheus and Amirani flows (Lopes et al 2001, 2004), and high spatialresolution images (Keszthelyi et al 2001) suggest that these large Ionian flows were similarto terrestrial inflated pahoehoe flows (Hon et al 1994), a repetitive process in which lava isinjected underneath cool crust, thickening the flow until a new lobe is formed. Other lava flowson Io appear to have been emplaced in a somewhat different manner, through lava tubes. Lavatubes also provide insulation of a cooled crust, allowing lavas to travel greater distances thanon open channels. Lava tubes are seen on Io’s Culann Patera volcano, one of them meanderingfor over 50 km (Phillips 2000, Geissler 2003).

A major question about Ionian volcanism after Voyager was the nature of volcanism—whether sulfur or silicates were predominant. Although temperature measurements fromGalileo clearly showed that many hot spots have temperatures far too high for sulfur, thepossibility that some sulfur flows occur on the surface cannot be ruled out. The colourful flowsaround Ra Patera that were argued by Sagan (1979) to be sulfur had been covered over bynew eruptions before Galileo’s first observations of the area in 1996 (McEwen et al 1998a).However, other locations may have sulfur flows. While most Ionian flows appear dark, a fewlocations show pale yellow or white flows that may well have been molten sulfur. Williamset al (2001a) proposed that flows radiating from Emakong Patera may be sulfur and that lowtemperature liquid sulfur heated to 450 K could explain many of the morphological featuresseen around Emakong, such as a meandering channel 105 km in length that appears to feed agrey-white flow some 270 km in length. Temperature measurements using NIMS (Lopes et al2001, 2004) indicate temperatures less than 400 K inside Emakong caldera, and much cooler


(below the instrument’s detection capabilities) over the flows. An active sulfur eruption mayalso have been detected by NIMS on the bright flow field Tsui Goab Fluctus in the Culann–Tohilregion late in the Galileo mission (Williams et al 2004). However, Galileo instruments couldnot distinguish between sulfur flows or cooled silicates coated by bright sulfurous materialsafter erupting. One possibility suggested by Greeley et al (1984) based on studies of a yellowsulfur flow at Mauna Loa, Hawaii, is that rising silicate magma may melt sulfur-rich countryrock as it nears the surface, producing ‘secondary’ sulfur flows (as opposed to ‘primary’ flowsthat originate from molten magmas at depth). The widespread occurrence of sulfurous depositsdetected around Ionian hot spots as seen by Galileo provide more evidence that this may bethe case. Therefore, the presence of sulfur flows on Io remains open.

The most common type of volcanic feature on Io is the patera (plural paterae, meaning‘saucer-like crater’). Although the origin of paterae is still somewhat uncertain, they arethought to be similar to terrestrial volcanic calderas, formed by collapse over shallow magmachambers following partial removal of magma. Some paterae show angular shapes that suggestsome structural control, indicating that they may be structural depressions that were later usedby magma to travel to the surface. At least 400 Ionian paterae have been mapped. Theiraverage diameter is ∼40 km but Loki, the largest patera known in the solar system, is over200 km in diameter. In contrast, the largest caldera on Earth, Yellowstone, is ∼80 km by 50 kmin size. The larger sizes of the Ionian features probably reflect the much larger sizes of magmachambers, which are thought to be relatively shallow (Leone and Wilson 2001).

Mountains are major structural landforms on Io and tower over the surrounding plains.Ionian mountains are defined as steep-sided landforms rising more than ∼1 km over the plains(McEwen et al 2004). At least 115 mountains have now been identified and mapped (Carr et al1998, Schenk et al 2001, Jaeger et al 2003). Io’s mountains rise, on average, about 6 km high,with the highest rising 17 km above the surrounding plains. Galileo images revealed that manymountains are partly or completely surrounded by debris aprons, plateaus and layered plains(Turtle et al 2001, McEwen et al 2004). The mountains appear to be unstable and are thoughtto be relatively short-lived features. The mountains are not active volcanoes and their originis still uncertain. Various models have been proposed to explain the origin of the mountains(e.g. McKinnon et al (2001), Schenk et al (2001), Turtle et al (2001)). Their asymmetricalshapes suggest the uplift and rotation of crustal blocks, implying that compressional uplift isprobably the dominant mechanism.

Neither the volcanic features nor the mountains appear to follow a distinct global pattern,suggesting that any surface expression of internal dynamics (convection) is subtle. Activehot spots appear to be randomly distributed (Lopes-Gautier et al 1999). The distributionof mountains and calderas (including those which have not been observed to be active) is,however, not random, as both features are concentrated towards lower latitudes and follow abimodal distribution with longitude. The greatest frequency of mountains occurs in two largeantipodal regions near the equator at about 65˚ and 265˚ (Schenk et al 2001). In contrast, thevolcanic patera follow a similar distribution but 90˚ out of phase with that of the mountains(Radebaugh et al 2001).

The bimodal distribution pattern for paterae and other volcanic centres matches theexpected pattern of heat flow from asthenospheric tidal heating (Ross et al 1990) andthe pattern of internal convection within the mantle predicted from simulations (Tackleyet al 2001). The anti-correlation in the distribution of mountains and volcanic centresis further evidence that the two are not related. Schenk et al (2001) presented severalhypotheses for the anti-correlation: (i) mountains are short-lived in regions of more activeresurfacing due to more rapid burial or collapse; (ii) tall (and longer-lived) mountains formless frequently in regions of higher heat flow and a thinner lithosphere (where the volcanic


centres are concentrated); or (iii) an additional stress mechanism is superimposed on theglobal compressive stresses. In the last case, the mechanisms could be stresses induced onthe lower crust by mantle convection plumes, or by nonsynchronous rotation of Io’s lithospherewith respect to the interior. Alternatively, the model of McKinnon et al (2001) proposes thatIo’s crust is inherently unstable if Io’s volcanism is episodic on regional scales. If volcanismslows down, increased heating at the base of the crust and associated thermal expansioncould increase horizontal compressive loads enough to fracture the crust and trigger thrustfaulting.

4.2.2. Surface changes. A consequence of Io’s prodigious volcanism is that surface changescan be detected even on the timescales of spacecraft observations. Notable changes occurredin the years between the Voyager and Galileo observations (1979–95); however, many surfacechanges at much shorter timescales have also been detected. Changes in appearance of plumedeposits were apparent in the 4 months between the Voyager 1 and 2 flybys (Smith et al1979b), and in Galileo images taken in different orbits, which were typically months apart(e.g. McEwen et al (1998a), Keszthelyi et al (2001)). Surface changes are mostly due to newvolcanic deposits, most importantly, volcanic plumes and deposits inferred to be pyroclastics(ash and tephra). Other changes include new lava flows, increase in area of existing flows andchanges in surface colour. Volcanic materials have been observed to fade or disappear due toburial, alteration or erosion (Geissler et al 2004). Most surface changes have been detected atvisible wavelengths; however, within individual volcanic centres changes in temperature andSO2 coverage have been detected at infrared wavelengths (e.g. Spencer et al (2000), Lopes et al(2001), Doute et al (2002)). At visible wavelengths, detecting of changes can be a complextask because of the varying illumination and viewing angles of different images (Simonelli et al(2001)). Io’s surface materials vary significantly in their light-scattering behaviour, producingsharp colour differences and contrast reversals that alter the appearance of the surface dependingon whether it is illuminated obliquely or viewed with the sun more directly behind the observer(Geissler et al 2004).

Although Io produces notable surface changes at short timescales, most of these arelocalized and take place inside dark volcanic paterae (caldera-like depressions) that cover1.4% of Io’s surface, or are ephemeral volcanic plume deposits that fade on timescales of afew months to years. Contrary to expectations (based on changes between the two Voyagerflybys), more than 90% of Io’s surface remained unchanged between Voyager (1979) and theend of the prime Galileo mission (1999). Two dramatic Galileo-era eruptions were particularlyuseful in the study of surface changes. The eruption of the Pillan volcanic centre in 1997 left aconspicuous ‘black eye’ on Io’s surface, covering an area of about 200 000 km2 and reachingdistances up to 260 km from the source (Keszthelyi et al 2001, Williams et al 2001). Laterobservations from Galileo’s SSI showed an absorption at 0.9 µm in these and other darkmaterials on Io, confirming their silicate composition (most likely Mg-rich orthopyroxene:Geissler et al (1999)). The dark deposit at Pillan slowly faded between 1997 and 1999 as itwas covered by red sulfurous deposits from nearby Pele.

The other major surface change occurred at the Tvashtar Catena, a chain of volcaniccraters in the northern sub-polar region. Galileo detected active eruptions in November 1999,February 2000 and December 2000, with each eruption a different ‘style’ (lava fountain, lavaflow or lava lake and explosive plume, respectively: Keszthelyi et al (2001), Turtle et al (2004)).The variation in eruption styles and the colours of explosive products at Tvashtar and otherhot spots on Io suggests that there is a complex, intimate, time-varying interaction betweensilicate magmas and sulfur- and/or sulfur dioxide-bearing volatile materials that influence Io’svolcanism.


4.2.3. High-temperature volcanism. The detection of high temperature volcanism on Io isconsidered by the Galileo team to be one of the most important discoveries at Io during thecourse of the mission. With no direct measurements of lava composition, the temperaturesdetected at active hot spots provide the best clues to magma composition. Temperatures canbe calculated by measurements made from two Galileo instruments, the SSI and the NIMS.The wavelength range used for the temperature determinations was 1.1–5.1 µm. Temperaturesdetermined by remote sensing data depend on the spatial resolution and wavelength range, andbecause of the rapid cooling of lava, they have to be considered minimum temperatures (e.g.Lopes et al (2001), Kargel et al (2003)). One Ionian hot spot, Pillan, showed temperaturesabove 1800 K during an eruption in 1997 which was detected by both NIMS and SSI (McEwenet al 1998b, Davies et al 2001, Williams et al 2001a). This temperature far exceeds basaltic lavatemperatures, which are in the range 1400–1550 K. Another hot spot on Io, Pele, also showshigh temperatures; however, the error margins were such that these measurements could notdistinguish between basaltic and ultramafic compositions (Lopes et al 2001, Radebaugh et al2004). A particularly violent eruption from the Surt hot spot was detected from ground-basedobservations in February 2001 (Marchis et al 2002), but the temperature measurements (lowerlimit 1470 K) did not place these lavas above the basaltic range.

At present, it is not known whether all lavas on Io are erupted at very high temperaturesor not, since it is difficult to detect sufficiently large areas where very fresh (and therefore veryhot) materials are exposed. It is also not known what is causing the very high temperatures.Three main hypotheses have been put forward: ultramafic volcanism (McEwen et al 1998b,Williams et al 2000), superheated basaltic volcanism (McEwen et al 1998b, Kargel et al 2003)and ‘ceramic’ volcanism (Kargel et al 2003).

The ultramafic (komatiite-like) composition hypothesis, first proposed by McEwen et al(1998a,b), is currently favoured by most Io researchers. Komatiites are ultramafic volcanicrocks on Earth with >18% MgO, often containing olivine or clinopyroxene. Colour data on Io’sdark volcanic materials obtained from SSI and NIMS (Geissler et al 1999) indicate the presenceof orthopyroxene very rich in Mg, requiring ultramafic bulk compositions (but apparentlylacking in olivine). The only occurrence of orthopyroxene phenocryst-bearing komatiiteson earth is in the Commondale greenstone belt, South Africa, which had inferred liquidus(eruption) temperatures of about 1884 K (Williams et al 2000). These are the closest likelyanalogues to the lavas erupted at Pillan and perhaps at other Ionian hot spots identified thus far.

A second hypothesis to explain Io’s hottest eruptions is superheating, which was alsodiscussed by McEwen et al (1998b). Magma can be superheated by rapid ascent from a deep,high pressure source. Melting temperatures of dry silicate rocks increase with pressure, andtherefore the erupted lava can be significantly hotter than its melting temperature at surfacepressure. The difficulty with this explanation is that, for lava at 1870 K to have a basalticcomposition (with liquidus temperatures of about 1400–1550 K at 1 bar), the magma wouldhave to ascend rapidly from a depth of nearly 1000 km, with little cooling on the way up(Kargel et al 2003). No record of such an eruption is known on Earth; however, magma mayhave erupted from nearly such depths on the Earth’s moon.

The third hypothesis, vapour distillation and ‘ceramic’ volcanism, was put forward byKargel et al (2003). Lava outside the basalt-peridotite–komatiite system is conceivable onIo due to the moon’s history of intense, high temperature magmatic processing. The partialvapour pressures of SiO, Mg, Fe, FeO and other gases of common oxide and silicate systemsexceed nanobars at 1500 K and approach microbars at 1900 K (Stolyarova and Semenov 1994).Vapourization of metals from hot lava can be significant when exposed on flow surfaces andthrough cracks in solidifying crusts, especially if fluxed by sulfurous volatiles. Modelling byKargel et al (2003) showed that, if metal and oxide vapours can be lost from Io entirely or


else cold-trapped at stable sites, compositional evolution of evaporated lava residues wouldaffect igneous phase equilibria and the temperature of subsequent eruptions. Refractory lavaresidues lack the amounts of SiO2 and FeO in common silicate lavas and may have very highliquidus temperatures. However, the physical properties, emplacement dynamics and natureof volatile interactions of these residues have not yet been thoroughly investigated.

4.2.4. Styles of active volcanism. Volcanic eruptions on Io can be classified into three broadstyles: Promethean, Pillanian and Lokian (figure 7). Promethean eruptions (Keszthelyi et al2001) originate from either paterae or fissures, and produce extensive compound lava flow fieldsthrough repeated small breakouts of lava, similar to the slowly emplaced, inflationary flow fieldsat Kilauea in Hawaii. The temperatures associated with terrestrial basaltic volcanism are mostlycorrelated with Promethean events. Promethean eruptions are long-lived, steady eruptions thatcan last years at a time, and often include small (<200 km high) explosive plumes of vaporizedsulfurous country materials that erupt from the edges of flow fronts. Pillanian eruptions(Keszthelyi et al 2001) originate from either paterae or fissures, and produce extensive darklava flow fields and dark pyroclastic deposits through short-lived, high effusion rate, vigorousoutbursts of material during discrete (less than a couple of months) events. The temperaturesassociated with terrestrial ultramafic volcanism were mostly correlated with Pillanian events.Pillanian events may or may not include eruption episodes with large (>200 km high) explosiveplumes due to interaction of silicate magma with either juvenile or meteoric sulfurous volatiles,resulting in large (∼1200 km diameter) red rings of short-chain sulfur around the sourceregions. Lokian eruptions (Lopes et al 2004) are confined within paterae, and are thoughtto involve lava lakes, some of which are possibly overturning, with or without associatedplume eruptions. Many if not most active volcanoes on Io show evidence of producing bothexplosive and effusive deposits, and many volcanoes can have more than one eruption style.For example, the volcano Pele typically produces both Lokian and Pillanian events, whereasthe volcano Tvashtar was observed by Galileo to produce apparently all three types of eruptionstyles.

4.2.5. Surface colours and relationship to composition. The colours observed on Io’s surfacecan be correlated with physical units and composition (e.g. Geissler et al (1999)). Essentially,Io’s surface has four primary colour units: red, yellow, white-grey and black. The red materialsare interpreted to be composed of short-chain sulfur molecules (S3, S4) resulting from eitherbreakdown of cyclo-S8 by charged particle irradiation (in polar regions), or by condensationfrom S2-rich volcanic gases in the plumes of active vents (in the equatorial rings). Theyellow materials (40% of surface) are interpreted to be the sulfur allotrope cyclo-S8 (Geissleret al 1999), with or without a covering of SO2 frosts deposited by plumes, or alternativelypolysulfur oxide and S2O without large quantities of elemental sulfur (Hapke 1989). Greenish-yellow patches exist in or near active vents, and are thought to be composed of either sulfurcompounds contaminated by iron, or silicates such as olivine or pyroxene with or withoutsulfur-bearing contaminants. White (to grey) materials (27% of surface) are interpreted to becomposed of coarse- to moderate-grained SO2 resulting from plume fallout and subsequentrecrystallization. Black areas (1.4% of surface) mostly correlate with active hot spots andoccur as patera floors, as lava flow fields or as dark diffuse materials near or surroundingactive vents. These materials are most consistent with Mg-rich orthopyroxene (enstatite orbronzitehypersthene), indicative of silicate lava flows or lava lakes (within paterae) or diffusesilicate pyroclastic deposits near paterae, of mafic to ultramafic composition.

One of the advantages derived from the repeated Galileo spacecraft’s flybys of Io is thatthey occurred at different sun-angle geometries, such that colour images were obtained at a


Figure 7. Three major eruption styles found on Io: (a) Lokian eruptions are confined to paterae, andoccur as intermittent to periodic outbursts that are thought to represent the resurfacing of pateraefloors by lava flows or overturning lava lakes. The type locality is Loki (inset upper left, Voyagerimage). In this montage, Galileo NIMS images track the temperature changes across the floorof Loki. (b) Promethean eruptions are relatively long-lived (months to years), and are associatedwith high (mafic) temperatures, relatively low (<200 km) SO2-rich flow-front plumes, and slowly-emplaced flows through multiple small breakouts (similar to Hawaiian pahoehoe flow fields). Inthis montage, Galileo SSI views of the Prometheus plume (centre) are surrounded by increasinglyhigher resolution views of the Prometheus flow field. (c) Pillanian eruptions occur in relatively brief(few months or less), intense outbursts that produce very high (possibly ultramafic) temperatures,plumes and rapidly-emplaced long (>50 km) lava flows. The plumes are typically >100–500 kmhigh, are known to contain SO2 gas, and have produced black (Pillan), red (Pele, Tvashtar) andwhite (Thor) rings over source vents hundreds of kilometres in diameter. These compositions arethought to be associated with silicate, sulfur and SO2 pyroclastic materials, respectively. Imagecredit: NASA.


Figure 7. (Continued.)

variety of resolutions and phase angles. Simonelli et al (1997) and Geissler et al (1999, 2000)discuss some of the complexities involved in assessing Io’s colour variations in terms of imagegeometry and photometry. Of key importance is recognition that surface colours on Io changewith increasing phase angle: polar deposits and some light-coloured plume deposits aroundactive volcanic centres brighten, whereas materials in the equatorial band darken (Geissleret al 2000). These colouration changes were attributed to the presence of thin, fine-grainedSO2 frosts that are transparent under normal low-phase illumination but that become visibleunder high-phase illumination (Geissler et al 2000). In addition to photometric affects, thecolours of Io’s surface materials also change as they age. By ageing, we refer to exposure tosolar radiation and the Jovian magnetic field. Sulfur allotropes undergo considerable colourchange (e.g. cyclo-S8 develops metastable impurities S3 and S4, with colours ranging frombrownish-yellow to greyish-yellow to orange: Nash and Fanale (1977), Nelson and Hapke(1978), Steudel et al (1986), Hapke and Graham (1989), Nelson et al (1990)). The red sulfurmaterials are very ephemeral at some vents, and tend to fade over a period of a few months.Dark materials tend to brighten with time, as they develop coatings of sulfurous materialsfrom plume deposits (Nash et al 1986). Bright materials tend to darken over time, as theyinteract with underlying or superposed materials. In addition to brightening and darkening,some colour units can completely change, the best example of which is the red material on thefloor of Pillan Patera which changed to a greenish colour over several months (Phillips 2000,Keszthelyi et al 2001). Phillips (2000) interpreted this colour change as due to a reaction ofshort-chain red sulfur (S3, S4) molecules with warm, dark silicate lava.

The correlation between colour variations and distribution of SO2 is not straightforward,as shown by the analysis of NIMS high spatial resolution data (e.g. Lopes-Gautier et al (2000)).Most of Io’s surface is covered by SO2 frost, with the exception of the vicinity of hot spots(e.g. Carlson et al (1997b)). The global distribution of SO2 on Io’s surface is a result ofvolcanic venting of gaseous SO2 into the plumes and atmosphere. SO2 is subsequentlydeposited onto the colder surfaces as frost. Doute et al (2001a) showed that SO2 is mostprevalent in several large areas at middle latitudes. Extended areas depleted in SO2 are


present, particularly in the longitude range 270–320˚, for reasons still unknown, but whichprobably include higher local temperatures preventing the condensation of SO2 frost. Theregions with abundant SO2 (surface coverage >60%) show a longitudinal correlation withplumes located close to the equator. Doute et al (2001) proposed that most of the SO2 gasfrom the plumes (which are located mainly in the equatorial regions) flows towards coldersurfaces (at higher latitudes) and condenses there. A small fraction of the SO2 condensesonto cold equatorial regions. The spectral signature of SO2 in the equatorial regions isconsistent with larger grain sizes, which is explained as sunlight causing condensed SO2 frostto undergo slow metamorphism and sublimation, creating deposits with larger effective grainsizes than at higher latitudes. SO2 deposits at medium and high latitudes may be opticallythin (Simonelli et al 1997, Geissler et al 2001). The weaker SO2 spectral signature at higherlatitudes (where surface temperatures are colder and more frost might be expected to condense)may be due to rapid radiolytic destruction of condensed SO2 (Wong and Johnson 1996,Carlson et al 2001).

The higher spatial resolution NIMS data provided the opportunity to map SO2 distributionat local scales (e.g. Lopes-Gautier et al (2000), Lopes et al (2001), Doute et al (2002),Doute et al (2004)). A nearly pure SO2 region (corresponding to a 90–100% concentration ofSO2, Lopes et al 2001) was found to be topographically confined within the Baldur caldera(see figure 4). In this case, a high concentration of SO2 corresponds to a bright white region atvisible wavelengths; however, higher concentrations of SO2 do not necessarily correspond tobright white regions seen at low phase angles, as shown by other studies such as of the plumedeposition ring from the Prometheus plume (Lopes-Gautier et al 2000, Doute et al 2002), inwhich higher concentrations of SO2 were found outside the bright white ring seen at moderatephase angles. However, the area with high concentration of SO2 matched well with the ringseen at moderate phase angles in visible wavelengths (Geissler et al 2001). Therefore, thecorrelation of surface colour and composition remains complex.

Sulfur as a free element is thought to be common on Io’s surface (e.g. Fanale et al (1982),Kargel et al (1999), Spencer et al (2000a)). Apart from plume deposits (discussed above), twoother spectral units on Io may be composed of sulfur in some form. Small patches of greenishmaterials, first identified by Geissler et al (1999) have spectra which are not characteristicof either sulfur or SO2. The green colour may be due to contamination or alteration of redmaterials by warm silicate lavas (Keszthelyi et al 2001, McEwen et al 2004).

Other compounds on Io have not been positively identified. A broad absorption featureof unknown origin in the 1–1.3 µm range (Carlson et al 1997b, Pollock et al 1978) was foundto be anti-correlated with dark regions of recent lavas (Lopes et al 2001) but appears to bestronger in dark polar regions (Carlson et al 2001). Suggestions for its origin include sulfur–iron compounds such as pyrite (Kargel et al 1999), short-chain sulfur allotropes or sulfanesproduced by the radiolysis of SO2 (Carlson et al 2001) and sulfur contaminated by traceelements (Doute et al 2001b).

A few other absorption features have been attributed to compounds other than SO2,including H2S (Nash and Howell 1989), SO3 (Nelson and Smythe 1986, Khanna et al 1995),H2O (Salama et al 1990) and Cl2 SO2 (Schmitt and Rodriguez 2003). Io is thought to havelost nearly all its hydrogen (Zolotov and Fegley 1999); however, the detection of hydrogenpickup ions by Galileo’s plasma analyser (Frank and Patterson 2001) led to the interpretationthat Io may provide a flux of hydrogen from the surface to the atmosphere as hydrogenescapes rapidly to space. It is also possible that the Jovian magnetosphere, and not Io, isthe source of the hydrogen detected by Galileo. It is likely that a new mission to the Joviansystem will be necessary before any of these suggested compounds are confirmed to existon Io.


4.2.6. Heat flow. Io’s heat flow results from tidal and orbital interactions with Jupiter, Europaand, to a lesser extent, Ganymede (Peale et al 1979, Yoder and Peale 1981, Hussmann andSpohn 2004). Due to Laplace resonance, the orbital periods of Io, Europa and Ganymede areclose to the ratio 4 : 2 : 1 (Io orbits Jupiter four times for every two orbits of Europa and one ofGanymede), and their forced eccentricities are maintained over long periods of time (Sinclair1975, Yoder 1979, Yoder and Peale 1981, Greenberg 1982, Peale 1986). Io gains orbital energyfrom the reservoir of Jupiter’s rotational energy through tidal torques exerted by the tidal bulgeon Jupiter. Part of this energy is passed on to Europa and Ganymede through gravitationalinteractions in the resonance (e.g. Hussmann and Spohn (2004)). Another part of the energy isdissipated in Io’s interior, causing rampant volcanism and high heat flow. Understanding Io’sheat flow is essential for constraining tidal heating models, for probing Io’s interior structureand for providing constraints on resurfacing rates (e.g. McEwen et al (2004)). Io’s heat flowis so high that it can be determined by remote sensing measurements of surface temperaturesfrom orbit and even from ground-based telescopes. Galileo’s main contribution to the study ofheat flow was the improved determination of heat flow using the photopolarimeter radiometerobservations (e.g. Spencer et al (2000b), Rathbun et al (2004)). In addition, Galileo alsoobtained measurements relevant to models of interior structure, which in turn are needed formodelling Io’s tidal dissipation. Independent measurements of the quadrupole coefficientsJ2 and C22 were obtained from three Galileo flybys (Anderson et al 2001) and from Galileoimaging measurements of Io’s limb (Thomas et al 1988).

Among the important questions about Io that the heat flow measurements aimed to answerwere (i) what is Io’s total heat flow and (ii) what is the distribution of heat flow. These, inturn, lead to what is the relationship between the heat flow and the active volcanic centres.Recent results indicate that active volcanism can account for most of Io’s heat flow, butconducted heat from large lava flows may also be significant as suggested before (Stevensonand McNamara 1988).

Determination of Io’s heat flow must take into account the ‘passive’ component (causedby re-radiated sunlight) and this component must be removed in order to isolate the volcaniccontribution to Io’s total heat flow. Determinations of Io’s heat flow must also take intoaccount the uniqueness of Loki, Io’s most powerful and time-variable hot spot. The firstestimate of Io’s heat flow was 2 ± 1 W m−2 (Matson et al 1981). Later work by McEwenet al (1996) using Voyager IRIS data yielded a minimum global heat flow of 1.85 W m−2,with likely additional contribution from conducted heat and widespread low-temperature hotspots. Veeder et al (1994) obtained the most comprehensive picture so far of Io’s heat flowby synthesizing 10 years of ground-based photometric observations in the range 5–20 µm andestimated a minimum average heat flow of 2.5 W m−2.

Galileo observations through the photopolarimeter radiometer (PPR) obtained the firsthemispheric maps of broadbased night-time emission from Io (Spencer et al 2000b). Theseobservations (from orbit I27) showed that nighttime temperatures away from obvious hotspots are near 95 K, and are remarkably independent of both latitude and local time. Higherresolution PPR observations obtained in orbit I31 (in 2001), which mapped Io pole topole at 60 km spatial resolution, showed a small decrease in nighttime temperatures athigh latitudes, but less than what would be expected from a cos1/4 law. The most likelyexplanation for the surprisingly warm nighttime poles is that excess endogenic heat is beingemitted from the polar regions (Rathbun et al 2004). In contrast, studies of the globaldistribution of hot spots (discussed above) imply that either the distribution of volcanismwith latitude is uniform or, possibly, there are fewer hot spots at high latitudes. This apparentdiscrepancy could be explained by the presence of a different style of volcanism at highlatitudes, perhaps less frequent volcanic activity leading to large, cooling areas, which would


be harder to detect at shorter wavelengths such as the range covered by Galileo NIMSand SSI.

Spencer et al’s (2000b) estimate of Io’s heat flow from I27 data was 1.7 W m−2. Thelatest estimates of Io’s heat flow to date are from Veeder et al (2004) of 3 ± 1 W m−2

and Spencer et al (2002a) of 2.2 ± 0.9 W m−2; the latter brackets the previous value of1.7 W m−2 from PPR data and is also in close agreement with the first estimate of Matsonet al (1981). Further refinements of Io’s heat flow are expected to come forward, butuncertainties in the heat flow estimate cannot account for the discrepancy between observedvalue and theoretical estimates expected from steady-state tidal heating models over thelast 4.5 Ga.

The current estimates of heat flow from observations are about twice the value predicted byPeale (1999) of ∼0.8 W m−2. If the theoretical estimates of steady-state heat flow are correct,then Io’s heat flow must have varied over time due to its orbital evolution. Other studies alsosuggest that Io’s current heat flow and tidal heating rate exceed the long-term equilibriumvalue. According to Aksnes and Franklin (2001), recent secular orbital accelerations suggestthat Io is now spiralling slowly inwards, losing more energy from internal dissipation thanit gains from Jupiter’s tidal torque. Furthermore, Wienbruch and Spohn (1995) arguedthat if Io’s thermal history varies strongly with time, then the absence of core convectiondriving a self-sustaining magnetic field (see Kivelson et al (2004)) requires that the mantle berelatively hot.

The most recent study to date is that of Hussmann and Spohn (2004), which took intoaccount Galileo results and presented two different scenarios of the coupled thermal–orbitalevolution of Io and Europa: in one scenario, tidal dissipation within Europa’s silicate layerwas considered, in the second, dissipation was assumed to be within Europa’s ice layer.Both models are consistent with the present eccentricity and mean motion of Io, Europaand Ganymede. While neither model could explain Io’s observed heat flow, Hussman andSpohn proposed two major ways of obtaining a higher heat flow in the Io models. The firstis to assume a lower value for the Jovian dissipation factor, QJ, thereby increasing the rateof energy transfer from Jupiter to Io and Io’s interior heating. The second way is to assumethat Io is at present far away from an equilibrium between heat production and heat flow.Disequilibrium occurs during oscillations in orbital acceleration (such as suggested by Aknesand Franklin (2001)). It is unclear at present which of these scenarios is more realistic, orif there are other possible factors contributing to the discrepancy between Io’s observed heatflow and the dissipation rate in Jupiter. The resolution of this discrepancy will have importantimplications not only for understanding the evolution of Io, but also that of Europa andGanymede.

4.2.7. Lithosphere, mantle and interior structure. With the completion of the initial analysesof all of the Io data from the Galileo mission, a post-Galileo model for the structure andthermal state of Io’s interior has been developed (Keszthelyi et al 2004). The key discoveriesfrom the Galileo mission about Io include the following: (1) the discovery of widespread hightemperature, silicate (possibly ultramafic) volcanism (McEwen et al 1998b, Geissler et al1999); (2) the detection of both S2 and SO2 gas in Ionian plumes (Spencer et al 2000b) andthe variation between Pillanian eruption styles plumes (originating from central vents) andPromethean eruption style plumes (originating from silicate lava flow fronts), suggesting thepossibility of extensive volatile reservoirs of sulfur and sulfur dioxide in the crust; (3) therelationship between mountains and paterae, in which many paterae are formed as magmapreferentially ascends along tectonic faults associated with mountain building (Jaeger et al2003); the conclusion is that the mountains are primarily driven upwards to compensate for


Figure 8. (a) One current post-Galileo model of volcanic processes and the interior of Io. Seesection 4.2.7 for description of model. (b) Cartoon of a leading patera formation model. Silicatemagma (red) ascends through solid silicate rock (black) but lacks the buoyancy to rise througha volatile-rich zone (dark blue) near the surface. The heat from the intrusion melts surroundingsulfur (yellow) and SO2 (light blue) as originally suggested by Greeley et al (1984), which migrateinto the permeable surroundings. Where SO2 is vaporized, it erupts on the surface, sometimesentraining sulfur. As this process continues, a caldera-like depression forms over the intrusion. Ifthe silicate magma flux is sustained, it may become locally or completely unroofed, forming a darklava lake. Diagrams after Keszthelyi et al (2004).

the compression caused as materials at the surface of Io are pushed down (Schenk and Bulmer1998, Jaeger et al 2003), which is augmented by expansion caused by heating of the rocks asthey approach the base of the lithosphere (McKinnon et al 2001, Jaeger et al 2003); and (4) theimplication from geophysical data that Io has a large Fe–FeS core, whose radius is slightlyless than half the radius of Io (Anderson et al 2001), and that the lack of an intrinsic magneticfield (Kivelson et al 2001) implies that the core is either completely solid or completely liquid(Keszthelyi et al 2004).

These key data have led to the following post-Galileo model (figure 8(a)) for thestructure and thermal state of Io’s interior (Keszthelyi et al 2004). This model, which isconsistent with all the new Galileo data, includes the following: (1) a completely moltencore; (2) an undifferentiated mantle that undergoes a high degree of partial melting (20–50%molten), that would produce lavas dominated by Mg-rich orthopyroxene in a ‘mushy magmaocean’ (Keszthelyi et al 1999); and (3) a rigid crust 20–30 km thick (Carr et al 1998, Jaegeret al 2003), that is cold away from volcanic heat sources (O’Reilly and Davies 1980). A keyconsequence of this model relates to patera formation, as shown in figure 8(b). Rising silicatemagma ascends through solid silicate rock but stalls under the near-surface volatile-rich zonedue to lack of buoyancy. The heat from the intrusion melts surrounding sulfur and SO2

materials a la Greeley et al (1984), which migrate into the permeable surroundings. WhereSO2 is vaporized, it will erupt to the surface, sometimes entraining sulfur with it. As thisprocess continues, a substantial caldera-like depression can form over the intrusion, initiallycontaining bright sulfurous materials. If the flux of silicate magma is sustained, it may becomelocally, or even completely, unroofed, forming a patera with a floor covered in dark silicate


flows. Currently, tests of these models for Io’s interior are being considered, and other modelsconsistent with the new Galileo data are being developed (e.g. Moore (2003)).

4.3. Io in the Jupiter environment

One of the primary goals of the Galileo mission was to better understand the relationshipbetween Io, its plasma torus and Jupiter’s magnetic field. Galileo’s magnetometer and other‘fields and particles’ instruments made a variety of observations of Io and Jupiter’s magneticfield during the course of the mission. This was particularly true in December 2000, whenjoint observations were made by Galileo (inside the magnetic field) and the Cassini spacecraft(outside the magnetic field) during the latter’s Jupiter gravity assist flyby on the way to Saturn.In this section, we attempt to summarize some of the key findings made during the Galileoera of Io’s plasma torus–atmosphere–surface interactions and their relationship to Jupiter’smagnetic field. This section will include a discussion of Io’s aurora, sodium neutral clouds,plasma torus and flux tube, and other phenomena.

Io orbits Jupiter at a distance of 5.9RJ (1RJ = 71 492 km), which is deep within the Jovianmagnetopause (∼60–100RJ along the Jupiter–sun direction: Joy et al (2002)). Thus there isconsiderable interaction of Io with the Jovian magnetic field. Io has both a patchy atmosphereand an ionosphere, in which Io’s atmospheric density is greater around sites of known, activevolcanic plumes. Since the time of the Voyager flybys, Io has been known to produce volcanicplumes hundreds of kilometres high, which serve as an efficient delivery mechanism for gasand dust particles into the magnetosphere and the space surrounding Io.

4.3.1. Io’s colourful aurora. Io’s aurora was an important new discovery made duringGalileo’s nominal mission. The aurora (figure 9) was detected through colour eclipse imagingwith the SSI camera while Io was in Jupiter’s shadow. The vivid colours detected (red, green andblue) are caused by collisions between Io’s atmospheric gases and energetic charged particlestrapped in Jupiter’s magnetic field. The green and red emissions are probably produced bymechanisms similar to those in Earth’s polar regions that produce terrestrial aurorae. Thegreen (actually yellow) glow comes from emission from sodium ions, whereas the red glow isassociated with oxygen ions. The bright blue glows mark the sites of dense plumes of volcanicvapour, and may represent the locations where Io is electrically connected to Jupiter via a fluxtube (see figure 11). The darker blue glow at right corresponds to the location of the Acala hotspot, which was known since Voyager times as a site of a ‘stealth plume’ (Johnson et al 1995),i.e. a location on Io where surface changes by plume deposits have been observed, but whereno actual plume was seen.

4.3.2. Io’s sodium cloud. Io’s cloud of neutral sodium atoms was discovered throughtelescopic observations during the mid-1970s. The cloud moves with Io around its orbit(Brown 1974, Smyth and McElroy 1977), and has a disc hundreds of RJ in size (Mendillo et al1990). Galileo’s SSI camera successfully obtained the first spacecraft images of the sodiumcloud early in the nominal mission (figure 10). It appears as a diffuse yellowish emissionproduced by scattered light from volcanic plumes and Io’s lit crescent. This emission comesfrom neutral sodium atoms within Io’s extensive material halo that scatter sunlight at the yellowwavelength of about 589 nm. Although neutral sodium atoms are most easily detectable byspectroscopy, it has been determined through extensive Earth-based telescopic studies thatsodium is a minor component of the neutral material escaping from Io. The primary neutralelements in the cloud are oxygen and sulfur, which are thought to have dissociated from sulfurdioxide gas (SO2) expelled from many of Io’s active volcanos at a rate of ∼1 ton per day


Figure 9. Io’s aurora. Bright blue glows are SO2 excited by electrical currents flowing betweenIo and Jupiter in the flux tube (see figure 11). Blue glow at right is over Acala, site of a ‘stealthplume’. Red and green (actually yellow) glows are atomic O and Na, respectively. Credit: NASA.

(Hill et al 1983). Some of the O and S neutral atoms become charged by the processes of UVphotoionization, electron impact ionization and charge exchange (Kivelson et al 2004). Muchof the ionized material ends up in the plasma torus.

4.3.3. Io’s plasma torus. The Io plasma torus is a doughnut-shaped ring of sulfur and oxygenions, derived from Io’s active volcanic plumes and transported around Jupiter by its magneticfield (figure 11). It was discovered from Voyager spacecraft data in the late 1970s (Bridgeet al 1979, Broadfoot et al 1979). Neutral atoms ejected by Io’s volcanos become ionizedand subject to electrodynamic forces that accelerate the newly-formed plasma to a relativevelocity of 57 km s−1 (the difference between the plasma torus’ rotation rate around Jupiter of74 km s−1 and Io’s rotation rate around Jupiter of 17 km s−1). The torus is about two Jupiterradii in width (about 143 000 km), and is centred on Io’s orbit around Jupiter at a distance of5.9 Jupiter radii from the planet (about 421 600 km).

Measurements made by the Galileo spacecraft during its close flybys showed that theplasma in the torus is slowed by Io’s ionosphere, redirected around Io and then reacceleratedin Io’s wake (Saur et al 2004). The measured temperature of the plasma ranges from ∼105 Kin the ionosphere and wake to ∼106 K in the torus. Additionally, the Galileo close flybys of Ioenabled a definitive answer to the question of whether Io has its own internal magnetic field:the polar flybys showed that Io has no internal magnetic field, but rather strongly perturbs theJovian magnetic field (Kivelson et al 2001). These perturbations vary with time, suggestingthat Io’s variable volcanic activity influences the density of the plasma torus and the strengthof its interactions with the Jovian magnetic field (Saur et al 2004). Because Jupiter’s magneticfield is tilted in relation to its spin axis, the Io torus appears to wobble as Jupiter rotates. Duringthe Galileo–Cassini Jupiter millennium flyby, Cassini had the first opportunity to image thewhole Io plasma torus by spacecraft over an extended period of time and to study changes inthe torus while Jupiter rotates. This was enabled by Cassini’s relatively distant flyby distancefrom Jupiter, in which the closest approach occurred at a distance of ∼138RJ. From a studyof the torus movie it became clear that the evening side of the Io torus (on the right side of


Figure 10. False colour image of Io and the surrounding sodium cloud. This image was takenat 5 h 30 min Universal Time on 9 November 1996 by the Galileo SSI experiment, using a clearfilter whose wavelength range was approximately 400–1100 nm. Sunlight is scattered from Io’silluminated crescent and from Prometheus’ sunlit plume. The green–yellow diffuse sky emissionis coming from Io’s sodium cloud. The large roundish red spot corresponds to thermal emissionfrom the volcano Pele. Credit: NASA.

the image in figure 11) is brighter than the morning side. This appears to be associated with adiscernible clump of material in the torus at a longitude of 210˚. The Cassini spacecraft alsodetected narrowband kilometric (wavelengths of about 1000 m) radio emissions, which aregenerated by interaction of energetic particles in the outer edge of the Io plasma torus withJupiter’s magnetic field.

4.3.4. Other phenomena. Galileo and Cassini observations have been used to understandbetter other energetic phenomena in Jupiter’s atmosphere. The detection of UV, IR anddecametric (wavelengths of about 10 m) radio emissions in Jupiter’s ionosphere directlybeneath Io in the Jovian auroral zone were interpreted as a ‘footprint’ of a force linkingIo and Jupiter via magnetic field lines. This force was identified as an electron flux tube, whichallows the transfer of charged particles between the two bodies (Bhardwaj et al 2001). Muchweaker flux tube footprints linking Europa and Ganymede to Jupiter have also been detectedin the UV. These footprint emissions vary with time.

4.4. Monitoring Io from Earth: HST and adaptive optics

After the end of the Galileo mission, monitoring of Io’s prodigious volcanic activity andstudies of Io’s atmosphere are continuing using Earth-orbiting observatories such as HST(figure 12) and, in the future, possibly the Spitzer space telescope. The recent introductionof the space telescope imaging spectrograph (STIS) instrument on HST, which combinesspectroscopy with imaging, has been of great value to the study of Io’s atmosphere, in particularthe spatial distribution of SO2 (e.g. Roesler et al (1999), Feldman et al (2000), Jessup et al(2004)). However, the STIS instrument on HST ceased operating on 3 August 2004, with noreplacement planned. In a recent study, Jessup et al (2004) detected the Prometheus plume andnearby regions in Io’s anti-Jovian hemisphere using STIS and concluded that Io’s atmosphereis denser and more widespread on the anti-Jovian hemisphere than at other longitudes. This isconsistent with studies of the distribution of SO2 frost on the surface (e.g. Nelson et al (1980),Doute et al (2001a)). In terms of latitudinal distribution, Jessup et al (2004)) found that, at leastfor latitudes less than ±30˚ on the anti-Jovian hemisphere, where there are extensive depositsof optically-thick SO2 frost (Doute et al 2001a), Io’s atmosphere is probably supported bysublimation of surface frost.


JJ2 2 R

Figure 11. The diagram (top) shows the location of the torus relative to Jupiter. The torus is abouttwo Jupiter radii in width (about 143 000 km or 88 700 miles wide), and is centred on Io’s orbitaround Jupiter at a distance of 5.9 Jupiter radii from the planet (about 421 600 km or 261 400 miles).Because the magnetic field is tilted in relation to Jupiter’s spin axis, the torus appears to wobbleas it rotates. The image (bottom) is one frame from a movie of the Io plasma torus imaged bythe ultraviolet imaging spectrograph (UVIS) on NASA’s Cassini spacecraft. The full movie can befound at the following web site: http://www.jpl.nasa.gov/jupiterflyby/gallery/gallery index.html.This image and the movie show a characteristic observed previously from NASA’s Galileo andVoyager spacecraft, and from Earth: the evening side of the Io torus (on the right side of the image)is brighter than the morning side. A moving dot tracks Jupiter’s rotation speed and indicates thata particular longitude (210˚) appears to be associated with a discernible clump of material in thetorus. Cassini’s millennium flyby of Jupiter afforded the first opportunity to image the whole Ioplasma torus by a spacecraft. The Cassini spacecraft also detected narrowband kilometric radioemissions, which are generated by interaction of energetic particles in the Io plasma torus withJupiter’s magnetic field. North is up in this image, which is one of 235 frames collected by theUVIS in November 2000, when Cassini was about 50 million kilometres (30 million miles) fromJupiter. Credit: NASA.

The volcanic activity at Io’s brighter hot spots can be monitored using ground-basedtelescopes with IR instruments. In particular, the use of adaptive optics techniques has beenvery successful. The software and equipment of the adaptive optics technology corrects for thedistortions of the Earth’s atmosphere and allows much higher spatial- and spectral-resolutionimages to be obtained of planetary objects. Adaptive optics observations of Io supplementedGalileo observations during the mission (e.g. Marchis et al (2000, 2001)), allowed the detectionof previously unknown hot spots and have been able to detect major outbursts of specific hotspots (e.g. Marchis et al (2003), de Pater et al (2004)).

5. Outstanding questions and future missions

Study of the data obtained at Io during the Galileo mission has revolutionized our understandingof the nature of geologic processes that function on its surface. However, as often occurs in


Figure 12. A series of false colour HST images of Io as it swept across Jupiter’s face on 22 July1997. The inset shows a 200 km high, SO2-rich plume from the 1997 Pillan eruption, which wasclosely studied by the Galileo spacecraft. Because a dark pyroclastic deposit was imaged at Pillanby Galileo in September 1997, a change in eruption conditions must have occurred during theeruption, which was first detected by Galileo in June 1997. Credit: NASA.

science, many new questions have arisen that challenge our understanding of how planetsoperate. Perhaps the most perplexing is the nature of ultramafic volcanism on Io, if that is trulythe correct interpretation of the limited Galileo observations. Specifically, how can relativelyprimitive ultramafic volcanism (typically associated with the ancient Archean (3.8–2.5 Ga) onEarth) persist on such a dynamically active body like Io where extreme differentiation shouldbe expected (Keszthelyi and McEwen 1997)? At Io’s current rate of volcanic activity, extendedover the age of the solar system, a volume of lava ∼140 times the volume of Io should have beenproduced (Keszthelyi et al 2004). Either the current style of volcanic activity is a geologicallyrecent phenomenon (i.e. Io has only recently attained its resonant orbit with resulting tidalheating), or the response of Io’s lithosphere–mantle to tidal heating has prevented extremedifferentiation (see Keszthelyi et al (2004) for one theory). Equally strange is the apparentrelationship between the formation of mountains and the formation of paterae, in which recentstudies show that 41% of mountains are found to be adjacent to paterae (Jaeger et al 2003).What does this relationship tell us about the stress environment in Io’s crust and its effecton volcano–tectonic activity? The presence of multi-hundred kilometre-high eruption plumesthat produce various coloured pyroclastic deposits on the surface suggests complex interactionsbetween magma and multiple volatile reservoirs, including sulfur, sulfur dioxide and possiblyhalides and others. Are magma–volatile interactions restricted to the shallow crust, or isthere deeper assimilation of volatiles by ascending magmas? Finally, the Galileo and Cassiniflybys have provided much new information on the interaction between Io and the Jovianmagnetosphere, particularly the recognition of the flux tube that allows the transfer of chargedparticles between the two bodies. But what are the sources of the atoms, neutrals and ionsthat are released into the plasma torus and magnetosphere, and what physical processes allow


them to escape? These are just some of the key questions that have developed about Io afterGalileo that require further data and study.

With the demise of the Galileo spacecraft in 2003, further observations of Io are restrictedto telescopic observations from Earth. These include observations from the surface, from theHST, and potentially from the newly-launched Spitzer infrared telescope facility (SIRTF). Asseen in section 4.4, the techniques of adaptive optics when applied to Earth-based telescopescan produce resolutions similar to those of the Galileo NIMS global observations. Thus,in lieu of having an Io-dedicated spacecraft in the Jovian system, a robust Earth-centred Ioobservation programme is a cost-effective means of monitoring Io’s active volcanos in theshort term. However, further long term advances in understanding Io’s geology will requireadditional close-up studies by spacecraft.

In 2001, at the request of the US Office of Management and Budget, the National Academyof Sciences commissioned the National Research Council to do a study to assess the highestpriority objectives in solar system exploration for the next decade, 2003–13. This study,published in 2002 and commonly referred to as the Planetary Decadal Survey (Space StudiesBoard 2003), solicited input from throughout the planetary science community and the generalpublic. A series of white papers were submitted to the survey panel on many of the objects inthe solar system and why they were good candidates for future exploration (Sykes 2002). Onesuch paper was submitted on the future of Io exploration (Spencer et al 2002b), which wascoauthored by the authors of this paper. Although an Io-dedicated spacecraft mission did notmake the cut in terms of highest priority missions in the next decade (partly due to engineeringand technology development issues), a future Io-dedicated mission was encouraged for thefollowing decade (2013–23) after certain technologies (e.g. radiation-hardened circuitry,advanced propulsion and communications) are developed for other missions. Whether anIo-dedicated mission is developed in the next decade may depend upon the success of the nextmajor mission currently in development for the Jovian system, the Jupiter Icy Moons Orbiter(JIMO). This mission came out of the recommendation in the Planetary Decadal Survey thatNASA undertake a flagship (>$1 billion) mission to send an orbiter to Europa to performglobal mapping of the satellite and assess the depth/nature of its sub-surface ocean. Ascurrently envisioned, JIMO is the first mission of Project Prometheus, NASA’s nuclear spacetechnologies initiative. JIMO would utilize advanced radiation-hardened computer systems,would be powered by a space-rated nuclear fission reactor, which is capable of providingmuch more power (megawatts) for multiple high energy science instruments than present solarcells or radioisotope thermoelectric generators (RTGs) can provide and would utilize nuclear–electric ion propulsion to produce high thrust with low mass requirements. JIMO would enterJovian orbit and alternately cycle into orbits of Callisto, then Ganymede, then Europa, spendingmonths orbiting each moon and obtaining global data sets for each satellite. Among proposedinstruments include ice-penetrating radar, to assess the thicknesses of the icy crusts overlyingthe liquid water layers on these moons. It is worth noting that, depending upon the quality ofthe imaging systems onboard JIMO, distant Io observations could be obtained by JIMO fromEuropa orbit, with resolutions of the order of 200 m per pixel of the antijovian hemisphere(Smythe et al 2003, Spencer et al 2003). Thus, potentially useful Io data could be obtainedby the non-Io-dedicated JIMO mission if it is actually developed for the next decade.

In this decade, there is one mission currently in advanced stages of development thathas the potential to obtain further Io observations. This is New Horizons (NH), the firstmission of NASA’s New Frontiers programme of medium-sized (∼$700 million) missionsrecommended by the Planetary Decadal Survey. NH is the long-awaited Pluto–Kuiper beltflyby. Assuming a scheduled launch in January/February 2006, NH would perform a gravity-assist Jupiter flyby in January/February 2007 on its way to a 2015 flyby of Pluto. NH flyby


distance would be ∼32RJ (∼2.29 million km), which is further than typical Galileo distantIo flybys (∼20–25RJ), but much closer than the Cassini Jupiter flyby of December 2000(∼138RJ: Porco et al (2003)). The Cassini spacecraft at that distance was capable of imagingvolcanic hot spots and multi-hundred kilometre high eruption plumes on Io, which suggeststhat NH should be capable of producing better quality images of Io (all else equal) at its closerflyby distance. Currently, as the NH spacecraft development nears its end, work is underway to assess the capabilities of NH’s instruments (which were optimized for study of Plutoand Kuiper Belt Objects) to obtain the complete range of desirable distant Io observations:daytime global colour observations for surface changes, nighttime eclipse observations for hotspot temperatures and plume monitoring observations.

Acknowledgments

Rosaly M C Lopes and David A Williams are partially funded by NASA’s Planetary Geologyand Geophysics Programme. We thank an anonymous referee for helpful comments.

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Io after Galileo - pdfs.semanticscholar.org · his telescope, which he named the Medici stars after his sponsor. After a few observations, After a few observations, however, Galileo