Volcanism on Iovolcanism was continued from Earth, by astronomers using infrared detectors on telescopes. These observations showed that brightenings and fadings of hot spots occur,

Chapter 43

Volcanism on Io

Rosaly M.C. LopesEarth and Space Sciences Division, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

David A. WilliamsSchool of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA

Chapter Outline

1. Introduction 747

2. Interior and Heat Flow 750

3. Surface Expressions of Volcanism 751

4. Surface Changes, Color, and Composition 753

5. Hot Spot Temperatures and Composition of Magma 754

6. Styles of Volcanic Activity 756

7. Global Geology and Distribution of Volcanism 757

8. Atmosphere and Torus 760

9. Future Studies 761

Further Reading 762

GLOSSARY

differentiation The process by which planets and satellites develop

layers or zones of different chemical and mineralogical

composition.

flyby Term used to describe the close approach of a spacecraft to a

body, without the spacecraft going into orbit around that body.

Galilean satellites The four largest satellites of Jupiter (Io, Europa,

Ganymede, Callisto), named after their discoverer Galileo Galilei.

heat flow Heat that comes out of a body from its interior and is ul-

timately radiated to space.

hot spots Regions of enhanced thermal emission on Io, a sign of

volcanic activity. The term does not imply a particular eruption

mechanism, and does not have the same meaning as hot spots on

Earth.

IRIS Acronym for Infrared Interferometer Spectrometer, an instru-

ment aboard Voyager.

NIMS Acronym for Near-Infrared Mapping Spectrometer, an in-

strument aboard Galileo that obtained spectra in the wavelength

range 0.7e5.2 mm. The spectra are used to analyze the compo-

sition of Io’s surface and to measure the temperatures of its vol-

canic regions.

Patera A collective term for a variety of saucer-shaped, shallow

volcanic constructs that often have a central caldera-like

depression.

pyroclastic materials Fragmented materials ejected during a volca-

nic eruption, including ash, pumice, and rock fragments.

SSI Acronym for solid state imaging system, the CCD camera aboard

the Galileo spacecraft.

sulfur allotropes Sulfur cooled rapidly from different temperatures,

resulting in different colors.

thermal emission Electromagnetic radiation produced by a body due

to its temperature.

volatiles Chemical compounds or elements contained in magmas that

are generally released as gases to the atmosphere during a vol-

canic eruption.

1. INTRODUCTION

Volcanism is a fundamental process that has affected allsolid planets and most moons in the solar system and no-where is this more clearly shown than on Jupiter’s moon Io(Figure 43.1). Prior to the Voyager 1 and 2 spacecraft ob-servations of Io in 1979, the Earth was the only planetknown to have active volcanic activity. Our views ofplanetary volcanism were dramatically changed whenVoyager 1 revealed active volcanoes on Io, a body about thesame size as the Earth’s Moon which, according to priorthinking, should have cooled enough to be volcanicallydead. Images from Voyager 1 showed plumes up to 300 kmin height and a vividly colored surface dominated by largecalderas and lava flows.

Io’s significance in science dates back centuries. Io isthe innermost of the four large satellites of the planetJupiter, which are known as the Galilean satellites. Theywere discovered by the Italian astronomer Galileo Galilei(1564e1642) in 1610, who soon realized that the objects heinitially thought to be stars were, in fact, bodies orbitingJupiter. These observations became the cornerstone of

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evidence that confirmed the Copernican theory, whichstated that the Earth and other planets revolved around theSun, challenging the prevalent view at the time that theEarth was the center of the universe.

Prior to observations from spacecraft, knowledgeabout Io’s surface was inferred from telescopic observa-tions. These showed that Io’s brightness varied accordingto its position in its orbit, suggesting that the moon alwayskeeps one face towards Jupiter. In the 1960s and 1970s,telescopic observations showed that Io differed from theother Galilean satellites because of the absence of waterbands in its spectra, and that Io was the reddest objectknown in the solar system. Ground-based measurementsin the mid-1970s also showed the existence of ionizedsulfur emission in the inner Jovian magnetosphere; sub-sequent studies revealed this to be a plasma torus. Theflyby of the Jupiter system by Pioneer 10 in 1973 markedthe beginning of observations of Io by spacecraft. Thesespace observations detected an ionosphere and thin at-mosphere around Io, and a cloud of neutral gasses (so-dium, potassium, sulfur, and oxygen) along Io’s orbitalpath. Astronomers now know that Io’s volcanic activity isresponsible for these phenomena. Nowhere else in thesolar system is the effect of volcanism as widespread andfar-reaching as on Io.

Io’s size and bulk density (Table 43.1) are similar tothose of the Earth’s Moon and suggest a compositionpredominantly of silicates. We might have expected Io tobe a cratered, dead world much like the Moon. However,even before the first close-up images of Io were returnedby Voyager in 1979, there were hints that Io was remark-ably different from the Moon. Earth-based telescopic

observations made in 1974 first showed that the overallspectrum of Io closely matched that of sulfur, suggestingthat sulfur and sulfur compounds might be abundant on thesurface. It was also known that Io’s spectrum lacked waterice, unlike those of the other Galilean satellites. Just priorto the Voyager encounters in 1979, two events presagedthe discovery of active volcanism. F. Witteborn and col-leagues reported a telescopic observation of an intensetemporary brightening of Io in the infrared wavelengthsfrom 2 to 5 mm. They explained it, although with someskepticism, as thermal emission caused by part of Io’ssurface being at a temperature of about 600 K, much hotterthan the average expected daytime temperature of about130 K. A few days before the Voyager encounters, aseminal theoretical paper by Stan Peale and colleagueswas published in the journal Science. They had studied thetidal stresses generated within Io as a result of the gravi-tational fields of Jupiter, Europa, and Ganymede. Theircalculations showed that the possible heat generated bytidal stresses was in the order of 1013 Wdmuch greaterthan heat that could be released from normal radioactivedecay. Their prediction was that Io might have “wide-spread and recurrent volcanism.”

Active volcanoes were not immediately obvious in thefirst images returned by Voyager 1. The most strikingaspect of Io shown in the first images was its colorfulsurface, with yellows, oranges, reds, and blacks. Scientistson the imaging team nicknamed Io the “pizza moon” andspeculated about the presence of large quantities of sulfuron the surface. Another surprising aspect was the absenceof impact craters. The obvious conclusion was that Io’ssurface was very young and the craters must have been

FIGURE 43.1 Io imaged by Galileo’s camera Solid State Imaging instrument (SSI) in 1996 (left) and by Galileo Near-Infrared Mapping Spectrometer

(NIMS) in 2001 (right). Left: Io’s colorful appearance of results from its constant volcanic activity, depositing sulfur compounds and silicates on the

surface. The reds and blacks are thought to be the most recent volcanic deposits, probably no more than a few years to decades old. Just to the right of the

image center is the Prometheus volcano, one of the most active hot spots on Io. Notice the dark flow and the bright annulus surrounding the volcano,

formed by plume deposits. The dark spot on the left edge just below the equator is the volcano Pele, which is surrounded by large red deposits. Jupiter is in

the background and appears blue, because it is seen through the camera’s infrared filter. On the right, active volcanoes are seen as “hot spots” in this false

color image taken using the instrument’s 5.0 mm wavelength, showing nearly the same view as the SSI image. Seventy one previously unknown active

volcanoes were detected by NIMS during the Galileo mission. (NASA PIA00494 and PIA03535).

748 PART | V Extraterrestrial Volcanism

obliterateddbut how? The answer came soon after, when anavigation engineer at the Jet Propulsion Laboratory, LindaMorabito, noticed a peculiar umbrella-shaped featureemanating from Io’s limb in one of the images that wastaken to aid navigation of the spacecraft (Figure 43.2). Thepattern turned out to be an eruption plume rising about260 km above the surface. A second plume was found onthe same image, and more plumes were seen upon closeexamination of various other images. Additional evidencefor active volcanism came from another of Voyager’s in-struments, the infrared interferometer spectrometer(IRIS), which detected enhanced thermal emission fromparts of Io’s surfacedsome areas had temperatures of about400 K, much higher than the rest of the surface, which hasnoontime equatorial temperatures of about 107e124 K.When one of the hot areas was found to coincide with oneof the plumes, there was no doubt that active volcanism wastaking place.

Eighteen weeks after Voyager 1’s dramatic discovery,the companion spacecraft, Voyager 2, flew close to Io.

Intense activity was still taking place, but significantchanges had occurred between the two flybys, includingthe cessation of the largest plume, Pele, and the alteredshape of the deposits associated with this plume. An areaof about 10,000 km2 had been filled in, presumably byfresh material falling down from the plume (Figure 43.3).It became evident that dramatic changes of Io’s surfacecould occur over short timescales. Initial analysis of theVoyager data showed nine plumes and nine “hot spots,”though not all plumes coincided with hot spots and viceversa. Hot spot is a term used by Io researchers to define aregion of enhanced thermal emission, a sign of activevolcanismdnot the same meaning as a hot spot on Earth.The Voyager IRIS experiment did not observe all of Io’ssurface, so it was suspected that other active volcanoesexisted.

After the two Voyager spacecraft left the Jupiter systemon their way to Saturn and beyond, the study of Io’svolcanism was continued from Earth, by astronomers usinginfrared detectors on telescopes. These observationsshowed that brightenings and fadings of hot spots occur,indicating variations in the level of volcanic activity. Theseobservations showed that Io’s most powerful hot spot, Loki,has brightenings that switch on in 1 month or less and lastseveral months before fading. Telescopic observations werealso used to analyze the reflected light from Io’s surface todetermine surface composition, confirming that it wasdominated by sulfur dioxide (SO2). Io was also observed by

TABLE 43.1 Major Io Characteristics

Orbital period 1.769 days

Rotational period Synchronous with orbit

Mean radius 1821.6� 0.5 km

Bulk density 3528� 3 kg m�3

Mass 8.9320� 0.0013� 1022 kg

Surface gravity 1.80 m s�2

Global average heat flow >2.5 W m�2

Radius of core If pure iron, 656 km

If iron and iron sulfidemixture, 947 km

Geometric albedo 0.62

Local topographic relief Up to w17 km

Active volcanic centers >166

Hot spot temperatures Up to at least 1500 K

Active plumes 16 observed

Surface composition SO2 frost mantles surface,other S compounds, silicates

Typical surfacetemperatures

Away from hot spots: 85 K(night) to 140 K (day)

Crustal thickness (estimated) 30e50 km

Atmospheric pressure 10�9 bar or less, higher atlocations of plumes

Atmospheric composition SO2 (main), SO, S2, Na

FIGURE 43.2 Volcanism on Io was first seen on this Voyager 1 image

taken on March 8, 1979. The image shows two plumes. The Pele plume is

seen on the edge of the disk against the dark sky and it rises about 260 km

above the surface. The other plume, Loki, is seen as a bright spot on the

nightside of the terminator (the boundary between day and night). The

plume is reaching above the darkness of the disk and catching the rays of

the rising sun. (NASA PIA00379).

749Chapter | 43 Volcanism on Io

the International Ultraviolet Explorer satellite and by theHubble Space Telescope.

The first spacecraft to orbit Jupiter was Galileo, from1995 to 2003. Galileo’s instruments were able to image Ioand monitor its volcanic activity, revealing variations involcanic activity, mapping global distribution of volcanism,characterizing eruption styles, measuring lava tempera-tures, and constraining interior structure, including infer-ring the existence of a magma ocean under the crust. In2000, the Cassini spacecraft observed Io on its way toSaturn and, in 2007, the New Horizons spacecraft alsomade valuable observations of Io on its way to Pluto. Sincethen, the study of Io’s remarkable volcanism has continued

using observations by telescopes both on the ground andorbiting Earth.

2. INTERIOR AND HEAT FLOW

Studies of telescopic and spacecraft data coupled withgeophysical modeling of Io suggest that it is differentiatedinto a metallic core (550e900 km radius if FeeFeeS, or350e650 km radius if Fe), silicate mantle, and crust(w30 km thick), which likely includes an asthenosphere.Volcanism on Io is powered by tidal heating, induced by aLaplace resonance with Jupiter’s moon Europa and Gany-mede. For every orbit of Ganymede around Jupiter, Europaorbits twice, and Io orbits four times. This 4:2:1 orbitalresonance results in a forced eccentricity in Io’s orbitaround Jupiter, which causes tidal flexing in Io’s crust andmantle (w100 m every 1.77 days), producing internalfriction of such a magnitude that rocks melt to magma.Indeed, this tidal heating results in Io’s very high meanglobal heat flow, which is estimated to range fromw1.5 to4.0 W/m2 with a mean value of 2.24� 0.45 W/m2. This isw20 times larger than the Earth’s heat flow. The dominantpost-Voyager model for heat transport in Io’s interior pro-poses that, rather than undergoing conduction through thelithosphere, heat from Io’s interior is advected to the sur-face via ascending silicate magma in a heat-pipe mecha-nism that delivers magma to surface hot spots that arecontained within a relatively cold lithosphere. This modelassumes no global magma ocean within Io, although suchoceans have been proposed in the early histories of othersolar system bodies like the Moon. However, more recentresearch by K. Khuruna and colleagues that has reexaminedGalileo magnetometer data from close flybys of Io in1999e2001 suggests that Io does have global subsurfacemagma layer >50 km thick (i.e., a magma ocean) with arock melt fraction of �20%. A magma ocean such as thiswould be akin to a “sponge” with at least 20% silicate meltwithin a matrix of slowly deformable rock, rather than acompletely fluid layer.

Another problem related to understanding Io’s interiorand heat flow involves comparing the distribution of vol-canic features to models of tidal heating. A statistical studyof the locations of Ionian patera (volcano-tectonic de-pressions) and hot spots (Figure 43.4) was undertaken by C.Hamilton and colleagues following the publication of thefirst global geological map of Io. Results showed that thelocations of sites of active volcanism are most consistentwith asthenospheric heating models, but that volcanic ac-tivity is shifted 30e60� E from where models predict. Thissystematic eastward offset between observed and predictedvolcano locations cannot be reconciled with any existingsolid body tidal heating models, suggesting that currentunderstanding of Io’s tidal heat production and its rela-tionship to surface volcanism is incomplete. Possibilities to

FIGURE 43.3 Io’s eruptions cause dramatic changes on the surface. Top:

changes in the bright red deposits around the Pele plume. The larger image

was taken by the Galileo camera in June 1996 and shows Pele just below

the center of Io’s disk. The upper inset is an image from Voyager 1 taken in

March 1979, while the lower inset was taken by Voyager 2 in July 1979.

Note the dramatic change in the shape of the annular deposit in the 4

months between the two Voyager encounters. The differences in the shape

of the deposit are thought to reflect the changing shape of the vent from

which the plume emerged. (NASA PIA00718). Bottom: changes at Pillan

Patera volcano. The image on the left was taken in April 1997. The middle

image shows the same area in September 1997 after a huge eruption

occurred. The eruption produced the large, dark deposit just above and to

the right of the center. The deposit, which is 400 km in diameter, surrounds

Pillan Patera and covers part of the bright red ring, which is the deposit

from Pele’s plume. The image on the right, acquired in July 1999, shows

that the red material from Pele has started to cover the dark deposit. This

image also shows that a volcano to the right of Pillan has erupted,

depositing dark material surrounded by a yellow ring. (NASA PIA02501).


explain the offset include: (1) a faster than expected rota-tion for Io; (2) an interior structure that permits magma totravel significant distances to surface eruption sites fromwhere the most heating occurs; or (3) a missing componentin existing tidal heating models, like fluid tides from asubsurface magma ocean. The existence of this discrepancydemonstrates the needs for better imaging of Io’s surfacefeatures and better determination of Io’s interior structure.

3. SURFACE EXPRESSIONS OFVOLCANISM

Volcanism on Io manifests itself in several discrete forms ofsurface features: (1) patera (pl. paterae, caldera-like fea-tures), (2) lava flows and flow fields, (3) volcanic “hills,”including domes, cones, and possibly shields, and (4)

diffuse deposits (DD) of tephra and frosts derived fromcondensed volcanic gasses. Paterae are volcano-tectonicdepressions that morphologically and structurallyresemble the pit craters that occur at the tops of terrestrialshield volcanoes, although on Io they are larger in size(mean diameter 41 km, some >200 km) and more than90% lack the surrounding shield-like edifice. Thesecaldera-like features are the primary form of volcanic venton Io, and range in shape from subcircular to very irregular,in which the irregular margins suggest tectonic faults mayhave influenced their formation. They are called paterae onIo rather than calderas because the connection to subsur-face magma chambers typical of terrestrial calderas cannotbe confirmed from spacecraft images alone. Nevertheless,high spatial resolution images of Io’s paterae (Figure 43.5)suggest they are similar to terrestrial calderas with floorsthat contain confined lava flows, lava ponds, or lava lakes.

FIGURE 43.4 Map of all hot spots on Io detected by telescopes and the Voyager, Galileo, Cassini, and New Horizonsmissions. Based on data by Lopes

and Spencer (2007) and Williams et al. (2011).

FIGURE 43.5 Two major volcanoes on Io, both of which show a patera (caldera-like structure), which contains dark, active lavas, and “islands,” which

have cold surfaces where sulfur dioxide is deposited. Left: Loki patera (imaged by Voyager) is about 200 km in diameter and shows dramatic, quasi-

periodic, infrared brightenings. Left: Tupan patera (imaged by Galileo) illustrates the result of lava interacting with sulfur-rich materials. Tupan is about

75 km across and is surrounded by cliffs about 0.9 km tall. The central “island” is, like Loki’s, presumably higher than the dark floor, as it is not covered by

lavas. Much of the area is coated with a diffuse red deposit that is thought to have condensed from sulfur gas escaping from volcanic vents. The floor of

Tupan is covered with a surreal pattern of dark black (active lavas), green, red, and yellow materials. The yellow is presumed to be a mix of sulfurous

compounds, and the green appears to form where red sulfur has interacted with the dark lavas. (NASA PIA02599).


The variation in color of the floors of Io’s paterae (rangingfrom bright white to yellow-orange to black, often withmultiple colored deposits within one patera) suggests thatthe compositions of patera floor materials include mixes ofsilicates and various sulfur-bearing compounds, includingdetection by Galileo’s near-infrared mapping spectrom-eter (NIMS) of very pure sulfur dioxide deposits at somelocalities.

Lava flows and flow fields (Figure 43.6) are typified bytheir generally elongated morphology (lengths>>widths),crenulate to lobate edges in planetary images, and sharpcontacts with the other units. Flows on Io are generallycharacterized using color and albedo as bright (possiblycomposed of sulfur), dark (presumably composed of sili-cates), and undivided (uncertain composition). Albedovariations in the flows are generally thought to be indicativeof age on the surface: the freshest dark flows are the darkestblack in color; the freshest bright flows are the brightestyellow in color (i.e., brighter than all plains units). Radia-tion exposure and superposition by DD of various com-positions tend to homogenize flowmaterials to a gray-greencolor. In fact, it is unclear whether the majority of brightflows are bright because they are composed of sulfur(which cools to a yellow color that changes to white fromradiation exposure), or whether they are simply old, coldsilicate flows covered by bright, sulfurous plume fallout(there are clear examples of both on Io). Morphologically,the highest resolution Galileo images of flow materials

suggest that they are reminiscent of terrestrial compoundpahoehoe flow fields (e.g., Kilauea, Hawaii) or platy ridgedlava flows (e.g., Laki, Iceland).

Volcanic hills on Io are called tholi (sing.: tholus, Latinfor “dome”), which is a generalized term for any positiverelief volcanic mountain. They are very rare on Io: theVoyager spacecraft imaged two low shield-like features(Apis and Inachus Tholi) in 1979, and several much smallerhills were identified during the Galileo mission. The rela-tive rarity of volcanic edifices compared to lava flow fieldsis attributed to the inferred low dynamic viscosities ofIonian mafic to ultramafic lavas, whose emplacement fa-vors widespread, low relief flows rather than the steeperedifices produced by more viscous lavas.

DD refer to the accumulations of frosts (derived fromcondensed volcanic gasses) and tephra that are found nearor around vents and flow margins and that mantle under-lying topography. Explosive eruption plumes that havebeen imaged by spacecraft (Figure 43.7) show that theyhave shapes, which range from umbrella-like in profile,producing circular haloes around vents, to asymmetric inprofile, which produce irregularly shaped deposits thatbroaden away from the vent or flow margin. DD are thesource of the widest range of color variations on Io, inwhich red, yellow, white, black, and green varietieshave been observed. These colors are interpreted to beindicative of key chemical constituents of the deposits: reddeposits are interpreted to be composed of short-chain

FIGURE 43.6 Lava flow fields on Io imaged by Galileo. Left: the 90-km long Prometheus flow field is surrounded by a bright plume deposit, the result

of an apparently ever-present sulfur dioxide plume, centered at the distal end of the flow field. The plume is thought to result from the interaction of hot

lava eroding through to a liquid sulfur dioxide substrate, with explosive results. To the right of the image, the main Prometheus patera, shaped like a half-

moon, can be seen. Galileo Near-Infrared Mapping Spectrometer (NIMS) thermal data show that the flow is likely tube-fed, and hotter areas are seen near

the main vent and at the distal edges. (NASA PIA02565). Right: the Amirani flow field is the longest active flow field in the solar system. The most recent

lavas appear darkest because their surfaces are too hot to enable deposition by sulfur compounds from plumes. Thermal maps from the Galileo NIMS

instrument show that fresh, hot lavas are coming out of several areas at the northern end of the flow. However, the main vent of the flow field appears to be

near the southern end, where a half-moon shaped patera is located. The liquid lava travels under a frozen layer of older lava, breaking out onto the surface

only after traveling hundreds of kilometers from the vent. The mosaic shows an area 500 km long and 180 km wide. Amirani (NASA PIA02567).


sulfur� sulfur chlorides, yellow deposits are interpreted tobe composed of sulfur� contaminants, white-gray depositsare interpreted to be composed of sulfur dioxideþ conta-minants, dark or black deposits are interpreted to becomposed of silicate ash, and green deposits are interpretedto be composed of products of silicateesulfur alteration.Although DD are often ephemeral, fading after a fewmonths when emplaced from a discrete eruption episode,long-term accumulation of red diffuse, yellow diffuse, andwhite DD from repeated or periodic eruptions could lead tothe formation of red-brown plains, yellow bright plains, andwhite bright plains materials, respectively.

4. SURFACE CHANGES, COLOR,AND COMPOSITION

Comparison between Voyager and Galileo images takennearly two decades apart showed many areas of surfacechanges, but not as many as might have been expectedgiven Io’s high rate of volcanism. Surprisingly, volcanoeswhere vigorous activity is known to have occurred, such asLoki, showed little surface change, presumably because theactivity is mostly confined to the interior of the caldera.One of the most striking cases of surface change happenedon Ra Patera (Figure 43.8), a region that so far has beenidentified only as an active plume site and not yet as a hotspot, though this could be due to poor spatial resolution ofthe thermal data over this region. Voyager images showedRa to have a dark caldera surrounded by narrow flows. Theflows showed a sequence of colors along their lengths thatwas consistent with the interpretation of Carl Sagan thatIo’s surface was covered by different sulfur allotropes,with the supposedly cooler materials further away from thevent. However, this interpretation of composition based oncolor was criticized on a variety of grounds, including thefact that the exact colors of sulfur can be drastically alteredby even small amounts of other materials. Ra Paterabecame even more of a puzzle when observations from theHubble Space Telescope showed that a significant bright-ening occurred in this region between 1994 and 1995.When images of Ra were returned after Galileo’s first orbit,they showed that the flows had been covered over by newdeposits and that a plume was now erupting from Ra.

The most significant surface changes detected by Gal-ileo were localized changes due to major eruptions. ThePillan eruption of 1997 left a conspicuous “black eye” onIo’s surface (Figure 43.3), covering an area of about200� 103 km2 and spreading about 260 km from thesource. This dark deposit slowly faded between 1997 and1999, as it was covered by red deposits from the nearbyPele plume. The Tvashtar eruption that started in 1999created a large annular red deposit as the result of a plumeand the Thor eruption that started in 2001 and resulted inthe largest plume so far observed from Io (w500 km) left awhite-gray plume deposit on the surface.

Galileo results brought new insights into what causesthe vivid colors of Io’s surface, including the colors of theflows that are now gone. Surface colors are the most easilyobserved expressions of surface changes and can provideinsight into composition, however, different illuminationangles affect how colors appear in images, so the analysis isnot straightforward. Galileo’s repeated flybys allowed ob-servations at various illumination angles, and differentcolor deposits were identified. Io’s surface has four majorcolor units. About 40% of the surface shows yellowcoloration, about 30% appears red to orange, 27% appearswhite to gray, and black deposits (w2%) are seen mostly in

FIGURE 43.7 Volcanic plumes on Io imaged by Galileo (top and left)

and New Horizons (bottom right). Top: the Pillan plume is seen against the

bright limb in the larger image and also in the upper inset. This plume rose

120 km above the caldera of Pillan Patera volcano in 1997. The second

plume in the larger image is erupting from the Prometheus volcano and

seen near the terminator, rising about 75 km. The lower inset shows the

shadow of this plume extending to the right of the volcano (NASA

PIA00703). Bottom left: the Masubi plume is ejected some 100 km above

the surface. This plume was seen to erupt from different locations within

the Masubi plume, and is thought to be similar to the Prometheus plume,

erupting from near the distal edges of the lava flow (NASA PIA02502).

Bottom right: the New Horizons spacecraft, which flew by Io in 2007,

captured this image of the Tvashtar plume rising some 330 km above the

surface near Io’s north pole. Io’s dayside was deliberately overexposed in

this image taken by the Long Range Reconnaissance Imager instrument to

bring out details on the nightside and in any volcanic plumes that might be

present. Hot, glowing lava at the source of the plume is the bright point of

light on the sunlit side of the terminator. (NASA PIA09360).


or near active areas. There are also some latitudinal dif-ferences. Red and orange materials are thought to be de-posits of short-chain sulfur molecules (S3, S4) and arefound mostly at latitudes higher than 30� N and S, wherethey are thought to result from the breakdown of sulfur(cyclo-S8) by irradiation from charged particles. Althoughthe time range of observations is limited, it appears that reddeposits last longer at higher latitudes, perhaps because thecolor is the result of irradiation and therefore the origin ofthe red deposits may be different at higher latitudes. Atlower latitudes, red deposits appear to be ephemeral, areassociated with hot spots and plumes and probably causedby condensation and fallout of material from sulfur-richplumes. In some locations, these deposits have been seento fade to yellow over several years. The vast expanses ofequatorial plains are dominated by yellow, white, and graymaterials that are generally anticorrelated with active vol-canoes, while the black deposits are associated with vol-canic activity, occurring in isolated patches, both as DDthat are probably pyroclastic in nature and as continuousunits confined to caldera interiors.

Sulfur and sulfur compounds are the most viablecompositional candidates for the variegated colors of Io,though silicate materials are thought to be exposed in theblack areas associated with volcanic activity. Some of thevery dark materials that are associated with calderas andactive hot spots show an apparent spectral absorptionfeature at 0.9 mm, seen in the Galileo imaging data andinterpreted by P. Geissler and colleagues as being due toorthopyroxene. Although further results are needed toconfirm the presence of this spectral feature and to providea unique mineral identification, the presence of orthopyr-oxene on Io, a magnesium-rich silicate mineral common interrestrial mafic and ultramafic rocks, seems likely.

Yellow deposits are interpreted either as cyclo-S8,possibly with a thin covering of SO2 frosts deposited by

plumes, or polysulfur oxide and SO2 without large quan-tities of elemental sulfur. White and gray materials areinterpreted as coarse to moderate grained SO2 thatcondensed from plumes and later recrystallized. Perhapsthe most intriguing materials on Io’s surface are thegreenish-yellow deposits seen in a few isolated spots(Figure 43.9). Unlike other terrains on Io, these areas showa negative near-infrared spectral slope, strongly suggestingthat a nonsulfur component is present. These green mate-rials could be made up of sulfur contaminated by iron, assuggested by J. Kargel and colleagues, however, the factthat an active hot spot is seen close to some of these areas(such as Balder and Tohil) is consistent with the alternativeexplanation of silicate deposits such as olivine or clino-pyroxene.

The poor spatial resolution of the observations of Ioavailable so far and the thin coating of SO2 that seems to beprevalent on much of the surface have made it difficult toidentify the compositions of the different deposits on Io.The Galileo NIMS instrument detected a broad absorptioncentered at about 1 mm, but it is still not known what theabsorption is due to, a clue is that it is anticorrelated withrecently emplaced lavas. Unfortunately, radiation damageto the NIMS instrument during Galileo’s close flybys of Io,where high spatial resolution data were obtained, signifi-cantly diminished the spectral resolution of the instrumentand many questions remain about the composition of Io’ssurface.

5. HOT SPOT TEMPERATURES ANDCOMPOSITION OF MAGMA

Shortly after the Voyager mission, the major controversyabout Io’s volcanism after results from the Voyager missionconcerned the nature of the volcanism: sulfur or silicates?

FIGURE 43.8 Possible sulfur flows on Io. Top:

Voyager 1 image of the Ra Patera volcano on Io taken

in 1979. Narrow, lobate lava flows are seen coming out

of a caldera-like patera. The colors of the flows were

interpreted as colors of different sulfur allotropes.

When Galileo imaged Io in 1996, those flows had been

covered over by younger deposits. Image is 1000 km

across (NASA PIA00361). Right: Emakong patera and

bright, channelized flows imaged by Galileo in 1999.

The top image, covering an area of about 120 by 40 km,

shows a bright flow with a distinct dark channel in the

middle. The winding channel and serrated margins

suggest a fast-moving flow over shallow slopes. The

unusual bright color of the flow may indicate that it is

sulfur rather than silicate. The Emakong caldera-like

patera is the dark, heart-shaped feature on the bottom

part of the picture, about 55 km across. (NASA

PIA02518).


Io’s surface colors were initially interpreted as sulfur de-posits. The temperatures of the hot spots measured by theVoyager IRIS instrument were relatively lowdmostlyaround 400 Kdand could be consistent with either moltensulfur or silicates. Sulfur volcanism could produce tem-peratures up to w700 K, while basaltic lavas on Earthrange mostly from 1300 to 1450 K. Between the Voyagerobservations in 1979 and the Galileo observations thatstarted in 1996, several of Io’s hot spots were detected byground-based telescopes. Temperature measurements usinginfrared detectors mounted on telescopes showed highertemperatures than had been measured by Voyagerdup to1500 K were reported. These measurements are consistentwith silicate magmas but not with sulfur volcanism. Sulfurboils vigorously on Io’s surface at about 700 K, hence thesemeasurements strongly suggested silicate magmas. Galileoobservations, and those that have followed since, showedtemperatures consistent with silicate volcanism, although itis possible that sulfur volcanism may be happening in afew areas.

Galileo carried instruments more capable of detectingsmaller areas at higher temperatures, including the solidstate imaging (SSI) instrument, sensitive from w0.4 to1.0 mm and the NIMS (sensitive from 0.7 to 5.2 mm). Theseinstruments soon showed temperatures consistent with ba-salts, however, in 1997, measurements of the violent Pillaneruption showed surprising results. A. McEwen and col-leagues showed that temperatures exceeded 1500 K atseveral hot spots and, in the case of Pillan, for which NIMSand SSI data were combined, lava temperatures were about1800 K, suggesting ultramafic compositions. This expla-nation was supported by the discovery of the 0.9 mmspectral absorption feature discussed above, which isinterpreted as magnesium-rich orthopyroxene. However,more recent reanalysis of the Pillan data by L. Kesthelyiand colleagues suggest lower temperatures, about 1600 K,which could be due either to ultramafic lavas or to unusu-ally hot (superheated) basalts. Measurements from Cassiniand New Horizons at other hot spots showed temperaturesin the basaltic range.

FIGURE 43.9 Colorful volcanic deposits and variety of paterae on Io. The region known as ChaaceCamaxtli was imaged by Galileo in 2000. Chaac

patera is the green-floored depression on the upper left. To the right of Chaac is Balder patera with a distinct bright white floor. The Near-Infrared Mapping

Spectrometer instrument on Galileo detected revealed that the white is nearly pure SO2. The middle image shows orange and white materials presumed to

be sulfurous deposits, and what may be thin plates of crust that have broken off and rafted over a lava lake. Between these two patera are bright flows that

may be sulfur flows. The top right patera, Camaxtli, shows dark deposits indicative of silicates. (Bottom image is PIA02566).


The composition of Io’s lavas remains unresolved, it isagreed that the Pillan eruption temperature, the highestrecorded, could be due to superheated basalts or ultramafic(komatiite-like) lavas that had cooled by a couple of hun-dred degrees Kelvin at the time the measurements weremade. Because there are no direct measurements of thecomposition of Io’s lavas, the question remains open,particularly because small areas of high temperature lavaswould not have easily been detected given the spatial res-olution of the available observations.

6. STYLES OF VOLCANIC ACTIVITY

Io is a dynamic bodydthe volcanic plumes turn on and off,the outpourings of magma vary in temperature and extent,and sometimes large-scale changes are seen to occur in amatter of months, completely changing the appearance of aregion. Not all volcanic activity on Io produces plumes andplume deposits, or large lava flows. Eruptions can begenerally classified as effusive or explosion-dominated,and a classification scheme evolved from observations ofindividual volcanoes, much like the system used forterrestrial eruptions (e.g., Hawaiian, Strombolian). Themajor eruption types on Io are flow-dominated (alsoreferred to as “Promethean” after the Prometheus volcano,and accompanying plumes are referred to as “Prometheus-type”), explosion-dominated (or “Pillanian” after the Pillanvolcano, and accompanying plumes often referred to asPele-type plumes), and intra-patera (or “Lokian” after theLoki volcano). Two other types are much rarer, and mayinvolve the eruption of sulfur or SO2.

Flow-dominated eruptions originate either from fissuresor paterae (calderas) and produce extensive compound lavaflow fields, probably through repeated breakouts of lavasimilar to compound inflationary flows commonlyobserved on Hawaiian eruptions on Earth. The two beststudied flow fields on Io are Amirani and Prometheus(Figure 43.6). Observations taken about 3 months apartusing Galileo’s SSI and NIMS instruments showed freshbreakouts of lava in these two flow fields, and temperatureprofiles obtained by NIMS showed that the flow fields wereconsistent with largely insulated flows. A curious phe-nomenon observed first at Prometheus is the movement ofthe eruption plume. Between Voyager and Galileo obser-vations, the Prometheus plume source and associated de-posit had movedw90 km to the east, advancing with a lavaflow. Unlike most terrestrial plumes, these Io plumes do noterupt from the vent but from the distal edges of the flow, aresult of hot silicates bury the icy, SO2-rich substrate asproposed by S. Kieffer and colleagues. Smaller jets at theactive margins of the flow can also be seen in Galileo im-ages. Although there is no good terrestrial analogue, theplume-generating process on Io is similar to the formationof pseudocraters as lava advances over soggy ground. On

Io, this type of plume shuts off if the lava flow stopsadvancing, such as happened at the Maui plume and flow.The plumes of these flow-dominated eruptions are rela-tively small (w100 km high), optically dense, andcomposed mostly of dust and SO2 gas rather than S gas. Animage of the shadow of Prometheus taken by Galileo showsa dense vertical column of dust topped by a mushroom-shaped canopy. The central part may be populated by fineparticulates. The resulting plume deposit is white-gray,SO2-rich deposits, rather than the giant red deposits(S-rich) seen at Pele and other locations.

Explosion-dominated eruptions differ from the flow-dominated type in that most of the energy of the eruptionis directed into a short-lived, vigorous event that lasts daysto weeks. They can originate from either paterae or fissures,but are discreet events compared to the more or lesscontinuous flow-dominated eruptions such as Prometheus.Explosion-dominated eruptions can produce extensive py-roclastic deposits and lava flow fields, and typically a large(>200 km high) plume, thought to originate from theinteraction of silicate magma with sulfurous volatiles.These large plumes often create large red ring depositsaround the source regions (like at Tvashtar and Pele)though the Pillan event of 1997 (Figure 43.3) produced adark deposit as discussed above. These eruptions often alsoproduce extensive flow fields, but over a shorter period oftime than the flow-dominated eruptions. For example, the1997 Pillan eruption produced a flow field w3100 km2 inarea, formed in a relatively short period of time (estimatedrange 52e167 days). The estimated volumetric flow rate isw1740e7450 m3 s�1, similar to those calculated for the1783 Laki eruption. Williams and colleagues analyzed highresolution images (20e30 m/pixel) of the Pillan flow andsuggested that the exceptionally rough, disrupted, and platyupper surface was the result of rapidly emplaced flows,possibly turbulent. In contrast, volumetric eruption ratesestimated for the Amirani and Prometheus flow fields,based on observed surface changes, range from 50 to500 m3 s�1.

The most common type of eruption on Io is the inter-patera, which are confined within paterae (calderas).These eruptions occur with or without associated plumes,and are thought to be lava lakes in a number of cases. Lavalakes on Io were discovered from the thermal signatures ofhigh resolution NIMS images, which revealed hot marginsat the edges of paterae (Figure 43.10), similar to terrestriallava lakes where the crust is broken up as the crusted-oversurface of the lava lake hits the crater walls. Most of Io’sactive hot spots coincide with the interiors of paterae,suggesting that most of the lava resurfacing on Io isconfined within these depressions. Io’s most powerfulvolcano, Loki, produces eruptions of this type, which havebeen monitored from Earth as well as from spacecraft overseveral decades, revealing reoccurring, almost periodic


brightenings of Loki in the infrared. Surprisingly, theappearance of Loki did not significantly change in the yearsbetween Voyager and Galileo observations, despite manypowerful eruptions. The eruption mechanism has beeninterpreted as either repeated foundering of the crust of alava lake or, alternatively, repeated flooding of the calderafollowed by drainback, in a manner similar to the eruptionsfrom the East Pacific Rise on Earth. If either of thesemechanisms is taking place on other paterae, it must be on amuch reduced scale, not leading to the large fluctuations inpower output as happens at Loki. Other paterae appear tocontain lava lakes, however, the formation mechanism forpaterae themselves is not well understood, particularly theformation of “islands” in the interior of several of thepaterae such as Loki and Tupan. These “islands” are coldregions covered by SO2 frost that are partially orcompletely surrounded by lavas and appear to remainremarkably stable over decades.

It is possible that nonsilicate eruptions are taking placeon Io, but they are thought to be secondary in nature, due tothe remelting and remobilization of crustal sulfurous ma-terials by adjacent silicate heat sources. Examples include,bright flows surrounding smaller volume dark flows at Sobopatera and a yellow and white-gray, 290 km long flowextending from Emakong patera (Figure 43.9). The bestevidence for active sulfur volcanism, but not conclusive, isa weak hot spot detected by NIMS at Tsui Goab Fluctus, abright yellow flow field adjacent to a small, apparently

inactive shield volcano in the CulanneTohil region. Thetemperature measured using data from Galileo NIMS fallsin the range of molten sulfur (w530 K� 95 K) and thelocation coincided with the yellow flow, not dark silicates.

Another possible eruption type of Io is eruption ofliquid SO2 from the substrate. The NIMS instrumentdetected a strong signature of pure SO2 confined inside tothe floor of Balder patera (Figure 43.9), which visible im-ages show has a homogeneous white floor. It is unclear whythe floor should be so SO2-rich relative to surrounding re-gions and it is possible that an SO2 glacial-like flow mayhave erupted and flooded the floor of the patera. Anotherpossible location for this type of volcanism is near Tohilpatera. If these deposits were indeed emplaced by SO2

coming from the substrate, most would vaporize whenexposed to Io’s tenuous atmosphere but, given sufficientlylarge quantities, it is estimated that some could freeze andform a layer at the bottom of paterae.

7. GLOBAL GEOLOGY ANDDISTRIBUTION OF VOLCANISM

A global assessment of volcanic features on Io was ob-tained through production of the first complete globalgeologic map of Io, which was based on a series of com-bined Galileo and Voyager image mosaics that cover Io’ssurface at 1 km/pixel spatial resolution. A version of the

FIGURE 43.10 Thermal emission detected from Loki patera by Galileo’s Near-Infrared Mapping Spectrometer (NIMS) instrument. Top left is an image

of thew200 km across Loki patera from Voyager 1 (see Figure 43.5), bottom left is Loki imaged by Galileo’s Solid State Imaging instrument indicating

the area at the bottom and most active part of the patera imaged by NIMS in 2001. Top right shows a NIMS image at 2.5 mm. Bottom right is a one

temperature fit to the NIMS multiple wavelength data, showing that the hottest lavas are seen near the edges of the patera, indicating a lava lake. (Lopes

et al., 2004).


map is shown in Figure 43.11. At the global scale, Io’ssurface can be subdivided into four major classes of units,each with three subclasses: (1) paterae (caldera-like,volcano-tectonic depressions, whose floors are bright, in-termediate, or dark); (2) lava flow fields (which are bright,intermediate, or dark); (3) mountains (which are linear,undivided, or mottled); and (4) plains (which are white,yellow, or red-brown). A fifth unit, DD, occur in five colors(red, white, yellow, green, and black) and mantle the otherunits. Other units that are found on Io include layered plains(which serve as a transition between mountains and plainsunits) and tholi (which are positive relief features that arevolcanic domes or cones). Table 43.2 shows the percentcoverage of Io by these units in terms of surface area.

Plains units are thought to be composed of Io’s under-lying silicate crust overlain by sulfur and sulfur dioxide(SO2) emplaced via effusive and explosive volcanic pro-cesses. Mapping shows that geography constrains the dis-tribution of these plains units, in which red-brown plainsoccur >� 30� latitude, white plains occur mostly in theequatorial antijovian region (� 30�, 90�e230� W), andyellow plains everywhere else. The red-brown plains arethought to result from enhanced alteration of other unitsinduced by radiation coming in from the poles. White plainsare thought to be dominated by SO2þ contaminants, andtheir restriction to one region is suggestive of a regional coldtrap.Yellowplains are thought to be composed of cyclo-octal

sulfur (S8) that accumulates from sulfur-rich plume deposits.However, there are outliers of white, yellow, and red-brownplains in other areas of Io that appear to result from long-termaccumulation of white, yellow, and red DD, respectively.

Bright lava flow fields, possibly composed of sulfur-richcompounds, make up 30% more lava flow fields than dark,presumably silicate lava flow fields (56.5% vs 43.5%).Furthermore, only 18% of bright flow fields occur withinclose proximity (e.g., within 10 km) of dark flow fields,which would be expected if sulfur flows result from meltingof crustal sulfur from adjacent silicate magma chambers(i.e., secondary sulfur volcanism). These results suggestthat primary sulfur volcanism (i.e., eruptions from sulfurmagma chambers) could be an important component of Io’srecent volcanism. An unusual concentration of bright flowsatw45e75� N,w60e120� W could be indicative of moreextensive primary sulfur volcanism in the recent past.However, it remains unclear whether most bright flows arebright because they are composed of sulfur flows, orbecause they are composed of cold silicate flows covered insulfur-rich particles from plume fallout. Nevertheless,about 29% of all of Io’s flow fields are very bright or verydark flows (i.e., young and fresh), suggesting that activelava flow emplacement is occurring in less than one-third ofIo’s flow fields at the present time.

Global mapping identified 425 paterae (volcano-tectonic depressions), up from 417 previously identified.

FIGURE 43.11 Mollweide projection of the 1:15,000,000 global geologic map of Io, centered on the antijovian point (0�, 180� W), fromWilliams et al.

(2011). The areal distribution of map units is given in Table 43.2. Map unit abbreviations: pbw, white bright plains; pby, yellow bright plains; prb, red-brown

plains; pl, layered plains; fd, dark flows; fb, bright flows; fu, undivided flows; pfd, dark patera floors; pfb, bright patera floors; pfu, undivided patera floors; t,

tholi; ml, lineated mountains; mm, mottled mountains; mu, undivided mountains; pr, region of poor resolution (likely red-brown plains). Diffuse deposits

are not displayed in this version of the map.


Although paterae cover only 2.5% of Io’s surface, theycorrelate with 64% of all detected hot spots. In particular,45% of Io’s hot spots correlate with dark patera floors,which are interpreted to be lava lakes or lava pondscomposed of mafic to ultramafic materials, and demon-strate the dominance of active silicate volcanism to Io’sheat flow. Of the w3% of Io’s surface that consists of

mountains, only 0.1% are tholi, interpreted to be volcanicshields, domes, or cones.

About 18% of Io’s surface at any given time is coveredby DD, which are thought to be composed of ash and/orfrosts derived from condensed volcanic gases from Io’smany plume eruptions. About 47% of these DD (by area)are red, presumably deriving their color from condensed

TABLE 43.2 Distribution of Geologic Material Units as Percentage of Io’s Surface Area

Unit Label Unit Name Area (km2) Area (%)

Plains Deposits

prb Red-brown plains material 1.41� 107 33.4

pby Yellow bright plains material 7.68� 106 18.4

pbw White bright plains material 3.75� 106 8.9

pl Layered plains material 1.84� 106 4.4

Region of poor resolution (likely red-brown plains material) 7.20� 105 1.7

Total plains 2.81� 107 66.6

Mountain Deposits

ml Lineated mountains material 6.40� 105 1.5

mm Mottled mountains material 8.05� 104 0.2

mu Undivided mountain material 5.54� 105 1.3

Tholi (domes) 5.25� 104 0.1

Total mountain material 1.33� 106 3.1

Patera Floor Deposits

pfb Bright patera floor material 1.84� 105 0.4

pfd Dark patera floor material 1.93� 105 0.5

pfu Undivided patera floor material 6.75� 105 1.6

Total patera floor material 1.05� 106 2.5

Flow Deposits

fb Bright flow material 1.80� 106 4.3

fd Dark flow material 1.23� 106 2.9

fu Undivided flow material 8.70� 106 20.6

Total flow material 1.17� 107 27.8

Diffuse Deposits

dby Yellow bright diffuse material 8.76� 105 2.1

dbw White bright diffuse material 2.90� 106 6.9

dr Red diffuse material 3.61� 106 8.6

dd Dark diffuse material 2.68� 105 0.6

dg Green diffuse material 4.09� 103 0.01

Note: Diffuse deposits are superposed on all other materials, and cover 18.2% of Io’s surface. Io’s surface Area¼ 4.17� 107 km2.From Williams et al. (2011).


sulfur (S2) gas that recrystallizes to short-chain sulfur(S3eS4). In contrast, about 38% of DD are white, pre-sumably dominated by condensed SO2. The much greaterareal extent of gas-derived DD (redþwhite, 85% of allDD) compared to presumably pyroclast-bearing DD (dark(silicate tephra) þ yellow (sulfur-rich tephra), 15% of allDD) indicates that there is effective separation between thetransport of tephra and gas in many Ionian explosiveeruptions.

Mountains are the dominant structural landforms thatare visible on Io and are recognized as steep-sided edificesrising more than w1 km above the plains, covering w3%of the surface. Approximately 150 mountains have beenidentified and mapped. Io’s mountains typically riseapproximately 6 km in height; the highest (BoosauleMontes) rises >17 km above the surrounding plains. Gal-ileo images show that many mountains are partly orcompletely surrounded by plateaus, layered plains, anddebris aprons. Schenk and Bulmer suggested that a hori-zontal lithospheric compressive stress is generated becauseof Io’s rapid resurfacing rate, which results in uplift ofcrustal blocks via thrust faulting. Alternatively, McKinnonand others suggested that sustained reduction in Io’s vol-canic activity at local, regional, or global scales results inlithospheric heating that causes a large compressive stressat the base of the lithosphere. This stress causes, over time,fluctuating thermally induced stress as resurfacing rateswax and wane, leading to alternating episodes of tensileand compressive faulting. Such repeated episodes ofnormal and reverse faulting might produce coherent crustalblocks (mountains) that float in a matrix of highly disruptedmaterial, similar to the chaos terrain of Europa. Regardless,the asymmetrical shapes of Io’s mountains suggest upliftalong thrust faults, implying that compressional uplift isprobably the dominant mechanism. Several studies haveshown statistical evidence that about 40% of Ionian pateraeand mountains are in direct contact with each other, morethan random chance would allow (probability w0.1%).This result suggests a possible genetic link; perhaps themagma exploits the weakness in the lithosphere created bydeep orogenic faults. Although a global anticorrelationbetween the areas with somewhat higher concentrations ofobserved mountains and the areas with somewhat higherconcentrations of observed volcanic centers has beenidentified, the areas of low mountain concentration alsocorrelate with areas with poor imaging for detecting topo-graphic features. Therefore, it is not yet confirmed that thisanticorrelation is not an observational bias.

8. ATMOSPHERE AND TORUS

SO2 is not only common over the surface of Io but is alsothe major component of Io’s tenuous atmosphere. SO2 andsulfur are thought to be the volatiles that drive explosive

volcanism on Io, as H2O and CO2 (the dominant volatilesdriving explosive volcanism on Earth) are either absent orhighly depleted. SO2 in the atmosphere is supplied largelyby the plumes, but a lesser amount comes from the evap-oration of the SO2 frost deposits on the surface. Someplumes, like Pele, also supply sulfur (S3 and S4 have beendetected) in addition to SO2. Observations show that Io’satmosphere is patchy, the average density is low (about10�9 bar) but the density is greater at the location of theactive plumes. Io’s large volcanic plumes serve as an effi-cient delivery mechanism for gas and dust particles intoIo’s atmosphere and beyond. The dynamics of Io’s plumesare complex as models have to take into account the verylow atmospheric pressure and poorly constrained variables,such as the relative amount of dust and gas in the variousplume types. The plumes associated with eruption types,such as at Prometheus and Pele, have been discussed above,but the existence of a third type of plume has been inferredand later supported by Galileo observations. Observationsof Io’s atmosphere at millimeter wavelengths showed thatthe amount of SO2 gas detected would require about 30e50large plumes to be erupting at one time. Since the twoVoyager spacecraft had only detected nine, the question of“missing plumes” arose. Spacecraft observations detectplumes because sunlight is scattered by the small particlesof SO2 within the plumes. The existence of stealth plumeswas proposed, that is, plumes composed almost entirely ofgas, which would be hard to detect, but that modeling worksuggested were possible. These stealth plumes are thoughtto occur when a hot, molten silicate intrusion encounters areservoir of liquid SO2 buried at depths of at least 1.5 km.The high temperature and pressure at the subsurface(>1400 K, >40 bars) results in an eruption that is geyser-like, in which particles do not form through condensa-tion. The existence of stealth plumes has been supported byGalileo observations, including those from the camera. Theplume over the hot spot Acala is not seen in sunlight,presumably because of lack of particles in the plume, but itcan be seen in observations of Io in eclipse, in which the gasappears to glow (Figure 43.12).

Volcanic outgassing and SO2 evaporation continuouslyreplenishes Io’s atmosphere, but the moon’s low gravityallows some material to escape into space, estimated to beat a rate of 1 t s�1. The material forms a corona, neutralclouds, and the Io torus. The corona is a low density shellwithin Io’s gravitational pull populated by atoms andmolecules from Io. Neutral clouds of oxygen, sodium, andsulfur extend from the corona to distances of many timesthe radius of Jupiter. The cloud was discovered from ob-servations by the Pioneer spacecraft that flew by Jupiter in1973. The most easily observed of the clouds is the sodiumcloud, populated by sodium atoms escaping Io atw2.6 km s�1. Although neutral sodium atoms are moreeasily detected, Earth-based telescopic studies indicate that


sodium is a minor constituent of the neutral clouds. Theprimary neutral elements are oxygen and sulfur, which arethought to be dissociated from SO2. So far, sodium has notbeen detected either on Io’s surface or plumes, but its ex-istence on Io is known from the material in the neutralclouds.

Io’s orbit is deep within the Jovian magnetopause, andthe interaction of the materials escaping Io with Jupiter’smagnetic field is unique in the solar system. The Io plasmatorus is a doughnut-shaped trail about 143,000 km widealong Io’s orbital path, made up almost exclusively ofvarious charged states of sulfur and oxygen derived fromthe breakup of volcanic SO2 and S2. The ionized particlesare held within the torus by Jupiter’s magnetic field, in asimilar way to the mechanism that holds charged particlesin the Van Allen radiation belts around the Earth. It isthought that Io’s variable volcanic activity influences thedensity of the plasma torus and the strength of its in-teractions with the Jovian magnetic field.

A consequence of the complex interactions between Ioand Jupiter’s magnetosphere is the existence of an auroraon Io, first detected from observations taken by the Galileocamera while Io was in Jupiter’s shadow (Figure 43.12).The vivid colors detected (red, green, and blue) are causedby collisions between Io’s atmospheric gases and energeticparticles trapped in Jupiter’s magnetic field. Observationsof Io’s aurora were also made by the New Horizonsspacecraft in 2007, which observed Io as it went in and outof Jupiter’s shadow, showing variations in aurora brightnessand morphology.

The effects of Io’s volcanic activity are felt throughoutthe Jovian system, which is a unique interaction in our solarsystem. Although new in situ observations will only happenwhen another spacecraft gets to the Jovian system, progressis being made using telescopic observations. Variations inthe torus and neutral clouds, particularly of sodium andsulfur, can be measured from Earth and these are thought torespond to Io’s variations in volcanic activity, though thenature of the correlations is not simple and still notunderstood.

9. FUTURE STUDIES

NASA’sGalileo spacecraft completed its observations of Ioin 2002, and the only subsequent spacecraft observations ofIo occurred during the February 2007 flyby of NASA’s NewHorizons spacecraft on its way to Pluto. Earth-based tele-scopic observations of Io continue, but only intermittently.However, new research continues on the existing Io datasets which led, for example, to the publication of the firstcomplete global geologic map of Io in 2012. The globalmapping of Io’s volcanic materials and the resultinggeographic and spatial information is now feeding newstudies to investigate Io’s interior processes.

The community of Io scientists has produced a blueprintof future Io exploration, which was submitted in the form ofa pair of white papers for the 2011 Planetary ScienceDecadal Survey, Visions and Voyages, which includedrecommendations for future missions with Io as the majortarget. However, Earth-based telescopic observations andobservations from the Hubble Space Telescope continue toyield results that help our understanding of Io. The onlymission for which plans are underway to observe Io’s sur-face is the European Jupiter Icy Moons Explorer.

Many important questions remain open about the natureof Io’s volcanism. Perhaps the most fundamental is thecompositional range of the magma. If ultramafic magmasare indeed present, it raises the question of how theseprimitive magmas could persist on such a dynamicallyactive body where extreme differentiation should be ex-pected. The question of whether sulfur flows exist on Io isstill open and, if they do, are they emplaced due toremelting of sulfur deposits by hotter magmas? Highspatial resolution composition and thermal measurementsare needed to answer this question. Many others questionsabout how Io’s powerful volcanoes work remain. The for-mation mechanism of the most widespread volcanic land-form, the patera (caldera-like depressions) is notunderstood. Nearly half are found adjacent to mountains,indicating a genetic link between mountains and paterae.Many patera interiors have “islands,” which have persistedover decades despite constant and vigorous activity insidethe paterae. How the magma is supplied to replenisheruptions, which last for years or decades? Is there indeed a

FIGURE 43.12 Io in eclipse (in Jupiter’s shadow) showing glowing

gases at visible wavelengths (red, green, and violet). The colors, caused by

collisions between Io’s atmospheric gases and energetic charged particles

trapped in Jupiter’s magnetic field, had not previously been observed.

Bright blue glows mark the sites of dense plumes, and may be places where

Io is electrically connected to Jupiter. (NASA PIA01637).


magma ocean under the lithosphere? Most of all, studies ofIo’s volcanoes can provide insight into how volcanismbehaves in an extreme environment, perhaps similar toconditions in the early Earth.

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Hamilton, C.W., Beggan, C.D., Still, S., Beuthe, M., Lopes, R.M.C.,

Williams, D.A., Radebaugh, J., Wright, W., 2013. Spatial distribution

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Volcanism on Iovolcanism was continued from Earth, by astronomers using infrared detectors on telescopes. These observations showed that brightenings and fadings of hot spots occur,