Fast neutral particles as probes of the extent of Io's exosphere

The spatial extent of ion cyclotron waves at Io has been interpreted as requiring a multistep acceleration and transport process: exospheric ions are accelerated outward (relative to Jupiter) due to the corotation electric field, neutralized due to charge exchange in the surrounding exosphere, and then reionized after traveling far across magnetic field lines, at which point they generate the waves. The trajectories of the particles away from Io are sensitive to the location of their initial ionization. This paper examines the spatial distributions of fast neutrals produced under varying conditions in order to provide constraints on the possible structure and nature of the Io exosphere. While a rapid onset of cyclotron waves at a specific location around Io can be modeled with a single, point-source region of ions, such as might occur over a volcano, the regional extent of the waves suggests multiple or distributed sources.     

High-Resolution Keck Adaptive Optics Imaging of Violent Volcanic Activity on Io

Io, the innermost Galilean satellite of Jupiter, is a fascinating world. Data taken by Voyager and Galileo instruments have established that it is by far the most volcanic body in the Solar System and suggest that the nature of this volcanism could radically differ from volcanism on Earth. We report on near-IR observations taken in February 2001 from the Earth-based 10-m W. M. Keck II telescope using its adaptive optics system. After application of an appropriate deconvolution technique (MISTRAL), the resolution, ∼100 km on Io's disk, compares well with the best Galileo/NIMS resolution for global imaging and allows us for the first time to investigate the very nature of individual eruptions. On 19 February, we detected two volcanoes, Amirani and Tvashtar, with temperatures differing from the Galileo observations. On 20 February, we noticed a slight brightening near the Surt volcano. Two days later it had turned into an extremely bright volcanic outburst. The hot spot temperatures (>1400 K) are consistent with a basaltic eruption and, being lower limits, do not exclude an ultramafic eruption. These outburst data have been fitted with a silicate-cooling model, which indicates that this is a highly vigorous eruption with a highly dynamic emplacement mechanism, akin to fire-fountaining. Its integrated thermal output was close to the total estimated output of Io, making this the largest ionian thermal outburst yet witnessed.     

Ground-based observations of time variability in multiple active volcanoes on Io

Since before the beginning of the Galileo spacecraft’s Jupiter orbital tour, we have observed Io from the ground using NASA’s Infrared Telescope Facility (IRTF). We obtained images of Io in reflected sunlight and in-eclipse at 2.3, 3.5, and 4.8 μm. In addition, we have measured the 3.5 μm brightness of an eclipsed Io as it is occulted by Jupiter. These lightcurves enable us to measure the brightness and one-dimensional location of active volcanoes on the surface. During the Galileo era, two volcanoes were observed to be regularly active: Loki and either Kanehekili and/or Janus. At least 12 other active volcanoes were observed for shorter periods of time, including one distinguishable in images that include reflected sunlight. These data can be used to compare volcano types and test volcano eruption models, such as the lava lake model for Loki.     

Implications from Galileo Observations on the Interior Structure and Chemistry of the Galilean Satellites

Data from the recent gravity measurements by the Galileo mission are used to construct wide ranges of interior structure and composition models for the Galilean satellites of Jupiter. These models show that mantle densities of Io and Europa are consistent with an olivine-dominated mineralogy with the ratios of Mg to Fe components depending on mantle temperature for Io and on ice shell thickness for Europa. The mantle density and composition depend relatively little on core composition. The size of the core is largely determined by the core's composition with core radius increasing with the concentration of a light component such as sulfur. For Io, the range of possible core sizes is between 38 and 53% of the satellite's radius. For Europa, there is also a substantial effect of the thickness of the ice layer which is varied between 120 and 170 km on the core size. Core sizes are between 10 and 45% of Europa's radius. The core size of Ganymede ranges between one-quarter and one-third of the surface radius depending on its sulfur content and the thickness of the ice shell. A subset of the Ganymede models is consistent with an olivine-dominated mantle mineralogy. The thickness of the silicate mantle above the core varies between 900 and 1100 km. The outermost ice shell is about 900 km in thickness and is further subdivided by pressure-induced phase transitions into ice I, ice III, ice V, and ice VI layers. Callisto should be differentiated, albeit incompletely. It is proposed that this satellite was never molten at a large scale but differentiated through the convective gradual unmixing of the ice and the metal/rock component. Bulk iron-to-silicon ratios Fe/Si calculated for the inner pair of satellites, Io and Europa, are less than the CI carbonaceous chondrite value of 1.7±0.1, whereas ratios for the outer pair, Ganymede and Callisto, cover a broad range above the chondritic value. Although the ratios are uncertain, in particular for Ganymede and Callisto, the values are sufficiently distinct to suggest a difference in composition between these two pairs of satellites. This may indicate a difference in iron-silicon fractionation during the formation of both classes of satellites in the protojovian nebula.     

The Dual Sources of Io's Sodium Clouds

Io's sodium clouds result mostly from a combination of two atmospheric escape processes at Io. Neutralization of Na⁺ and/or NaX⁺ pickup ions produces the “stream” and the “jet” and results in a rectangular-shaped sodium nebula around Jupiter. Atmospheric sputtering of Na by plasma torus ions produces the “banana cloud” near Io and a diamond-shaped sodium nebula. Charge exchange of thermal Na⁺ with Na in Io's atmosphere does not appear to be a major atmospheric ejection process. The total ejection rate of sodium from Io varied from 3×10²⁶ to 25×10²⁶ atoms/s over seven years of observations. Our results provide further evidence that Io's atmospheric escape is driven from collisionally thick regions of the atmosphere rather than from the exosphere.     

Ridges and tidal stress on Io

Sets of ridges of uncertain origin are seen in twenty-nine high-resolution Galileo images, which sample seven locales on Io. These ridges are on the order of a few kilometers in length with a spacing of about a kilometer. Within each locale, the ridges have a consistent orientation, but the orientations vary from place to place. We investigate whether these ridges could be a result of tidal flexing of Io by comparing their orientations with the peak tidal stress orientations at the same locations. We find that ridges grouped near the equator are aligned either north-south or east-west, as are the predicted principal stress orientations there. It is not clear why particular groups run north-south and others east-west. The one set of ridges observed far from the equator (52° S) has an oblique azimuth, as do the tidal stresses at those latitudes. Therefore, all observed ridges have similar orientations to the tidal stress in their region. This correlation is consistent with the hypothesis that tidal flexing of Io plays an important role in ridge formation.     

Lava lakes on Io: observations of Io's volcanic activity from Galileo NIMS during the 2001 fly-bys

Galileo's Near-Infrared Mapping Spectrometer (NIMS) obtained its final observations of Io during the spacecraft's fly-bys in August (I31) and October 2001 (I32). We present a summary of the observations and results from these last two fly-bys, focusing on the distribution of thermal emission from Io's many volcanic regions that give insights into the eruption styles of individual hot spots. We include a compilation of hot spot data obtained from Galileo, Voyager, and ground-based observations. At least 152 active volcanic centers are now known on Io, 104 of which were discovered or confirmed by Galileo observations, including 23 from the I31 and I32 Io fly-by observations presented here. We modify the classification scheme of Keszthelyi et al. (2001, J. Geophys. Res. 106 (E12) 33 025-33 052) of Io eruption styles to include three primary types: promethean (lava flow fields emplaced as compound pahoehoe flows with small plumes <200 km high originating from flow fronts), pillanian (violent eruptions generally accompanied by large outbursts, >200 km high plumes and rapidly-emplaced flow fields), and a new style we call “lokian” that includes all eruptions confined within paterae with or without associated plume eruptions). Thermal maps of active paterae from NIMS data reveal hot edges that are characteristic of lava lakes. Comparisons with terrestrial analogs show that Io's lava lakes have thermal properties consistent with relatively inactive lava lakes. The majority of activity on Io, based on locations and longevity of hot spots, appears to be of this third type. This finding has implications for how Io is being resurfaced as our results imply that eruptions of lava are predominantly confined within paterae, thus making it unlikely that resurfacing is done primarily by extensive lava flows. Our conclusion is consistent with the findings of Geissler et al. (2004, Icarus, this issue) that plume eruptions and deposits, rather than the eruption of copious amounts of effusive lavas, are responsible for Io's high resurfacing rates. The origin and longevity of islands within ionian lava lakes remains enigmatic.     

A post-Galileo view of Io's interior

We present a self-consistent model for the interior of Io, taking the recent Galileo data into account. In this model, Io has a completely molten core, substantially molten mantle, and a very cold lithosphere. Heat from magmatic activity can mobilize volatile compounds such as SO₂ in the lithosphere, and the movement of such cryogenic fluids may be important in the formation of surface features including sapping scarps and paterae.     

Alkali and Chlorine Photochemistry in a Volcanically Driven Atmosphere on Io

Observations of the Io plasma torus and neutral clouds indicate that the extended ionian atmosphere must contain sodium, potassium, and chlorine in atomic and/or molecular form. Models that consider sublimation of pure sulfur dioxide frost as the sole mechanism for generating an atmosphere on Io cannot explain the presence of alkali and halogen species in the atmosphere—active volcanoes or surface sputtering must also be considered, or the alkali and halide species must be discharged along with the SO₂ as the frost sublimates. To determine how volcanic outgassing can affect the chemistry of Io's atmosphere, we have developed a one-dimensional photochemical model in which active volcanoes release a rich suite of S-, O-, Na-, K-, and Cl-bearing vapor and in which photolysis, chemical reactions, condensation, and vertical eddy and molecular diffusion affect the subsequent evolution of the volcanic gases. Observations of Pele plume constituents, along with thermochemical equilibrium calculations of the composition of volcanic gases exsolved from high-temperature silicate magmas on Io, are used to constrain the composition of the volcanic vapor. We find that NaCl, Na, Cl, KCl, and K will be the dominant alkali and chlorine gases in atmospheres generated from Pele-like plume eruptions on Io. Although the relative abundances of these species will depend on uncertain model parameters and initial conditions, these five species remain dominant for a wide variety of realistic conditions. Other sodium and chlorine molecules such as NaS, NaO, Na₂, NaS₂, NaO₂, NaOS, NaSO₂, SCl, ClO, Cl₂, S₂Cl, and SO₂Cl₂ will be only minor constituents in the ionian atmosphere because of their low volcanic emission rates and their efficient photochemical destruction mechanisms. Our modeling has implications for the general appearance, properties, and variability of the neutral sodium clouds and jets observed near Io. The neutral NaCl molecules present at high altitudes in atmosph eres generated by active volcanoes might provide the NaX⁺ ion needed to help explain the morphology of the high-velocity sodium “stream” feature observed near Io.     

Mapping of Io's thermal radiation by the Galileo photopolarimeter-radiometer (PPR) instrument

Between 1999 and 2002, the Galileo spacecraft made 6 close flybys of Io during which many observations of Io's thermal radiation were made with the photopolarimeter-radiometer (PPR). While the NIMS instrument could measure thermal emission from hot spots with T>200 K, PPR was the only Galileo instrument capable of mapping the lower temperatures of older, cooling lava flows, and the passive background. We tabulate all data taken by PPR of Io during these flybys and describe some scientific highlights revealed by the data. The data include almost complete coverage of Io at better than 250 km resolution, with extensive regional coverage at higher resolutions. We found a modest poleward drop in nighttime background temperatures and evidence of thermal inertia variations across the surface. Comparison of high spatial resolution temperature measurements with observed daytime SO₂ gas pressures on Io provides evidence for local cold trapping of SO₂ frost on scales smaller than the 60 km resolution of the PPR data. We also calculated the power output from several hot spots and estimated total global heat flow to be about 2.0-2.6 W m⁻². The low-latitude diurnal temperature variations for the regions between obvious hot spots are well matched by a laterally-inhomogeneous thermal model with less than 1 W m⁻² endogenic heat flow.     

The final Galileo SSI observations of Io: orbits G28-I33

We present the observations of Io acquired by the Solid State Imaging (SSI) experiment during the Galileo Millennium Mission (GMM) and the strategy we used to plan the exploration of Io. Despite Galileo's tight restrictions on data volume and downlink capability and several spacecraft and camera anomalies due to the intense radiation close to Jupiter, there were many successful SSI observations during GMM. Four giant, high-latitude plumes, including the largest plume ever observed on Io, were documented over a period of eight months; only faint evidence of such plumes had been seen since the Voyager 2 encounter, despite monitoring by Galileo during the previous five years. Moreover, the source of one of the plumes was Tvashtar Catena, demonstrating that a single site can exhibit remarkably diverse eruption styles—from a curtain of lava fountains, to extensive surface flows, and finally a ∼400 km high plume—over a relatively short period of time (∼13 months between orbits I25 and G29). Despite this substantial activity, no evidence of any truly new volcanic center was seen during the six years of Galileo observations. The recent observations also revealed details of mass wasting processes acting on Io. Slumping and landsliding dominate and occur in close proximity to each other, demonstrating spatial variation in material properties over distances of several kilometers. However, despite the ubiquitous evidence for mass wasting, the rate of volcanic resurfacing seems to dominate; the floors of paterae in proximity to mountains are generally free of debris. Finally, the highest resolution observations obtained during Galileo's final encounters with Io provided further evidence for a wide diversity of surface processes at work on Io.     

Io's volcanic control of Jupiter's extended neutral clouds

Dramatic changes in the brightness and shape of Jupiter's extended sodium nebula are found to be correlated with the infrared emission brightness of Io. Previous imaging and modeling studies have shown that varying appearances of the nebula correspond to changes in the rate and the type of loss mechanism for atmospheric escape from Io. Similarly, previous IR observational studies have assumed that enhancements in infrared emissions from Io correspond to increased levels of volcanic (lava flow) activity. In linking these processes observationally and statistically, we conclude that silicate volcanism on Io controls both the rate and the means by which sodium escapes from Io's atmosphere. During active periods, molecules containing sodium become an important transient in Io's upper atmosphere, and subsequent photochemistry and molecular-ion driven dynamics enhance the high speed sodium population, leading to the brightest nebulas observed. This is not the case during volcanically quiet times when omni-present atmospheric sputtering ejects sodium to form a modest, base-level nebula. Sodium's role as a “trace gas” of the more abundant species of sulfur (S) and oxygen (O) is less certain during volcanic episodes. While we suggest that volcanism must also affect the escape rates of S and O, and consequently their extended neutral clouds, the different roles played by lava and plume sources for non-sodium species are far too uncertain to make definitive comparisons at this time.     

Keck AO survey of Io global volcanic activity between 2 and 5 μm

We present in this Keck AO paper the first global high angular resolution observations of Io in three broadband near-infrared filters: Kc (2.3 μm), Lp (3.8 μm), and Ms (4.7 μm). The Keck AO observations are composed of 13 data sets taken during short time intervals spanning 10 nights in December, 2001. The MISTRAL deconvolution process, which is specifically aimed for planetary images, was applied to each image. The spatial resolution achieved with those ground-based observations is 150, 240, and 300 km in the Kc, Lp, and Ms band, respectively, making them similar in quality to most of the distant observations of the Galileo/NIMS instrument. Eleven images per filter were selected and stitched together after being deprojected to build a cylindrical map of the entire surface of the satellite. In Kc-band, surface albedo features, such as paterae (R>60 km) are easily identifiable. The Babbar region is characterized by extremely low albedo at 2.2 μm, and shows an absorption band at 0.9 μm in Galileo/SSI data. These suggest that this region is covered by dark silicate deposits, possibly made of orthopyroxene. In the Lp-Ms (3-5 μm) bands, the thermal emission from active centers is easily identified. We detected 26 hot spots in both broadband filters over the entire surface of the minor planet; two have never been seen active before, nine more are seen in the Ms band. We focused our study on the hot spots detected in both broadband filters. Using the measurements of their brightness, we derived the temperature and area covered by 100 brightness measurements. Loki displayed a relatively quiescent activity. Dazhbog, a new eruption detected by Galileo/NIMS in August 2001, is a major feature in our survey. We also point out the fading of Tvashtar volcanic activity after more than two years of energetic activity, and the presence of a hot, but small, active center at the location of Surt, possibly a remnant of its exceptional eruption detected in February 2001. Two new active centers, labeled F and V on our data, are detected. Using the best temperature and the surface area derived from the L and M band intensities, we calculated the thermal output of each active center. The most energetic hot spots are Loki and Dazhbog, representing respectively 36 and 19% of the total output calculated from a temperature fit of all hot spots (20.6×¹⁰¹² W). Based on the temperature derived from hot spots (∼400 K), our measurement can unambiguously identify the contribution to the heat flux from the silicate portion of the surface. Because the entire surface was observed, no extrapolation was required to calculate that flux. It is also important to note that we measured the brightness of the individual hot spots when they were located close to the Central Meridian. This minimizes the line-of-sight effect which does not follow strictly a classical cosine law. Finally, we argue that despite the widespread volcanic activity detected, Io was relatively quiescent in December 2001, with a minimum mean total output of 0.4-1.2 W m⁻². This output is at least a factor of two lower than those inferred from observations made at longer wavelengths and at different epochs.     

Observations and temperatures of Io's Pele Patera from Cassini and Galileo spacecraft images

Pele has been the most intense high-temperature hotspot on Io to be continuously active during the Galileo monitoring from 1996-2001. A suite of characteristics suggests that Pele is an active lava lake inside a volcanic depression. In 2000-2001, Pele was observed by two spacecraft, Cassini and Galileo. The Cassini observations revealed that Pele is variable in activity over timescales of minutes, typical of active lava lakes in Hawaii and Ethiopia. These observations also revealed that the short-wavelength thermal emission from Pele decreases with rotation of Io by a factor significantly greater than the cosine of the emission angle, and that the color temperature becomes more variable and hotter at high emission angles. This behavior suggests that a significant portion of the visible thermal emission from Pele comes from lava fountains within a topographically confined lava body. High spatial resolution, nightside images from a Galileo flyby in October 2001 revealed a large, relatively cool (<800 K) region, ringed by bright hotspots, and a central region of high thermal emission, which is hypothesized to be due to fountaining and convection in the lava lake. Images taken through different filters revealed color temperatures of 1500±80 K from Cassini ISS data and 1605±220 and 1420±100 K from small portions of Galileo SSI data. Such temperatures are near the upper limit for basaltic compositions. Given the limitations of deriving lava eruption temperature in the absence of in situ measurement, it is possible that Pele has lavas with ultramafic compositions. The long-lived, vigorous activity of what is most likely an actively overturning lava lake in Pele Patera indicates that there is a strong connection to a large, stable magma source region.     

The Io-controlled decametric radiation

The (magnetic) amplitude of the Alfvén waves emitted by Io is related to the growth rate of coherent cyclotron radiation. The growth rate is large only in the dense parts of the Jovian ionosphere. The amplitude varies as a function of sub-Io longitude. This together with the beaming of the cyclotron radiation is used to explain the observed emission pattern of Io-controlled decametric radiation from Jupiter.     

Photochemistry of a Volcanically Driven Atmosphere on Io: Sulfur and Oxygen Species from a Pele-Type Eruption

To determine how active volcanism might affect the standard picture of sulfur dioxide photochemistry on Io, we have developed a one-dimensional atmospheric model in which a variety of sulfur-, oxygen-, sodium-, potassium-, and chlorine-bearing volatiles are volcanically outgassed at Io's surface and then evolve due to photolysis, chemical kinetics, and diffusion. Thermochemical equilibrium calculations in combination with recent observations of gases in the Pele plume are used to help constrain the composition and physical properties of the exsolved volcanic vapors. Both thermochemical equilibrium calculations (Zolotov and Fegley 1999, Icarus141, 40-52) and the Pele plume observations of Spencer et al. (2000; Science288, 1208-1210) suggest that S₂ may be a common gas emitted in volcanic eruptions on Io. If so, our photochemical models indicate that the composition of Io's atmosphere could differ significantly from the case of an atmosphere in equilibrium with SO₂ frost. The major differences as they relate to oxygen and sulfur species are an increased abundance of S, S₂, S₃, S₄, SO, and S₂O and a decreased abundance of O and O₂ in the Pele-type volcanic models as compared with frost sublimation models. The high observed SO/SO₂ ratio on Io might reflect the importance of a contribution from volcanic SO rather than indicate low eddy diffusion coefficients in Io's atmosphere or low SO “sticking” probabilities at Io's surface; in that case, the SO/SO₂ ratio could be temporally and/or spatially variable as volcanic activity fluctuates. Many of the interesting volcanic species (e.g., S₂, S₃, S₄, and S₂O) are short lived and will be rapidly destroyed once the volcanic plumes shut off; condensation of these species near the source vent is also likely. The diffuse red deposits associated with active volcanic centers on Io may be caused by S₄ radicals that are created and temporarily preserved when sulfur vapor (predominantly S₂) condenses around the volcanic vent. Condensation of SO across the surface and, in particular, in the polar regions might also affect the surface spectral properties. We predict that the S/O ratio in the torus and neutral clouds might be correlated with volcanic activity—during periods when volcanic outgassing of S₂ (or other molecular sulfur vapors) is prevalent, we would expect the escape of sulfur to be enhanced relative to that of oxygen, and the S/O ratio in the torus and neutral clouds could be correspondingly increased.     

Composition of jovian dust stream particles

The Cassini spacecraft encountered Jupiter in late 2000. Within more than 1 AU of the gas giant the Cosmic Dust Analyser onboard the spacecraft recorded the first ever mass spectra of jovian stream particles. To determine the chemical composition of particles, a comprehensive statistical analysis of the dataset was performed. Our results imply that the vast majority (>95%) of the observed stream particles originate from the volcanic active jovian satellite Io from where they are sprinkled out far into the Solar System. Sodium chloride (NaCl) was identified as the major particle constituent, accompanied by sulphurous as well as potassium bearing components. This is in contrast to observations of gas in the ionian atmosphere, its co-rotating plasma torus, and the neutral cloud, where sulphur species are dominant while alkali and chlorine species are only minor components. Io has the largest active volcanoes of the Solar System with plumes reaching heights of more than 400 km above the moons surface. Our in situ measurements indicate that alkaline salt condensation of volcanic gases inside those plumes could be the dominant formation process for particles reaching the ionian exosphere.     

Galileo observations of volcanic plumes on Io

Io's volcanic plumes erupt in a dazzling variety of sizes, shapes, colors and opacities. In general, the plumes fall into two classes, representing distinct source gas temperatures. Most of the Galileo imaging observations were of the smaller, more numerous Prometheus-type plumes that are produced when hot flows of silicate lava impinge on volatile surface ices of SO₂. Few detections were made of the giant, Pele-type plumes that vent high temperature, sulfur-rich gases from the interior of Io; this was partly because of the insensitivity of Galileo's camera to ultraviolet wavelengths. Both gas and dust spout from plumes of each class. Favorably located gas plumes were detected during eclipse, when Io was in Jupiter's shadow. Dense dust columns were imaged in daylight above several Prometheus-type eruptions, reaching heights typically less than 100 km. Comparisons between eclipse observations, sunlit images, and the record of surface changes show that these optically thick dust columns are much smaller in stature than the corresponding gas plumes but are adequate to produce the observed surface deposits. Mie scattering calculations suggest that these conspicuous dust plumes are made up of coarse grained “ash” particles with radii on the order of 100 nm, and total masses on the order of 10⁶ kg per plume. Long exposure images of Thor in sunlight show a faint outer envelope apparently populated by particles small enough to be carried along with the gas flow, perhaps formed by condensation of sulfurous “snowflakes” as suggested by the plasma instrumentation aboard Galileo as it flew through Thor's plume [Frank, L.A., Paterson, W.R., 2002. J. Geophys. Res. (Space Phys.) 107, doi:10.1029/2002JA009240. 31-1]. If so, the total mass of these fine, nearly invisible particles may be comparable to the mass of the gas, and could account for much of Io's rapid resurfacing.