Flyby Of Io Research Articles

Io's Near‐Field Alfvén Wings and Local Electron Beams Inferred From Juno/Waves

AbstractJuno conducted two close Io flybys on 30 December 2023 and 03 February 2024, both at a minimum altitude of 1,500 km. Filamentary structures in the electric and magnetic field spectra indicate Juno crossed the Alfvén wing, the magnetic structure connecting Io to Jupiter's polar ionosphere. We show that the first pass took Juno diametrically through the northern Alfvén wing, while the second pass had Juno graze the southern Alfvén wing boundary, enabling extended measurements of the transition region between Io's vicinity and the Jovian magnetosphere. Of note, evidence of local electron beams is inferred from whistler‐mode emissions. We demonstrate that their energies are sub‐keV, are sourced from Io's ionosphere or local torus, and are part of a distributed current system connecting Io to Jupiter. Finally, upper hybrid resonances indicate electron densities are significantly elevated in Io's polar region (∼28,000 cm−3) compared to the local Io torus (∼2,000 cm−3).

Ion Precipitation Into Io's Poles Driven by a Strong Sub‐Alfvénic Interaction

AbstractJuno performed two close flybys of Io and found enhanced field‐aligned proton fluxes are absorbed by Io. These protons are absorbed at mass input rates comparable to previous estimates for hydrogen losses from Io, hence Jupiter is likely the source of hydrogen at Io. The conditions necessary for this to occur are: (a) formation of Alfvén waves at Io, (b) wave‐particle coupling to energize protons, (c) anti‐planetward transport of ions due to the magnetic mirror force and/or parallel acceleration, and (d) strong sub‐Alfvénic interaction slowing the flow connected to Io's fluxtube allowing for sufficient travel time for energized ions to transit to Io. The derived slowdown of ≤12% the upstream value is linked to filamentation within the Alfvén wing. This mechanism is likely operating at all strongly interacting satellites and provides an avenue to transfer material from a planetary body to its satellites, including exoplanets and brown dwarfs.

Energetic Charged Particle Measurements During Juno's Two Close Io Flybys

AbstractOn days 2023‐364 and 2024‐034, the Juno spacecraft made close passages of Jupiter's moon Io, at altitudes of about 1,500 km. Data obtained from the first flyby, when the spacecraft was on magnetic field lines connected to both Jupiter and Io, revealed deep flux decreases. In addition, Juno's energetic particle detectors observed tens to hundreds of keV electron and proton beams. Such beams could be generated near Jupiter on field lines associated with Io. The second encounter occurred in the plasma wake and a more modest flux decrease was observed. Furthermore, data from both encounters suggest a spatially extensive decrease in >1 MeV electrons that includes regions inward of Io's orbit. In the immediate vicinity of Io, signatures of absorption likely dominate the data whereas diffusion and wave‐particle interactions are expected to be needed to understand MeV electron data in the wider spatial region around Io.

Energetic Proton Losses Reveal Io's Extended and Longitudinally Asymmetrical Atmosphere

AbstractAlong the I24, I27, and I31 flybys of Io (1999–2001), the Energetic Particle Detector (EPD) onboard the Galileo spacecraft observed localized regions of energetic protons losses (155–1,250 keV). Using back‐tracking particle simulations combined with a prescribed atmospheric distribution and a magnetohydrodynamics (MHD) model of the plasma/atmosphere interaction, we investigate the possible causes of these depletions. We focus on a limited region within two Io radii, which is dominated by Io's SO2 atmosphere. Our results show that charge exchange of protons with the SO2 atmosphere, absorption by the surface and the configuration of the electromagnetic field contribute to the observed proton depletion along the Galileo flybys. In the 155–240 keV energy range, charge exchange is either a major or the dominant loss process, depending on the flyby altitude. In the 540–1,250 keV range, as the charge exchange cross sections are small, the observed decrease of the proton flux is attributed to absorption by the surface and the perturbed electromagnetic fields, which divert the protons away from the detector. From a comparison between the modeled losses and the data we find indications of an extended atmosphere on the day/downstream side of Io, a lack of atmospheric collapse on the night/upstream side as well as a more global extended atmospheric component (>1 Io radius). Our results demonstrate that observations and modeling of proton depletion around the moon constitute an important tool to constrain the electromagnetic field configuration around Io and the radial and longitudinal atmospheric distribution, which is still poorly understood.

Joint analysis of JUICE and Europa Clipper tracking data to study the Jovian system ephemerides and dissipative parameters

Context. Jupiter and its moons form a complex dynamical system that includes several coupling dynamics at different frequencies. In particular the Laplace resonance is fundamental to maintaining the energy dissipation that sustain Io’s volcanic activity and Europa’s subsurface ocean; studying its stability is thus crucial for characterizing the potential habitability of these moons. The origin and evolution of the Laplace resonance is driven by the strong tidal interactions between Jupiter and its Galilean moons, and the future planetary exploration missions JUICE and Europa Clipper could bring new light to this unsolved mechanism. During the Jupiter tours of both missions and JUICE’s Ganymede orbital phase, two-way radiometric range and Doppler data will be acquired between Earth ground stations and the spacecraft, which will be processed to recover the static and time-varying gravity field of the moons. Moreover, range and Doppler data will improve the orbit accuracy of the moons, providing precise measurements of Jupiter’s tidal parameters. Aims. This work presents a covariance analysis of the joint orbit determination of JUICE and Europa Clipper, aimed at quantifying the expected uncertainties on the main parameters that characterize the dynamics of the Jupiter system. Methods. We simulated radio science data from JUICE and Clipper missions under conservative noise assumptions, using a multi-arc approach to estimate the ephemerides and dissipation in the system. Results. Even though JUICE and Europa Clipper will not perform flybys of Io, the strong coupling with Europa and Ganymede will allow an improvement of our knowledge of the Jupiter-Io dissipation parameters thanks to JUICE and Europa Clipper radiometric data. Moreover, the expected uncertainty in Jupiter’s dissipation at the frequency of Callisto could unveil a potential resonance locking mechanism between Jupiter and Callisto.

FIRE - Flyby of Io with Repeat Encounters: A conceptual design for a New Frontiers mission to Io

A conceptual design is presented for a low complexity, heritage-based flyby mission to Io, Jupiter’s innermost Galilean satellite and the most volcanically active body in the Solar System. The design addresses the 2011 Decadal Survey’s recommendation for a New Frontiers class mission to Io and is based upon the result of the June 2012 NASA-JPL Planetary Science Summer School. A science payload is proposed to investigate the link between the structure of Io’s interior, its volcanic activity, its surface composition, and its tectonics. A study of Io’s atmospheric processes and Io’s role in the Jovian magnetosphere is also planned. The instrument suite includes a visible/near-IR imager, a magnetic field and plasma suite, a dust analyzer, and a gimbaled high gain antenna to perform radio science. Payload activity and spacecraft operations would be powered by three Advanced Stirling Radioisotope Generators (ASRG). The primary mission includes 10 flybys with close-encounter altitudes as low as 100km. The mission risks are mitigated by ensuring that relevant components are radiation tolerant and by using redundancy and flight-proven parts in the design. The spacecraft would be launched on an Atlas V rocket with a delta-v of 1.3km/s. Three gravity assists (Venus, Earth, Earth) would be used to reach the Jupiter system in a 6-year cruise. The resulting concept demonstrates the rich scientific return of a flyby mission to Io.

A novel technology for measuring the eruption temperature of silicate lavas with remote sensing: Application to Io and other planets

The highly variable and unpredictable magnitude of thermal emission from evolving volcanic eruptions creates saturation problems for remote sensing instruments observing eruptions on Earth and on Io, the highly volcanic moon of Jupiter. For Io, it is desirable to determine the temperature of the erupting lavas as this measurement constrains lava composition. One method of determining lava eruption temperature is by measuring radiant flux at two or more wavelengths and fitting a blackbody thermal emission function. Only certain styles of volcanic activity are suitable, those where detectable thermal emission is from a restricted range of surface temperatures close to the eruption temperature. Volcanic processes where this occurs include large lava fountains; smaller lava fountains common in active lava lakes; and lava tube skylights. Problems that must be overcome to obtain usable data are: (1) the rapid cooling of the lava between data acquisitions at different wavelengths, (2) the unknown magnitude of thermal emission, which has often led to detector saturation, and (3) thermal emission changing on a shorter timescale than the observation integration time. We can overcome these problems by using the HOT-BIRD detector and a novel, advanced digital readout circuit (D-ROIC) to achieve a wide dynamic range sufficient to image lava on Io without saturating. We have created an instrument model that allows various instrument parameters (including mirror diameter, number of signal splits, exposure duration, filter band pass, and optics transmissivity) to be tested to determine the detectability of thermal sources on Io's surface. We find that a short-wavelength infrared instrument on an Io flyby mission can achieve simultaneity of observations by splitting the incoming signal for all relevant eruption processes and still obtain data fast enough to remove uncertainties in accurate determination of the highest lava surface temperatures. Observations at 1 and 1.5μm are sufficient for this purpose. Even with a ten-way beam split, instrument throughput generates acceptable signal-to-noise values. Accurate constraints on lava eruption temperature are also possible with a visible wavelength detector so long as data at different wavelengths are obtained simultaneously and integration time is very short. Fast integration times are important for examining the thermal emission from lava tube skylights due to rapidly changing viewing geometry during close flybys. The technology described here is applicable to instruments observing terrestrial volcanism and for investigating proposed volcanic activity on Venus, where lava composition is not known.

Broad-search algorithms for the spacecraft trajectory design of Callisto–Ganymede–Io triple flyby sequences from 2024 to 2040, Part II: Lambert pathfinding and trajectory solutions

Triple-satellite-aided capture employs gravity-assist flybys of three of the Galilean moons of Jupiter in order to decrease the amount of ΔV required to capture a spacecraft into Jupiter orbit. Similarly, triple flybys can be used within a Jupiter satellite tour to rapidly modify the orbital parameters of a Jovicentric orbit, or to increase the number of science flybys. In order to provide a nearly comprehensive search of the solution space of Callisto–Ganymede–Io triple flybys from 2024 to 2040, a third-order, Chebyshev's method variant of the p-iteration solution to Lambert's problem is paired with a second-order, Newton–Raphson method, time of flight iteration solution to the V∞-matching problem. The iterative solutions of these problems provide the orbital parameters of the Callisto–Ganymede transfer, the Ganymede flyby, and the Ganymede–Io transfer, but the characteristics of the Callisto and Io flybys are unconstrained, so they are permitted to vary in order to produce an even larger number of trajectory solutions. The vast amount of solution data is searched to find the best triple-satellite-aided capture window between 2024 and 2040.

Multiple-satellite-aided capture trajectories at Jupiter using the Laplace resonance

Satellite-aided capture is a mission design concept used to reduce the delta-v required to capture into a planetary orbit. The technique employs close flybys of a massive moon to reduce the energy of the planet-centered orbit. A sequence of close flybys of two or more of the Galilean moons of Jupiter may further decrease the delta-v cost of Jupiter orbit insertion. A Ganymede-Io sequence can save 207 m/s of delta-v over a single Io flyby. A phase angle analysis based on the Laplace resonance is used to find triple-satellite-aided capture sequences involving Io, Europa, and Ganymede. Additionally, the near-resonance of Callisto and Ganymede is used to find triple-satellite-aided capture sequences involving Callisto, Ganymede, and another moon. A combination of these techniques is used to find quadruple-satellite-aided capture sequences that involve gravity-assists of all four Galilean moons. These sequences can save a significant amount of delta-v and have the potential to benefit both NASA’s Jupiter Europa orbiter mission and ESA’s Jupiter Ganymede orbiter mission.

Europa Orbiter tour design with Io gravity assists

For a Europa Orbiter mission, the strategy for designing Jovian System tours that include Io flybys differs significantly from schemes developed for previous versions of the mission. Assuming that the closest approach distance of the incoming hyperbola at Jupiter is below the orbit of Io, then an Io gravity assist gives the greatest energy pump-down for the least decrease in perijove radius. Using Io to help capture the spacecraft can increase the savings in Jupiter orbit insertion ΔV over a Ganymede-aided capture. The tour design is guided by Tisserand graphs overlaid with a simple and accurate radiation model so that tours including Io flybys can maintain an acceptable radiation dosage. While Io flybys increase the duration of tours that are ultimately bound for Europa, they offer ΔV savings and greater scientific return, including the possibility of flying through the plume of one of Io's volcanoes.

Electrodynamic Tether at Jupiter—II: Fast Moon Tour After Capture

An electrodynamic bare-tether mission to Jupiter, following the capture of a spacecraft (SC) into an equatorial highly elliptical orbit with perijove at about 1.3 times the Jovian radius, is discussed. Repeated applications of the propellantless Lorentz drag on a spinning tether, at the perijove vicinity, can progressively lower the apojove at constant perijove, for a tour of Galilean moons. Electrical energy is generated and stored as the SC moves from an orbit at 1 : 1 resonance with a moon, down to resonance with the next moon; switching tether current off, stored power is then used as the SC makes a number of flybys of each moon. Radiation dose is calculated throughout the mission, during capture, flybys and moves between moons. The tour mission is limited by both power needs and accumulated dose. The three-stage apojove lowering down to Ganymede, Io , and Europa resonances would total less than 14 weeks, while 4 Ganymede, 20 Europa, and 16 Io flybys would add up to 18 weeks, with the entire mission taking just over seven months and the accumulated radiation dose keeping under 3 Mrad (Si) at 10-mm Al shield thickness.

Are Io's Alfvén wings filamented? Galileo observations

The interaction of Io with the Jovian magnetosphere generates auroral and radio emissions. The underlying electron acceleration process is not understood and few observations exist to constrain the theoretical models. The source of energy for the electron acceleration is in all likelihood supplied from the Alfvén wings that stretch out from both poles of Io into the two Jovian hemispheres. The form of the current system associated with the Alfvén wings has been disputed, some suggesting that the greatly slowed flow near Io implies that a steady current loop links Io to Jupiter's ionosphere, others arguing that the return waves appear only downstream of Io and others suggesting that both forms develop. Given the finite inclination of the Alfvén wings implied by the finite value of the Alfvén Mach number and the strong reflection that occurs at the boundary of the Io torus, we argue that no steady current loop can be invoked between Io and Jupiter's ionosphere. However, the energetics of the auroral and radio emissions imply that most of the energy in the Alfvén wings is transformed into electron acceleration at high-latitudes, that is, outside the Io torus. The dilemma then is to understand how a large fraction of the power penetrates the reflecting boundary. We present data from Galileo's multiple flybys of Io that suggest that the coupling with the Jovian ionosphere is mediated by filamentary Alfvén wings associated with electromagnetic waves propagating out of the torus. In particular, we report on the systematic observation, within the cross-section of Io's Alfvén wings and in their immediate vicinity, of intense electromagnetic waves at frequencies up to several times the proton gyrofrequency. We interpret these “high-frequency/small-scale” waves as the signature of a strong filamentation/fragmentation of the Alfvén wings before they reflect off of the sharp boundary gradient of the Io torus. As a consequence, we suggest that most of the primary energy is converted into “high-frequency/small-scale” electromagnetic waves that can propagate out from the torus toward Jupiter's ionosphere. Reaching high-latitudes, these waves are able to accelerate electrons to almost relativistic speeds.

Geology and activity around volcanoes on Io from the analysis of NIMS spectral images

The last two successful flybys of Io by Galileo in 2001 (orbits I31, I32) allowed the Near Infrared Mapping Spectrometer to enrich its collection of IR spectral image cubes of the satellite. These data cover hemispheric portions of Io, several volcanic centers as well as their surroundings with a spatial resolution ranging from 2 to 93 km pixel −1. They map thermal emission from the hot-spots and the distribution of solid SO 2 in the 1.0–4.7 μm spectral range. We obtain maps of SO 2 abundance and granularity from the NIMS data using the method of Douté et al. (2002, Icarus 158, 460–482). The maps are correlated to distinguish four different physical units that indicate zones of SO 2 condensation, metamorphism and sublimation. We relate these information with visible images from Galileo's Solid State Imaging System and with detailed mapping of the thermal emission produced by Io's surface. Our principal goal is to understand the mechanisms controlling how lava, pyroclastics and gas are emitted by different types of volcanoes and how these products evolve. The 800 km diameter white ring of fallout created by a violent “Pillanian” eruption during summer of 2001 is at least partly composed of solid SO 2 and has enriched preexisting regional deposits. Orange materials have been recently or are currently emplaced 240 km south from the main eruption site, possibly as sulfur flows. A similar event may have taken place in the past at Ababinili Patera (12.5° N, 142° W). Carefull study of SO 2 maps covering the Emakong region also suggests that sulfur forms the bright channel-fed flow emerging from the south eastern side of the caldera. Within the main caldera of Tvashtar Catena completely cooled patches of crust exist. Elsewhere, the caldera is still cooling from previous episodes of flooding. We confirm that Amirani emits constantly large amount of SO 2 gas by interaction of fresh lava with the volatiles of the underlying plains. Nevertheless SO 2 frost is not the major component of the bright white ring seen in the SSI images. Over the whole Gish Bar region, SO 2 frost seems barely stable and is constantly regenerated. The stability increases along gray filamentary structures which could be faults filled with materials having peculiar thermal properties. Northwest of Gish Bar Patera, a localized bright deposit shows an unusual spectral signature potentially indicative of H 2O molecules forming ice crystals or being trapped in a nonidentified matrix. The Chaac region may present a thickened old crust reducing the geothermal flux to levels lower than 0.5 W m −2 and thus creating a cold trap for SO 2. Looking at the abundance and degree of metamorphose of SO 2, we establish the relative age of different flows and ejecta for the Sobo Fluctus. Finally the assumption that the white patches in visible images indicate SO 2 rich deposits is once again challenged. In the Camaxtli region we identify a topographically controlled compact white deposit showing only moderate SO 2 abundance. In contrast, we detect two spots of quite pure SO 2 ice on the gray flanks of Emakong. Furthermore, the close association of fumarolic SO 2 and red S 2 already noted for several volcanic centers is observed at Tupan.

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 SO 2 gas pressures on Io provides evidence for local cold trapping of SO 2 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.

Project Galileo: surviving Io, meeting Cassini

Galileo has completed the long awaited Io flybys of the Galileo Europa Mission and the Galileo Millennium Mission, and is now pumping up its apojove in each succeeding orbit in preparation for the chance to perform a dual spacecraft observation of the Jupiter System. Making up for the missed opportunity at the beginning of Galileo's primary mission, a total of three close passes by Io have given us an up close and personal look at this highly volcanic body. The results presented include new and exciting information about Io's interactions with Jupiter's magnetosphere, its interior structure, and amazing volcanoes. Galileo Millennium Mission results from a magnetic field study of Europa, and its likely impact on the question of a European ocean are provided. In addition the engineering challenges of operating the spacecraft during the past year are explored, as well as a brief examination of future challenges. The spacecraft has now experienced more than three times the radiation dose it was designed to, and this exposure is contributing to a number of spacecraft problems and concerns. The engineering data being generated by these continuing radiation-induced anomalies, and the ability of the spacecraft to survive these doses, will prove invaluable to designers of future spacecraft to Jupiter and its satellites.

Interpretation of Galileo's Io plasma and field observations: I0, I24, and I27 flybys and close polar passes

We interpret plasma and magnetic field observations taken during the Galileo spacecraft Io flybys I0 in December 1995, I24 in October 1999, and I27 in February 2000, and we give predictions for a generic pass over Io's northern pole with a three‐dimensional, two‐fluid plasma model. We show that all previous field and plasma observations by the Galileo spacecraft can be explained without the assumption of an internal magnetic field of Io in contrast to claims by Kivelson et al. [1996a, 1996b] and Khurana et al. [1997]. We are also able to reproduce both the magnitude and the structure of the double‐peak magnetic field signature of the I0 flyby. The origin of this structure can be attributed to diamagnetic and inertia currents. Observations by a polar flyby should answer Io's internal magnetic field question decisively. We also study the effect of different neutral atmosphere models on Io's electrodynamic interaction. Our analysis suggests that Io's atmosphere is longitudinally asymmetric with the scale height on the upstream side smaller than on the downstream side due to the drag force of the flowing plasma on Io's atmosphere. We also show how the Hall effect in Io's ionosphere generates rotated Alfvén wings. In addition, the high‐energy electrons observed by Williams et al. [1996, 1999] and Frank and Paterson [1999] might play an important role for the formation of Io's downstream wake.

Io’s Volcanos: Latest Results from Galileo and from the Earth

AbstractThree recent close flybys of Io by the Galileo spacecraft, and new observations from the Hubble Space Telescope and ground-based telescopes, have greatly advanced our understanding of Jupiter’s volcanic moon Io. Io’s volcanos are much hotter than previously suspected, perhaps requiring exotic silicate magma compositions. Despite much new data, Io’s largest volcano, Loki, is still poorly understood. New data on Io’s plumes suggest the existence of two types of plumes: primary plumes, relatively rich in S2 gas, which are emitted where magma first reaches the surface, and secondary plumes, more SO2 rich, which result from interaction of lava flows with a volatile-rich substrate.

Introduction to the Special Section: Geology and Geophysics of Io

Io is about the size and density of Earth's Moon, but it has a dramatically different thermal state and geologic history. Tidal heating leads to intense volcanic and tectonic activity, so Io is an accelerated experiment in planetary geology and geophysics. Here we can witness volcanic activity on very large scales and at very high temperatures, perhaps providing insights into the early histories of the terrestrial planets.Io has always been a high‐priority objective of Galileo, but her proximity to Jupiter's intense radiation belt has meant saving the best for last. An Io flyby occurred during Jupiter orbit insertion in December 1995, but a tape recorder anomaly led to cancellation of remote‐sensing observations. No further close flybys of Io were planned until the final two orbits (I24 and I25) of the Galileo Europa Mission (GEM) late in 1999 in order to minimize radiation damage to spacecraft and instrument electronics. I24 was an exciting encounter, with a spacecraft safing event and recovery just hours before Io closest approach. Most of the Io data were acquired, but not without problems. The majority of images from the Solid State Imager (SSI) were acquired in a mode that failed due to the cumulative radiation damage, producing garbled images, although reconstruction has produced some useful pictures [Keszthelyi et al., this issue]. Also, the grating of the Near‐Infrared Mapping Spectrometer (NIMS) became stuck leading to loss of spectral sampling [Lopes et al., this issue]. These instrument problems were mitigated by the use of different observation modes or strategies in subsequent orbits. The 125 polar pass included another safing event, this time eliminating the recording of the highest‐resolution observations. However, the medium‐resolution observations were of excellent quality and provided a surprise look at a new fissure eruption at Tvashtar Catena. GEM was followed by the Galileo Millennium Mission (GMM) including another close Io flyby in orbit I27, a fully successful encounter. Three additional Io flybys are anticipated in August 2001 (I31), October 2001 (I32), and January 2002 (I33). The groundtracks for all six Io encounters are shown in Plate 1.

A survey of Io's volcanism by adaptive optics observations in the 3.8‐μm thermal band (1996–1999)

In this study, we analyze a series of images of Io obtained with the European Southern Observatory adaptive optics system (Adaptive Optics Near Infrared System, ADONIS) at 3.8 μm from 1996 to 1999, with particular emphasis on the observations carried out in late 1999 in support of the Galileo flybys of Io. Use of a new myopic deconvolution method, Myopic Iterative Step Preserving Algorithm (MISTRAL), especially designed for planetary objects, significantly improves the quality and the reliability of the reconstructed images. Using a simulation of artificial images of Io, we estimate the extent to which this algorithm is able to prevent noise amplification, to better restore sharp edges, and to preserve the initial photometry. Once this algorithm has been applied to our data and the solar‐reflected background has been subtracted, we have 89 images of Io available for search and temporal survey of the hot spots over 4 years. In most cases, the data, acquired during two consecutive nights, provide an almost global view of the surface of Io. We identify 20 hot spots for which we determine the coordinates and the brightness at 3.8 μm (as a function of time). More than half of the hot spots were detected on all the images and were considered as persistent. Particular emphasis has been put on the brightest two hot spots, Loki and Pele, for which we obtain a center‐to‐limb variation curve (a cosine curve for Loki and a curve decreasing slightly faster than a cosine law for Pele with possible physical interpretations). The temporal variations of their activity have been derived over these 4 years and, for Loki, compared with the results of other ground‐based surveys. The variability of the other, fainter, sources has also been derived but with less accuracy. We have also identified a few new hot spots, some of which are transient, such as a diffuse emission seen in September and November 1999 at the location of the upcoming Tvashtar outburst. The data are based on observations (58.F‐1003, 63.S‐0260, and 64.S‐0661) collected at the European Southern Observatory in La Silla, Chile.

Evaluation of sulfur flow emplacement on Io from Galileo data and numerical modeling

Galileo images of bright lava flows surrounding Emakong Patera have been analyzed and numerical modeling has been performed to assess whether these flows could have resulted from the emplacement of sulfur lavas on Io. Images from the solid‐state imaging (SSI) camera show that these bright, white to yellow Emakong flows are up to 370 km long and contain dark, sinuous features that are interpreted to be lava conduits, ∼300–500 m wide and >100 km long. Near‐Infrared Mapping Spectrometer (NIMS) thermal emission data yield a color temperature estimate of 344 K ± 60 K (≤131°C) within the Emakong caldera. We suggest that these bright flows likely resulted from either sulfur lavas or silicate lavas that have undergone extensive cooling, pyroclastic mantling, and/or alteration with bright sulfurous materials. The Emakong bright flows have estimated volumes of ∼250–350 km3, similar to some of the smaller Columbia River Basalt flows. If the Emakong flows did result from effusive sulfur eruptions, then they are orders of magnitude greater in volume than any terrestrial sulfur flows. Our numerical modeling results show that sulfur lavas on Io could have been emplaced as turbulent flows, which were capable of traveling tens to hundreds of kilometers, consistent with the predictions of Sagan [1979] and Fink et al. [1983]. Our modeled flow distances are also consistent with the measured lengths of the Emakong channels and bright flows. Modeled thermal erosion rates are ∼1–4 m d−1 for flows erupted at ∼140–180°C, which are consistent with the melting rates of Kieffer et al. [2000]. The Emakong channels could be thermal erosional in nature; however, the morphologic signatures of thermal erosion channels cannot be discerned from available images. There are planned Galileo flybys of Io in 2001 which provide excellent opportunities to obtain high‐resolution morphologic and color data of Emakong Patera. Such observations could, along with further modeling, provide additional information to better constrain whether sulfur lavas produced the Emakong flows.