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.     

Filamentation of volcanic plumes on the Jovian satellite Io

Volcanic plumes on the Jovian satellite Io may be a visible manifestation of a plasma-arc discharge phenomenon. The amount of power in the plasma arc (10¹¹ W) is not enough to account for all the energy dissipated by the volcanoes. However, once a volcano is initiated by tidal and geologic processes, the dynamics of the volcanic plumes can be influenced by the plasma arcs. As initially pointed out by Gold (1979), plasma arcs are expected because of 10⁶ A currents and 400 kV potentials generated by the flow past Io of a torus of relatively dense magnetospheric plasma. We utilize our experience with laboratory plasma arcs to investigate the plume dynamics. The filamentation in the plume of the volcano Prometheus and its cross-sectional shape is quantitatively consistent with theories developed from laboratory observation.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988.     

Simulation of gas dynamics and radiation in volcanic plumes on Io

Modeling results of volcanic plumes on Jupiter’s moon Io are presented. Two types of low density axisymmetric SO₂ plume flows are modeled using the direct simulation Monte Carlo (DSMC) method. Thermal radiation from all three vibrational bands and overall rotational lines of SO₂ molecules is modeled. A high resolution computation of the flow in the vicinity of the vent was obtained by multidomain sequential calculation to improve the modeling of the radiation signature. The radiation features are examined both by calculating infrared emission spectra along different lines-of-sight through the plume and with the DSMC modeled emission images of the whole flow field. It is found that most of the radiation originates in the vicinity of the vent, and non-LTE (non-local-thermodynamic equilibrium) cooling by SO₂ rotation lines exceeds cooling in the v₂ vibrational band at high altitude.In addition to the general shape of the plumes, the calculated average SO₂ column density (∼10¹⁶ cm⁻²) over a Pele-type plume and the related frost-deposition ring structure (at R ∼ 500 km from the vent) are in agreement with observations. These comparisons partially validate the modeling. It is suggested that an observation with spatial resolution of less than 30 km is needed to measure the large spatial variation of SO₂ near a Pele-type plume center. It is also found that an influx of 1.1 × 10²⁹ SO₂ s⁻¹ (or 1.1 × 10⁴ kg s⁻¹) is sufficient to reproduce the observed SO₂ column density at Pele. The simulation results also show some interesting features such as a multiple bounce shock structure around Prometheus-type plumes and the frost depletion by plume-induced erosion on the sunlit side of Io. The model predicts the existence of a canopy shock, a ballistic region inside the Pele-type plume, and the negligible effect of surface heating by plume emission.     

Near-surface flow of volcanic gases on Io

Significant near-surface flow of gas several hundred kilometers from Pele (Plume 1) on Io is indicated by a series of bright, elongate albedo markings. Particles produced at small, local vents are apparently carried as much as 70 km farther “downwind” from Pele. The gas densities and velocities necessary to suspend 0.1 to 10 μm particles at such a distance imply mass flow rates of 10⁷ to 10⁹ g/sec. Such flow rates are consistent with other estimates of mass transport by the plume. The large flow rates so far from the source allow an estimate of the rate of resurfacing of Io by lava flows and pyroclastics that is independent of estimates based on meteorite flux or on the amount of solids carried within the plumes themselves.     

Alkali and halogen chemistry in volcanic gases on Io

We use chemical equilibrium calculations to model the speciation of alkalis and halogens in volcanic gases emitted on Io. The calculations cover wide temperature (500-2000 K) and pressure (10⁻⁶ to 10⁺¹ bars) ranges, which overlap the nominal conditions at Pele (T=1760 K, P=0.01 bars). About 230 compounds of 11 elements (O, S, Li, Na, K, Rb, Cs, F, Cl, Br, I) are considered. The elemental abundances for O, S, Na, K, and Cl are based upon observations. CI chondritic elemental abundances relative to sulfur are used for the other alkalis and halogens (as yet unobserved on Io). We predict the major alkali species in Pele-like volcanic gases and the percentage distribution of each alkali are LiCl (73%), LiF (27%); NaCl (81%), Na (16%), NaF (3%); KCl (91%), K (5%), KF (4%); RbCl (93%), Rb (4%), RbF (3%); CsCl (92%), CsF (6%), Cs (2%). Likewise the major halogen species and the percentage distribution of each halogen are NaF (88%), KF (10%), LiF (2%); NaCl (89%), KCl (11%); NaBr (89%), KBr (10%), Br (1%); NaI (61%), I (30%), KI (9%). We predict the major halogen condensates and their condensation temperatures at P=0.01 bar are NaF (1115 K), LiF (970 K); NaCl (1050 K), KCl (950 K); KBr (750 K), RbBr (730 K), CsBr (645 K); and solid I₂ (200 K). We also model disequilibrium chemistry of the alkalis and halogens in the volcanic plume. Based on this work and our prior modeling for Na, K, and Cl in a volcanic plume, we predict the major loss processes for the alkali halide gases are photolysis and/or condensation onto grains. Their estimated photochemical lifetimes range from a few minutes for alkali iodides to a few hours for alkali fluorides. Condensation is apparently the only loss process for elemental iodine. On the basis of elemental abundances and photochemical lifetimes, we recommend searching for gaseous KCl, NaF, LiF, LiCl, RbF, RbCl, CsF, and CsCl around volcanic vents during eruptions. Based on abundance considerations and observations of brown dwarfs we also recommend a search of Io's extended atmosphere and the Io plasma torus for neutral and ionized Li, Cs, Rb, and F.     

Io: Heat flow from dark volcanic fields

Dark flow fields on the jovian satellite Io are evidence of current or recent volcanic activity. We have examined the darkest volcanic fields and quantified their thermal emission in order to assess their contribution to Io’s total heat flow. Loki Patera, the largest single source of heat flow on Io, is a convenient point of reference. We find that dark volcanic fields are more common in the hemisphere opposite Loki Patera and this large scale concentration is manifested as a maximum in the longitudinal distribution (near ∼200 °W), consistent with USGS global geologic mapping results. In spite of their relatively cool temperatures, dark volcanic fields contribute almost as much to Io’s heat flow as Loki Patera itself because of their larger areal extent. As a group, dark volcanic fields provide an asymmetric component of ∼5% of Io’s global heat flow or ∼5 × 10¹² W.     

Speckle imaging of volcanic hotspots on Io with the Keck telescope

Using speckle imaging techniques on the 10-m W.M. Keck I telescope, we observed near-infrared emission at 2.2 μm from volcanic hotspots on Io in July-August 1998. Using several hundreds of short-exposure images we reconstructed diffraction-limited images of Io on each of three nights. We measured the positions of individual hotspots to ±0.004″ or better, corresponding to a relative positional error of ∼20 km on Io's surface. The sensitivity of normal ground-based images of Io is limited by confusion between overlapping sources; by resolving these multiple points we detected up to 17 distinct hotspots, the largest number ever seen in a single image.During the month-long span of our 1998 observations, several events occurred. Loki was at the end of a long brightening, and we observed it to fade in flux by a factor of 2.8 over the course of one month. At the 3-sigma level we see evidence that Loki's position shifts by ∼100 km. This suggests that the brightening may not have been located at the “primary” Loki emission center but at a different source within the Loki caldera. We also see a bright transient source near Loki. Among many other sources we detect a dim source on the limb of Io at the latitude of Pele; this source is consistent with 2.7% of the thermal emission from the Pele volcano complex being scattered by the Pele plume, which would be the first detection of a plume through scattered infrared hotspot emission.     

Numerical modeling of ionian volcanic plumes with entrained particulates

Volcanic plumes on Jupiter's moon Io are modeled using the direct simulation Monte Carlo (DSMC) method. The modeled volcanic vent is interpreted as a “virtual” vent. A parametric study of the “virtual” vent gas temperature and velocity is performed to constrain the gas properties at the vent by observables, particularly the plume height and the surrounding condensate deposition ring radius. Also, the flow of refractory nano-size particulates entrained in the gas is modeled with “overlay” techniques which assume that the background gas flow is not altered by the particulates. The column density along the tangential line-of-sight and the shadow cast by the plume are calculated and compared with Voyager and Galileo images. The parametric study indicates that it is possible to obtain a unique solution for the vent temperature and velocity for a large plume like Pele. However, for a small Prometheus-type plume, several different possible combinations of vent temperature and velocity result in both the same shock height and peak deposition ring radius. Pele and Prometheus plume particulates are examined in detail. Encouraging matches with observations are obtained for each plume by varying both the gas and particle parameters. The calculated tangential gas column density of Pele agrees with that obtained from HST observations. An upper limit on the size of particles that track the gas flow well is found to be ∼10 nm, consistent with Voyager observations of Loki. While it is certainly possible for the plumes to contain refractory dust or pyroclastic particles, especially in the vent vicinity, we find that the conditions are favorable for SO₂ condensation into particles away from the vent vicinity for Prometheus. The shadow cast by Prometheus as seen in Galileo images is also reproduced by our simulation. A time averaged frost deposition profile is calculated for Prometheus in an effort to explain the multiple ring structure observed around the source region. However, this multiple ring structure may be better explained by the calculated deposition of entrained particles. The possibility of forming a dust cloud on Io is examined and, based on a lack of any such observed clouds, a subsolar frost temperature of less than 118 K is suggested.     

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.     

The geothermal gradient of Io: Consequences for lithosphere structure and volcanic eruptive activity

We solve numerically the equations describing the transfer of heat through the lithosphere of Io by a mixture of conduction and volcanic advection as proposed by O’Reilly and Davies (O’Reilly, T.C., Davies, G.F. [1981]. Geophys. Res. Lett. 8, 313-316), removing the requirement that average material properties must be used. As expected, the dominance of advective heat transfer by volcanic eruptions means that Io’s geothermal gradient well away from volcanic centres is very small, of order 1 K km⁻¹. This result is independent of any reasonable assumptions about the radiogenic heating rate in the lithosphere. The lithosphere temperature does not increase greatly above the surface temperature until the base of the lithosphere is approached, except in limited areas around shallow magma bodies. As a consequence, solid volatile sulphur compounds mobilized by volcanic processes and re-deposited on the surface of Io commonly remain solid until they reach great depths as they are progressively buried by ongoing activity. For current estimates of the volcanic heat transfer rate, melting of SO₂ does not begin until a depth of ∼20 km and sulphur remains solid to a depth of ∼26 km in a 30 km thick lithosphere. Rising magmas can incorporate fluids from these deep sulphur compound aquifers, and we quantify the major influence that this can have on the bulk density of the magma and hence the resulting possible intrusion and eruption styles.     

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.     

Two dynamical classes of Centaurs

The Centaurs are a transient population of small bodies in the outer Solar System whose orbits are strongly chaotic. These objects typically suffer significant changes of orbital parameters on timescales of a few thousand years, and their orbital evolution exhibits two types of behaviors described qualitatively as random walk and resonance-sticking. We have analyzed the chaotic behavior of the known Centaurs. Our analysis has revealed that the two types of chaotic evolution are quantitatively distinguishable: (1) the random walk type behavior is well described by so-called generalized diffusion in which the rms deviation of the semimajor axis grows with time t as ∼t^H, with Hurst exponent H in the range 0.22-0.95, however (2) orbital evolution dominated by intermittent resonance sticking, with sudden jumps from one mean motion resonance to another, has poorly defined H. We further find that these two types of behavior are correlated with Centaur dynamical lifetime: most Centaurs whose dynamical lifetime is less than ∼22 Myr exhibit generalized diffusion, whereas most Centaurs of longer dynamical lifetimes exhibit intermittent resonance sticking. We also find that Centaurs in the diffusing class are likely to evolve into Jupiter-family comets during their dynamical lifetimes, while those in the resonance-hopping class do not.     

Two luminosity classes of quasars

Observations of quasars suggest that compact quasars with flat radio spectra belong to the class of dwarf galaxies and extended quasars with steep spectra to the one of bright galaxies. This can be seen also in the associations of quasars with clusters of galaxies, compact quasars occurring in nearby clusters and extended quasars in distant clusters.     

Subsidence of topography on Io

Topographic features on Io tend to subside because their underlying roots are softened and eroded by contact with hot mantle. This can be offset by crustal thickening, due primarily to ongoing volcanism, but observations suggest that this is ≲1 cm year⁻¹ at current topographic highs. Since crustal thinning occurs at ∼50 cm year⁻¹ if the underlying material is a pure magma ocean, we conclude that Io has no global magma ocean. Viscosities in excess of ∼10¹⁰ P are implied for Io's interior.     

Sulfur volcanism on Io

In February 2003, March 2003 and January 2004 Pele plume transmission spectra were obtained during Jupiter transit with Hubble's Space Telescope Imaging Spectrograph (STIS), using the 0.1″ wide slit and the G230LB grating. The STIS spectra covered the 2100-3100 Å wavelength regions and extended spatially along Io's limb encompassing the region directly above and northward of the vent of the Pele volcano. The S₂ and SO₂ absorption signatures evident in these data indicate that the gas signature at Pele was temporally variable, and that an S₂ absorption signature was present ∼12° from the Pele vent near 6±5 S and 264±15 W, suggesting the presence of another S₂ bearing plume on Io. Contemporaneous with the spectral data, UV and visible-wavelength images of the plume were obtained in reflected sunlight with the Advanced Camera for Surveys (ACS) prior to Jupiter transit. The dust scattering recorded in these data provide an additional qualitative measure of plume activity on Io, indicating that the degree of dust scattering over Pele varied as a function of the date of observation, and that there were several other dust bearing plumes active during the observations. We present constraints on the composition and variability of the gas abundances of the Pele plume as well as the plumes detected by ACS and recorded within the STIS data, as a function of time.     

Silicon tetrafluoride on Io

Silicon tetrafluoride (SiF₄) is observed in terrestrial volcanic gases and is predicted to be the major F-bearing species in low-temperature volcanic gases on Io [Schaefer, L., Fegley Jr., B., 2005b. Alkali and halogen chemistry in volcanic gases on Io. Icarus 173, 454-468]. SiF₄ gas is also a potential indicator of silica-rich crust on Io. We used F/S ratios in terrestrial and extraterrestrial basalts, and gas/lava enrichment factors for F and S measured at terrestrial volcanoes to calculate equilibrium SiF₄/SO₂ ratios in volcanic gases on Io. We conclude that SiF₄ can be produced at levels comparable to the observed NaCl/SO₂ gas ratio. We also considered potential loss processes for SiF₄ in volcanic plumes and in Io's atmosphere including ion-molecule reactions, electron chemistry, photochemistry, reactions with the major atmospheric constituents, and condensation. Photochemical destruction (t_chem ∼266 days) and/or condensation as Na₂SiF₆ (s) appear to be the major sinks for SiF₄. We recommend searching for SiF₄ with infrared spectroscopy using its 9.7 μm band as done on Earth.     

Io: A volcanic flow model for the hot spot emission spectrum and a thermostatic mechanism

The hot spots of Io are modeled as a steady state of active areas at 600°K, continuing creation of new lava flows and calderas, cooling off of recent flows and calderas, and the cessation of radiation of old flows and calderas from the accumulation of insulation added by resurfacing. There are three adjustable parameters in this model: the area of active sources at 600°K, the rate of production of new area that is cooling, and the temperature of cessation of emission as the result of resurfacing. The resurfacing rate sets constraints on this last parameter. The emission spectrum computed with reasonable values for these parameters is an excellent match to the spectrum from recent observations. A thermostatic mechanism is described whereby the volcanic activity is turned on for a long period of time and is then turned off for a nearly equal period. As a result the presently observed internal heat flow of ～ 1.5 W m^?2 may be as much as twice the rate of production of internal heat. Thus the restrictions placed on theories of tidal dissipation by the observed heat flow may be partially relieved.     

Stability of sulfur slopes on Io

The mechanical properties of elemental sulfur are such that the upper crust of Io cannot be primarily sulfur. For heat flows in the range 100–1000 ergs cm^?2, sec^?1, sulfur becomes ductile within several hundred meters of the surface and would prevent the formation of calderas with depths greater than this. However, the one caldera for which precise depth data are available is 2 km deep, and this value may be typical. A study of the mechanical equilibrium of simple slopes shows that the depth to the zone of rapid ductile flow strongly controls the maximum heights for sulfur slopes. Sulfur scarps with heights greater than 1 km will fail for all heat flows greater than 180 ergs cm^?2 sec^?1 and slope angles greater than 22.5°. The observed relief on Io is inconsistent with that anticipated for a predominantly sulfur crust. However, a silicate crust with several percent sulfur included satisfies both the mechanical constraints and the observed presence of sulfur on Io.     

Effects of Io ejecta on Europa

We examine the effects of Io ejecta on the surface and environment of Europa. We find that the observed sulfur on the trailing side of Europa, when interpreted as a deposit in equilibrium between implanation of, and sputtering by, corotating Io ejecta, implies a slow loss of material from Europa by sputtering. From this we infer that the spectrum of particles sputtered from water ice is soft. The quantity of observed sulfur and its confinement to the trailing hemisphere appear to exclude significant implantation and sputtering by energetic heavy ions. We also conclude that the contribution from Europa to the magnetospheric plasma (even at Europa itself) is negligible compared to the matter ejected from Io.     

No sulfur flows on Io

Physical and chemical properties of elemental sulfur are incompatible with the suggestion that the colored flows associated with volcanoes on Io are quenched unstable allotropes of sulfur. Either the volcanic flows are not sulfur, or some mechanism other than quenching is required to produce colored forms of sulfur in them. The properties of sulfur are unsuited to the production and survival of colored unstable allotropes on Io. The color of this object is probably due to some other material, possibly iron compounds.