The polar contribution to the heat flow of Io

Polar brightness temperatures on Io are higher than expected for any passive surface heated by absorbed sunlight. This discrepancy implies large scale volcanic activity from which we derive a new component of Io's heat flow. We present a ‘Three Component’ thermal background, infrared emission model for Io that includes active polar regions. The widespread polar activity contributes an additional ∼0.6 W m⁻² to Io's heat flow budget above the ∼2.5 W m⁻² from thermal anomalies. Our estimate for Io's global average heat flow increases to ∼3±1 W m⁻² and ∼1.3±0.4×10¹⁴ watts total.     

Volcanic activity at Tvashtar Catena, Io

Galileo's Solid State Imager (SSI) observed Tvashtar Catena four times between November 1999 and October 2001, providing a unique look at a distinctive high latitude volcanic complex on Io. The first observation (orbit I25, November 1999) resolved, for the first time, an active extraterrestrial fissure eruption; the brightness temperature was at least 1300 K. The second observation (orbit I27, February 2000) showed a large (∼500 km²) region with many, small, hot, regions of active lava. The third observation was taken in conjunction with Cassini imaging in December 2000 and showed a Pele-like, annular plume deposit. The Cassini images revealed an ∼400 km high Pele-type plume above Tvashtar Catena. The final Galileo SSI observation of Tvashtar (orbit I32, October 2001), revealed that obvious (to SSI) activity had ceased, although data from Galileo's Near Infrared Mapping Spectrometer (NIMS) indicated that there was still significant thermal emission from the Tvashtar region. In this paper, we primarily analyze the style of eruption during orbit I27 (February 2000). Comparison with a lava flow cooling model indicates that the behavior of the Tvashtar eruption during I27 does not match that of simple advancing lava flows. Instead, it may be an active lava lake or a complex set of lava flows with episodic, overlapping eruptions. The highest reliable color temperature is ∼1300 K. Although higher temperatures cannot be ruled out, they do not need to be invoked to fit the observed data. The total power output from the active lavas in February 2000 was at least ¹⁰¹¹ W.     

Io: Heat flow from dark paterae

Dark paterae on the jovian satellite Io are evidence of recent volcanic activity. Some paterae appear to be entirely filled with dark volcanic material, while others have only partially darkened floors. Dark paterae have area and heat flow longitudinal distributions that are bimodal as well as anti-correlated with the longitudinal distribution of mountains on Io at a global scale. As part of our study of Io’s total heat flow, we have examined the darkest paterae and quantified their thermal emission in order to assess their contribution. This is the first time that the areas of the dark material in these paterae have been measured with such precision and correlated with their thermal emission. Dark paterae yield a significantly larger contribution to Io’s heat flow than dark volcanic fields. Dark paterae (including Loki Patera) yield at least ∼4 × 10¹³ W or ∼40% of Io’s total heat flow. In comparison, dark flow fields yield ∼10¹³ W or ∼10% of Io’s total heat flow. Of the total heat loss from dark paterae, Loki Patera alone yields ∼10¹³ W or ∼10% of Io’s total thermal emission.     

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.     

关于木卫一上火山羽的气体动力学结构

本文从气体动力学分析出发,解释了“旅行者1号”观察到的木卫一上火山羽的特点,如伞状外形和丝状结构等,给出了激波和羽顶之间流场的解析形式。     

Volcanism on Io: New insights from global geologic mapping

We produced the first complete, 1:15 M-scale global geologic map of Jupiter’s moon Io, based on a set of monochrome and color Galileo-Voyager image mosaics produced at a spatial resolution of 1 km/pixel. The surface of Io was mapped into 19 units based on albedo, color and surface morphology, and is subdivided as follows: plains (65.8% of surface), lava flow fields (28.5%), mountains (3.2%), and patera floors (2.5%). Diffuse deposits (DD) that mantle the other units cover ∼18% of Io’s surface, and are distributed as follows: red (8.6% of surface), white (6.9%), yellow (2.1%), black (0.6%), and green (∼0.01%). Analyses of the geographical and areal distribution of these units yield a number of results, summarized below. (1) The distribution of plains units of different colors is generally geographically constrained: Red-brown plains occur >±30° latitude, and are thought to result from enhanced alteration of other units induced by radiation coming in from the poles. White plains (possibly dominated by SO₂ + contaminants) occur mostly in the equatorial antijovian region (±30°, 90-230°W), possibly indicative of a regional cold trap. Outliers of white, yellow, and red-brown plains in other regions may result from long-term accumulation of white, yellow, and red diffuse deposits, respectively. (2) Bright (possibly sulfur-rich) flow fields make up 30% more lava flow fields than dark (presumably silicate) flows (56.5% vs. 43.5%), and only 18% of bright flow fields occur within 10 km of dark flow fields. These results suggest that secondary sulfurous volcanism (where a bright-dark association is expected) could be responsible for only a fraction of Io’s recent bright flows, and that primary sulfur-rich effusions could be an important component of Io’s recent volcanism. An unusual concentration of bright flows at ∼45-75°N, ∼60-120°W could be indicative of more extensive primary sulfurous volcanism in the recent past. However, it remains unclear whether most bright flows are bright because they are sulfur flows, or because they are cold silicate flows covered in sulfur-rich particles from plume fallout. (3) We mapped 425 paterae (volcano-tectonic depressions), up from 417 previously identified by Radebaugh et al. (Radebaugh, J., Keszthelyi, L.P., McEwen, A.S., Turtle, E.P., Jaeger, W., Milazzo, M. [2001]. J. Geophys. Res. 106, 33005-33020). Although these features cover only 2.5% of Io’s surface, they correspond to 64% of all detected hot spots; 45% of all hot spots are associated with the freshest dark patera floors, reflecting the importance of active silicate volcanism to Io’s heat flow. (4) Mountains cover only ∼3% of the surface, although the transition from mountains to plains is gradational with the available imagery. 49% of all mountains are lineated and presumably layered, showing evidence of linear structures supportive of a tectonic origin. In contrast, only 6% of visible mountains are mottled (showing hummocks indicative of mass wasting) and 4% are tholi (domes or shields), consistent with a volcanic origin. (5) Initial analyses of the geographic distributions of map units show no significant longitudinal variation in the quantity of Io’s mountains or paterae, in contrast to earlier studies. This is because we use the area of mountain and patera materials as opposed to the number of structures, and our result suggests that the previously proposed anti-correlation of mountains and paterae (Schenk, P., Hargitai, H., Wilson, R., McEwen, A., Thomas, P. [2001]. J. Geophys. Res. 106, 33201-33222; Kirchoff, M.R., McKinnon, W.B., Schenk, P.M. [2011]. Earth Planet. Sci. Lett. 301, 22-30) is more complex than previously thought. There is also a slight decrease in surface area of lava flows toward the poles of Io, perhaps indicative of variations in volcanic activity. (6) The freshest bright and dark flows make up about 29% of all of Io’s flow fields, suggesting active emplacement is occurring in less than a third of Io’s visible lava fields. (7) About 47% of Io’s diffuse deposits (by area) are red, presumably deriving their color from condensed sulfur gas, and ∼38% are white, presumably dominated by condensed SO₂. The much greater areal extent of gas-derived diffuse deposits (red + white, 85%) compared to presumably pyroclast-bearing diffuse deposits (dark (silicate tephra) + yellow (sulfur-rich tephra), 15%) indicates that there is effective separation between the transport of tephra and gas in many Ionian explosive eruptions. Future improvements in the geologic mapping of Io can be obtained via (a) investigating the relationships between different color/material units that are geographically and temporally associated, (b) better analysis of the temporal variations in the map units, and (c) additional high-resolution images (spatial resolutions ∼200 m/pixel or better). These improvements would be greatly facilitated by new data, which could be obtained by future missions.     

Formation of mountains on Io: Variable volcanism and thermal stresses

Thermal stresses are potentially important drivers of Io's tectonics and mountain building. It has been hypothesized that sustained local or regional shut down of heat-pipe volcanism on Io could lead to deep crustal heating and large compressive stresses [McKinnon, W.B., Schenk, P.M., Dombard, A.J., 2001. Geology 29, 103-106]. Such large stresses would then be relieved by thrust faulting and uplifting of crustal blocks, producing mountains like those observed on Io. Here we analyze the tectonic consequences of the heat-pipe model in detail, considering both the initial thermal stress state of a basalt or peridotite crust created by heat-pipe volcanism, and relative roles of subsidence stresses (due to burial of preexisting layers) and thermal stresses arising from variable volcanism and changes in crustal (∼lithosphere) thickness. We limit the magnitude of the potential subsidence stresses in our study, because the magnitude of subsidence stresses can be quite large, if not dominant. Results indicate that for a fixed crustal thickness, the region of failure and faulting moves closer to the surface as eruption rate decreases and time increases. When the crust melts at its base as volcanism decreases (as might occur under steady state tidal heating), resulting in crustal thinning, the region of failure is brought even closer to the surface. Naturally, when compressive, subsidence stresses are included, the vertical extent of crust in brittle failure thickens to include most of the lithosphere. In contrast, increases in eruption rate cause the extent of the region in compressional failure to decrease and be driven very deep in the crust (in the absence of sufficient subsidence stress). Therefore, regions of declining volcanism are more likely to produce mountains, whereas regions of extensive or increasing volcanism are less likely to do so. This is consistent with the observation of a global anticorrelation between mountains and volcanic centers on Io. Finally, we find that the choice of crustal composition/rheology (dry basalt vs. dry peridotite) has little effect on our results implying that basalt, peridotite and komatiite are all similarly “stiff” in the Io environment.     

Geologic mapping of the Amirani-Gish Bar region of Io: Implications for the global geologic mapping of Io

We produced the first geologic map of the Amirani-Gish Bar region of Io, the last of four regional maps generated from Galileo mission data. The Amirani-Gish Bar region has five primary types of geologic materials: plains, mountains, patera floors, flows, and diffuse deposits. The flows and patera floors are thought to be compositionally similar, but are subdivided based on interpretations regarding their emplacement environments and mechanisms. Our mapping shows that volcanic activity in the Amirani-Gish Bar region is dominated by the Amirani Eruptive Center (AEC), now recognized to be part of an extensive, combined Amirani-Maui flow field. A mappable flow connects Amirani and Maui, suggesting that Maui is fed from Amirani, such that the post-Voyager designation “Maui Eruptive Center” should be revised. Amirani contains at least four hot spots detected by Galileo, and is the source of widespread bright (sulfur?) flows and active dark (silicate?) flows being emplaced in the Promethean style (slowly emplaced, compound flow fields). The floor of Gish Bar Patera has been partially resurfaced by dark lava flows, although other parts of its floor are bright and appeared unchanged during the Galileo mission. This suggests that the floor did not undergo complete resurfacing as a lava lake as proposed for other ionian paterae. There are several other hot spots in the region that are the sources of both active dark flows (confined within paterae), and SO₂- and S₂-rich diffuse deposits. Mapped diffuse deposits around fractures on mountains and in the plains appear to serve as the source for gas venting without the release of magma, an association previously unrecognized in this region. The six mountains mapped in this region exhibit various states of degradation. In addition to gaining insight into this region of Io, all four maps are studied to assess the best methodology to use to produce a new global geologic map of Io based on the newly released, combined Galileo-Voyager global mosaics. To convey the complexity of ionian surface geology, we find that a new global geologic map of Io should include a map sheet displaying the global abundances and types of surface features as well as a complementary GIS database as a means to catalog the record of surface changes observed since the Voyager flybys and during the Galileo mission.     

Free-convective flow of non-Newtonian fluid with heat sources

An analytical study is performed to examine the effects of temperature-dependent heat source on free-convective flow of non-Newtonian fluid (Walters's liquidB). The expression for the velocity field has been obtained by the Laplace transform technique. The influence of the various parameter entering into the problem is extensively discussed.     

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.     

Supersonic meteorology of Io: Sublimation-driven flow of SO2

The horizontal flow of SO₂ gas from day side to night side of Io is calculated. The surface is assumed to be covered by a frost whose vapor pressure at the subsolar point is orders of magnitude larger than that on the night side. Temperature of the frost is controlled by radiation. The flow is hydrostatic and turbulent, with velocity and entropy per particle independent of height. The vertically integrated conservation equations for mass, momentum, and energy are solved for atmospheric pressure, temperature, and horizontal velocity as functions of solar zenith angle. Formulas from boundary layer theory govern the interaction between atmosphere and surface. The flow becomes supersonic as it expands away from the subsolar point, as in the theory of rocket nozzles and the solar wind. Within 35° of the subsolar point atmospheric pressureis less than the frost vapor pressure, and the frost sublimes. Elsewhere, atmospheric pressure is greater than the frost vapor pressure, and the frost condenses. The two pressures seldom differ by more than a factor of 2. The sublimation rate at the subsolar point is proportional to the frost vapor pressure, which is a sensitive function of temperature. For a subsolar temperature of 130°K, the sublimation rate is 10¹⁵ molecules/cm²/sec. Diurnally averaged sublimation rates at the equator are comparable to the 0.1 cm/year resurfacing rate required for burial of impact craters. At the poles where both the vapor pressures and atmospheric pressures are low, the condensation rates are 100 times smaller. Surface pressures near the terminator are generally too low to account for the ionosphere discovered by Pioneer 10. The possibility of a noncondensable gas in addition to SO₂ must be seriously considered.     

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.     

Unsteady free convection flow past an infinite plate with constant suction and heat sources

An analysis of a two-dimensional unsteady free convective flow of an incompressible viscous fluid past an infinite vertical porous plate has been carried out under the following conditions: (i) constant suction, (ii) the plate temperature oscillating in time about a constant non-zero mean, (iii) presence of the temperature-dependent sources in the fluid. Approximate solutions have been derived for the mean velocity and temperature fields, the transient velocity and temperature fields, the amplitude and the phase of the skin-friction and the rate of heat transfer. It is shown that an increase inS (the source-strength), leads to an increase in the value of |B| (the amplitude of the skin-friction) and |Q| (the amplitude of the rate of heat transfer), in case of air, but in case of water |B| and |Q| decrease.     

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.     

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.     

Io: Energy constraints and plume volcanism

Observational and theoretical considerations, including near-surface energy constraints, suggest a model of Io that features a surface layer of sulfur overlying an active silicate crust. Such a model would imply frequent contact between silicate magma intrusions and the sulfur layer. This contact could produce volcanic plumes driven by high-temperature sulfur vapor. Plumes driven by sulfur vapor meet observationall constraint for a wide range of possible conditions, in contrast to the special conditions required for plume generation by SO₂. Characteristics of the two models are compared, and it is suggested that high-resolution infrared radiometry could identify the driving volatile.     

Mass pickup in sub-Alfvénic plasma flow: A case study for Io

Mass pickup may be an important process in the vicinity of the Jovian satellite Io and could be largely responsible for the slowing and deviation of the magnetospheric plasma flow in Io's vicinity. Theoretical treatments hitherto have treated a flow dominated by mass pickup near Io in a manner exactly analogous to the case where Io's immediate vicinity is treated as a simple electrical conductor. The analogy although instructive is not exact for it does not take account of the increased mass density in the flow downstream of Io. We point out that when correctly treated, mass pickup gives rise to a distinct wake structure behind Io. Observationally there is no way at present of distinguishing between dominant mechanisms using the Voyager 1 data; a wake pass is needed.     

Electron density and temperature in the Io plasma torus from Ulysses thermal noise measurements

During the Ulysses flyby of Jupiter, the spacecraft crossed the outer part of the Io plasma torus along a basically North-to-South trajectory at a Jovicentric distance of about 8R_J. The quasi-thermal noise measured by the Unified Radio and Plasma Wave (URAP) experiment is used to deduce the electron density and temperature along the trajectory. The density is deduced from the upper hybrid frequency line and the temperature from the spin modulation of Bernstein waves. These results are used to build a simplified Gaussian model of the torus. The density profile is roughly symmetric with respect to the centrifugal equator, with a scale height of about 0.9R_J. The density at equator crossing is twice as large as that expected from the Divine-Garrett Voyager-based model at the same radial distance. The density scale height is lower than that found by Voyager 1; it is consistent with an ion temperature of about 5 × 10⁵K, assuming an effective mass of about 20 proton masses. The fitting of the pressure distribution, symmetric with respect to the centrifugal equator, yields a cold electron temperature of about 1.4 × 10⁵K at the equator, which is of the same order of magnitude as found by Voyager 1.     

Io: Geochemistry of sulfur

The evidence from Voyager imaging, Earth-based spectral reflectivity studies, and thermal emission measurements combine to suggest an extremely fresh, volcanically recycled sulfur-rich crust for Io, with very shallow large-scale melting. Two present styles of volcanism are possible, depending on the thickness of local deposits of sulfur: shallow liquid sulfur magma generation with quiescent flooding, and high-temperature volcanism with violent eruption of a sulfur-iron magma driven by SO₂. Evolutionary considerations preclude direct derivation of Io's lithosphere from any metal-bearing chondritic source material. Metal-free C3V- or C2M-type parent material of either primary or secondary origin is the most plausible direct antecedent of the present sulfur-rich crust. Sulfates are almost certainly important constituents of the mantle, and can participate in the recycling of reduced, dense sulfide species to prevent total extraction of sulfur into the core.     

Io: Thermal models and chemical evolution

A combined thermal and chemical evolution model of Io is presented, outlining limits on the possible starting materials, heating history, chemical history, and present state of Io. Our best scenario starts with Io being accreted from material in a proto-Jovian nebula which condensed between 400–600°K. Radionuclides and tidal heating would lead to large-scale convection within Io and chemical reactions leading to the outgassing of water and methane. Reactions between Fe⁰FeS and water, at least near the surface, go to completion, resulting in all Fe being oxidized with elemental sulfur producing a low-conductivity crust. In the deep interior, these reactions may not completely exhaust Fe metal, and an FeS-rich core may be formed.