The atmosphere of Io

Observations of Io suggest that it may have an atmosphere in which sodium vapor, ammonia, and nitrogen are important constituents. Several atmospheric models consisting of these gases are treated here. These are tested as a function of total content against the Pioneer 10 observations and for stability against escape. The results suggest that the atmosphere is very tenuous and that the interpretation of the ionosphere detected by Pioneer 10 by a static model may be inconsistent with the sodium cloud observations. It is postulated that ionization may also be escaping and that sodium may be comparable in content in the atmosphere with some molecular constituent such as NH₃ or N₂. Sodium and this molecular component then dominate the atmosphere. It is also suggested that particle precipitation contributes to heating of the atmosphere and to the production of ionization; furthermore, the difference between day- and nighttime ionospheres and possible trailing and leading side effects may relate to the nature of the particle energy distributions. These distributions may be the result of the peculiar interaction of Io with the Jovian magnetosphere.     

The post-eclipse brightening of Io

We review the photometric work on eclipse reappearances of Io. New observations of eclipse reappearances of Io confirm the post-eclipse brightness anomaly reported by Binder and Cruikshank (1964) but testify to its intermittent nature. A post-eclipse anomaly of approximately 0.07 mag was observed on two occasions in 1972, while observations of Europa and Ganymede showed no brightness anomaly greater than 0.01 mag. The atmospheric condensation model for the anomaly on Io is reviewed in terms of the quantity of frost required to produce the effect and the corresponding amount of gas liberated to the atmosphere upon sublimation. The observational data and the results from a stellar occultation are in general accord with the theoretical predictions of the stability of heavy gases on Io, while both observational and theoretical criteria are satisfied by a tenuous atmosphere of a heavy gas such as methane or ammonia having a surface pressure ～10^?7 bar.     

The electrical conductivity of Io

If Io has a thin crust of ice [this possibility has recently been suggested by Lewis (1971)] then the electrical resistance of the satellite is determined by an outer layer of thickness ∼8 km and is higher by a factor ∼10¹⁵ than that needed to account for the modulation of Jupiter's decametric radio emission in the unipolar inductor model of Goldreich and Lynden-Bell. The modulation, however, could possibly be accounted for if the surface composition of Io is chondritic or if it has an ionosphere.     

The rotation of Io with a liquid core

In a previous paper (Henrard, Celest. Mech. Dyn. Astron. 178, 144–153, 2005c) we have developed an analytical theory of the rotation of the Galilean satellite Io, considered as a rigid body and based on a synthetic theory of its orbital motion due to Lainey (Théorie Dynamique des Satellites Galiléens. PhD dissertation, Observatoire de Paris, 2002) (see also Lainey et al., A&A, 420, 1171–1183 2004a; A&A, 427, 371–376, 2004b). One of the most important causes of departure of the actual rotation from the rigid theory is thought to be the existence of a liquid core, the size of which is unknown but would be an important piece of information concerning the structure of the interior of the satellite. In this contribution we develop the analytical theory of a liquid core contained in a cavity filled by an inviscid fluid of constant uniform density and vorticity. The theory is based on Poincaré (Bull. Astron. 27, 321–356, 1910) model and is developed by a Lie transform perturbation method, very much like in our previous contribution. Our main conclusion is that the addition of a degree of freedom (the spin of the core) with a frequency close to the orbital frequency multiplies the possibility of resonances and that for some particular size of the core one may expect a (possibly small) region of chaotic behaviour in the vicinity of the Cassini state.     

The ion mass loading rate at Io

The Io plasma torus, composed of mostly heavy ions of oxygen and sulfur, is sustained by an Iogenic mass loading rate of ∼10³⁰ amu s⁻¹ = 1.6 × 10²⁸ SO₂ s⁻¹ or approximately 10³ kg s⁻¹(A.L. Broadfoot et al., 1979, Science 204, 979-982). We argue on the basis of available power sources, reanalysis of F. Bagenal (1997, Geophys. Res. Lett. 24, 2111-2114), HST UV remote sensing, and detailed model calculations that at most 20% of this mass leaves Io in the form of ions, i.e., ≤3 × 10²⁷ × (n_e,0/3600 cm⁻³) ions s⁻¹, where n_e,0 is the average torus electron density. For the Galileo spacecraft Io pass in December 1995, the ion mass loading rate was ≤3 × 10²⁷ ions s⁻¹, whereas for the Voyager epoch with lower n_e,0 (=2000 cm⁻³), this rate would be ≤1.7 × 10²⁷ ions s⁻¹, consistent with the D.E. Shemansky (1980, Astrophys. J. 242, 1266-1277) mass loading limit of ≤1 × 10²⁷ ions s⁻¹. We investigate the processes that control Io’s large scale electrodynamic interaction and find that the elastic collision rate exceeds the ionization/pickup rate by at least a factor of 5 for all atmospheric column densities considered (10¹⁶-10²¹ m⁻²) and by a factor of ∼100 for the most realistic column density. Consequently, elastic collisions are mostly responsible for Io’s high conductances and thus generate Io’s large scale electrodynamic interaction such as the generation of Io’s electric current system and the slowing of the plasma flow. The electrodynamic part of Io’s interaction is thus best described as an ionosphere-like interaction rather than a comet-like interaction. An analytic expression for total electron impact rates is derived for Io’s atmosphere, which is independent of any particular model for the 3D interaction of torus electrons with its atmosphere.     

The sodium and hydrogen gas clouds of Io

Models are developed to describe the spatial distribution of gases emitted by Io and are applied to recent observations which indicate extensive gas clouds of hydrogen and sodium in orbit around Jupiter. Hydrogen and sodium atoms are emitted from Io with velocities in the range 2 to 3 km sec^?1, with fluxes of about 10¹⁰ and 10⁸cm^?2sec^?1 for hydrogen and sodium respectively. Hydrogen atoms may be formed by photodecomposition of gases such as NH₃ or H₂S released from the satellite surface and may escape thermally from an exosphere whose temperature is about 500 K. Sodium may be ejected from the surface by energetic particles or by ultraviolet radiation and it appears that a non-thermal mechanism drawing energy from Jupiter's magnetic field is required in order to account for its release to space.     

Mass-injection rate from Io into the Io plasma torus

Theoretical arguments have been presented to the effect that both plasma and energy are supplied to the Jovian magnetosphere primarily from internal sources. If we assume that Io is the source of plasma for the Jovian magnetosphere and that outward flow of plasma from the torus is the means of drawing from the kinetic energy of rotation of Jupiter to drive magnetospheric phenomena, we can obtain a new, independent estimate of the rate of mass injection from Io into the Io plasma torus. We explicitly assume the solar wind supplies neither plasma nor energy to the Jovian magnetosphere in significant amounts. The power expended by the Jovian magnetosphere is supplied by torus plasma falling outward through the corotational-centrifugal-potential field. A lower limit to the rate of mass injection into the torus, which on the average must equal the rate of mass loss from the torus, is therefore derivable if we adopt a value for the power expended to drive the various magnetospheric phenomena. This method yields an injection rate of at least 10³ kg/sec, a value in agreement with the results obtained by two other independent methods of estimating mass injection rate. If this injection rate from Io and extraction of energy from Jupiter's kinetic energy of rotation has been maintained over geologic time, then approximately 0.1% of Io's mass (principally in the form of sulfur and oxygen) has been lost to the Jovian magnetosphere, and Jupiter's spin rate has been reduced by less than 0.1%.     

Magmatic Differentiation of Io

If Io has been volcanically active through much of its history, it must be highly differentiated. We present an initial attempt to quantify the differentiation of the silicate portion of Io. We suggest that, on average, each part of Io has undergone about 400 episodes of partial melting. We employ a widely used thermodynamic model of silicate melts to examine the effect of such repeated differentiation. Despite many caveats, including a grossly oversimplistic model of the differentiation process, uncertainties in the initial composition of the mantle, and the failure to model more than four episodes of partial melting, we are able to make some robust conclusions. Io should have a roughly 50 km thick, low density (2600–2900 kg m⁻³), alkali-rich, siliceous crust composed primarily of feldspars and nepheline. The crustal magmas should have relatively low melting temperatures (<1100 °C). The bulk of the mantle should be essentially pure forsterite (magnesian olivine). It is possible that the denser iron- and calcium-rich materials are segregated into a lower mantle and thus no longer involved in surface processes. These model predictions are generally consistent with the observations of Io. The enrichment of the crust in alkalis may help to explain the composition of the neutral clouds around Io. The failure to detect silicates at the surface of Io to date might be due in part to the difficulty in detecting Fe-poor minerals such as nepheline, feldspars, and forsterite via near-IR spectroscopy. Many hot spot temperatures are too high for sulfur alone but are in line with silica-rich melts. The mountains on Io could be manifestations of large buoyant plutons. The highest temperature lavas may be the result of melts from the depleted mantle making their way to the surface from great depths.     

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.     

Voyager disk-integrated photometry of Io

Voyager full-disk images of Io, available at solar phase angle of α = 2?29° and 101?159°, allow comparisons of the satellite's near-opposition photometric behavior with Earth-based results and the determination of the phase curve out to very high phase angles. The near-opposition data were reduced iteratively for self-consistent phase and rotation curves in each Voyager filter; the resulting phase coefficients, geometric albedos, and rotational lightcurves are consistent with Earth-based findings, except for a previously noted tendency for Voyager to yield somewhat redder spectral information. The derived near-opposition phase coefficients, ranging between 0.016 and 0.024 mag/ deg, decrease with increasing wavelength, a trend weakly noted in some Earth-based observations. The full, α = 2?159° phase curves allow the first direct determination of the phase integral of Io at several wavelengths: q rises from ≈0.7 in the ultraviolet to ≈0.8 in the orange. Combination of the Voyager phase integrals with Earth-based albedo information leads to a best estimate of the bolometric Bond albedo of 0.50 ± 0.10, a value consistent with, but slightly below, previous estimates.     

The reflection spectrum of liquid sulfur: Implications for Io

The spectral reflectance from 0.38 to 0.75 μm of a column of liquid sulfur has been measured at several temperatures between the melting point (～118°C) and 173°C. Below 160°C the spectral reflectance was observed to vary reversibly as a function of temperature, independent of the previous thermal history of the column. Once the temperature exceeded 160°C, the spectrum would not change given a subsequent decrease in temperature. The spectral reflectance of the liquid-sulfur column at all temperatures was very low (10–19%). Combining this information with Voyager spectrophotometry of Jupiter's satellite Io, it is concluded that liquid sulfur at any temperature on Io's surface would be classified as a “black area” according to the standards used by the Voyager imaging team in their spectrophotometric analysis (L. Soderblom, T. V. Johnson, D. Morrison, E. Danielson, B. L. Smith, J. Veverka, A. Cook, C. Sagan, P. Kupferman, D. Pieri, J. Mosher, C. Avis, J. Gradie, and T. Clancy (1980). Geophys. Res. Lett.7, 963–966).     

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.     

The role of critical velocity ionization phenomena in the atmosphere of Io

The Alfvén's critical ionization velocity (CIV) have been observed in a number of laboratory and space experiments. In the Io-torus system, relative velocity of the plasma species in the torus with respect to the neutral species in the Io's atmosphere and neutral cloud exceeds the critical velocity required for CIV. Townsand condition is satisfied up to 6r _io , in the neutral cloud when Io passes through the torus. In this paper it is shown that during the passage of Io through the plasma torus, apart from critical velocity and Townsand condition, a number of other requirements are also satisfied. Therefore, it is concluded that, the CIV mechanism must play an important role in ionizing the neutral cloud and enriching the plasma torus.     

The atmosphere of Io from Pioneer 10 radio occultation measurements

The occultation of the Pioneer 10 spacecraft by Io (JI) provided an opportunity to obtain two S-band radio occultation measurements of its atmosphere. The dayside entry measurements revealed an ionosphere having a peak density of about 6 × 10⁴ elcm^?3 at an altitude of about 100 km. The topside scale height indicates a plasma temperature of about 406 K if it is composed of Na⁺ and 495 K if N₂⁺ is principal ion. A thinner and less dense ionosphere was observed on the exit (night side), having a peak density of 9 × 10³ elcm^?3 at an altitude of 50 km. The topside plasma temperature is 160 K for N₂^? and 131 K for Na⁺. If the ionosphere is produced by photoionization in a manner analogous to the ionospheres of the terrestrial planets, the density of neutral particles at the surface of Io is less than 10¹¹?10¹² cm³, corresponding to a surface pressure of less than 10^?8 to 10^?9 bars. Two measurements of its radius were also obtained yielding a value of 1830 km for the entry and 192 km for the exit. The discrepancy between these values may indicate an ephemeris uncertainty of about 45 km. The two measurements yield an average radius of 1875 km, which is not in agreement with the results of the Beta Scorpii stellar occultation.     

Sodium remote from Io

We find that faint sodium emission originating in the middle Jupiter magnetosphere has two distinct kinematical components. The “normal” signature of atoms on bound orbits with large apojoves seems always to be present, and we suggest these atoms are an extension of the bright, near-Io sodium cloud. The “fast” signature, with speeds up to at least 100 km sec^?1, is seen only occasionally, and we suggest it is due to an interaction of the near-Io sodium cloud with the corotating, heavy-ion plasma. Both elastic and charge-exchange collisions seem consistent with the observed kinematical and temporal signatures. Elastic collisions seem marginally more capable of producing the high observed sodium atom speeds. We predict observable occurences of the fast component in the hours following passage of the Io sodium cloud through the plasma centrifugal symmetry surface if Io is at a favorable orbital longitude. Between 10 and 20 R_J we find an atomic sodium density ～10^?2 cm^?3. If the photoionization lifetime applies, an Io source of at least 10²⁶ sodium atoms sec^? is required to maintain this remote sodium population.     

Models of the Io Torus

Three-dimensional models of the Io torus are employed to analyze the spectroscopic data reported by J.S. Morgan (1985, Icarus62, 389–414). These models are used to compare Morgan's ground-based spectroscopic data with R.J. Oliversen's (1983, The Io Plasma Torus: Its Structure and Sulfur Emission Spectra. Ph.D. thesis, University of Wisconsin-Madison) nearly simultaneous [SII] images and with the in situ measurements made by Voyager 1. The models are also used to investigate whether the observed [SII] longitudinal intensity variations were caused by intrinsic or geometric effects, and to test the hypothesis that the observed optical east-west variations are consistent with the convective motions suggested by D.D. Barbosa and M.G. Kivelson (1983, Geophys. Res. Lett.10, 210–213) and W.-H. Ip and C. K. Goertz (1983, Nature302, 232–233). Oliversen's images are found to be in good agreement with Morgan's spectroscopic measurements. Three significant differences exist between these data and the torus described in the Voyager 1 experiments: (1) the torus beyond ～5.7R_J was found to be at least 1.5 to 2 times denser in 1981 than at the time of the Voyager 1 measurements in 1979, (2) the outer torus SII ion temperatures were approximately two times cooler than those measured by Voyager 1, and (3) in 1981, the outer torus OII mixing ratios were lower than were suggested by the Voyager 1 experiments. The 1981 ground-based OII/SII intensity ratios are found to be consistent with a radial peak near 6.0R_J in the ratio of oxygen to sulfur. At its maximum this ratio is ～2, and it falls to ～1 within ～0.5R_J inside and outside of this radius. Viewing geometry variations were found to be inadequate to account for the longitudinal variations observed by Morgan (1984). Intrinsic longitudinal intensity changes of about a factor of 2 are required to match the 1981 observations. Convective motions were found to adequately explain the observed optical east-west intensity asymmetry, but problems in interpreting the [OII] doublet line ratios still remain. It is suggested that systematic errors are present in the measurements of the [OII] line ratios.