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.     

Sputtering of sulfur by kiloelectronvolt ions: Application to the magnetospheric plasma interaction with Io

Measurements of total yields, temperature dependences, mass spectra, and energy spectra of molecules sputtered from condensed sulfur (S₈) at low temperatures by keV ions are reported and results are given for Jovian plasma ion bombardment of Io. A change in the reflectance of the sulfur, which can be removed by annealing, is produced by the most penetrating ions and may be connected with the darker, colder polar regions on Io. The measured sputtering yields are much lower than those estimated earlier for room temperature sulfur films but are comparable to previous measurements of keV ion sputtering of SO₂ at low temperatures. The corrected mass spectrum indicates that ≈66% of the total yield corresponds to S₂ ejection while only 5 and 16% correspond to S and S₃, respectively. Therefore, if ions reach the surface of Io its atmosphere will have a non-negligible sulfur component of primarily S₂. The ejection of S and S₂ is temperature independent for temperatures characteristic of most of the surface of Io. The energy spectrum for S has an approximate 1/E² dependence at high ejection energies, whereas S₂ and S₃ fall off more rapidly. Assuming 50% coverage of both sulfur and SO₂ and a thin atmosphere (e.g., nightside and polar region) the direct sputter injection of sulfur atoms and molecules into the Jovian plasma torus and the indirect injection due to coronal processes are estimated. These injection rates for sulfur are compared to those for SO₂ showing that injection from sulfur deposits contributes 13% to the total mass injection rate of ∼2–3 × 10²⁹ amu/sec.     

Magnetosphere,exosphere, and surface of mercury

The discovery of an atomic sodium exosphere at Mercury raises the question of whether Mercury, like Io at Jupiter, can maintain a heavy ion magnetosphere. We suggest that it does, and that heavy ions (mainly Na⁺) from the exosphere are typically accelerated to keV energies and make important or dominant contributions to the mass (∼300 g sec⁻¹) and energy (∼3 × 10⁹W) budgets of the magnetosphere. The sodium supply to the exosphere is largely from within Mercury itself, with external sources like meteroid infall and the solar wind being relatively unimportant. Therefore Mercury is in the process of losing its semivolatiles. Photosputtering dominates charged particle sputtering and can maintain an adequate rate of Na ejection from the surface.     

Properties of filamentary sublimation residues from dispersions of clay in ice

During vacuum sublimation experiments on simulated Martian polar deposits and cometary dirty ices, a fluffy filamentary sublimate residue material with unique physical properties was produced. The silica-to-silica bonds that we believe join the particles together are the result of conditions that may exist in some Martian polar deposits and on some cometary surfaces. Submicron particles of montmorillonite clay thinly dispersed (1: 1000 clay/water) and not contacting one another in water ice can form very-low-density structures (density as low as 10⁻³ g cm⁻³) during sublimation of the ice. The lightweight constructs, when viewed in scanning electron microscopy micrographs, are composed of long network chains of the clay particles. The material is sufficiently electrically conductive to drain away the scanning electron microscopy charge. It is also resistant (no change in electronic properties are apparent) to scanning electron microscopy electron-beam heating for hours in vacuo. Infrared spectra and X-ray diffraction patterns of the sublimate residues show little difference from spectra and patterns of the original minerals. Heating in an oven, in air, to 370°C produces little change in the structure of the sublimate residual material. The particle bonding forces are strong and produce a resilient, elastic lightweight material. The particle bonding will allow vapors to diffuse through it, and its thermal conductivity is very low. These properties produce a high-performance vacuum insulation. This material may have applications for insulating ice bodies (solid cryogens) in space. The incoming heat is partially carried away by the out-flowing water vapor.     

The condensation and vaporization behavior of ices containing SO2, H2S, and CO2: implications for Io

In an extension of previously reported work on ices containing CO, CO2, H2O, CH3OH, NH3, and H2, measurements of the physical and infrared spectral properties of ices containing molecules relevant to Jupiter's moon Io are presented. These include studies on ice systems containing SO2, H2S, and CO2. The condensation and sublimation behaviors of each ice system and surface binding energies of their components are discussed. The surface binding energies can be used to calculate the residence times of the molecules on a surface as a function of temperature and thus represent important parameters for any calculation that attempts to model the transport of these molecules on Io's surface. The derived values indicate that SO2 frosts on Io are likely to anneal rapidly, resulting in less fluffy, "glassy" ices and that H2S can be trapped in the SO2 ices of Io during night-time hours provided that SO2 deposition rates are on the order of 5 micrometers/hr or larger.     

Galileo plasma wave observations of iogenic hydrogen

The Galileo plasma wave instrument has detected intense electromagnetic wave emissions approximately centered on the second and fourth harmonics of the local proton gyrofrequency during the close equatorial flyby of Io on 7 December 1995. Their frequencies suggest these emissions are likely generated locally by an instability driven by non thermal protons. Given that this process occurs close to Io, we suggest that hydrogen-bearing compounds, escaping from Io, are broken up/ionized near this moon, thereby releasing protons. Newly-created protons are thus injected in the Jovian corotating plasma with the corotation velocity, leading to the formation of a ring in velocity space. Several electromagnetic wave–particle instabilities can be driven by a ring of newborn protons. Given that the corotating plasma is sub-Alfvénic relative to Io, the magnetosonic mode cannot be destabilized by this proton ring. The full dispersion relation is studied using the WHAMP program (Rönmark, 1982. Rep. 179. Kiruna Geophys. Inst., Kiruna, Sweden) as well as a new algorithm that allows us to fit the distribution function of newborn protons in a more realistic way. This improvement in the ring model is necessary to explain the relative narrowness of the observed spectral peaks. The measured E/B ratio is also used to identify the relevant instability and wave mode: this mode results from the coupling between the ion Bernstein and the ion cyclotron mode (IBCW). To our knowledge this mode has not yet been studied. From the instability threshold an estimate of the density of newborn protons around Io is thus given; at about 2 Io radii from the surface and 40°W longitude from the sub-Jupiter meridian, this density is found to be ≥0.5% of the local plasma density (∼4000 cm⁻³), namely ≥20 cm⁻³. Assuming a stationary pickup process and a r⁻ⁿ distribution of pickup protons within several Io radii of Io’s wake, this implies that more than 10²⁶ protons/s are created around Io. The ultimate origin of these protons is an open issue.     

On the surface composition of Io

The wavelength dependence of the reflectivity of Io indicates the presence of two materials on the surface of this satellite of Jupiter. These materials are sulfur and an unspecified material (R₁) which shows a wavelength dependence of its reflectivity for 0.3 μm < λ < 1.0 μm similar to the non-H₂O frost spectrum of the rings of Saturn. A 60/40 admixture of these two spectra matches the observed reflection spectrum of Io from 0.3μm–3 μm, if the spectrum of R₁ is featureless for λ > 1 μm. Sulfur will give rise to a posteclipse brightening. The variation with wavelength of the temperature dependence of the reflectivity of sulfur will allow an observational confirmation of the presence of sulfur on Io. The material R₁ should show a large geometrical albedo. The translucency of sulfur is consistent with the polarization-phase curve to Io. The material R₁ is also required to be translucent. The thermal conductivity of a cooled sulfur powder under vacuum was measured and found to agree with the value determined for the upper layer of Io from observations at 10 μm. It is shown that this agreement is not necessarily meaningful.     

Spectral reflectance of solid sulfur trioxide (0.25–5.2 μm): Implications for Jupiter's satellite Io

We have measured the reflection spectrum of solid sulfur trioxide and we have compared this spectrum to the spectral geometric albedo of Jupiter's satellite Io. We find that the laboratory spectrum of solid SO₃ has very strong absorption features at 3.38, and 4.08 μm. The 3.38- and 3.70-μm absorptions are present very weakly (if indeed at all) in the spectral geometric albedo of Io. This suggests that solid SO₃, if present at all, could exist only as a very minor component of Io's surface. We note that studies involving particle bombardment of SO₂ (a known Io surface constituent) produce SO₃ (Moore, 1984, Icarus 31, 40–80). Sulfur trioxide, once formed on Io's surface, would be extremely stable; however, it would not be expected to accumulate to levels detectable from Earth-based instruments. While it may be possible that the constant resurfacing of Io by volcanic ejecta may cover any SO₃ formed, the area subject to such extensive resurfacing on short time scales (∼ 1 year) is at best ∼10%. Therefore, we would expect that condensed SO₂ remote from volcanos should develop a small but significant SO₃ concentration that could be detected by instruments such as the near-infrared mapping spectrometer on the Galileo spacecraft.     

Mid-infrared detection of large longitudinal asymmetries in Io's SO2 atmosphere

We have observed about 16 absorption lines of the ν₂ SO₂ vibrational band on Io, in disk-integrated 19-μm spectra taken with the TEXES high spectral resolution mid-infrared spectrograph at the NASA Infrared Telescope Facility in November 2001, December 2002, and January 2004. These are the first ground-based infrared observations of Io's sunlit atmosphere, and provide a new window on the atmosphere that allows better longitudinal and temporal monitoring than previous techniques. Dramatic variations in band strength with longitude are seen that are stable over at least a 2 year period. The depth of the strongest feature, a blend of lines centered at 530.42 cm⁻¹, varies from about 7% near longitude 180° to about 1% near longitude 315° W, as measured at a spectral resolution of 57,000. Interpretation of the spectra requires modeling of surface temperatures and atmospheric density across Io's disk, and the variation in non-LTE ν₂ vibrational temperature with altitude, and depends on the assumed atmospheric and surface temperature structure. About half of Io's 19-μm radiation comes from the Sun-heated surface, and half from volcanic hot spots with temperatures primarily between 150 and 200 K, which occupy about 8% of the surface. The observations are thus weighted towards the atmosphere over these low-temperature hot spots. If we assume that the atmosphere over the hot spots is representative of the atmosphere elsewhere, and that the atmospheric density is a function of latitude, the most plausible interpretation of the data is that the equatorial atmospheric column density varies from about 1.5×¹⁰¹⁷ cm⁻² near longitude 180° W to about 1.5×¹⁰¹⁶ cm⁻² near longitude 300° W, roughly consistent with HST UV spectroscopy and Lyman-α imaging. The inferred atmospheric kinetic temperature is less than about 150 K, at least on the anti-Jupiter hemisphere where the bands are strongest, somewhat colder than inferred from HST UV spectroscopy and millimeter-wavelength spectroscopy. This longitudinal variability in atmospheric density correlates with the longitudinal variability in the abundance of optically thick, near-UV bright SO₂ frost. However it is not clear whether the correlation results from volcanic control (regions of large frost abundance result from greater condensation of atmospheric gases supported by more vigorous volcanic activity in these regions) or sublimation control (regions of large frost abundance produce a more extensive atmosphere due to more extensive sublimation). Comparison of data taken in 2001, 2002, and 2004 shows that with the possible exception of longitudes near 180° W between 2001 and 2002, Io's atmospheric density does not appear to decrease as Io recedes from the Sun, as would be expected if the atmosphere were supported by the sublimation of surface frost, suggesting that the atmosphere is dominantly supported by direct volcanic supply rather than by frost sublimation. However, other evidence such as the smooth variation in atmospheric abundance with latitude, and atmospheric changes during eclipse, suggest that sublimation support is more important than volcanic support, leaving the question of the dominant atmospheric support mechanism still unresolved.     

Sputtering of sulfur: Experiments and consequences for Io

Experimental results on the sputtering of sulfur by MeV He ions are presented. It is shown that, for most of an Io surface, supposedly covered by sulfur, sputtering dominates sublimation in removing material. Sputtering cannot, however, be the sole supply of neutral sulfur for the Io torus.     

The Winchcombe fireball—That lucky survivor

On February 28, 2021, a fireball dropped ∼0.6 kg of recovered CM2 carbonaceous chondrite meteorites in South-West England near the town of Winchcombe. We reconstruct the fireball's atmospheric trajectory, light curve, fragmentation behavior, and pre-atmospheric orbit from optical records contributed by five networks. The progenitor meteoroid was three orders of magnitude less massive (∼13 kg) than any previously observed carbonaceous fall. The Winchcombe meteorite survived entry because it was exposed to a very low peak atmospheric dynamic pressure (∼0.6 MPa) due to a fortuitous combination of entry parameters, notably low velocity (13.9 km s⁻¹). A near-catastrophic fragmentation at ∼0.07 MPa points to the body's fragility. Low entry speeds which cause low peak dynamic pressures are likely necessary conditions for a small carbonaceous meteoroid to survive atmospheric entry, strongly constraining the radiant direction to the general antapex direction. Orbital integrations show that the meteoroid was injected into the near-Earth region ∼0.08 Myr ago and it never had a perihelion distance smaller than ∼0.7 AU, while other CM2 meteorites with known orbits approached the Sun closer (∼0.5 AU) and were heated to at least 100 K higher temperatures.     

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.     

Spectral behavior of the symbiotic nova AG Pegasi observed with IUE and HST

Ultraviolet spectra from the International Ultraviolet Explorer (IUE) and from the Hubble Space Telescope (HST) of the symbiotic novae AG Peg during the period 1978–1996 are analyzed. Some spectra showing the modulations of spectral lines at different times are presented. We determined the reddening from the 2200 Å feature, finding that E(B−V) = 0.10 ± 0.02. We studied N IV] at 1486 Å, C IV 1550 Å, and O III] at 1660 Å, produced in the fast wind from the hot white dwarf. The mean wind velocity of the three emission lines is 1300 km s⁻¹ (FWHM). The mean wind mass loss rate is ∼6 × 10⁻⁷ M_⊙ yr⁻¹. The mean temperature is ∼6.5 × 10⁵ K. The mean ultraviolet luminosity is ∼5 × 10³³ erg s⁻¹. The modulations of line fluxes in the emitting region at different times are attributed to the variations of density and temperature of the ejected matter as a result of variations in the rate of mass loss.     

Sulfur flows of Ra Patera,Io

Voyager 1 imaging data have been used to investigate the color and morphology of several radial flow-like features at Ra Patera, a broad volcanic structure at approximately 8° latitude and 325° longitude on the Galilean satellite Io (J1). It was found that downstream progressions of flow color and morphology are consistent with lava of a predominately sulfur composition cooling radiatively and erupting in the range of 470 to 520°K at effusion rates at 10¹⁰ to 10¹¹ cm³/sec. This implies global resurfacing rates by volcanic flows on Io of the order of 1 cm/year. Calculated energy content and effusion rates for flows at Ra Patera, using the physical parameters of sulfur, are of the order of the largest known terrestial basaltic eruptions and are consistent with calculations of globally available energy.     

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.     

Observational tests for sulfur allotropes on Io

One of the intrinsic properties of particulate sulfur allotropes is a change in UV-visible reflectivity with temperature change of the material. The surface of Io experiences temperature changes during eclipse which are sufficient to cause a detectable change in the spectral reflectivity of sulfur; thus, if the surface of Io is composed primarily of sulfur allotropes, a change in reflectivity at certain wavelengths should be observable shortly after eclipse reappearance. We observed four eclipse reappearances during July and August of 1983 and saw no posteclipse brightening effects in filter bands selected for sensitivity to color changes in sulfur. Our model of the brightness change for S₈ (“yellow” sulfur) implies that this material covers less than 50% of Io's surface. Negative posteclipse brightening observations were also obtained with a filter chosen for the high contrast between SO₂ frost and the average albedo of the surface of Io at that wavelength. We conclude that no significant condensation of optically thick SO₂ occurred on the surface of Io during these eclipses.     

Optical properties of solid and liquid sulfur at visible and infrared wavelengths

Knowledge of the optical constants of elemental sulfur has potential applications to Venus, Jupiter, Io, Amalthea, and the Earth. The real part, n, of the index of refraction of liquid sulfur (at 133°C) and of solid orthorhombic sulfur (at 25°C) for the wavelength range 0.4–2.0 μm were measured ellipsometrically. The imaginary part, k, of the refractive index of liquid sulfur was obtained by transmittance measurements at the same temperature and wavelength range. The reflectance of semi-infinite slabs of solid and liquid sulfur is calculated using the measured n and k values. We confirm that sulfur melts on Io would be classified as “black” by the Voyager imaging system.     

New upper limits for atmospheric constituents on Io

A spectrum of the satellite of Jupiter, Io, from 0.86 to 2.7 μm at a resolution of 3.36 cm^?1 and a signal to rms noise ratio of 120 is presented. No absorptions due to any atmospheric constituents on Io could be found on the spectrum. Upper limits of 0.12 cm-atm for NH₃, 0.12cm-atm for CH₄, 0.4cm-atm for N₂O, and 24cm-atm for H₂S were determined. Laboratory spectra of ammonia frosts as a function of temperature were compared with the spectrum of Io and showed as a frost not to be present at the surface of Io. A search for possible resonance lines of carbon, silicon, and sulfur as well as the 1.08μm line of helium proved negative and upper emission limits of 60, 18, 27, and 60 kilorayleighs, respectively, were established for these lines.     

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.