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Io's atmosphere, with an average equatorial column density of ≥1020m−2, exhibits significant density variations with latitude and longitude. We apply a 3‐D magnetohydrodynamic model to investigate the effects of atmospheric asymmetries, both locally from volcanic plumes and globally, on the plasma and magnetic field environment of Io. The model takes into account collisions between ions and neutrals, plasma production and loss due to electron impact ionization and dissociative recombination, and the ionospheric Hall effect. Our simulation results show that volcanic plumes influence the plasma interaction locally, generating Alfvén winglets within Io's global Alfvén wing. Signals from individual plumes can however barely be probed by magnetic field measurements during spacecraft flybys at Io. In contrast, the surface number density, scale height, the longitudinal and latitudinal variations of the global atmosphere are crucial factors for modeling and understanding magnetic field and plasma perturbations. Comparing our model results with the magnetic field data from the I24 and I27 flybys of the Galileo spacecraft, we find that the measured perturbations can be primarily caused by the plasma interaction with the longitudinally asymmetric atmosphere. This implies that a significant magnetic induction signal from a partially molten magma ocean is not necessarily required to explain the Galileo magnetometer data. A near‐surface magma ocean is not necessarily required to explain the Galileo MAG dataAtmospheric asymmetries crucially affect Io's magnetic field and plasma environmentVolcanic plumes in Io's global atmosphere generate Alfvén winglets within Io's global Alfvén wings

Recent observations of interplanetary medium atomic hydrogen Lyman‐α emission in the outer solar system, made with the Alice ultraviolet spectrograph on New Horizons, are presented. The observations include regularly spaced great circle scans of the sky and pointed observations near the downstream and upstream flow directions of interstellar H atoms. The New Horizons Alice data agree very well with the much earlier Voyager UVS results, after these are reduced by a factor of 2.4 in brightness, in accordance with recent reanalyses. In particular, the falloff of interplanetary medium Lyman‐α brightness in the upstream‐looking direction as a function of spacecraft distance from the Sun is well matched by an expected 1/rdependence, but with an added constant brightness of ~40 Rayleighs. This additional brightness is a possible signature of the hydrogen wall at the heliopause or of a more distant background. Ongoing observations are planned at a cadence of roughly twice per year. Long‐term observations made with the Alice instrument on the New Horizons spacecraft confirm measurements made 30 years earlier with the Voyager spacecraft. Both sets of data are best explained if the observed ultraviolet light is not only a result of the scattering of sunlight by hydrogen atoms within the solar system but includes a substantial contribution from a distant source. This distant source could be the signature of a “wall” of hydrogen, formed near where the interstellar wind encounters the solar wind, or could be more distant. Similar future observations from New Horizons are planned about twice each year. New Horizons Alice observations of interplanetary medium Lyman alpha are presentedThe observed brightness falloff with distance from the Sun matches well with Voyager results and indicates a substantial external background contribution

Pluto’s atmosphere is cold and hazy. Recent observations have shown it to be much colder than predicted theoretically, suggesting an unknown cooling mechanism. Atmospheric gas molecules, particularly water vapour, have been proposed as a coolant; however, because Pluto’s thermal structure is expected to be in radiative–conductive equilibrium, the required water vapour would need to be supersaturated by many orders of magnitude under thermodynamic equilibrium conditions. Here we report that atmospheric hazes, rather than gases, can explain Pluto’s temperature profile. We find that haze particles have substantially larger solar heating and thermal cooling rates than gas molecules, dominating the atmospheric radiative balance from the ground to an altitude of 700 kilometres, above which heat conduction maintains an isothermal atmosphere. We conclude that Pluto’s atmosphere is unique among Solar System planetary atmospheres, as its radiative energy equilibrium is controlled primarily by haze particles instead of gas molecules. We predict that Pluto is therefore several orders of magnitude brighter at mid-infrared wavelengths than previously thought—a brightness that could be detected by future telescopes.

We observed Io with the James Webb Space Telescope (JWST) while the satellite was in eclipse, and detected thermal emission from several volcanoes. The data were taken as part of our JWST‐ERS program #1373 on 15 November 2022. Kanehekili Fluctus was exceptionally bright, and Loki Patera had most likely entered a new brightening phase. Spectra were taken with NIRSpec/IFU at a resolving power R≈ 2,700 between 1.65 and 5.3 µm. The spectra were matched by a combination of blackbody curves that showed that the highest temperature, ∼1,200 K, for Kanehekili Fluctus originated from an area ∼0.25 km2in size, and for Loki Patera this high temperature was confined to an area of ∼0.06 km2. Lower temperatures, down to 300 K, cover areas of ∼2,000 km2for Kanehekili Fluctus, and ∼5,000 km2for Loki Patera. We further detected the a1Δ ⇒ X3Σ−1.707 µm rovibronic forbidden SO emission band complex over the southern hemisphere, which peaked at the location of Kanehekili Fluctus. This is the first time this emission has been seen above an active volcano, and suggests that the origin of such emissions is ejection of SO molecules directly from the vent in an excited state, after having been equilibrated at temperatures of ∼1,500 K below the surface, as was previously hypothesized. We observed Io with JWST in November 2022 while the satellite was in Jupiter's shadow, and glowing volcanoes show up without being (partially) obscured by reflected sunlight. We detected the volcanoes Loki Patera and Kanehekili Fluctus; the latter was exceptionally bright, and Loki Patera had likely entered a new brightening phase. Both volcanoes show erupting lavas at temperatures of at least 1,200 K, originating at a vent of ∼0.25 km2in size for Kanehekili Fluctus and <0.1 km2for Loki Patera. In addition to lava, Kanehekili Fluctus spews out gases, and we detected, for the first time, SO emission at 1.707 μm right over the volcano. This is the first time this emission has been seen above an active volcano, and suggests that such emissions are produced by SO molecules immediately upon leaving the vent. James Webb Space Telescope observations detected an energetic eruption at Kanehekili Fluctus, and a new brightening event at Loki PateraThe erupting lavas have a temperature of at least 1,200 K over an area of ∼0.25 km2or lessWe detected, for the first time, a clear association of the 1.707 micron forbidden SO emissions with an active volcano James Webb Space Telescope observations detected an energetic eruption at Kanehekili Fluctus, and a new brightening event at Loki Patera The erupting lavas have a temperature of at least 1,200 K over an area of ∼0.25 km2or less We detected, for the first time, a clear association of the 1.707 micron forbidden SO emissions with an active volcano

We study the morphology of Ganymede's FUV aurora by analyzing spectral images obtained over the past two decades by the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. The observations cover the eastern and western elongation as well as various magnetic latitudes of Ganymede within the Jovian plasma sheet. We find both asymmetries in the spatial distribution of auroral brightness on the observed moon disk and temporal variation correlated to Ganymede's changing magnetic latitude. The total disk brightness is on average 1.42 ± 0.07 times brighter on the leading side (95.4 ± 2.1 R) than on the trailing side (67.2 ± 2.9 R). The brightness ratio of the sub‐Jovian hemisphere to the anti‐Jovian hemisphere is 1.81 ± 0.06 on the leading side and 1.41 ± 0.14 on the trailing side, respectively. Inside the Jovian current sheet, the brightness of the auroral ovals increases by a factor of 1.45 ± 0.02 on the leading side and decreases by a factor of 0.80 ± 0.02 on the trailing side. At the current sheet center, the auroral ovals shift 4.1° ± 0.7°latitude toward Ganymede's planetographic equator on the leading side and 2.9° ± 1.5° toward the poles on the trailing side. Both effects, the variation of brightness and the movement of the ovals are correlated to a stronger interaction of Jupiter's magnetospheric plasma with Ganymede's minimagnetosphere inside the current sheet. Finally, we calculate the latitudinal difference of the northern and southern ovals from Ganymede's magnetic equator. The result suggests a farther westward orientation of Ganymede's dipole magnetic moment at approximately 47° + 58°/−43°west longitude compared to previous estimates. Brightness and locations of Ganymede's auroral ovals change periodically with position in Jupiter's magnetosphereAurora is brighter on the Jovian facing side and on the leading side of GanymedeAuroral observations can help to constrain Ganymede's dynamo magnetic field

The morphology of Io's aurora is dominated by bright spots near the equator that oscillate up and down in approximate correlation with the oscillating orientation of the Jovian magnetospheric field. Analyzing Hubble Space Telescope images, we find that the auroral spots oscillate in phase with the time‐variable Jovian magnetic field at Io and that the amplitude of the spot oscillations is reduced by 15% (±5%) with respect to the amplitude of the magnetic field oscillation. We investigate the effects of Io's plasma interaction and magnetic induction in the moon's interior on the magnetic field topology and the aurora oscillations using a magnetohydrodynamic (MHD) simulation and an analytical induction model. The results from the MHD simulation suggest that the plasma interaction has minor effects on the oscillations, while the magnetic induction generally reduces magnetic field oscillations near the surface. However, the analytical model shows that induction in any near‐surface layer for which the skin depth is larger than the thickness—like a conductive magma ocean—would induce a phase shift, in conflict with the observations. Under the assumption that the spot oscillations represent the magnetic field oscillation, we constrain the conductance of a near‐surface layer to 1 × 103S or lower. A magma ocean with conductances of 104S or higher as derived from Galileo magnetometer measurements would cause overly strong attenuation of the amplitude in addition to the irreconcilable phase shift. The observed weakly attenuated, in‐phase spot oscillation is consistent with induction in a deep, highly conductive layer like Io's metallic core. The amplitude of Io's auroral spot oscillations is reduced by 15% with respect to the amplitude of the Jovian magnetic field oscillationA conductive magma ocean should induce strong attenuation and a phase shift in the spot oscillations and appears inconsistent with the dataThe reduction of the spot oscillations is consistent with magnetic induction from a deep, highly conductive layer like Io's metallic core

We report far ultraviolet observations of Ceres obtained with the Cosmic Origin Spectrograph (COS) of the Hubble Space Telescope in the search for atomic emissions from an exosphere. The derived brightnesses at the oxygen lines at 1304 Å and 1356 Å are consistent with zero signals within the 1σpropagated statistical uncertainties. The OI 1304 Å brightness of 0.12 ± 0.20 Rayleighs can be explained by solar resonant scattering from an atomic oxygen column density of (8.2 ± 13.4) × 1010cm−2. Assuming that O is produced by photodissociation of H2O, we derive an upper limit for H2O abundance and compare it to previous observations. Our upper limit is well above the expected O brightness for a tenuous sublimated H2O exosphere, but it suggests that H2O production with a rate higher than 4 × 1026molecules s−1was not present at the time of the COS observation. Additionally, we derive an extremely low geometric albedo of ∼1% in the 1300 Å to 1400 Å range. New unprecedented far ultraviolet observations at Ceres with the Hubble Space TelescopeUpper limits for oxygen emission are derived and related to upper limits for oxygen abundanceThe found upper limit for O suggests that H2O production is not higher than during previous observations

We report results of Hubble Space Telescope observations from Ganymede's orbitally trailing side which were taken around the flyby of the Juno spacecraft on 7 June 2021. We find that Ganymede's northern and southern auroral ovals alternate in brightness such that the oval facing Jupiter's magnetospheric plasma sheet is brighter than the other one. This suggests that the generator that powers Ganymede's aurora is the momentum of the Jovian plasma sheet north and south of Ganymede's magnetosphere. Magnetic coupling of Ganymede to the plasma sheet above and below the moon causes asymmetric magnetic stresses and electromagnetic energy fluxes ultimately powering the auroral acceleration process. No clear statistically significant timevariability of the auroral emission on short time scales of 100s could be resolved. We show that electron energy fluxes of several tens of mW m−2are required for its OI 1,356 Å emission making Ganymede a very poor auroral emitter. Jupiter's moon Ganymede is the largest moon in the solar system and the only known moon with an intrinsic magnetic field and two auroral ovals around its north and south poles. Earth also possesses two auroral ovals, which are bands of emission around its poles. This emission is also referred to as northern and southern lights. We use the Hubble Space Telescope to observe Ganymede's aurora around the time when NASA's Juno spacecraft had a close flyby at Ganymede. We find that the brightness of the northern and southern ovals alternate in intensity with a period of 10 hr. Additionally, we derive that an energy flux of several tens of milli‐Watt per square meter is necessary to power the auroral emission. This energy flux comes from energetic electrons accelerated in the vicinity of Ganymede. Hubble Space Telescope observations of Ganymede's orbitally trailing hemisphere on 7 June 2021 in support of Juno flybyBrightness ratio of northern and southern auroral ovals oscillates such that the oval facing the Jovian plasma sheet is brighterOscillation suggests the aurora is driven by magnetic stresses coupling the moon's magnetic field to the surrounding Jovian plasma sheet Hubble Space Telescope observations of Ganymede's orbitally trailing hemisphere on 7 June 2021 in support of Juno flyby Brightness ratio of northern and southern auroral ovals oscillates such that the oval facing the Jovian plasma sheet is brighter Oscillation suggests the aurora is driven by magnetic stresses coupling the moon's magnetic field to the surrounding Jovian plasma sheet

We report the detection of atomic chlorine emissions in the atmosphere of Io using Hubble Space Telescope observations with the Goddard High Resolution Spectrograph (GHRS). The Cl I l1349 dipole allowed and Cl I] l1386 forbidden transition multiplets are detected at a signal-to-noise ratio of 6 and 10, respectively, in a combined GHRS spectrum acquired from 1994 through 1996. Oxygen and sulfur emissions are simultaneously detected with the chlorine, which allows for self-consistent abundance ratios of chlorine to these other atmospheric species. The disk-averaged ratios are Cl/O = 0.017 +- 0.008, Cl/S = 0.10 +- 0.05, and S/O = 0.18 +- 0.08. We also derive a geometric albedo of 1.0% +- 0.4% for Io at 1335 A, assuming an SO2 atmospheric column density of 1 x 1016 cm-2.

The spectrum of the Io plasma torus in the range 905-1187 A was recorded at 0.26 A resolution by the Far Ultraviolet Spectroscopic Explorer (FUSE) on 2001 January 14. Five orbits of data were obtained with the west ansa of the torus centered and tracked in the 30'' x 30'' apertures of the FUSE spectrographs for a total observation time of 9740 s. This region of the spectrum is dominated by transitions of S II, S III and S IV, whose multiplet structure is nearly completely resolved. We confirm our earlier detection of emission from resonance multiplets of Cl III and Cl II and derive an abundance of Cl+2 of 2.1% relative to S2+, leading to an overall chlorine ion abundance in the torus of slightly less than 1%. A number of features near 990 A remain unidentified, and C III l977 is detected in two independent channels at the 3 s level. The inferred relative ion abundance of C2+ relative to S2+ is 3.7 x 10-4. We also present spectra at 0.085 A resolution taken on 2001 October 19 and 21 with the 4'' x 20'' aperture. In these spectra, the observed lines are resolved, and their widths correspond to ion temperatures of 60-70 eV for all three sulfur ions.

The J0-ingress radio occultation of the Galileo orbiter by Jupiter exhibits a system of well-defined, regularly spaced electron layers in the altitude range where the presence of gravity waves have been previously inferred. Based on the terrestrial analog of sporadic E and spread F ionospheric layers, we argue that the observed layers are a result of dynamical processes rather than chemistry. We consider the impact of upward propagating gravity waves on the plasma distribution in a H+ dominated ionosphere. The relevant physics is discussed and illustrated with an analytic, small-amplitude model. A time-dependent, 2D, large-amplitude model is developed to simulate the observed large excursions in the J0-electron density profile. We show that gravity waves with parameters consistent with the thermal structure of Jupiter's upper atmosphere are capable of creating large peaks in the electron density similar to the observed ones. The ionospheric response is extremely anisotropic with respect to the direction of wave propagation. We demonstrate that the location of the J0-ingress radio occultation on Jupiter favors large ionospheric response for waves propagating along the magnetic meridian. A wave driven plasma flux results in plasma removal above the altitude of maximum ionospheric response and plasma deposition in the region below, significantly modifying the initial steady state electron density profile.

We analyze the ultraviolet aurorae observed on Ganymede by means of the Hubble Space Telescope and compare them to similar phenomena on Earth. We find that the tenuous nature of Ganymede's atmosphere precludes excitation of the aurora by high-energy electrons and requires a local acceleration mechanism. We propose the following as plausible mechanisms for generating both the continuous background emission and the intense auroral bright spots:

An analysis and interpretation of reflected solar Lyman α intensity data acquired with the Hubble Space Telescope (HST) implies an equatorially confined atmosphere with SO2column densities ∼ 1–2 × 1016cm-2. Poleward of 30° the SO2density must decrease sharply reaching an asymptotic polar value of < 1015cm-2at 45° to achieve the observed 2 kR intensity peaks. The corresponding surface reflectivities must be either a constant 0.047 for higher equatorial SO2or a variable reflectivity of 0.027 with lower SO2densities at the equator increasing to a polar value of ∼ 0.05. The average residence time for an atmospheric SO2molecule is ∼ 2–3 days for the canonical mass loading rate of the Io plasma torus = 1030amu s-1. With atomic hydrogen in the atmosphere and corona constrained by the HST observations, it is estimated that a pickup proton density ratio of 0.25–0.4% can be sustained by a supply of Io plasma torus protons neutralized in Io's atmosphere/exosphere, if protons constitute 7% of the total torus ion density, which is close to the Chust et al.(1999) pickup proton density ratio and under the widely quoted 10% proton content of the torus.

We report new observations of the spectrum of Ganymede in the spectral range 1160-1720 A made with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) on 1998 October 30. The observations were undertaken to locate the regions of the atomic oxygen emissions at 1304 and 1356 A, previously observed with the Goddard High Resolution Spectrograph on HST, that Hall et al. claimed indicated the presence of polar aurorae on Ganymede. The use of the 2'' wide STIS slit, slightly wider than the disk diameter of Ganymede, produced objective spectra with images of the two oxygen emissions clearly separated. The O I emissions appear in both hemispheres, at latitudes above |40|deg, in accordance with recent Galileo magnetometer data that indicate the presence of an intrinsic magnetic field such that Jovian magnetic field lines are linked to the surface of Ganymede only at high latitudes. Both the brightness and relative north-south intensity of the emissions varied considerably over the four contiguous orbits (5.5 hr) of observation, presumably because of the changing Jovian plasma environment at Ganymede. However, the observed longitudinal nonuniformity in the emission brightness at high latitudes, particularly in the southern hemisphere, and the lack of pronounced limb brightening near the poles are difficult to understand with current models. In addition to observed solar H I Lya reflected from the disk, extended Lya emission resonantly scattered from a hydrogen exosphere is detected out to beyond two Ganymede radii from the limb, and its brightness is consistent with the Galileo UVS measurements of Barth et al.

Heating Jupiter's thermosphere by viscous dissipation of upward propagating gravity waves is evaluated with correct formulations of total energy conservation and the total wave induced vertical energy flux. In contrast to the results of L. A. Young et al.(1997, Science276, 108–111), our calculations, with their wave amplitudes and parameters, yield a maximum thermospheric temperature of T=380 K at 552 km above the 1-bar level in comparison to the Galileo probe inferred temperature of T=900 K and therefore gravity waves may not be solely responsible for the observed steep temperature gradient just above the homopause. The large sensible heat flux associated with dissipating gravity waves generates net heating of the lower regions and net cooling of the upper regions of wave dissipation due to energy redistribution. The transition from net heating to net cooling occurs at the level of constant wave amplitude. In regions of substantial wave dissipation the local cooling rate due to sensible heat flux divergence can exceed the local heating due to convergence of the Eliassen–Palm flux to produce (1) net cooling of and (2) a distinct temperature decrease (≈65K) in the topside thermosphere. To simulate Jupiter's thermospheric temperature profile inferred from the Galileo probe data with (1) gravity wave heating only, (2) 100% conversion of wave energy to internal energy, and (3) radiative cooling by H+3near-IR emission ∼0.1 erg cm−2s−1, gravity waves must deposit their energy high in the thermosphere with peak heating occurring near ∼500 and ∼1000 km with near saturation amplitudes at very high altitudes (>1100 km).

We have marginally detected two CO absorption bands in the UV reflection spectrum of Neptune measured by the HST Faint Object Spectrograph (FOS). They are the (1, 0) and (0,0) Cameron bands at 1992 and 2063 Å. The interpretation of these features with a radiative transfer model—including Rayleigh, Raman, and particulate scattering processes—implies a CO mole fraction of (2.7 ± 1.8) × 10−6in the 30–800 mbar pressure range. The total uncertainty takes into account the statistical error bars as well as systematic effects (correction for straylight in the FOS data, temperature dependence of the CO absorption). Our CO mole fractions are in agreement with previous determinations made from millimeter measurements of CO rotational lines both in the stratosphere and in the troposphere. Carbon monoxide thus appears to be well-mixed in Neptune's atmosphere, which would confirm its internal origin. We have also examined the possible influence of NH3and HC3N on Neptune's spectrum in the 1950–2150 Å range, but could only estimate upper limits for these species (respectively 3 × 10−4and 6 × 10−3cm-am) that are much larger than the abundances predicted by current photochemical models.

A review of the important constraints on gravity wave induced diffusion of chemical tracers, heat, and momentum is given. Ground-based microwave spectroscopy measurements of H2O and CO and rocket-based mass spectrometer measurements of Ar constrain the eddy diffusion coefficient for constituent transport (Kzz) to be (1–3)×105 cm2s-1 in the upper mesosphere. Atomic oxygen data also limitsKzz to a comparable value at the mesopause. From the energy balance of the upper mesosphere the eddy diffusion coefficient for heat transport (DH) is, at most 6×105 cm2s-1 at the mesopause and decreasing substantially with decreasing altitude. The available evidence for mean wind deceleration and the corresponding eddy diffusion coefficient for momentum stresses (DM) suggests that it is at least 1×106 cm2s-1, in the upper mesosphere. Consequently the eddy Prandtl number for macroscopic scale lengths is >3.

Ultraviolet spectra of Titan obtained with the Faint Object Spectrograph of the Hubble Space Telescope in October 1991 and August 1992 have yielded a disk-average geometric albedo of 0.02–0.044 from 1800–3300 Å. These results are in excellent agreement with previous UV measurements from 2200–3300 Å spanning two decades, but have a significantly higher signal to noise ratio and spectral resolution. We add ∼400 Å of new spectral information from 1800–2200 Å, a wavelength region in which Titan has not been detected previously. The albedo above 2200 Å is a factor of 1.18 higher in August 1992 than October 1991, which is most likely due to the 4.3° difference in Titan's phase angle between the two dates. Below 2200 Å the albedo decreases only modestly with decreasing wavelength and does not show unambiguous evidence for discrete spectral features characteristic of several of the known hydrocarbons, particularly acetylene (C2H2). Comparison of the albedo with best-fit models provides constraints on the optical properties and distribution of the small particle component of Titan's haze and on the C2H2mole fraction. The small particle haze seems to be darker in the UV than the “Titan” tholins analog produced in the laboratory, and the vertical extent limited to altitudes above ∼120–150 km. The acetylene distribution is consistent with the Voyager IRIS determination in the 130- to 180-km altitude range, where the mole fraction is of order 2 × 10−6.

A radiative-thermal conduction model for the vertical thermal structure of Io's atmosphere is developed with solar heating by SO2absorption in UV and near-IR bands and non-LTE cooling by SO2v1, v2, v3, vibrational bands and rotational lines. The model predicts the existence of a mesopause in Io's atmosphere when the surface pressure exceeds ∼10 nbar. The radiative time constant for establishing a mesosphere/mesopause on Io is only ∼20 min, whereas the thermaspheric radiative time constant is about 1 hr. These time constants are significantly shorter than the diurnal time scale and competitive with dynamical time scales. In the thermosphere when solar UV heating dominates, the asymptotic thermospheric temperature is ∼270 K, only 140 K greater than the surface temperature because at high altitudes non-LTE cooling by SO2rotation lines exceeds cooling in the v2vibrational band. Solar-heating-only models are incapable of generating warm enough atmospheres to satisfy the observational inferences from UV and especially millimeter-wave measurements. Joule heating driven by the penetration of Jupiter's corotational electric field into Io's conducting ionosphere is demonstrated to be the dominant heating mechanism in the subnanobar regions of Io's atmosphere with temperatures ranging from 150 to 1000 K as a function of decreasing pressure from 1 to 0.1 nbar. The asymptotic thermaspheric temperature can attain a value as high as 1800 K.

We present an updated model for the photochemistry of Io's atmosphere and ionosphere and use this model to investigate the sensitivity of the chemical structure to vertical transport rates. SO2is assumed to be the dominant atmospheric gas, with minor molecular sodium species such as Na2S or Na2O released by sputtering or venting from the surface. Photochemical products include SO, O2, S, O, Na, NaO, NaS, and Na2. We consider both “thick” and “thin” SO2atmospheres that encompass the range allowed by recent HST and millimeter-wave observations, and evaluate the possibility that O2and/or SO may be significant minor dayside constituents and therefore likely dominant nightside gases. The fast reaction between S and O2limits the column abundance of O2to ∼104less than that calculated by Kumar (J. Geophys. Res.87, 1677–1684, 1982; 89(A9), 7399–7406, 1984) for a pure sulfur/oxygen atmosphere. If a significant source of NaO2or Na2O were supplied by the surface and mixed rapidly upward, then oxygen liberated in the chemical reactions which also liberate free Na would provide an additional source of O2. Fast eddy mixing will enhance the transport of molecular sodium species to the exobase, in addition to increasing the vertical transport rate of ions. Ions produced in the atmosphere will be accelerated by the reduced corotation electric field penetrating the atmosphere. These ions experience collisions with the neutral gas, leading to enhanced vertical ion diffusion. The dominant ion, Na+, is lost primarily by charge exchange with Na2O and/or Na2S in the lower atmosphere and by diffusion through the ionopause in the upper atmosphere. The atmospheric column abundance of SO, O2, and the upper atmosphere escape rates of Na, S, O, and molecular sodium species are all strong functions of the eddy mixing rate. Most atmospheric escape, including that of molecular sodium species, probably occurs from the low density “background” SO2atmosphere, while a localized high density “volcanic” SO2atmosphere can yield an ionosphere consistent with that detected by the Pioneer 10 spacecraft.

We use recent measurements of OH made with the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) along with satellite observations of water vapor abundances, to study odd‐hydrogen photochemistry in the mesosphere. The MAHRSI data sampled primarily the morning part of the diurnal variation of OH, and here we focus on one orbit of data that is representative of the MAHRSI observations during the mission. Our approach is to use a photochemical model, with input H2O abundances fixed to observed values, to simulate the diurnal variation of mesospheric HOxspecies. Models of OH using standard recommended HOxchemical rate coefficients are found to substantially overpredict OH between 5065 km. Proposed modifications to HOxchemistry that produce lower OH and higher HO2and O3, yield better agreement in the lower mesophere but worsen the agreement at the observed OH peak near∼70 km. We find that neither standard HOxchemistry, nor models incorporating proposed HOxmodifications, adequately reproduce the observed OH density profile over the entire 50–75 km altitude range for any of the observed early to mid morning local times.

A radiative–conductive model for the vertical thermal structure of Pluto's atmosphere is developed with a non-LTE treatment of solar heating in the CH43.3 μm and 2.3 μm bands, non-LTE radiative exchange and cooling in the CH47.6 μm band, and LTE cooling by CO rotational line emission. The model includes the effects of opacity and vibrational energy transfer in the CH4molecule. Partial thermalization of absorbed solar radiation in the CH43.3 and 2.3 μm bands by rapid vibrational energy transfer from the stretch modes to the bending modes generates high altitude heating at sub-microbar pressures. Heating in the 2.3 μm bands exceeds heating in 3.3 μm bands by approximately a factor of 6 and occurs predominantly at microbar pressures to generate steep temperature gradients ∼10–20 K km−1forp> 2 μbar when the surface or tropopause pressure is ∼3 μbar and the CH4mixing ratio is a constant 3%. This calculated structure may account for the “knee” in the stellar occultation lightcurve. The vertical temperature structure in the first 100 km above the surface is similar for atmospheres with Ar, CO, and N2individually as the major constituent. If a steep temperature gradient ∼20 K km−1is required near the surface or above the tropopause, then the preferred major constituent is Ar with 3% CH4mixing ratio to attain a calculated ratio ofT/M(= 3.5 K amu−1) in agreement with inferred values from stellar occultation data. However, pure Ar and N2ices at the same temperature yield an Ar vapor pressure of only ∼0.04 times the N2vapor pressure. Alternative scenarios are discussed that may yield acceptable fits with N2as the dominant constituent. One possibility is a 3 μbar N2atmosphere with 0.3% CH4that has 106 K isothermal region (T/M= 3.8 K amu−1) and ∼8 K km−1surface/tropopause temperature gradient. Another possibility would be a higher surface pressure ∼10 μbar with a scattering haze forp> 2 μbar. Our model with appropriate adjustments in the CH4density profile to Triton's inferred profile yields a temperature profile consistent with the UVS solar occultation data (Krasnopolsky, V. A., B. R. Sandel, and F. Herbert 1992.J. Geophys. Res.98, 3065–3078.) and ground-based stellar occultation data (Elliot, J. L., E. W. Dunham, and C. B. Olkin 1993.Bull. Am. Astron. Soc.25, 1106.).

Simultaneous ultraviolet spectra and images of the north polar aurora of Jupiter were obtained on 1995 March 9 with the Hopkins Ultraviolet Telescope (HUT) and the HubbleSpaceTelescope(HST). This unique data set contains a well-calibrated flux measurement that includes all of the H2Lyman and Werner band emission from 900 to 1650 Å, as well as information about the spatial extent of the aurora. For these observations, we oriented the 10'' × 57'' slit of the HUT spectrograph parallel to the equator of Jupiter so that it was underfilled by the aurora and minimally contaminated by extraneous reflected solar light from low to mid latitudes. The aurora was imaged simultaneously using the HSTWide Field and Planetary Camera 2 (WFPC2). Images were taken through a Wood's filter and also through a Wood's filter/CaF2filter combination. These images have recorded the aurora in the light of atomic and molecular hydrogen with relatively low reflected solar contamination. The molecular hydrogen aurora is found to extend over 4.5 ± 0.5 × 10-10sr, with an average brightness of 106 ± 12 kR integrated over the band of wavelengths from 900 to 1650 Å, excluding H I?1216. We measure a 13% larger emitting area (5.1 ± 0.5 × 10-10sr) and significantly different morphology for the combined atomic and molecular hydrogen aurora (Ly? + H2), although these differences could be a result of time variability. Color ratios derived from the data indicate a total auroral energy deposition rate of 13 ergs s-1cm-2by 3-30 keV electrons. We find little evidence for ions as primary auroral particles in the HUT spectra. Our analysis places upper limits on sulfur ion emissions that are an order of magnitude lower than those from the IUEobservations, and, therefore, the emissions could be entirely from the Io torus in the background of the planet.

Far-ultraviolet spectra of Jupiter obtained on 1994 July 20 in the region of the Shoemaker-Levy 9 G-fragment impact site contain features that differ markedly from the typical Jovian dayglow spectrum. These features are identified as fluorescence of H2due to pumping by solar Ly? photons, which requires the presence of a substantial population of H2in the second excited vibrational state; normal atmospheric temperatures preclude such an elevated H2(v'' = 2) population in thermal equilibrium. The observed fluorescence includes approximately 600 rayleighs (R) of Ly?-induced fluorescence in addition to the 500 R of solar fluorescence estimated for typical Jovian dayglow at the time of the observations. The weaker (14, v'') transitions of the CO fourth positive band system are also excited primarily by resonant and fluorescent scattering of solar Ly? photons. As a result of their low-oscillator strengths and the large Ly? flux, these bands remain unsaturated up to column densities ?1017cm-2, therefore providing reliable upper limits for the CO band emission produced by the (14, 3), (14, 4), and (14, 5) transitions at 1317, 1354, and 1392 Å, respectively. Comparing the derived upper limits to estimates inferred from millimeter-wave data, we demonstrate that there is no contradiction between observations in the two spectral regions.

The Voyager 1 Ultraviolet Spectrometer (UVS) solar occultation data and a spectrum of the North Polar Region dayglow obtained during the Titan flyby are analyzed to infer an upper limit on the tropopause argon mixing ratio as a function of the tropopause CH₄ mixing ratio, ƒ_CH4. The spectroscopic upper limit on the tropopause Ar mixing ratio (mole fraction), ƒ_Ar, varies from 0.15 at ƒ_CH4 = 0.01 to 0.1 at ƒ_CH4 = 0.026 and decreasing to 0.08 at ƒ_CH4 = 0.05. When combined with the mean molecular mass constraint of E. Lellouch, A. Coustenis, D. Gautier, F. Raulin, N. Dubouloz, and C. Frere (1989, Icarus 79, 328-349), ƒ_Ar &lt; 0.17 for any ƒ_CH4. The best fits to the solar occultation data require ƒ_CH4 ≥ 0.026; thus ƒ_Ar ≤ 0.1. Copyright 1993, 1999 Academic Press

The Voyager 2 Ultraviolet Spectrometer (UVS) stellar and solar occultation data obtained at Uranus have been analyzed to infer smooth thermospheric temperature profiles and heat source characteristics. Temperatures inferred in the upper thermosphere from these occultations are much higher (850 ± 100 K) than can be explained by solar extreme ultraviolet heating alone. Our analysis yields one heat source peaking at ∼1 × 10-5μbar (range 8 × 10-7-2 × 10-5micro;bar) with an integrated heating rate of approximately 0.06 (+0.02, -0.04) erg cm-2sec-1as the least-squares best fit to all occultation data. The best inferred temperature for the stellar entrance occultation (63.7° S) is at least 50 K cooler near 50 μbar than for the stellar exit occultation (69.7° N), whereas at about 0.1 μbar this temperature contrast is less than 10 K. A constant temperature on pressure surfaces at low altitudes, however, cannot be excluded by the data.

The photochemistry of atmospheric constituents in the cold atmospheres of Jupiter, Saturn and Titan is reviewed: H2, CH4, NH3, PH3, H20, N2, etc. A detailed discussion of major dissociation paths and essential chemical reactions is given with the emphasis on a first-order description of the concentrations of simple molecules in the photochemical regions of these atmospheres. The composition as measured by Voyager spacecraft instruments, IUE and ground-based telescopes is compared with the predictions of aeronomical models and generally acceptable agreement is found for Jupiter, provided the C2H6 mixing ratio is ∼ 1 × 10-6. More complex molecules do not exceed ppb mixing ratios as a consequence of large H-atom concentrations and the efficiency of H-atom cracking reactions. Even on Titan with lower H-atom concentrations these reactions are apparently effective in suppressing the concentrations of C4H2, C3H4, HC3N, and C2N2.

Partial frequency redistribution (PRD), describing the formation of the line profile, has negligible observational effects for optical depths smaller than [?]103, at the resolving power of most current instruments. However, when the spectral resolution is sufficiently high, PRD modeling becomes essential in interpreting the line shapes and determining the total line fluxes. We demonstrate the effects of PRD on the H2 line profiles observed at high spectral resolution by the Far-Ultraviolet Spectroscopic Explorer (FUSE) in the atmospheres of Jupiter and Saturn. In these spectra, the asymmetric shapes of the lines in the Lyman (v''-6) progression pumped by the solar Lyb are explained by coherent scattering of the photons in the line wings. We introduce a simple computational approximation to mitigate the numerical difficulties of radiative transfer with PRD and show that it reproduces the exact radiative transfer solution to better than 10%. The lines predicted by our radiative transfer model with PRD, including the H2 density and temperature distribution as a function of height in the atmosphere, are in agreement with the line profiles observed by FUSE. We discuss the observational consequences of PRD and show that this computational method also allows us to include PRD in modeling the continuum pumped H2 fluorescence treating about 4000 lines simultaneously.