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The atmosphere of Jupiter's volcanic moon Io consists of mainly sulfur dioxide (SO2), and this main constituent has been studied with a variety of observing techniques across many wavelengths over the years. Here we study absorption by SO2 at the hydrogen Ly-α line (1216 A) in a large set of images taken by the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST) between 1997 and 2018. An advanced statistical analysis using a Monte-Carlo trial method is applied to derive the SO2 column density from the Ly-α intensity, which includes the uncertainties of the used variables such as solar and background flux. Our analysis produces a probability distribution function of the SO2 column density and highlights some short-comings of the observing technique. Most importantly, the HST/STIS images of the surface-reflected Ly-α flux are only sensitive to SO2 column densities between ~1015 cm−2 and ~5×1016 cm−2. Due to strong non-linearity in the relationship between the SO2 abundance and the Ly-α flux at the low and high values of detected flux, SO2 abundance directly retrieved from the STIS images will generally fall within these boundaries. This explains the relatively low equatorial column density of about 1016 cm−2 reported by previous studies using the Ly-α images (e.g., Feldman et al., 2000; Feaga et al., 2009) compared to other studies (e.g., Spencer et al., 2005; Tsang et al., 2013; Jessup and Spencer, 2015; Lellouch et al., 2015), where the obtained column density is often 1017 cm−2. By assuming a log-normal probability distribution function for the column density, a new estimate of the SO2 column density is then fitted, indirectly accounting for abundances beyond the detectability limits. This method suggests slightly higher equatorial SO2 abundances and much larger upper-limits, revealing that the Ly-α observations are in fact consistent with the higher abundances found in other studies. We then investigate the SO2 abundances at three volcanic sites (Loki, Marduk, Thor), where plumes were observed before and where the sensitivity in our images is comparably high. The observations did not reveal transient changes due to local outgassing at any of the three sites. Finally, the heliocentric distance of Io changed from 4.95 AU to 5.45 AU between the observation dates, potentially allowing us to investigate the influence of solar intensity changes on the SO2 column density via surface frost sublimation. However, the derived error bars are significantly larger than the derived variability, preventing any firm conclusion on seasonal changes and local volcanic outgassing.

We analyzed two observations obtained in Jan. 2013, consisting of spatial scans of the jovian north ultraviolet aurora with the HST Space Telescope Imaging Spectrograph (STIS) in the spectroscopic mode. The color ratio (CR) method, which relates the wavelength-dependent absorption of the FUV spectra to the mean energy of the precipitating electrons, allowed us to determine important characteristics of the entire auroral region. The results show that the spatial distribution of the precipitating electron energy is far from uniform. The morning main emission arc is associated with mean energies of around 265 keV, the afternoon main emission (kink region) has energies near 105 keV, while the 'flare' emissions poleward of the main oval are characterized by electrons in the 50-85. keV range. A small scale structure observed in the discontinuity region is related to electrons of 232 keV and the Ganymede footprint shows energies of 157 keV. Interestingly, each specific region shows very similar behavior for the two separate observations.The Io footprint shows a weak but undeniable hydrocarbon absorption, which is not consistent with altitudes of the Io emission profiles (~900 km relative to the 1 bar level) determined from HST-ACS observations. An upward shift of the hydrocarbon homopause of at least 100 km is required to reconcile the high altitude of the emission and hydrocarbon absorption. The relationship between the energy fluxes and the electron energies has been compared to curves obtained from Knight's theory of field-aligned currents. Assuming a fixed electron temperature of 2.5 keV, an electron source population density of ~800 m-3 and ~2400 m-3 is obtained for the morning main emission and kink regions, respectively. Magnetospheric electron densities are lowered for the morning main emission (~600 m-3) if the relativistic version of Knight's theory is applied.Lyman and Werner H2 emission profiles, resulting from secondary electrons produced by precipitation of heavy ions in the 1-2 MeV/u range, have been applied to our model. The low CR obtained from this emission suggests that heavy ions, presumably the main source of the X-ray aurora, do not significantly contribute to typical UV high latitude emission.

We report on an investigation of the spatial distribution of Io’s atmosphere, and its diurnal variability, using Hubble’s Space Telescope Imaging Spectrograph (HST/STIS). From December 2010 to January 2012, we obtained spatially resolved limb-to-limb 2100–3100 A spectra at low latitudes (

From STIS observations of Uranus in 2012, we found that the methane volume mixing ratio declined from about 4% at low latitudes to about 2% at 60 deg N and beyond. This is similar to that found in the south polar regions in 2002, in spite of what appears to be strikingly different convective activity in the two regions. Keck and HST imaging observations close to equinox imply that the depletions were simultaneously present in 2007, suggesting they are persistent features. The depletions appear to be mainly restricted to the upper troposphere, with depth increasing poleward from about 30 deg N, reaching ~4 bars at 45 deg N and perhaps much deeper at 70 deg N. The latitudinal variations in degree and depth of the depletions are important constraints on models of meridional circulation. Our observations are qualitatively consistent with previously suggested circulation cells in which rising methane-rich gas at low latitudes is dried out by condensation and sedimentation of methane ice particles as the gas ascends to altitudes above the methane condensation level, then is transported to high latitudes, where it descends and brings down methane depleted gas. Since this cell would seem to inhibit formation of condensation clouds in regions where clouds are actually inferred from spectral modelling, it suggests that sparse localized convective events may be important in cloud formation. The small-scale latitudinal variations we found in the effective methane mixing ratio between 55 deg N and 82 deg N have significant inverse correlations with zonal mean latitudinal variations in cloud reflectivity in near-IR Keck images taken before and after the HST observations. If the CH4/H2 absorption ratio variations are interpreted as local variations in para fraction instead of methane mixing ratio, we find that downwelling correlates with reduced cloud reflectivity., Comment: 20 pages, 19 figures, 4 tables, on-line supplemental material available

Io’s sublimation atmosphere is inextricably linked to the SO 2 surface frost temperature distribution which is poorly constrained by observations. We constrain Io’s surface thermal distribution by a parametric study of its thermophysical properties in an attempt to better model the morphology of Io’s sublimation atmosphere. Io’s surface thermal distribution is represented by three thermal units: sulfur dioxide (SO 2 ) frosts/ices, non-frosts (probably sulfur allotropes and/or pyroclastic dusts), and hot spots. The hot spots included in our thermal model are static high temperature surfaces with areas and temperatures based on Keck infrared observations. Elsewhere, over frosts and non-frosts, our thermal model solves the one-dimensional heat conduction equation in depth into Io’s surface and includes the effects of eclipse by Jupiter, radiation from Jupiter, and latent heat of sublimation and condensation. The best fit parameters for the SO 2 frost and non-frost units are found by using a least-squares method and fitting to observations of the Hubble Space Telescope’s Space Telescope Imaging Spectrograph (HST STIS) mid- to near-UV reflectance spectra and Galileo PPR brightness temperature. The thermophysical parameters are the frost Bond albedo, α F , and thermal inertia, Γ F , as well as the non-frost surface Bond albedo, α NF , and thermal inertia, Γ NF . The best fit parameters are found to be α F ≈ 0.55 ± 0.02 and Γ F ≈ 200 ± 50 J m −2 K −1 s −1/2 for the SO 2 frost surface and α NF ≈ 0.49 ± 0.02 and Γ NF ≈ 20 ± 10 J m −2 K −1 s −1/2 for the non-frost surface. These surface thermophysical parameters are then used as boundary conditions in global atmospheric simulations of Io’s sublimation-driven atmosphere using the direct simulation Monte Carlo (DSMC) method. These simulations are unsteady, three-dimensional, parallelized across 360 processors, and include the following physical effects: inhomogeneous surface frosts, plasma heating, and a temperature-dependent residence time on the non-frost surface. The DSMC simulations show that the sub-jovian hemisphere is significantly affected by the daily solar eclipse. The simulated SO 2 surface frost temperature is found to drop only ∼5 K during eclipse due to the high thermal inertia of SO 2 surface frosts but the SO 2 gas column density falls by a factor of 20 compared to the pre-eclipse column due to the exponential dependence of the SO 2 vapor pressure on the SO 2 surface frost temperature. Supersonic winds exist prior to eclipse but become subsonic during eclipse because the collapse of the atmosphere significantly decreases the day-to-night pressure gradient that drives the winds. Prior to eclipse, the supersonic winds condense on and near the cold nightside and form a highly non-equilibrium oblique shock near the dawn terminator. In eclipse, no shock exists since the gas is subsonic and the shock only reestablishes itself an hour or more after egress from eclipse. Furthermore, the excess gas that condenses on the non-frost surface during eclipse leads to an enhancement of the atmosphere near dawn. The dawn atmospheric enhancement drives winds that oppose those that are driven away from the peak pressure region above the warmest area of the SO 2 frost surface. These opposing winds meet and are collisional enough to form stagnation point flow. The simulations are compared to Lyman-α observations in an attempt to explain the asymmetry between the dayside atmospheres of the anti-jovian and sub-jovian hemispheres. Lyman-α observations indicate that the anti-jovian hemisphere has higher column densities than the sub-jovian hemisphere and also has a larger latitudinal extent. A composite “average dayside atmosphere” is formed from a collisionless simulation of Io’s atmosphere throughout an entire orbit. This composite “average dayside” atmosphere without the effect of global winds indicates that the sub-jovian hemisphere has lower average column densities than the anti-jovian hemisphere (with the strongest effect at the sub-jovian point) due primarily to the diurnally averaged effect of eclipse. This is in qualitative agreement with the sub-jovian/anti-jovian asymmetry in the Lyman-α observations which were alternatively explained by the bias of volcanic centers on the anti-jovian hemisphere. Lastly, the column densities in the simulated average dayside atmosphere agree with those inferred from Lyman-α observations despite the thermophysical parameters being constrained by mid- to near UV observations which show much higher instantaneous SO 2 gas column densities. This may resolve the apparent discrepancy between the lower “average dayside” column densities observed in the Lyman-α and the higher instantaneous column densities observed in the mid- to near UV.

Conflicting observations regarding the dominance of either sublimation or volcanism as the source of the atmosphere on Io and disparate reports on the extent of its spatial distribution and the absolute column abundance invite the development of detailed computational models capable of improving our understanding of Io’s unique atmospheric structure and origin. Improving upon previous models, Walker et al. (Walker, A.C., Gratiy, S.L., Levin, D.A., Goldstein, D.B., Varghese, P.L., Trafton, L.M., Moore, C.H., Stewart, B. [2009]. Icarus) developed a fully 3-D global rarefied gas dynamics model of Io’s atmosphere including both sublimation and volcanic sources of SO 2 gas. The fidelity of the model is tested by simulating remote observations at selected wavelength bands and comparing them to the corresponding astronomical observations of Io’s atmosphere. The simulations are performed with a new 3-D spherical-shell radiative transfer code utilizing a backward Monte Carlo method. We present: (1) simulations of the mid-infrared disk-integrated spectra of Io’s sunlit hemisphere at 19 μm, obtained with TEXES during 2001–2004; (2) simulations of disk-resolved images at Lyman- α obtained with the Hubble Space Telescope (HST), Space Telescope Imaging Spectrograph (STIS) during 1997–2001; and (3) disk-integrated simulations of emission line profiles in the millimeter wavelength range obtained with the IRAM-30 m telescope in October–November 1999. We found that the atmospheric model generally reproduces the longitudinal variation in band depth from the mid-infrared data; however, the best match is obtained when our simulation results are shifted ∼30° toward lower orbital longitudes. The simulations of Lyman- α images do not reproduce the mid-to-high latitude bright patches seen in the observations, suggesting that the model atmosphere sustains columns that are too high at those latitudes. The simulations of emission line profiles in the millimeter spectral region support the hypothesis that the atmospheric dynamics favorably explains the observed line widths, which are too wide to be formed by thermal Doppler broadening alone.

An extensive set of HI Lyman-α images obtained with the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) from 1997–2001 has been analyzed to provide information about the spatial and temporal character of Io's SO2 atmosphere. An atmospheric distribution map derived from the observations reveals that the sunlit SO2 atmosphere is temporally stable on a global scale, with only small local changes. An anti-/sub-jovian asymmetry in the SO2 distribution is present in all 5 years of the observations. The average daytime atmosphere is densest on the anti-jovian hemisphere in the equatorial regions, with a maximum equatorial column density of 5.0 × 10 16 cm − 2 at 140° longitude. The SO2 atmosphere also has greater latitudinal extent on the anti-jovian hemisphere as compared to the sub-jovian. The atmospheric distribution appears to be best correlated with the location of hot spots and known volcanic plumes, although small number statistics for the plumes limits the correlation.

We report on the analysis of high spatial resolution visible to near-infrared spectral images of Titan at Ls=240°Ls=240° in November 2000, obtained with the Space Telescope Imaging Spectrograph instrument on board the Hubble Space Telescope as part of program GO-8580. We employ a radiative transfer fractal particle aerosol model with a Bayesian parameter estimation routine that computes Titan's absolute reflectivity per pixel for 122 wavelengths by modeling the vertical distribution of the lower atmosphere haze and tropospheric methane. Analysis of these data suggests that Titan's haze concentration in the lower atmosphere varies in strength with latitude. We find Titan's tropospheric methane profile to be fairly consistent with latitude and longitude, and we find evidence for local areas of a CH4–N2 binary saturation in Titan's troposphere. Our results suggest that a methane and haze profile at one location on Titan would not be representative of global conditions.

The Cassini spacecraft Ultraviolet Imaging Spectrograph (UVIS) obtained observations of Jupiter's auroral emissions in H-2 band systems and H Lyman-alpha from day 275 of 2000 (October 1), to day 81 of 2001 (March 22). Much of the globally integrated auroral variability measured with UVIS can be explained simply in terms of the rotation of Jupiter's main auroral arcs with the planet. These arcs were also imaged by the Space Telescope Imaging Spectrograph (STIS) on Hubble Space Telescope (HST). However, several brightening events were seen by UVIS in which the global auroral output increased by a factor of 2-4. These events persisted over a number of hours and in one case can clearly be tied to a large solar coronal mass ejection event. The auroral UV emissions from these bursts also correspond to hectometric radio emission (0.5-16 MHz) increases reported by the Galileo Plasma Wave Spectrometer (PWS) and Cassim Radio and Plasma Wave Spectrometer (RPWS) experiments. In general, the hectometric radio data vary differently with longitude than the UV data because of radio wave beaming effects. The 2 largest events in the UVIS data were on 2000 day 280 (October 6) and on 2000 days 325-326 (November 20-21). The global brightening events on November 20-21 are compared with corresponding data on the interplanetary magnetic field, solar wind conditions, and energetic particle environment. ACE (Advanced Composition Explorer) solar wind data was numerically propagated from the Earth to Jupiter with an MHD code and compared to the observed event. A second class of brief auroral brightening events seen in HST (and probably UVIS) data that last for similar to 2 min is associated with aurora] flares inside the main auroral ovals. On January 8, 2001, from 18:45-19:35 UT UVIS H-2 band emissions from the north polar region varied quasiperiodically. The varying emissions, probably due to amoral flares inside the main auroral oval, are correlated with low-frequency quasiperiodic radio bursts in the 0.6-5 kHz Galileo PWS data. (c) 2005 Elsevier Inc. All rights reserved.

We have analyzed the Cassini Ultraviolet Imaging Spectrometer (UVIS) observations of the Jupiter aurora with an auroral atmosphere two-stream electron transport code. The observations of Jupiter by UVIS took place during the Cassini Campaign. The Cassini Campaign included support spectral and imaging observations by the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS). A major result for the UVIS observations was the identification of a large color variation between the far ultraviolet (FUV: 1100–1700 A) and extreme ultraviolet (EUV: 800–1100 A) spectral regions. This change probably occurs because of a large variation in the ratio of the soft electron flux (10–3000 eV) responsible for the EUV aurora to the hard electron flux (∼15–22 keV) responsible for the FUV aurora. On the basis of this result a new color ratio for integrated intensities for EUV and FUV was defined ( 4 π I 1550 – 1620 A / 4 π I 1030 – 1150 A ) which varied by approximately a factor of 6. The FUV color ratio ( 4 π I 1550 – 1620 A / 4 π I 1230 – 1300 A ) was more stable with a variation of less than 50% for the observations studied. The medium resolution (0.9 A FWHM, G140M grating) FUV observations (1295–1345 A and 1495–1540 A) by STIS on 13 January 2001, on the other hand, were analyzed by a spectral modeling technique using a recently developed high-spectral resolution model for the electron-excited H2 rotational lines. The STIS FUV data were analyzed with a model that considered the Lyman band spectrum (B Σ u + 1 → X Σ g + 1 ) as composed of an allowed direct excitation component (X Σ g + 1 → B Σ u + 1 ) and an optically forbidden component (X Σ g + 1 → EF , GK , H H ¯ , … Σ g + 1 followed by the cascade transition Σ g + 1 → B Σ u + 1 ). The medium-resolution spectral regions for the Jupiter aurora were carefully chosen to emphasize the cascade component. The ratio of the two components is a direct measurement of the mean secondary electron energy of the aurora. The mean secondary electron energy of the aurora varies between 50 and 200 eV for the polar cap, limb and auroral oval observations. We examine a long time base of Galileo Ultraviolet Spectrometer color ratios from the standard mission (1996–1998) and compare them to Cassini UVIS, HST, and International Ultraviolet Explorer (IUE) observations.

Although methane is the dominant absorber in Titan's reflection spectrum, the amount of methane in the atmosphere has only been determined to an order of magnitude. We analyzed spectra from the Space Telescope Imaging Spectrograph, looking at both a bright surface region (700-km radius) and a dark surface region. The difference between the spectra of the two regions is attributed to light that has scattered off the surface, and therefore made a round-trip through all of Titan's methane. Considering only absorption, the shape of the difference spectrum provides an upper limit on methane abundance of 3.5 km-am. Modeling the multiple scattering in the atmosphere further constrains the methane abundance to 2.63±0.17 km-am. In the absence of supersaturation and with a simplified methane vertical profile, this corresponds to a surface methane-mole fraction near 3.8% and a relative humidity of 0.32. With supersaturation near the tropopause, the surface methane mole fraction could be as low as 3%.

Spatially resolved spectra in four 50-A FUV spectral windows were obtained across the jovian aurora with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope. Nearly simultaneous ultraviolet imaging makes it possible to correlate the intensity variations along the STIS slit with those observed in the images and to characterize the global auroral context prevailing at the time of the observations. Spectra at ∼1-A resolution taken in pairs included an unabsorbed window and a spectral region affected by hydrocarbon absorption. Both sets of spectra correspond to an aurora with a main oval brightness of about 130 kilorayleighs of H 2 emission. The far ultraviolet color ratios I (1550–1620 A)/ I (1230–1300 A) are 2.3 and 5.9 for the noon and morning sectors of the main oval, respectively. We use an interactive model coupling the energy degradation of incoming energetic electrons, auroral temperature and composition, and synthetic H 2 spectra to fit the intensity distribution of the H 2 lines. It is found that the model best fitting globally the spectra has a soft energy component in addition to a 10 erg cm −2 s −1 flux of 80 keV electrons. It provides an effective H 2 temperature of 540 K. The relative intensity of temperature-sensitive H 2 lines indicates differences between the auroral main oval and polar cap emissions. The amount of methane absorption across the polar region is shown to vary in a way consistent with temperature. For the second spectral pair, the polar cap shows a higher attenuation by CH 4 , indicating a harder precipitation along high-latitude magnetic field lines.

Post-perihelion observations of Comet Hale–Bopp (C/1995 O1) were made with the Hubble Space Telescope (HST) on 27 August 1997 ( r =2.48 AU; Δ=2.99 AU), 11 November 1997 ( r =3.38 AU; Δ=3.31 AU), and 19 February 1998 ( r =4.44 AU; Δ=4.40 AU) using the newly installed Space Telescope Imaging Spectrograph (STIS). The STIS CCD was used to image the comet, and useful spectroscopy was obtained between 2000 and 3190 A using the G230LB, G230MB, and G230L gratings. The morphology of the images closely resembled that obtained at similar heliocentric distances preperihelion, but the dust production rates may have been slightly lower post-perihelion. We find no evidence for any companions to the nucleus in the STIS images, but fairly bright objects (up to ∼20% of the brightness of the main nucleus) could be easily “hidden” within the strong coma jets. Emissions from OH and CS were observed in the spectra and were used to derive production rates for H 2 O and CS 2 . As with the dust, the gas production rates appear to be somewhat smaller post-perihelion than preperihelion. The two-dimensional STIS data allowed us to map the spatial distribution of the OH emission with a spatial resolution of ∼0.″1, and the ∼6-A spectral resolution of the G230MB grating permitted a detailed examination of the OH excitation. The relative intensities of the rotational lines in the OH(0,0) band are fairly well matched by a standard fluorescence excitation model. The OH spatial brightness profile was slightly asymmetric (∼20% brightness differences when comparing two different directions), and the intensity peak was offset by ∼3200 km from the continuum peak for both the August and November 1997 observations. Perhaps this offset may be explained by asymmetric ejection of H 2 O molecules from the nucleus, but we have not attempted to model this effect. Except for this offset, conventional models for the spatial distribution of OH provide a good match to the data from November 1997. The observed spatial profile for the August 1997 observation is well matched by our model at large cometocentric distances but is considerably flatter near the nucleus (within ∼2″ of the continuum peak) than predicted. The available evidence strongly suggests that an optical depth effect, rather than the production of OH from a population of icy grains in the coma, is responsible for the observed flattening of the OH spatial brightness profile.