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Author(s): Xi Zhang (corresponding author) [1]; Darrell F. Strobel [2]; Hiroshi Imanaka [3, 4] Plutos atmosphere is cold and hazy [1, 2, 3]. Recent observations [1] have shown it to [...]

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 km2 in 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 km2 for Kanehekili Fluctus, and ∼5,000 km2 for 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. Plain Language Summary: 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 km2 in size for Kanehekili Fluctus and <0.1 km2 for 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. Key Points: 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 km2 or lessWe detected, for the first time, a clear association of the 1.707 micron forbidden SO emissions with an active volcano [ABSTRACT FROM AUTHOR]

One of Professor Donald M. Hunten's lasting contributions to the field of planetary atmospheres was the principle of the (Hunten) limiting flux, where the escape of light species is limited by the rate at which they can diffuse through the atmosphere. While his limiting flux expression has been well tested for hydrogen's escape from the Earth's atmosphere (e.g., Hunten and Strobel (J. Atmos. Sci. 31, 305 (1974)); Hunten and Donahue (Ann. Rev. Earth Planet Sci. 4, 265 (1976))), it has not been tested for Titan's atmosphere, which was the original motivation for the principle. The Cassini-Huygens mission has provided sufficient data on the variation of the [H.sub.2] mole fraction with altitude to test its applicability and validity. Only in the vicinity of the homopause does the limiting flux expression yield the actual [H.sub.2] escape flux, because the mole fraction varies with altitude. This paper deals also with our current understanding of the three major constituents of Titan's atmosphere ([N.sub.2], C[H.sub.4], and [H.sub.2]) from the various measurements by instruments on the Cassini orbiter and the Huygens probe. Specific problems addressed are additional required sources of [H.sub.2], the C[H.sub.4] escape rate, and the possible role of energetic electron and ion precipitation from Saturn's magnetosphere. PACS Nos.: 96.12.J-, 96.12.Jt, 96.30.nd, 96.12.Bc, 96.15.Bc Une des contributions marquantes du professeur Donald M. Hunten au champ des atmospheres planetaires concerne le principe de limitation du flux (Hunten), oh la fuite vers l'espace des gaz legers est limitee par le taux avec lequel ils peuvent diffuser a travers l'atmosphere. Bien que cette expression limitant le flux est bien verifiee dans l'atmosphere terrestre, (par exemple Hunten and Strobel (J. Atmos. Sci. 31, 305 (1974)); Hunten and Donahue (Ann. Rev. Earth Planet Sci. 4, 265 (1976))), elle n'a pas ete testee pour l'atmosphere de Titan, qui etait le motivation initiale pour le developpement du principe. La mission Cassini-Huygens a fourni suffisamment de donnees sur la variation de la fraction molaire de [H.sub.2] avec l'altitude pour pouvoir verifier son applicabilite et sa validite. Seulement dans la region de l'homopause l'expression donne-t-elle le taux correct de fuite, parce la fraction molaire varie avec l'altitude. Nous analysons egalement ici notre comprehension actuelle des trois composantes majeures de l'atmosphere de Titan, ([N.sub.2]), C[H.sub.4] et [H.sub.2]) a partir de divers instruments a bord de l'orbiteur Cassini et de la sonde Huygens. Nous etudions particulierement les sources requises additionnelles de [H.sub.2], le taux de fuite de C[H.sub.4] et le role possible des ions et electrons energetiques provenant de la magnetosphere de Saturne. [Traduit par la Redaction], 1. Introduction Arguably, two of the most important and influential papers written by Prof. Donald Hunten were on the escape of molecular hydrogen from Titan [1] and the escape of [...]

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−2 are required for its OI 1,356 Å emission making Ganymede a very poor auroral emitter. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.08.021 Byline: Darrell F. Strobel Keywords: Pluto; Pluto; atmosphere; Aeronomy; Atmospheres; dynamics; Atmospheres; structure Abstract: Hydrodynamic escape of N.sub.2 molecules from Pluto's atmosphere is calculated under the assumption of a high density, slow outflow expansion driven by solar EUV heating by N.sub.2 absorption, near-IR and UV heating by CH.sub.4 absorption, and CO cooling by rotational line emission as a function of solar activity. At 30 AU, the N.sub.2 escape rate varies from (4-6.4)x10.sup.26 moleculess.sup.-1 in the absence of heating, but driven by an upward thermal heat conduction flux from the stratosphere, for lower boundary temperatures varying from 70-100 K. With solar heating varying from solar minimum to solar maximum conditions and a calculated lower boundary temperature, 88.2 K, the N.sub.2 escape rate range is (1.8-6.7)x10.sup.27 moleculess.sup.-1, respectively. LTE rotational line emission by CO reduces the net solar heat input by at most 35% and plays a minor role in lowering the calculated escape rates, but ensures that the lower boundary temperature can be calculated by radiative equilibrium with near-IR CH.sub.4 heating. While an upward thermal conduction heat flux at the lower boundary plays a fundamental role in the absence of heating, with solar heating it is downward at solar minimum, and is, at most, 13% of the integrated net heating rate over the range of solar activity. For the arrival of the New Horizons spacecraft at Pluto in July 2015, predictions are lower boundary temperature, T.sub.0[approximately equal to]81 K, and N.sub.2 escape rate [approximately equal to]2.2x10.sup.27 moleculess.sup.-1, and peak thermospheric temperature [approximately equal to]103 K at 1890 km, based on expected solar medium conditions. Author Affiliation: Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218-2687, USA Article History: Received 21 March 2007; Revised 20 July 2007

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.02.008 Byline: Conor Laver (a), Imke de Pater (a), Henry Roe (b), Darrell F. Strobel (c) Keywords: Io; Atmospheres; composition; Jupiter; satellites; Volcanism Abstract: We report observations of the ro-vibronic a.sup.1[DELTA][right arrow]X.sup.3[SIGMA].sup.- transition of SO at 1.707 [mu]m on Io. These data were taken while Io was eclipsed by Jupiter, on four nights between July 2000 and March 2003. We analyze these results in conjunction with a previously published night to investigate the temporal behavior of these emissions. The observations were all conducted using the near-infrared spectrometer NIRSPEC on the W.M. Keck II telescope. The integrated emitted intensity for this band varies from 0.8x10.sup.27 to 2.4x10.sup.27photons/s, with a possible link to variations in Loki's infrared brightness. The band-shapes imply rotational temperatures of 550-1000 K for the emitting gas, lending further evidence to a volcanic origin for sulfur monoxide. An attempt to detect the B.sup.1[SIGMA][right arrow]X.sup.3[SIGMA].sup.- transition of SO at 0.97 [mu]m was unsuccessful; simultaneous detection with the 1.707 [mu]m band would permit determination of the SO column abundance. Author Affiliation: (a) Department of Astronomy, 601 Campbell Hall, University of California, Berkeley, CA 94720, USA (b) Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA (c) Department of Earth and Planetary Sciences and Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA Article History: Received 28 March 2006; Revised 6 February 2007

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.12.015 Byline: Darrell F. Strobel Keywords: Titan; Tides; atmospheric; Atmospheres; dynamics; Atmospheres; structure; Satellites; atmospheres Abstract: Tidal waves driven by Titan's orbital eccentricity through the time-dependent component of Saturn's gravitational potential attain nonlinear, saturation amplitudes (|T.sup.'|10K, |u.sup.'|20ms.sup.-1, and |w.sup.'|5cms.sup.-1) in the upper atmosphere ([greater than or equal to]500 km) due to the approximate exponential growth as the inverse square root of pressure. The gravitational tides, with vertical wavelengths of [approximately equal to]100-150 km above 500 km altitude, carry energy fluxes sufficient in magnitude to affect the energy balance of the upper atmosphere with heating rates [greater than or equal to]10.sup.-9ergcm.sup.-3s.sup.-1 in the altitude range of 500-900 km. Author Affiliation: EPS/Olin 121, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218-2687, USA Article History: Received 9 June 2005; Revised 16 December 2005

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.01.016 Byline: Xun Zhu (a), Darrell F. Strobel (b) Keywords: Titan; Atmospheric dynamics; Meteorology Abstract: Titan's atmospheric winds, like those on Venus, exhibit superrotation at high altitudes. Titan general circulation models have yielded conflicting results on whether prograde winds in excess of 100 mas.sup.-1 at the 1 mbar level are possible based on known physical processes that drive wind systems. A comprehensive two-dimensional (2D) model for Titan's stratosphere was constructed to systematically explore the physical mechanisms that produce and maintain stratospheric wind systems. To ensure conservation of angular momentum in the limit of no net exchange of atmospheric angular momentum with the solid satellite and no external sources and sinks, the zonal momentum equation was solved in flux form for total angular momentum. The relationships among thermal wind balance, meridional circulation, and zonal wind were examined with numerical experiments over a range of values for fundamental input parameters, including planetary rotation rate, radius, internal friction due to wave stresses, and net radiative drive. The magnitude of mid-latitude jets is most sensitive to a single parameter, the planetary rotation rate and results from the conversion of planetary angular momentum to relative angular momentum by the meridional circulation, whereas the strength of meridional circulation is mainly determined by the magnitude of the radiative drive. For Titan's slowly rotating atmosphere, the meridional temperature gradient is vanishingly small, even when the radiative drive is enhanced beyond reasonable magnitudes, and can be inferred from zonal winds in gradient/thermal wind balance. In our 2D model large equatorial superrotation in Titan's stratosphere can be only produced through internal drag forcing by eddy momentum fluxes, which redistribute angular momentum within the atmosphere, while still conserving the total angular momentum of the atmosphere with time. We cannot identify any waves, such as gravitational or thermal tides, that are sufficiently capable of generating the required eddy forcing of >50 mas.sup.-1aTitan-day.sup.-1 to maintain peak prograde winds in excess of 100 mas.sup.-1 at the 1 mbar level. Author Affiliation: (a) The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723-6099, USA (b) Departments of Earth and Planetary Sciences and Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA Article History: Received 10 February 2004; Revised 20 January 2005

More than 500 images of Io in eclipse were acquired by the Cassini spacecraft in late 2000 and early 2001 as it passed through the jovian system en route to Saturn (Porco et al., 2003, Science 299, 1541-1547). Io's bright equatorial glows were detected in Cassini's near-ultraviolet filters, supporting the interpretation that the visible emissions are predominantly due to molecular S[O.sub.2]. Detailed comparisons of laboratory S[O.sub.2] spectra with the Cassini observations indicate that a mixture of gases contribute to the equatorial emissions. Potassium is suggested by new detections of the equatorial glows at near-infrared wavelengths from 730 to 800 nm. Neutral atomic oxygen and sodium are required to explain the brightness of the glows at visible wavelengths. The molecule [S.sub.2] is postulated to emit most of the glow intensity in the wavelength interval from 390 to 500 nm. The locations of the visible emissions vary in response to the changing orientation of the external magnetic field, tracking the tangent points of the jovian magnetic field lines. Limb glows distinct from the equatorial emissions were observed at visible to near-infrared wavelengths from 500 to 850 nm, indicating that atomic O, Na, and K are distributed across Io's surface. Stratification of the atmosphere is demonstrated by differences in the altitudes of emissions at various wavelengths: S[O.sub.2] emissions are confined to a region close to Io's surface, whereas neutral oxygen emissions are seen at altitudes that reach up to 900 km, or half the radius of the satellite. Pre-egress brightening demonstrates that light scattered into Jupiter's shadow by gases or aerosols in the giant planet's upper atmosphere contaminates images of Io taken within 13 minutes of entry into or emergence from Jupiter's umbra. Although partial atmospheric collapse is suggested by the longer timescale for post-ingress dimming than pre-egress brightening, Io's atmosphere must be substantially supported by volcanism to retain auroral emissions throughout the duration of eclipse. Keywords: Io; Aurorae; Satellites; Atmospheres

We present a model that describes Io's delayed electrodynamic response to a temporal change in Io's atmosphere. Our model incorporates the relevant physical processes involved in Io's atmosphere-ionosphere-magnetosphere electrodynamic interaction to predict the far-ultraviolet (FUV) radiation as Io enters Jupiter's shadow and re-emerges into sunlight. The predicted FUV brightnesses are highly nonlinear as the strength of the electrodynamic interaction depends on the ratios of ionospheric conductances to the torus Alfven conductance, but the former are functions of electrodynamics and the atmospheric density, which decays rapidly upon entering eclipse. Key factors governing the time evolution are the column density due to sublimation and the column density due to volcanoes, which maintain the background atmosphere during eclipse. The plasma interaction does not react instantaneously, but lags to a temporarily changing atmosphere. We find three qualitatively different scenarios with two of them including a post-eclipse brightening. The brightness ratio of in-sunlight/ in-eclipse coupled with the existence of a sub-jovian equatorial spot constrains the volcanic column density to several times [10.sup.18] [m.sup.-2], based on the currently available observations. Thus in sunlight, the sublimation driven part of Io's atmosphere dominates the volcanically driven contribution by roughly a factor of 10 or more. Keywords: Io's atmosphere; Io in eclipse; Satellite plasma interaction; Post-eclipse brightening

We have reanalyzed the Voyager 1 UVS solar occultations by Titan to expand upon previous analyses and to resolve inconsistencies that have been noted in the scientific literature. To do so, we have developed a detailed model of the UVS detector and improved both the data reduction methods and retrieval techniques. In comparison to the values previously determined by Smith et al. (1982, J. Geophys. Res. 87, 1351-1359) we find [N.sub.2] densities that are 25-60% higher, C[H.sub.4] densities that are smaller by a factor of 3-7, and [C.sub.2] [H.sub.2] densities that are roughly two orders of magnitude smaller. Our values for the thermospheric temperature are 153-158 K, which are approximately 20-40 K colder than previous estimates. We also report the first-ever determination from Voyager UVS data of density profile information [C.sub.2][H.sub.4], HCN, and H[C.sub.3]N Finally, we present a simple engineering model that is consistent with our new results in the upper atmosphere and merges smoothly with the model of Yelle et al. (1997, in: HUYGENS Science, Payload and Mission, in: ESA SP, vol. 1177, pp. 243-256) in the lower atmosphere. Our results provide improved constraints for photochemical models and offer scientists a better understanding of Titan's upper atmosphere as we head into the Cassini era in the exploration of the saturnian system. Keywords: Titan; Occultations; Aeronomy; Atmospheres, structure

The Io plasma torus, composed of mostly heavy ions of oxygen and sulfur, is sustained by an Iogenic mass loading rate of [~10.sup.30] amu [s.sup.-1] = 1.6 x [10.sup.28] S[O.sub.2] [s.sup.-1] or approximately [10.sup.3] kg [s.sup.-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., [less than or equal to] 3 X [10.sup.27] x ([n.sub.e,0]/3600 [cm.sup.-3]) ions [s.sup.-1], where [n.sub.e,0] is the average torus electron density. For the Galileo spacecraft Io pass in December 1995, the ion mass loading rate was [less than or equal to] 3 X [10.sup.27] ions [s.sup.-1], whereas for the Voyager epoch with lower [n.sub.e,0] (=2000 [cm.sup.-3]), this rate would be [less than or equal to] 1.7 X [10.sup.27] ions [s.sup.-1], consistent with the D.E. Shemansky (1980, Astrophys. J. 242, 1266-1277) mass loading limit of [less than or equal to] 1 x [10.sup.27] ions [s.sup.-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 ([10.sup.16]-[10.sup.21] [m.sup.-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 toms electrons with its atmosphere.

Thirteen lines of the CO band near 4.7 [micro]m have been observed on a jovian hot spot at a resolution of 0.045 [cm.sup.-1]. The measured line profiles indicate that the CO mole fraction is 1.0 [+ or -] 0.2 ppb around the 6-bar level and is larger in the upper troposphere and/or stratosphere. An external source of CO providing an abundance of [4.sup.+3.sub.-2] x [10.sup.16] molecules [cm.sub.-2] is implied by the observations in addition to the amount deposited at high altitude by the Shoemaker-Levy 9 collision. From a simple diffusion model, we estimate that the CO production rate is (1.5-10)x [10.sup.6] molecules [cm.sup.-2] [s.sup.-1] assuming an eddy diffusion coefficient around the tropopause between 300 and 1500 [cm.sup.2] [s.sup.-1]. Precipitation of oxygen atoms from the jovian magnetosphere or photochemistry of water vapor from meteoroidal material can only provide a negligible contribution to this amount. A significant fraction of the CO in Jupiter's upper atmosphere may be formed by shock chemistry due to the infall of kilometer- to subkilometer-size Jupiter family comets. Using the impact rate from Levison et al. (2000, Icarus 143, 415-420) rescaled by Bottke et al. (2002, Icarus 156, 399-433), this source can provide the observed stratospheric CO only if the eddy diffusion coefficient around the tropopause is 100-300 [cm.sup.2] [s.sup.-1]. Higher values, ~700 [cm.sup.2] [s.sup.-1], would require an impact rate larger by a factor of 5-10, which cannot be excluded considering uncertainties in the distribution of Jupiter family comets. Such a large rate is indeed consistent with the observed cratering record of the Galilean satellites (Zahnle et al. 1998, Icarus 13b, 202-222). On The other hand, the ~1 ppb concentration in the lower troposphere requires an internal source. Revisiting the disequilibrium chemistry of CO in Jupiter, we conclude that rapid vertical mixing can provide the required amount of CO at ~6 bar for a global oxygen abundance of 0.2-9 times the solar value considering the uncertainties in the convective mixing rate and in the chemical constants. Key Words: Jupiter, atmosphere; infrared observations; atmospheres, composition.

We report the discovery of the forbidden electronic [a.sup.1] [DELTA] [right arrow] [X.sup.3] [[SIGMA].sup.-] transition of the SO radical on Io at 1.7 [micro]m with the W. M. Keck II telescope on 24 September 1999 (UT), while the satellite was eclipsed by Jupiter. The shape of the SO emission band suggests a rotational temperature of ~1000 K; i.e., the gas is extremely hot. We interpret the observed emission rate of ~2 x [10.sup.27] photons [s.sup.-1] to be caused by SO molecules in the excited [a.sup.1] [DELTA] state being directly ejected from the vent at a thermodynamic quenching temperature of ~1500 K, assuming a SO/S[O.sub.2] abundance ratio of ~0.1 and a total venting rate of ~[10.sup.31] molecules [s.sup.-1] (Strobel and Wolven 2001, Astrophys. Space Sci. 277, 1-17). The shape of our complete (1.6-2.5 [micro]m) spectrum suggests that the volcano Loki contains a small (~2 [km.sup.2]) hot spot at 960 [+ or -] 12 K, as well as a larger (~50 [km.sup.2]) area at 640 [+ or -] 5 K. Key Words: Io; infrared observations; volcanism; atmosphere composition; atmosphere structure.

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 [Angstrom]. These results are in excellent agreement with previous UV measurements from 2200-3300 A spanning two decades, but have a significantly higher signal to noise ratio and spectral resolution. We add [approximately]400 [Angstrom] of new spectral information from 1800-2200 [Angstrom], a wavelength region in which Titan has not been detected previously. The albedo above 2200 [Angstrom] is a factor of 1.18 higher in August 1992 than October 1991, which is most likely due to the 4.3 [degrees] difference in Titan's phase angle between the two dates. Below 2200 [Angstrom] 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 ([C.sub.2][H.sub.2]). 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 [C.sub.2][H.sub.2] mole 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 [approximately]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 x [10.sup.-6]. Key Words: atmospheres, structure; abundances, atmospheres; Titan; satellites of Saturn.

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. S[O.sub.2] is assumed to be the dominant atmospheric gas, with minor molecular sodium species such as [Na.sub.2]S or [Na.sub.2]O released by sputtering or venting from the surface. Photochemical products include SO, [O.sub.2], S, O, Na, NaO, NaS, and [Na.sub.2]. We consider both 'thick' and 'thin' S[O.sub.2] atmospheres that encompass the range allowed by recent HST and millimeter-wave observations, and evaluate the possibility that [O.sub.2] and/or SO may be significant minor dayside constituents and therefore likely dominant nightside gases. The fast reaction between S and [O.sub.2] limits the column abundance of [O.sub.2] to [approximately][10.sup.4] less 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 Na[O.sub.2] or [Na.sub.2]0 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 [O.sub.2]. 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.sup.+], is lost primarily by charge exchange with [Na.sub.2]O and/or [Na.sub.2]S in the lower atmosphere and by diffusion through the ionopause in the upper atmosphere. The atmospheric column abundance of SO, [O.sub.2], 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' S[O.sub.2] atmosphere, while a localized high density 'volcanic' S[O.sub.2] atmosphere can yield an ionosphere consistent with that detected by the Pioneer 10 spacecraft.

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 C[H.sub.4] 3.3 [[micro]meter] and 2.3 [[micro]meter] bands, non-LTE radiative exchange and cooling in the C[H.sub.4] 7.6 [[micro]meter] band, and LTE cooling by CO rotational line emission. The model includes the effects of opacity and vibrational energy transfer in the C[H.sub.4] molecule. Partial thermalization of absorbed solar radiation in the C[H.sub.4] 3.3 and 2.3 [[micro]meter] 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 [[micro]meter] bands exceeds heating in 3.3 [[micro]meter] bands by approximately a factor of 6 and occurs predominantly at microbar pressures to generate steep temperature gradients [approximately]10-20 K [km.sup.-1] for p > 2/[micro]bar when the surface or tropopause pressure is [approximately]3 [micro]bar and the C[H.sub.4] mixing 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 [N.sub.2] individually as the major constituent. If a steep temperature gradient [approximately]20 K [km.sup.-1] is required near the surface or above the tropopause, then the preferred major constituent is Ar with 3% C[H.sub.4] mixing ratio to attain a calculated ratio of T/M(= 3.5 K [amu.sup.-1]) in agreement with inferred values from stellar occultation data. However, pure Ar and [N.sub.2] ices at the same temperature yield an Ar vapor pressure of only [approximately]0.04 times the [N.sub.2] vapor pressure. Alternative scenarios are discussed that may yield acceptable fits with [N.sub.2] as the dominant constituent. One possibility is a 3 [micro]bar [N.sub.2] atmosphere with 0.3% C[H.sub.4] that has 106 K isothermal region (T/M = 3.8 K [amu.sup.-1]) and [approximately]8 K [km.sup.-1] surface/tropopause temperature gradient. Another possibility would be a higher surface pressure [approximately]10 [micro]bar with a scattering haze for p > 2 [micro]bar. Our model with appropriate adjustments in the C[H.sub.4] density 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.).

A newly developed radiative-thermal conduction model deals with the vertical structure of Io's atmosphere and predicts the presence of mesopause in the atmosphere of Io. The model is developed by the application of the SO2 absorption lines in the near infra-red and ultraviolet bands consistent with the solar heating in the atmosphere. The mesopause in the atmosphere of Io is prominent when the pressure on the surface of Io is more than 10 nanobars.

An upper limit on the tropopause argon mixing ratio is a function of the tropopause CH4 mixing ratio f(CH4). This is inferred by an analysis of the Voyager 1 Ultraviolet Spectrometer solar occultation data, and a spectrum of the North Polar Region dayglow obtained by the Titan flyby. The upper limit varies from 0.15 to 0.08. The mean molecular mass is 0.17 for any f(CH4).

Ultraviolet spectrometer stellar and solar data obtained by the Voyager 2 deep space probe were analyzed to determine the smooth thermospheric temperature and heat source characteristics of Uranus. The results showed that the temperatures indicated by the occultations were much higher than can be explained by solar extreme ultraviolet heating. However, the effects of constant temperature on pressure surfaces at low altitudes cannot be excluded.

The problem of nonlinear saturation of baroclinic waves in two-layer models is studied and it is shown that Shepherd's rigorous bound on the wavy disturbance growth due to instabilities of parallel shear flow can be improved significantly, in some cases, by exact calculation of the averaged Arnol'd's invariant. Shepherd's bound for the Phillips' beta-plane two-layer model with constant potential vorticity gradient is achievable at the minimum critical shear as the supercriticality parameter epsilon right arrow 0. The underlying reason for such an achievable bound for the wavy disturbance is that the condition leading to the Arnol'd's stability theorem is both necessary and sufficient. Based on such an achievable bound, (2beta/3F)(super 1/2) is deduced as the maximum wave amplitude at the minimum critical shear as the supercriticality parameter epsilon right arrow 0. When Arnol'd's invariant is applied to an f-plane two-layer model, the bound derived from Arnol'd's invariant is not as powerful a constraint on the amplitude of the evolving wavy disturbance. The reason is that the opposite signs of potential vorticity gradients in upper and lower layers are not a sufficient condition for instability.