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The cloud structure of the jovian atmosphere at pressures less than 2 bars has previously been estimated using near-infrared observations such as those by both the Solid State Imager (SSI) and Near-Infrared Mapping Spectrometer (NIMS) instruments on board the Galileo spacecraft. Unfortunately, complete near-infrared spectra, such as those measured by NIMS, take a long time to be analyzed with multiple-scattering radiative transfer models and thus it has until now been rather difficult to use these data to produce wide-area cloud maps. In this paper we show how principal component analysis may be employed to isolate a small number of empirical orthogonal functions (EOFs) from spectra of Jupiter made by Galileo/NIMS. These EOFs may be used to represent the variance of real NIMS spectra to a high degree of accuracy and with good noise and 'drop-out' discrimination. Because of this, a small set of representative spectra may then be calculated using these EOFs and input into a retrieval model that generates a table of fitted cloud profiles for each case. This approach avoids the long times required for analyzing a large number of spectra with full multiple-scattering radiative transfer models and allows us to represent the variability of the 3000 spectra contained in the observations of the North Equatorial Belt (NEB) used in this study with only 75 representative spectra. The cloud structures fitted to these representative spectra were interpolated for the spectra found at individual locations in the measured NIMS data set to produce maps of cloud opacity and mean particle size. We find that the dominant opacity variation, anticorrelated with 5-[micro]m brightness, exists in the 1-2 bar pressure range. The distribution of the cloud at 0.72 bars is mapped and found to be more zonally diffuse than the lower clouds. We find at least one 2000-km-sized deep convective cloud in the NEB vertically extending to all the pressure levels that can be sensed by NIMS. Key Words: Jupiter; atmosphere; clouds; image processing.

Measurements from the Galileo probe suggest that the zonal winds are deep rooted. Jupiter's high rotation rate makes it likely that the whole outer molecular H/He layer is involved in these long-lived jet flows. Assuming that the primary flows are geostrophic, and that the banded surface structure stretches right through the molecular H/He layer, we examine the conditions for such flows to be stable. As a first step, the linear stability of some prescribed banded zonal flows in a rotating spherical shell is explored. Incompressibility is assumed for simplicity, and the boundary condition is stress-free. We compare solutions for two aspect ratios, appropriate for the molecular H/He layers of Jupiter and Saturn, and two Ekman numbers (E = [10.sup.-2] and E = [10.sup.-4]). Convective and shear flow instabilities compete in our system. The convective instabilities are of the well-known columnar structure. Shear flow instabilities for the larger Ekman number are similar to the Taylor-Couette instability in rotating annuli. At the lower Ekman number, shear flow instabilities adopt a geostrophic character, assuming the form of rotating columns, similar to the convective instabilities. While the convective instability always sets in outside the tangent cylinder, shear instability can become unstable inside the tangent cylinder. If even a weak zonal flow is present inside the tangent cylinder, the flow is unstable to shear instability. This offers an explanation why the jovian zonal jet structure is much weaker at the higher latitudes that correspond to locations inside the tangent cylinder. Key Words: Jupiter, interior; rotational dynamics.

The Copernican revolution began over 500 years ago with the realization that the Earth was not the center of the universe, but we still await its grand finale: The anticipated [...]

We present profiles of the line-of-sight ionospheric wind velocities in the northern auroral/polar region of Jupiter. Our velocities are derived from the measurement of Doppler shifting of the [H.sup.+.sub.3] [v.sub.2] Q(1,[0.sup.-]) line at 3.953 [micro]m. The data for this study were obtained using the facility high-resolution spectrometer CSHELL on the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii, during the nights of September 7-11, 1998 (UT). A detailed analysis, allowing for several effects on the measured wavelength, is presented. The velocity profiles finally derived are consistent with an [H.sup.+.sub.3] electrojet of ~1.2 to 1.5 km [s.sup.-1] flowing clockwise (as viewed from above the north rotational pole)--i.e., counter to the rotation of the planet--around the main auroral oval, for the night of September 11, and with somewhat lower velocities for September 8 and 10. In addition, we note two main regions inside (poleward of) the main auroral oval, which we call the Dark Polar Region (DPR) and the Bright Polar Region (DPR). These features correspond to the yin-yang structure noted by T. Satoh and J. E. P. Connerney (1999, Icarus 141, 236-252). The DPR appears to be strongly redshifted, with poleward winds in the line of sight up to -2.5 km [s.sup.-1]. The BPR may have weakly blueshifted emission. The detection of the auroral electrojet continuously over our observing run confirms the initial electrojet detection of D. Rego, N. Achilleos, T. Stallard, S. Miller, R. Prange, M. Dougherty, and R. D. Joseph (1999, Nature 399, 121-124), as well the mechanism proposed by T. W. Hill (1979, J. Geophys. Res. 84, 6554-6558) to explain how Jupiter's equatorial magnetospheric plasmasheet is kept in corotation with the planet. We discuss the results in light of recent proposals and analyses of the jovian middle magnetosphere, and suggest that our DPR winds may be under solar wind control. Key Words: atmospheres, dynamics; aurorae; infrared observations; jupiter, atmosphere; jupiter, magnetosphere.

We study the thermosphere and ionosphere effects of the July 1994 Comet P/Shoemaker-Levy 9 impacts with Jupiter using a one-dimensional transport model which includes detailed neutral-neutral and ion-neutral chemistry and diffusive vertical transport for a variety of sulfur, nitrogen, and oxygen molecular species observed at the time of the impacts. The model uses a background neutral atmosphere based on the temperature profile obtained from Galileo probe measurements and a basic ionospheric model which satisfies current observational constraints on electron and [H.sup.+.sub.3] densities. After the influx of impact-related neutral species into the ionosphere, new ion species such as [S.sup.+], [H.sub.3]C[S.sup.+], and N[H.sup.+.sub.3] (and in some cases [H.sub.3][O.sup.+], S[O.sup.+], and HC[O.sup.+]) appear along with the usual ionospheric species. Our model predicts that [S.sup.+] begins to control the density of the ionospheric peak some hours after impact. The [H.sup.+.sub.3] column density decreases by a factor ranging from 5 to 100, depending on the amount of water present. This is consistent with most nonauroral observations of [H.sup.+.sub.3] emission during and just after the impact period. The model predicts electron density column enhancements of up to a factor of 10 for post-impact times less than a few days as well as significant abundances of [S.sub.2] at the Galileo observational epoch, though at altitudes below ionospheric levels. Perhaps of more general interest is the compilation, presented here, of the ionospheric mechanisms involved in a comprehensive sulfur, nitrogen, and oxygen chemical model which may be of considerable use in future studies of ablation or particle precipitation in any hydrogen-dominated outer-planet ionosphere. Key Words: aeronomy; ionospheres; comets; SL-9; jovian planets; Jupiter.

We present BVR colors and magnitudes for four jovian irregular prograde satellites (Himalia J6, Elara J7, Lysithea J10, and Leda J13) and four irregular retrograde satellites (Pasiphae J8, Sinope J9, Carme J11, and Ananke J12). All eight have generally `solar' colors but the retrograde group has slightly redder and more diverse colors. The strikingly similar colors of the four prograde satellites suggest the parent planetesimal was likely very homogeneous. The four retrograde satellites show a diversity in color that suggests a heterogeneous progenitor and thus variations in precapture formation history. The absolute magnitudes and revised diameters are presented. We also report new colors and diameters for two uranian irregular satellites (Caliban (S/1997 U1) and Sycorax (S/1997 U2) that are slightly redder than any of these jovian satellites. Key Words: jovian; satellites; irregular; BVR colors.

Raman scattered features by molecular hydrogen have been detected in Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) observations of Jupiter. The measurements were obtained with the G190H grating and red detector combination spanning 158.0-232.0 nm at about 0.3 nm resolution. The data were corrected for scattered light, and careful modeling of the line spread function (LSF) of the instrument was performed to accurately degrade the solar spectrum obtained by SOLSTICE (solar-stellar irradiance comparison experiment) to the spectral resolution of the FOS. A cross-correlation method was used to align features in the planetary spectra to those in the SOLSTICE solar spectrum. At all latitudes longward of 210.0 nm, the resulting I/F displayed discrete features up to 20% of the continuum level that anticorrelate with the solar spectrum. A radiative transfer code was developed to include the effect of rotational and vibrational multiple Raman scattering for the first few lowest energy rotational states of molecular hydrogen under the approximation that the Raman component of the scattering phase function is isotropic. Simulations show not only that the detected features are indeed due to Raman scattering by H(sub 2), but are sensitive to its ortho-para ratio as well. An analysis of the equatorial spectrum reveals that the features are consistent with an equilibrium or normal population of H(sub 2) at 130 K. Key Words: Jupiter, atmosphere; data reduction techniques; spectroscopy; radiative transfer; ultraviolet observations.

Ultraviolet spectra of the northern and southern hemispheres of Jupiter taken with the Hubble Space Telescope Faint Object Spectrograph (FOS) in May 1992 and June 1993 have been used to derive the altitude profiles of NH(sub 3) in the vicinity of the tropopause. For a given pressure level, it is shown that the vertical profile of ammonia varies with latitude and the atmospheric feature being observed. The mixing ratio of ammonia present above the Great Red Spot (GRS) is 8 x 10(super -8) at 250 mbar, whereas it is four times greater in the nearby South Equatorial Belt at the same pressure level. Our findings agree with those of C. A. Griffith, B. Bezard, T. Owen, and D. Gautier (1992, Icarus 98, 82-93), who find NH(sub 3) to be depleted over the GRS with respect to the South Tropical Zone at the time of the Voyager encounters. Variations of the ammonia mixing ratio in the northern and southern hemispheres are found to be nonmonotonic in latitude, indicating local dynamical effects. The observed latitudinal variation of the altitude profile of NH(sub 3) is likely to be caused by variations in the vertical eddy mixing (K), which competes with the photolysis and condensation losses of NH(sub 3). We also find this vertical mixing to be nonmonotonic with latitude. Instead, it shows high variability depending on the region and feature observed, consistent with the values of eddy mixing derived by L.-M. Lara, B. Bezard, C. A. Giffith, J. H. Lacy, and T. Owen (1998, Icarus 131, 317-333). Values at 13 degrees N, 18 degrees N, at the equator, and over the Great Red Spot region range between 3 and 10 x 10(super 3)cm(super 2) s(super -1) at 250 mbar, while weaker mixing occurs at 20 degrees S, 33 degrees S, and 7 degrees N ( 300 mbar), followed by a drop to a minimum as we proceed to lower pressures, and finally an increase again with decreasing pressure. In this paper, we also update our previous results on NH(sub 3) and K following the recalibration of the FOS data using the most recent recommendations of the Space Telescope Science Institute. Key Words: Jupiter, atmosphere; photochemistry; radiative transfer; spectroscopy; ultraviolet observations.

In October and November of 1997 the Galileo Solid State Imager (SSI) detected lightning from 26 storms on the night side of Jupiter. More than half the surface area of the planet was surveyed. The data include images of lightning against moonlit clouds (illuminated by light from Io) and images of the same storm on the day and night sides. The spatial resolution ranged from 23 to 134 km per pixel, while the storms ranged in size up to approximately 1500 km. Most storms were imaged more than once, and they typically exhibit many flashes per minute. The storms occur only in areas of cyclonic shear and near the centers of westward jets. Latitudes near 50 degrees in both hemispheres are particularly active, although the northern hemisphere has more lightning overall. The greatest optical energy observed in a single flash was 1.6 x 10(super 10) J, which is several times larger than terrestrial superbolts. The average optical power per unit area is 3 x 10(super -7) W m(super -2), which is close to the terrestrial value. The limited color information is consistent with line and continuum emission from atomic hydrogen and helium. The intensity profiles of resolved lightning strikes are bell-shaped, with the half-width at half-maximum ranging from approximately 45 to 80 km. We used these widths to infer the depth of the strikes, assuming that the appearance of each is the result of light scattering from a point source below the cloud-tops. We conclude that lightning must be occurring within or below the jovian water cloud. The occurrence of lightning in regions of cyclonic shear has important implications for the dynamics of Jupiter's atmosphere. Key Words: atmosphere dynamics; Jupiter atmosphere; meteorology.

Local radiative-convective thermal structure is calculated on Jupiter and Saturn under the assumption that a low opacity zone exists near the 2000-K depth, as suggested by T. Guillot et al. (1994a, Icarus 112, 337-353). The opacity is uncertain, and a simple but qualitatively accurate model is used to explore a range of opacities. Under the assumption of local radiative-convective equilibrium, insolation gradients lead to latitudinal temperature gradients in the shell outside the stably stratified radiative zone near 2000 K, and to a latitudinal variation in the emission to space. Measurements of the net heat balance (solar and planetary radiation) do not yet extend to high enough latitudes to test these predictions. On Jupiter and especially Saturn, the preference for westerly surface motion may be due to equator-to-pole temperature gradients in this shell. It is speculated that the dynamical response to radiative forcing due to low opacity near the 2000-K level may limit latitudinal temperature gradients and may be an important drive for meteorology on the outer planets.

During the 1998 Jupiter-Sun conjunction,the 60-year-old white ovals BC and DE at 33 degrees south latitude merged into a larger oval 'BE' at the same latitude (J. Lecacheux, P. Drossart, E Colas, G. S. Orton, B. Fisher, A. Sanchez-Lavega, R. Hueso, J. E Rojas, and I. Miyazaki, IAU Circular No. 6942). Here we report our study of the motions, morphology, and reflectivity changes pre and postmerger of the ovals based on extensive and continuous Earth-based CCD imaging (April 1997-September 1998) in the visual range (400-800 nm) and in the methane band at 890 nm. In late August 1997, BC and DE started their mutual approach with a relative zonal velocity of 1 m/s. The longitude drift curves of BC, DE, and new BE suggest that a merger of BC and DE took place in February 1998 in the position of BC. A smaller apparently cyclonic oval called O1 was seen in the northwest side of BE at latitude -31 degrees. BE is about 12% larger than BC or DE as measured in the blue, red, and 890-nm methane band filters. However, if O1 is assumed to be a by-product of the merger, the sum of the areas of BC and DE is about that of BE and O1. The motion of BE is similar to that of the classic white oval spots. No significant center-to-limb reflectivity changes in the white oval spots at the 890-nm wavelength (sensitive to upper hazes and clouds) were noted before and after the merger. However the area-integrated blue-red color index changed slightly, with BE being bluer than BC or DE probably because the particles in BE have a higher single-scattering blue albedo. Using a radiative transfer model, the 890-nm data suggest that the upper cloud and haze (above 600 mbar) in BE are similar to those of BC and DE. In the limit, the upper cloud in BE could be higher (cloudtop at 110 mbar instead of 150 mbar) or have slightly larger optical depths (3.5 instead of 2.85) than that of BC or DE.

Low-resolution spectrophotometry of Jupiter by Karkoschka (1994, Icarus 111, 174-192) revealed a weak apparent absorption feature in the spectrum of Jupiter at 920-940 nm. This is the wavelength of the strong 2[v.sub.1] + [v.sub.3] band of [H.sub.2]O, and Karkoschka tentatively identified the feature as jovian [H.sub.2]O absorption. If this spectral feature is indeed due to [H.sub.2]O absorption, it would offer the exciting possibility of monitoring the jovian [H.sub.2]O with visible band spectroscopy. This feature has been seen in previous low-resolution spectroscopy of Jupiter such as that of Woodman et al. (1979, Icarus 37, 73-83), but was not identified as jovian in origin because of uncertainties in extinction corrections for the very strong telluric [H.sub.2]O absorption. Cochran et al. (1995, Bull. Am. Astron. Soc. 27, 66) tentatively identified the feature as due to [NH.sub.3], an identification that was supported by further low-resolution spectrophotometry by Karkoschka (1998, Icarus 133, 134-146). Here we present high-resolution spectra (R = 155,000) of Jupiter using the 2dcoude spectrometer on the 2.7-m Harlan Smith Telescope at McDonald Observatory, in order to make a definitive identification of this absorption feature. Observations were obtained when Jupiter was near quadrature in order to Doppler shift potential jovian [H.sub.2]O features away from the corresponding telluric lines. We demonstrate conclusively that the absorption band identified by Karkoschka is not, in fact, due to [H.sub.2]O. We present further evidence that it is most likely to be an absorption band of [NH.sub.3]. We searched our spectra carefully to attempt to identify absorption lines which are actually due to [H.sub.2]O, without success. Key Words: Jupiter, atmosphere; spectroscopy; atmospheres, composition.

High-resolution spectra obtained using the NASA Infrared Telescope Facility on Mauna Kea, Hawaii, have provided direct evidence for auroral electrojets around Jupiter's auroral ovals. These electrojets have winds near or in excess of the local speed of sound, and the energy they produce could heat the entire upper atmosphere, if the auroral regions link effectively with the rest of the planet. This research has presented strong confirmation of current understanding of the jovian magnetosphere.

We have measured the motions and wind velocity vectors in six jovian hot spot-plume areas (latitudes 0 [degrees] to + 12 [degrees]) using high resolution Voyager 1 and 2 images separated by one jovian rotation ([approximately]10 hr). Rapid changes in cloud forms take place in a few hours at scales from 100 to 2000 km. No large scale organized motions, e.g., diagonal convergence along the plume tails toward hot spots or circulation patterns, such as those recently reported using Galileo images separated by 1 hr, have been found. We show that particle evaporation and cloud element mixing in the upper ammonia cloud could be fast enough to dissipate the small cloud elements, such as those used as tracers by the Galileo SSI team, in less than 10 hr. Key Words: Atmospheres, dynamics; Jupiter, atmosphere.

The Galileo probe into Jupiter's atmosphere entered a 'hot spot' composed of dry, less dense air. Despite assertions that hot spots are regions of downwelling, no viable mechanism has been proposed to transport less dense air over a scale height into the deep atmosphere. Here, we present a numerical simulation of convection in Jupiter's atmosphere which indicates that less dense air from above the clouds may be carried into the deep atmosphere by strong convective entrainment. Key Words: Jupiter; Jupiter, atmosphere; atmospheres, dynamics; Galileo.

During the first six orbits of the Galileo spacecraft's prime mission, the Solid State Imaging (SSI) system acquired multispectral image mosaics of Jupiter's Great Red Spot, an equatorial belt/zone boundary, a '5-[[micro]meter] hot spot' similar to the Galileo Probe entry site, and two of the classic White Ovals. We present mosaics of each region, approximating their appearance at visible wavelengths and showing cloud height and opacity variations. The local wind field is derived by tracking cloud motions between multiple observations of each region with time separations of roughly 1 and 10 hr. Vertical cloud structure is derived in a companion paper by Banfield et al. (Icarus 135, 230-250). Galileo's brief, high-resolution observations complement Earth-based and Voyager studies and offer local meteorological context for the Galileo Probe results. Our results show that the dynamics of the zonal jets and large vortices have changed little since Voyager, with a few exceptions. We detect a cyclonic current within the center of the predominantly anticyclonic Great Red Spot. The zonal velocity difference between 0 [degrees] S and 6 [degrees] S has increased by 20 m [sec.sup.-1]. We measure a strong northeast flow approaching the hot spot. This flow indicates either massive horizontal convergence or the presence of a large anticyclonic vortex southeast of the hot spot. The current compact arrangement of two White Ovals and a cyclonic structure greatly perturbs the zonal jets in that region. Key Words: Jupiter atmosphere; atmospheric dynamics; image processing; spacecraft.

The two-dimensional dynamics of plasma motion in an idealized form of the corotating convection system of the jovian magnetosphere is investigated. Plasma pressure is ignored, and a dipole magnetic field is assumed, with the magnetic axis and spin axis aligned. The effects of microdiffusion are incorporated into an otherwise macroscopic magnetohydrodynamic model. By applying the mathematical technique (of Galerkin truncation) that was used to derive the famous Lorenz equations for convection in Earth's atmosphere, we reduce the governing partial differential equations to simple, low-order, ordinary differential form. On the basis of this extreme simplification, the plasma dynamics of the jovian magnetosphere is found to be controlled by the height-integrated Pedersen ionospheric conductivity [Sigma]; viz., if [Sigma] is greater than a critical value [[Sigma].sub.c] then plasma motion is steady-state convection, whereas if [Sigma] is less than [[Sigma].sub.c] then plasma motion is expected to be chaotic. The value [[Sigma].sub.c] is found to lie in the approximate range 0.7 mho [less than or equal to] [[Sigma].sub.c] [less than or equal to] 1.8 mho, which is within the accepted range of values of [Sigma] for Jupiter. This predicted form of control of magnetospheric plasma motion by the height-integrated Pedersen ionospheric conductivity is testable if sufficiently refined data become available. Lyapunov (fractal) dimensions of typical strange attractors are found to be in the range DL [similar to] 2.03-2.3. Whether a low-dimensional strange attractor does exist at certain times in the jovian magnetosphere can be reliably tested only by analyzing a sufficiently long time series constructed from plasma velocity data.

Photometrically calibrated grism spectra of Jupiter in the H (1.45-1.8 [[micro]meter]) and K (1.95-2.5 [[micro]meter]) bands with a resolution of about 100 were taken with the 5-m Hale telescope at Palomar in August of 1995. The spectra were obtained as meridional cuts at six different longitudes on the planet, with one cut crossing the Great Red Spot. The technique outlined in D. Banfield et al. (1996, Icarus 121, 389-410) for the retrieval of scatterer density with altitude from near-infrared spectra is used and refined. It is expected that this retrieval technique will find use in the interpretation of many atmospheric near-infrared reflection spectra, especially those from Galileo NIMS and Cassini VIMS. For the wavelengths and spectral resolution used in this study, the sensitivity of the inversions extends from pressure levels near 400 mbar up to about 20 mbar. Employing this inversion technique on the spectra yields well-resolved jovian cloud densities for [Tau] [approximately less than] 0.1, as a function of latitude and altitude. The density of scatterers is minimum at a height where the pressure is about 100 mbar and increases both upward and downward from this level. The minimum near 100 mbar can be explained by coagulation of settling particles, leading to an increase in fall speed. The results indicate that stratospheric haze particles are generated at heights where p [approximately less than] 20 mbar. Key Words: atmospheres, abundances and dynamics; jovian atmosphere; radiative transfer; spectrophotometry.

Interplanetary and interstellar dust grains entering Jupiter's magnetosphere form a detectable diffuse faint ring of exogenic material. This ring is composed of particles in the size range of 0.5 to 1.5 micrometers on retrograde and prograde orbits in a 4:1 ratio, with semimajor axes 3 [is less than] a [is less than] 20 jovian radii, eccentricities 0.1 [is less than] e [is less than] 0.3, and inclinations i ?? 20 degrees or i ?? 160 degrees. The size range and the orbital characteristics are consistent with in situ detections of micrometer-sized grains by the Galileo dust detector, and the measured rates match the number densities predicted from numerical trajectory integrations., The dust detector on the Galileo spacecraft (DDS) measures impacts of dust particles and gives estimates of the masses and the velocities of the grains (1). Grains were detected by [...]

The Galileo probe found the jovian abundance of [H.sub.2]S to be 30% solar at the 8 bar level, while the abundance of water was less than 3% solar at 12 bars. From 8 to 20 bars, [H.sub.2]S increased to three times solar, and water apparently increased as well. Since [H.sub.2]S and water condense at 2 and 5 bars, respectively, the probe probably entered a dry downdraft, wherein dry air above 2 bars is advected to 12 bars or deeper (Owen et al. 1996, Eos (Spring Suppl.) 77, S171). This is consistent with the fact that the probe entered the south edge of a 5-[[micro]meter] hot spot, a local region of Jupiter's atmosphere known from spectral modeling to be unusually low in cloud abundance (Orton et al. 1996, Science 272, 839). We use basic physical constraints to address three problems raised by Galileo probe data. First, it is unclear how the hypothesized downdraft remains dry, since simple models of convection preclude dessication below the 2- and 5-bar condensation levels. We suggest that to suppress moist plumes from below, the downdraft must be of low density below 5 bars and hence thermally indirect, requiring mechanical forcing from other parts of the atmosphere. Second, if geostrophic balance holds, the Galileo probe winds imply that the hot spot (north of the probe site) contains a stable layer from 1 to 5 bars; this is inconsistent with a downwelling, since downwellings should be adiabatic below 2 bars due to the low radiative flux divergence. We show that when the centripetal acceleration of curving parcel trajectories is included in the force balance, however, a variety of density profiles is possible within the hot spot (depending on the radius of curvature of the winds). The most plausible profile implies that the hot spot is nearly dry adiabatic and that the equatorial zone south of the probe site is stable from 2 to 6 bars, suggesting moist adiabatic upwellings with a water abundance of 1-2 times solar. This is consistent with Galileo and Voyager images suggesting upwelling at the equator. The profile further implies that from 1 to 5 bars the hot spot is denser than the equatorial zone south of the probe site. Third, probe data indicate that NH3 increased with depth below 1 bar and became constant by 8 bars, [H.sub.2]S began increasing below 8 bars and leveled off by 16 bars, while water only began increasing below 12 bars and was still increasing with depth at 20 bars. We propose that lateral mixing along isopycnals (surfaces of constant potential density) could produce the observed pattern; alternatively, the downwelling might consist of column stretching, so that the N[H.sub.3], [NH.sub.4]SH, and water lifting condensation levels were pushed to 8, 16, and >20 bars, respectively. In either case, the simplest form of this model requires the downdraft to be less dense than the surroundings from 0.5 to 20 bars. In its simplest form, this model is therefore incompatible with our favored interpretation of the winds; more detailed studies will be necessary to resolve the problem. Key Words: Jupiter (atmosphere); atmospheres (dynamics); atmospheres (structure); spacecraft.

The 'quench level' approximation for estimating the observed abundance of chemically reacting species in the presence of convective dynamics states that the chemical reaction is quenched at the level where the time scales for the chemical reaction, [t.sub.chem], and for convective dynamics, [t.sub.dyn] are equal. The dynamical time constant, [t.sub.dyn], can be computed using [t.sub.dyn] = [L.sup.2]/[K.sub.eddy], where L is a length scale and [K.sub.eddy] is the vertical eddy diffusion coefficient. Usually [K.sub.eddy], is left as a free parameter, and lacking any better information, L is taken to be the pressure scale height, H. Here it is shown that an effective length scale [L.sub.eff], can be estimated from the e-folding length scales of [t.sub.chem] and [t.sub.dyn] and the pressure dependence of the equilibrium value of the chemical abundance. The improved accuracy of using L = [L.sub.eff] instead of L = H is demonstrated by comparing the results of three different mathematical 'models' for convective dynamics. The three models are the quench level approximation, a second computation that uses diffusive mixing to describe convection, and a third computation that explicitly integrates the equations of motion and chemical equilibration. Each of the three models uses a different fundamental quantity in describing the strength or efficiency of mixing that depends on the length scale L in a different way. When L = H is used, the three models give three different results, but when L = [L.sub.eff] is used, results from the three models are virtually identical. The effective length scale, [L.sub.eff], is different for different chemical reactions and can be far from a scale height. As an example, for the isotropic enrichment of (D/H) in C[H.sub.4] over (D/H) in [H.sub.2], the effective length scale is [L.sub.eff] = 0.14H at a typical value of [K.sub.eddy] = [10.sup.8] [cm.sup.2] [sec.sup.-1]. Using L = 0.14H leads to an enrichment factor of 1.17 on Jupiter compared to 1.21 when L = H is used. Key Words: abundances, atmospheres; jovian planets, atmosphere.

The reaction of hydrogen sulfide with ground-state atomic carbon was examined with crossed molecular beams experiments and ab initio calculations. The thiohydroxycarbene molecule, HCSH, was the reactive intermediate, which fragmented into atomic hydrogen and the thioformyl radical HCS. This finding may account for the unassigned HCS source and an unidentified HCSH radical needed to match observed CS abundances from the collision of comet Shoemaker-Levy 9 into Jupiter. In the shocked jovian atmosphere, HCS could further decompose to H and CS, and CS could react with SH and OH to yield the observed [CS.sub.2] and COS., The discovery of comet D/Shoemaker-Levy 9 (SL9) initiated the only observation of the collision of two solar system bodies(1). From 16 to 22 July 1994, about 20 fragments of the [...]

During the impacts of the fragmented Comet D/Shoemaker-Levy 9, Jupiter's synchrotron radio emission at [Lambda]90 cm was monitored using the Very Large Array (VLA) and Westerbork Synthesis Radio Telescope. The dramatic infall of the comet onto Jupiter temporarily induced significant changes in the planet's radiation belts. The total flux density of Jupiter increased steadily and decreased later, reaching below pre-impact levels. The ratio of the emission peaks changed dramatically, particularly around July 19-20, indicating pronounced E/W asymmetries induced after the impacts of fragments K and L. Comparing with similar data at [Lambda]20 cm, the changes induced in the E/W ratios are essentially equal at both [Lambda]90 and [Lambda]20 cm. At [Lambda]90 cm, however, the VLA images obtained on July 19-20 also revealed, for the first time, the temporal evolution of a compact component in the radiation morphology. This compact feature might be associated with the infall of fragments K and L on 1994 July 19. The observations suggest that this feature appeared at a radial distance range of [approximately equal to]6.3-6.7[R.sub.j], assuming it was corotating with Jupiter while steadily diminishing in its brightness. Key Words: planets, general; Jupiter; Jupiter, magnetosphere; comets; radio observations; comets, dynamics.

High spectral resolution measurements of N[H.sub.3] and P[H.sub.3] lines on Jupiter in the 10.5- to 11.2-[[micro]meter] range are presented. Observations, recorded on January 21-23, 1991, cover the 10 [degrees]-40 [degrees] S latitude range and several longitudes including the Great Red Spot (GRS). Information on the temperature in the upper troposphere was retrieved from the continuum radiance at wavelengths around 12.8 and 17.8 [[micro]meter]. At all observed longitudes, the 200-mbar temperature field is minimum at latitudes of 20 [degrees]-25 [degrees] S near the location of the South Tropical Zone, in agreement with Voyager infrared retrievals. This minimum temperature is lower over the GRS than at other longitudes. The ammonia mixing ratio at [approximately]380 mbar is not significantly enhanced over the GRS. The phosphine abundance probed at [approximately]580 mbar is also not enhanced (within a precision of 10%), suggesting that this molecule is not a precursor of the reddish chromophores. The N[H.sub.3] abundance at 380 mbar varies highly with latitude and longitude, a possible consequence of the active jovian meteorology. At the resolution of our observations ([approximately]8000 km), the N[H.sub.3] humidity at this altitude ranges between 15 and 100% throughout the available data set. Above the cloud tops, the N[H.sub.3] mixing ratio in the 240-mbar region reaches a maximum near 15 [degrees]-18 [degrees] S and decreases by a factor of [approximately]40 at latitudes 30 [degrees]-35 [degrees] S. This variation is not correlated with the 200-mbar temperature. It can be explained by a decrease of the eddy mixing coefficient near 240 mbar from [approximately]4000 to [less than or equal to]400 [cm.sup.2] [sec.sup.-1] between the two latitude ranges. The [PH.sub.3] mixing ratio near 580 mbar lies between 1.7 and 2.6 x [10.sup.-7] in the observed regions. At all longitudes, P[H.sub.3] varies smoothly with latitude, decreasing by [approximately]30% from 10 [degrees] to 35 [degrees] S. This variation may also reflect a decrease in the strength of the eddy mixing near 580 mbar or at deeper levels in the atmosphere. Key Words: Jupiter, atmosphere; atmospheres, composition; atmospheres, structure; infrared observations.

Using the helium abundance measured by Galileo in the atmosphere of Jupiter andinterior models reproducing the observed external gravitational field, we derivenew constraints on the composition and structure of the planet. We conclude that,except for helium which must be more abundant in the metallic interior than in themolecular envelope, Jupiter could be homogeneous (no core) or could have a centraldense core up to 12 [M.sub.[symmetry]]. The mass fraction of heavy elements is lessthan 7.5 times the solar value in the metallic envelope and between 1 and 7.2 timessolar in the molecular envelope. The total amount of elements other than hydrogenand helium in the planet is between 11 and 45 [M.sub.[symmetry]]. Key Words: Jupiter; interiors; planets; chemical abundances.

We report measurements of the Jupiter brightness spectrum in the 850-[[micro]meter]and 1100-[[micro]meter] atmospheric windows with a spectral resolution of 125 MHz,obtained with a Fourier transform spectrometer on the James Clerk MaxwellTelescope. Three results were obtained. First, the predicted absorption featuresdue to the rotational lines of HCN at 266 and 354 GHz were not detected within ourerror limits of less than 1%. We establish new upper limits for the HCN abundancein the jovian troposphere for five assumed abundance distributions and for twoassumed [NH.sub.3] abundances. The upper limits are 1.7 to 13 times smaller thanthe abundance value obtained in the only reported detection of HCN in Jupiter priorto the impact of ShoemakerLevy 9. Second, the continuum brightness temperaturespectrum at 850 [[micro]meter] was determined and is in agreement with previousmeasurements, but has large error bars due to uncertainties in the photometriccalibration. We estimate the ammonia abundance in the 1-2 bar region to be 1.7times solar, but this result is tentative since scattering by [NH.sub.3] cloudparticles and absorption by gaseous [H.sub.2]S were neglected in our atmosphericmodel. Finally, the first rotational line of [PH.sub.3] at 267 GHz was notdetected, a result which we demonstrate is consistent with the statistical noiselevel in these measurements, with current values of the spectroscopic parameters,and with phosphine measurements at other wavelengths. Key Words: abundances, atmospheres; atmospheres, composition; data reductiontechniques; Jupiter; Jupiter, atmosphere; radiative transfer; spectroscopy.

An updated dynamical model for the circulation in the jovian stratosphere has been developed. For computation of the heating and cooling rates in the stratosphere, required to obtain the radiative flux convergence, the model makes use of exponential-sum derived coefficients from laboratory and correlated-k distributions from line listing data for the various atmospheric gases. The aerosol distribution at different latitudes was determined from comparisons of results from microphysical models and near-UV jovian images obtained by the Wide Field and Planetary Camera 2 on the Hubble Space Telescope. The largest aerosol content is found at latitudes south of 65 [degrees] S planetographic, the aerosols being produced at higher atmospheric levels and at higher rates than those at the corresponding northern latitudes. At midlatitudes, the aerosol production rate is less than about two orders of magnitude than that found at high latitudes. The distinct imaginary refractive index spectra of the aerosols found at the different latitudes reflect a compositional difference in the material being produced as a result of chemical reactions. This difference is evident in the shortest wavelength explored ([[Lambda].sub.eff], = 230 nm) between low to midlatitudes and the polar regions. The resulting heating rates are much higher in the stratosphere than those given by previous estimates. The corresponding radiative equilibrium temperatures are therefore hotter than previously reported, mainly above the 10-mbar level. A north-south asymmetry in the stratospheric aerosol distribution was found, which, in turn, produces a different pattern in the derived quantities from the dynamical model. In particular, the Eliassen-Palm flux divergence ([Epsilon]) shows a different pattern when compared with that inferred by Conrath et al. (1990, Icarus 83,255-281) and West et al. (1992, Icarus 100, 245-259). At the base of the stratosphere we inferred the same circulation pattern as found by Gierasch et al. (1986, Icarus 67, 456-483) and West et al. (1992) with upwelling over the zones and subsidence in belts, but with higher values in the magnitude of the vertical winds at low latitudes.

Spectroscopic observations of Jupiter at thermal-infrared wavelengths were performed on November 8-9 and December 14, 1994, 16 and 21 weeks after the fragments of comet Shoemaker-Levy 9 collided with Jupiter. Residual stratospheric ammonia was searched for in emission at 930.76 [cm.sup.-1] over the longitudes corresponding to the remnants of the D/G/R/S impact sites. We believe a small amount of ammonia may have been detected in November, but report an upper limit for both dates. The constant volume mixing ratio for pressure levels less than 10 mbar that best fits the observation on November 8 is no more than 3.6 [+ or -] 2.4 x [10.sup.-10]. This upper limit decreased to [less than or equal to]2.3 [+ or -] 3.0 x [10.sup.-10] on December 14. These values are equivalent to upper limits of 1.9 [+ or -] 1.2 x [10.sup.-10] on November 8 and [less than or equal to]1.2 [+ or -] 1.5 x [10.sup.-10] on December 14 if we adopt a constant volume mixing ratio above a base at 100 mbar. We also searched, without success, for stratospheric ammonia on November 9 over the K/W and L impact site remnants. Our upper limits place stronger constraints on the stratospheric ammonia lifetimes and photochemical depletion models.

The vertical structure of Jupiter's atmosphere is examined to better characterize the latitudinal region of the Galileo probe entry site: +5 [degrees]-10 [degrees] planetographic latitude. One-hundred Hubble Space Telescope images of Jupiter taken with the 893- and 953-nm filters on 13 and 17 February 1995, 5 October 1995, and 14 May 1996 are analyzed, and a two-cloud model of the jovian atmosphere (D. M. Kuehn and R. F. Beebe 1993, Icarus 101, 282-292) is used to interpret center-to-limb variations of bright plume heads and adjacent dark areas in the latitude region +2 [degrees]-10 [degrees]. For both the plume heads and dark areas, the top of the ammonia cloud deck reaches a pressure level of roughly 200 mbar, and the cloud has an optical depth of between 6 and 7. The optical depth of the stratospheric haze layer is 0.59 for the plumes and 0.34 for the dark regions, while the single scattering albedo of the ammonia cloud deck is 0.996 for the plumes and 0.950 for the dark regions. Sensitivity tests motivated by the preliminary results from the Galileo probe net flux radiometer and nephelometer experiments indicate that the ammonia cloud deck may have a patchy structure.

The atmospheres of the giant planets (Jupiter, Saturn, Uranus, and Neptune) and Earth reach high temperatures in the low-pressure region of the upper atmosphere known as the thermosphere (1). On […]

We analyze the properties of wave-like longitudinal temperature variations in Jupiter's upper troposphere and stratosphere, as revealed using thermal infrared (IR) ground-based observations taken yearly during the 1989-1993 period. These thermal waves are apparently the same as the 'slowly moving thermal features' identified in Voyager data. We conclude that they are ubiquitous at near-equatorial latitudes on Jupiter, and have a spatial scale which typically spans wavenumber [approximately equal to]2 to [approximately equal to]15. Their stratospheric (20 mbar) temperature amplitude, from 7.8 [[micro]meter] data, is approximately a factor of 3 larger than their amplitude in the upper troposphere ([approximately equal to]0.3 bars), as revealed using 18 [[micro]meter] data. These amplitudes are consistent with the [[Rho].sup.-1/2] growth expected for a vertically propagating Rossby wave. The phase at the two levels is the same to within 5 [degrees] of longitude. The temperature wave is not advected by the zonal winds, having a phase velocity of [approximately equal to]5 m [sec.sup.-1] in system III, but the temperature amplitude can decay on time scales [approximately equal to][10.sup.5] sec. Imaging of the thermal waves shows that they are extended in latitude to a greater extent than the belt/zone structure, or the zonal wind jets. An apparently related wave is seen in broad-bandwidth (7-13 [[micro]meter]) IR observations which are sensitive to clouds and aerosols. This structure is advected by the zonal wind, indicating that some cloud or aerosol component shares the wave-like structure and spatial scale of the temperature wave. We interpret these waves as stationary Rossby waves, represented by small latitude excursions in the streamlines of the zonal winds. The latitude excursions could be produced by either deep structure, or by the global effect of vortices such as the Great Red Spot. In either case, temperature fluctuations are produced via 'vortex stretching.' We calculate the required magnitude of the latitude excursions in the zonal winds to be [approximately equal to]1 [degrees] in order to produce the observed temperature amplitudes. This is a subarcsec scale for Earth-based observations, but might be detectable using observations from the Hubble space telescope.

Observations of the carbon monoxide fundamental vibration-rotation band near 4.7 [[micro]meter] before and after the impacts of the fragments of Comet Shoemaker-Levy 9 showed no detectable changes in the R5 and R7 lines, with one possible exception. Observations of the G-impact site 21 hr after impact do not show CO emission, indicating that the heated portions of the stratosphere had cooled by that time. The large abundances of CO detected at the millibar pressure level by millimeter wave observations did not extend deeper in Jupiter's atmosphere. Predicted upwelling of shocked, O-rich material from below also did not occur. Combined with evidence for upwelling of N- and S-rich gas, our observations indicate that the comet fragments may not have penetrated to the [H.sub.2]O cloud. We find that CO concentrations in Jupiter's stratosphere may be higher than previously suspected, suggesting that some of the CO detected after the impacts may already have been present in Jupiter's stratosphere.

A semianalytic model for the breakup of fragments of Comet Shoemaker-Levy 9 upon entry into Jupiter's atmosphere is presented. The model assumes that the impacting fragments behave as bodies of viscous fluid and that the dominant process in their breakup is the growth of hydrodynamic instabilities. It is shown that the size of the smallest instabilities that contribute to mass loss largely determines the depth of penetration in a way which is consistent with the changes in penetration depth obtained using numerical models with different resolutions. If the diameter of the impactor corresponds to 8 resolution elements then the penetration depths obtained are about [10.sup.2] km too large. To obtain penetration depths within one scale height ([approximately equal to]25 km) of the viscosity limited value, at least 25 resolution elements are required across the diameter of the impactor. This result is in agreement with the numerical studies of K. Zahnle and M.-M. Mac Low (1994, Icarus 108, 1-17). It is also shown that two different regimes of hydrodynamic mass loss exist, one caused by Kelvin-Helmholtz (KH) type instabilities and a later one caused by the onset of Rayleigh-Taylor (RT) type instabilities. These regimes can be identified in the numerical results of D. A. Crawford et al. (1994, Shock Waves 4, 47-50), where KH instabilities appear to be the major mass loss mechanism between 100 and 200 km below I bar and RT instabilities become dominant below 200 km below 1 bar. The upward velocity of material behind the shock caused by the expansion of the superheated gas in the Comet's wake is then calculated and shown to be about 12 km [sec.sup.-1] and, to a first approximation, independent of the size of the impacting fragment provided that the fragment is not significantly decelerated before it reaches the tropopause (100 mbar). This upward velocity implies a plume height of 3000 km above the 1-bar level, which agrees with Hubble Space Telescope observations. It is shown that for no significant deceleration to occur before the tropopause the impacting fragments that produced plumes must have had diameters larger than 0.3 km. This, in turn, implies a progenitor diameter of 1.6 km. It is then estimated that the time interval between impacts of 0.3 km diameter comets on Jupiter is approximately 500 years, whereas the interval between the impact of 1.6 km comets is about 6000 years.

We present high-resolution 11-[[micro]meter] N[H.sub.3] observations of the Shoemaker-Levy 9 sites that were conducted on May 15-18, 1995 UT at the NASA Infrared Telescope Facility. No emission from stratospheric N[H.sub.3] was detected at any location. The upper limit on the N[H.sub.3] column density above 40 mbar is 6 x [10.sup.15] molecule [cm.sup.-2], about 30 times less than that observed on July 20-31 1994 UT over the K site. Destruction of stratospheric N[H.sub.3] indicates that shielding from ultraviolet radiation was not efficient over a period of several months. Latitudinal variations observed in the N[H.sub.3] absorption line can be interpreted through variations in the tropospheric temperature field consistent with Voyager and ground-based observations. No change in tropospheric N[H.sub.3] around impact latitude needs to be invoked to explain these variations.

Atmospheric jets and vortices on Jupiter persist for decades in the presence of vigorous turbulence which might be expected to destroy organized flows. The penetration depth of the jets into the planet has been a major question, and new data from the NASA Galileo entry probe suggest that they are not shallow atmospheric phenomena. Numerical simulations and Galileo orbiter data promise further progress in the near future.

A one-dimensional code to study the microphysical processes affecting aerosol particles in the jovian stratosphere and upper troposphere has been developed. The algorithms are based on those developed by Turco et al.(Turco, R. P., P. Hamill, O. B. Toon, R. C. Whitten, and C. S. Kiang 1979. J. Atmos. Sci. 36, 699-717) and include the effects of particle sedimentation, coagulation, eddy diffusion, and growth by heteromolecular condensation. A set of Hubble Space Telescope images in the near-UV was used to retrieve the aerosol distribution in the equatorial and south polar regions of Jupiter. The results obtained are consistent with the picture in which in the polar regions the aerosols are formed from auroral-related phenomena, while in the equatorial region the largest component of the aerosols would be those particles formed after condensation of hydrazine onto condensation nuclei. Concerning the charge of the particles, the particles in the south polar region must be considerably more charged than at equatorial latitudes. The model solutions give particles smaller than about 0.05 [[micro]meter] in radius at the equator, and 0.07 [[micro]meter] at the polar regions, with total optical depths of 0.3 and 1.3 at the equator and the south polar region, respectively, at 410 nm wavelength. The differences in the derived imaginary refractive indices reflect a compositional change in the aerosols at low and high latitudes.

I have searched for absorptions from the molecule HBr in the 4 [[micro]meter] spectra of Jupiter and Saturn and find upper limits to the mole fraction of HBr to be q(HBr) [less than or equal to] 1.6 ppb in Saturn and q(HBr) [less than or equal to] 3 ppb in Jupiter, or two and four times the solar abundance of bromine in Saturn and Jupiter, respectively. The upper limit for Saturn is lower than the global abundance of bromine, if, as expected, bromine is enriched in Saturn relative to C1 carbonaceous chondrites. This deficiency is most likely the result of local sinks for HBr. Less likely is a global depletion of bromine, conceivably related to the low condensation temperature of bromine-containing solids in the presolar nebula. Searches for HCl in the 4 [[micro]meter] window are also possible, but require large enrichments of Cl for detection with existing data.

We study power spectra and search for planetary waves in images of Jupiter's cloud opacity. The observation wavelength of 4.9 [[micro]meter] senses thermal emission from the [approximately]5-bar level; overlying clouds attenuate the emission. Our companion paper (J. Harrington, T. E. Dowling, and R. L. Baron, 1996, Icarus 124, 22-31) describes 19 nights of observations (6 with 360 [degrees] longitude coverage) and new reduction techniques. Atmospheric seeing limits resolution to [approximately]2500 km. Zonal power spectral density at planetary wavenumbers higher than [approximately]25 follows a power law in the wavenumber. Eastward jet-power laws average -2.71 [+ or -] 0.07 and westward jet-power laws, excluding cloud-obscured regions, average -3.14 [+ or -] 0.12. Wavenumbers 1-24 roughly follow power laws near -0.7 for both jet directions, but with many superposed discrete features. The meridional spectrum similarly breaks around wavenumber 25, with power law trends of -0.36 and -3.27. However, a pattern of undulations is superposed over its linear trends. L. D. Travis (1978, J. Atmos. Sci. 35, 1584-1595) established an empirical correspondence between power spectra of atmospheric kinetic energy and those of cloud opacities for the Earth and analyzed Venus cloud data under this assumption. We do the same for Jupiter. If the Rossby deformation radius, [L.sub.d], were an energy input scale, as baroclinic instability theory predicts, one would expect energy and enstrophy cascades (power laws of -5/3 and -3, respectively) on opposite sides of the wavenumber corresponding to [L.sub.d]. If the top of our high-wavenumber power law is [L.sub.d], its value is [approximately]2100 km at 45 [degrees] latitude. Our spectra show persistent features with phases moving linearly over the 99-day observation period. Some of these can be identified with periodic features such as vortex chains and the equatorial plumes. The origin of others is less certain. We present a table of our best wave candidates.

This paper presents a new Jupiter data set and several new techniquesdeveloped for its reduction. A companion paper (J. Harrington, T. E.Dowling, and R. L. Baron, 1996, Icarus 124, 32-44) uses these data to studythe scales of energy deposition into Jupiter's atmosphere. We observed Jupiter's tropospheric thermal emission at a wavelength of 4.9 [[micro]meter] (1% band-pass) with the NASA Infrared Telescope Facility (Mauna Kea, HI). The ProtoCAM 62 x 58-pixel infrared array camera took a total of 229 full-disk 3 x 3- and 4 x 4-image mosaics on 19 nights spanning the period 11 Jan. through 19 Apr. 1992. Resolution was typically 0.5[inches]-0.75[inches]. We obtained full longitude coverage on six nights. This wavelength is sensitive to emission from the [approximately]5-bar level, near the top of the (putative) jovian water cloud level. Clouds located higher in the atmosphere attenuate this emission. The images show atmospheric structure at all resolved spatial scales and have a cloud optical depth range of [approximately]4.6. We developed image processing techniques for finding the optimal registration of images in mosaics and for locating low-contrast planetary limbs in images of planets with many high-contrast features. Both techniques are new and generally applicable and require no human interaction.

The atmosphere of Io has been detected at infrared, millimeter, and ultraviolet wavelengths. The observations indicate a predominantly [SO.sub.2] tenuous atmosphere on both the leading and trailing daysides, probably variable but permanently detectable. The observed [SO.sub.2] pressure is on the order of 1 nanobar. There are several indications that the atmosphere is patchy rather than spatially uniform. The observable atmosphere may be produced from sublimation of [SO.sub.2] frost or directly from volcanic output, while surface sputtering may produce a more tenuous atmospheric component in specific regions. The large horizontal pressure gradients probably drive a supersonic meteorology that affects the pressure distribution on both local and large scales and the sublimation/condensation exchanges. SO appears to be a significant minor component along with [SO.sub.2], but its spatial distribution (global or localized) remains uncertain. Fundamental issues are unresolved, as the detailed interpretation of some observations is still ambiguous. The surface pressure and areal extent of the atmosphere are only known to within factors of [approximately]10 and 5, respectively. The vertical structure is extremely poorly characterized. We do not know for sure whether Io's atmosphere is in hydrostatic equilibrium or if the gas is rapidly ejected by volcanic venting. The gas temperature is very difficult to constrain from observations, although detailed models are available. The dominant source of Io's atmosphere remains uncertain. Current data sets would suggest a preference for the concept of an atmosphere in dynamical equilibrium with volcanic sources, but this conclusion must be viewed as tentative. The existence, composition, thickness, and extent of the atmosphere on the nightside are unknown.

We present a new analysis of jovian auroral IUE spectra covering the period from 1981 to 1991. To extract integrated auroral H Ly-[Alpha] emission from these spectra we have developed a new extraction method that bins signal with wavelength while preserving the spatial information provided by the IUE imaging spectrograph. This separates auroral emission from background sources including the jovian dayglow, geocoronal H Ly-[Alpha] emission, emission from hydrogen in the interplanetary medium (IPH), and grating scattered light in the IUE spectrograph. Auroral H Ly-[Alpha] emission is compared with [H.sub.2] emission covering the bandpasses from 1230 to 1300 [Angstrom] and 1550 to 1620 [Angstrom]. Study of [H.sub.2] and H Ly-[Alpha] variability as a function of central meridian longitude (CML) and the results of linear correlative analysis indicates that these features are produced by a common or linked process with a similar spatial distribution. Further comparative analysis of the relative optical depth of the correlated emission using a modified form of the [H.sub.2] color ratio of Yung et al. (1982, Astrophys. J. 254, L65-L69) suggest that [H.sub.2] and H Ly-[Alpha] emissions are subject to similar variability in C[H.sub.4] extinction with CML. From this a conservative upper limit of 37% is derived for the amount of H Ly-[Alpha] emission that can be produced above the altitude where the bulk of the [H.sub.2] emission is produced. We also discuss how the implied radiative transfer environment for H Ly-[Alpha] production and the local H/C[H.sub.4] ratio can limit the altitude distribution of the emission, the auroral H Ly-[Alpha] intensity, and the shape and information available from the emergent line profile.

Reported here is the first experimental measurement of the frequency dependent refractive index of crystalline hydrazine. The method used in this study is based on the measurement of the infrared spectrum of laboratory generated aerosols. The optical constants determined here should be directly useful in remote sensing and radiative transfer models of the jovian atmosphere.

Recently the rate constants of the reactions [C.sub.2][H.sub.3] + [H.sub.2] [approaches] [C.sub.2][H.sub.4] + H and [C.sub.2][H.sub.3] + H [approaches] products have been measured by more direct methods than used previously. The new determination for the rate constant of [C.sub.2][H.sub.3] + [H.sub.2] is lower by a factor of 1000 at room temperature than the rate used by Allen et al. in their methane photochemical model (The relative abundance of ethane to acetylene in the jovian stratosphere, Icarus 100, 527-533 (1992)). A fast rate for this reaction is key to their proposed reaction scheme to convert [C.sub.2][H.sub.2] to [C.sub.2][H.sub.6] and explain the [C.sub.2][H.sub.6]/[C.sub.2][H.sub.2] ratio on Jupiter. However, their scheme becomes ineffective when a rate constant expression consistent with Fahr et al. is used (Experimental determination of the rate constant for the reaction of [C.sub.2][H.sub.3] with [H.sub.2] and implications for the partitioning of hydrogen in the atmospheres of the other planets, Icarus 116, 415-422 (1995)). The rate measurement for [C.sub.2][H.sub.3] + H by Monks et al. (Kinetics and products of the reaction between H and [C.sub.2][H.sub.3] of T = 213 and 298 K, J. Phys. Chem. 99, 17,151-17,159 (1993)) is consistent with recent measurements but they found [C.sub.2][H.sub.4] to be a significant product, in addition to [C.sub.2][H.sub.2] + [H.sub.2]. Only the latter product channel had been used in methane photochemical models. With the new lower rate constant for [C.sub.2][H.sub.3] + [H.sub.2], the reaction [C.sub.2][H.sub.3] + H [approaches] [C.sub.2][H.sub.4] becomes important in determining the [C.sub.2][H.sub.6]/[C.sub.2][H.sub.2] ratio on Jupiter. However, the new reaction scenario is less efficient in converting [C.sub.2][H.sub.2] to [C.sub.2][H.sub.6] than the one proposed by Allen et al. because it depends in part upon H, which is in scarce supply in the lower stratosphere where [C.sub.2][H.sub.3] + H [approaches] [C.sub.2][H.sub.4] is important.