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We present H$_2$O production rates for comet C/2012 S1 (ISON) derived from observations of [OI] and OH emission during its inbound leg, covering a heliocentric distance range of 1.8-0.44 AU. Our production rates are in agreement with previous measurements using a variety of instruments and techniques and with data from the various observatories greatly differing in their projected fields of view. The consistent results across all data suggest the absence of an extended source of H$_2$O production, for example sublimation of icy grains in the coma, or a source with spatial extent confined to the dimensions of the smallest projected field of view (in this case $ 1.2 AU, which then decreased to about half this value from R$_h$=1.2-0.9 AU. This was followed by a rapid increase in active area at about R$_h$=0.6 AU, corresponding to the first of three major outbursts ISON experienced inside of 1 AU. The combination of a detected outburst in the light curve and rapid increase in active area likely indicates a major nucleus fragmentation event. The 5-10 km$^2$ active area observed outside of R$_h$=0.6 AU is consistent with a 50-100% active fraction for the nucleus, larger than typically observed for cometary nuclei. Although the absolute value of the active area is somewhat dependent on the thermal model employed, the changes in active area observed are consistent among models. The conclusion of a 50-100+% active fraction is robust for realistic thermal models of the nucleus. However the possibility of a contribution of a spatially unresolved distribution of icy grains cannot be discounted., Comment: 26 pages, 5 figures, 5 tables, accepted for publication in Icarus

We apply a new shape-based thermophysical model, SHERMAN, to the near-Earth asteroid (NEA) 8567 (1996 HW1) to derive surface properties. We use the detailed shape model of Magri et al. (2011) for this contact binary NEA to analyze spectral observations (2–4.1 microns) obtained at the NASA IRTF on several different dates to find thermal parameters that match all the data. Visible and near-infrared (0.8–2.5 microns) spectral observations are also utilized in a self-consistent way. We find that an average visible albedo of 0.33, thermal inertia of 70 (SI units) and surface roughness of 50% closely match the observations. The shape and orientation of the asteroid is very important to constrain the thermal parameters to be consistent with all the observations. Multiple viewing geometries are equally important to achieve a robust solution for small, non-spherical NEAs. We separate the infrared beaming effects of shape, viewing geometry and surface roughness for this asteroid and show how their effects combine. We compare the diameter and albedo that would be derived from the thermal observations assuming a spherical shape with those from the shape-based model. We also discuss how observations from limited viewing geometries compare to the solution from multiple observations. The size that would be derived from the individual observation dates varies by 20% from the best-fit solution, and can be either larger or smaller. If the surface properties are not homogeneous, many solutions are possible, but the average properties derived here are very tightly constrained by the multiple observations, and give important insights into the nature of small NEAs.

SHERMAN, a new thermophysical modeling package designed for analyzing near-infrared spectra of asteroids and other solid bodies, is presented. The model’s features, the methods it uses to solve for surface and subsurface temperatures, and the synthetic data it outputs are described. A set of validation tests demonstrates that SHERMAN produces accurate output in a variety of special cases for which correct results can be derived from theory. These cases include a family of solutions to the heat equation for which thermal inertia can have any value and thermophysical properties can vary with depth and with temperature. An appendix describes a new approximation method for estimating surface temperatures within spherical-section craters, more suitable for modeling infrared beaming at short wavelengths than the standard method.

We present thermal properties and an improved shape model for potentially hazardous asteroid (162421) 2000 ET70. In addition to the radar data from 2000 ET70's apparition in 2012, our model incorporates optical lightcurves and infrared spectra that were not included in the analysis of Naidu et al. (2013, Icarus 226, 323-335). We confirm the general "clenched fist" appearance of the Naidu et al. model, but compared to their model, our best-fit model is about 10% longer along its long principal axis, nearly identical along the intermediate axis, and about 25% shorter along the short axis. We find the asteroid's dimensions to be 2.9 km $\times$ 2.2 km $\times$ 1.5 km (with relative uncertainties of about 10%, 15%, and 25%, respectively). With the available data, 2000 ET70's period and pole position are degenerate with each other. The radar and lightcurve data together constrain the pole direction to fall along an arc that is about twenty-three degrees long and eight degrees wide. Infrared spectra from the NASA InfraRed Telescope Facility (IRTF) provide an additional constraint on the pole. Thermophysical modeling, using our SHERMAN software, shows that only a subset of the pole directions, about twelve degrees of that arc, are compatible with the infrared data. Using all of the available data, we find that 2000 ET70 has a sidereal rotation period of 8.944 hours ($\pm$ 0.009 h) and a north pole direction of ecliptic coordinates $(52^{\circ}, -60^{\circ}) \pm 6^{\circ}$. The infrared data, acquired over several dates, require that the thermal properties (albedo, thermal inertia, surface roughness) must change across the asteroid's surface. By incorporating the detailed shape model and spin state into our thermal modeling, the multiple ground-based observations at different viewing geometries have allowed us to constrain the levels of the variations in the surface properties., Revised and submitted to Icarus; 22 pages, 13 figures; 51 supplementary figures (in a 25MB PDF) can be found at http://astro.cornell.edu/~seanm/2000et70/

We present our shape model of Amor class near-Earth asteroid (1627) Ivar. During Ivar's apparition in 2013, in which it had a minimum distance of 0.32 AU, we obtained both CCD photometry and radar observations. The radar data consist of echo power spectra and delay-Doppler imaging with 300 m resolution and were obtained using the Arecibo Observatory's 2380 MHz radar. Lightcurve data were gathered using the 0.35 m telescope at the Palmer Divide Station. Using these data, we constructed a detailed shape model of Ivar that reveals more surface detail than earlier models. Ivar was found to have a rotational period of 4.7951689 ± 0.0000026 hours with a pole located within 6° of ecliptic longitude and latitude 336° and 37° respectively. Ivar is an elongated asteroid with maximum extensions along the three body-fixed coordinates being 15.15 × 6.25 × 5.66 km ± 10%. The results of surface slope analysis suggest that Ivar is covered with a loose regolith. Ivar appears to reside in, or near, an optimum state with respect to its shape, spin, and bulk density, such that dynamic topography, surface slopes, and erosion rates on the body are near minimum levels and is therefore dynamically stable.

The Ultraviolet and Visible Spectrometer channel of the Mercury Atmospheric and Surface Composition Spectrometer instrument aboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft made near-daily observations of solar-scattered resonant emission from magnesium in Mercury's exosphere during the mission's orbital phase (March 2011–April 2015, ∼17 Mercury years). In this paper, a subset of these data (March 2013–April 2015) is described and analyzed to illustrate Mg's spatial and temporal variations. Dayside altitude profiles of emission are used to make estimates of the Mg density and temperature. The main characteristics of the Mg exosphere are (a) a predominant enhancement of emission in the morning (6 am–10 am) near perihelion, (b) a bulk temperature of ∼6000 K, consistent with impact vaporization as the predominant ejection process, (c) a near-surface density that varies from 5 cm −3 to 50 cm −3 and (d) a production rate that is strongest in the morning on the inbound leg of Mercury's orbit with rates ranging from 1 × 10 5 cm −2 s −1 to 8 × 10 5 cm −2 s −1 .

A systematic analysis of the mixing ratios with respect to H 2 O for eight species (CH 3 OH, HCN, NH 3 , H 2 CO, C 2 H 2 , C 2 H 6 , CH 4 , and CO) measured with high-resolution infrared spectroscopy in thirty comets between 1997 and 2013 is presented. Some trends are beginning to emerge when mixing ratios in individual comets are compared to average mixing ratios obtained for all species within the population. The variation in mixing ratios for all measured species is at least an order of magnitude. Overall, Jupiter-family comets are depleted in volatile species with respect to H 2 O compared to long-period Oort cloud comets, with the most volatile species showing the greatest relative depletion. There is a high positive correlation between the mixing ratios of HCN, C 2 H 6 , and CH 4 , whereas NH 3 , H 2 CO, and C 2 H 2 are moderately correlated with each other but generally uncorrelated or show only weak correlation with other species. CO is generally uncorrelated with the other measured species possibly because it has the highest volatility and is therefore more susceptible to thermal evolutionary effects. Most of these correlations appear to be independent of dynamical class with a few possible exceptions. Molecular mixing ratios for CH 3 OH, HCN, C 2 H 6 , and CH 4 show an expected behavior with heliocentric distance suggesting a dominant ice source, whereas there is emerging evidence that the mixing ratios of NH 3 , H 2 CO, C 2 H 2 , NH 2 , and CN may increase at small heliocentric distances, suggesting the possibility of additional sources related to the thermal decomposition of organic dust. Although this provides information on the composition of the most volatile grains in comets, it presents an additional difficulty in classifying comet chemistry because most comets within this dataset were only observed over a limited range of heliocentric distance. Although there is remarkable compositional diversity resulting in a unique chemical fingerprint for each comet, a hierarchical tree cluster analysis is used to determine a taxonomic classification system containing four groups and eleven subgroups. Optical and infrared comparisons indicate that mixing ratios of daughter species and potential parents from cometary ices are sometimes but not always consistent with one another. This suggests that in many comets there are significant sources of C 2 and/or CN from grains, and that the importance of these sources is variable within the comet population.

High-resolution infrared spectra of Jupiter-family comet 45P/Honda-Mrkos-Pajdusakova were obtained with NIRSPEC at the W. M. Keck Observatory on two post-perihelion dates (UT 2017 February 13 and 19), when the comet was at heliocentric distances of 1.01 and 1.10 AU, respectively. On UT February 13, H2O was measured simultaneously with six trace parent molecules: CH3OH, C2H6, HCN, NH3, C2H2, and H2CO. On UT February 19, CH4 and CO were also targeted in addition to the species measured on UT February 13. Abundances of CO, CH4, and C2H2 relative to H2O are consistent with values obtained from IRTF/iSHELL observations of 45P in early January 2017 just after perihelion when the heliocentric distance was 0.55–0.56 AU. Differences are seen in H2CO/H2O, C2H6/H2O, CH3OH/H2O, and HCN/H2O in February compared to January. Additionally, NH3 abundances appear highly variable during the February measurements, suggesting possible fluctuations of a factor of ten; however, there is significant uncertainty in quantifying NH3 owing to the marginal detections of only one or two lines on each of the two dates. Combining all infrared spectroscopic observations of 45P in January and February 2017, a post-perihelion relationship of Q(H2O) = (2.81 ± 0.25) × 1027 [Rh(−3.83±0.18)] molecules s−1 is derived. However, all measurements suggest significant variability in H2O production on timescales of hours and days. Compared to other comets, volatile abundances relative to H2O in 45P are as follows: CO (depleted relative to all measured comets), CH3OH (enriched/all comets), CH4 and C2H6 (typical/all comets, enriched/Jupiter-family comets), C2H2 (depleted/all comets, typical/Jupiter-family comets), H2CO (January: typical, February depleted/all comets), HCN (January: severely depleted, February: typical/Jupiter-family comets). The small geocentric distances of the comet in February 2017 provide high spatial resolution in the coma of 45P (~12 and 19 km/pixel on February 13 and 19, respectively). Overall, the spatial distributions of volatiles and dust suggest a relatively symmetric and uniform coma during the UT February 13 and 19 observations, with small spatial differences noted between some volatile species. Measured C2H2 and HCN abundances are consistent with HCN as the primary parent of CN and C2H2 as a significant but not primary parent for C2, based on C2 and CN production rate measurements from the 2017 and previous apparitions. Extracted spectra show an increase in the dust-to-gas ratio in 45P from February 13 to 19 in agreement with contemporaneous narrowband optical measurements.

We present analysis of high spectral resolution NIR spectra of CO and H$_2$O in comet C/2009 P1 (Garradd) taken during its 2011-2012 apparition with the CSHELL instrument on NASA's Infrared Telescope Facility (IRTF). We also present analysis of observations of atomic oxygen in comet Garradd obtained with the ARCES echelle spectrometer mounted on the ARC 3.5-meter telescope at Apache Point Observatory and the Tull Coude spectrograph on the Harlan J. Smith 2.7-meter telescope at McDonald Observatory. The observations of atomic oxygen serve as a proxy for H$_2$O and CO$_2$. We confirm the high CO abundance in comet Garradd and the asymmetry in the CO/H$_2$O ratio with respect to perihelion reported by previous studies. From the oxygen observations, we infer that the CO$_2$/H$_2$O ratio decreased as the comet moved towards the Sun, which is expected based on current sublimation models. We also infer that the CO$_2$/H$_2$O ratio was higher pre-perihelion than post-perihelion. We observe evidence for the icy grain source of H$_2$O reported by several studies pre-perihelion, and argue that this source is significantly less abundant post-perihelion. Since H$_2$O, CO$_2$, and CO are the primary ices in comets, they drive the activity. We use our measurements of these important volatiles in an attempt to explain the evolution of Garradd's activity over the apparition., 35 pages, 10 figures, 9 tables, accepted for publication in Icarus

The Mercury Atmospheric and Surface Composition Spectrometer (MASCS) Ultraviolet and Visible Spectrometer (UVVS) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft now orbiting Mercury provides the first close-up look at the planet's sodium exosphere. UVVS has observed the exosphere from orbit almost daily for over 10 Mercury years. In this paper we describe and analyze a subset of these data: altitude profiles taken above the low-latitude dayside and south pole. The observations show spatial and temporal variations, but there are no obvious year-to-year variations in most of the observations. We do not see the episodic variability reported by some ground-based observers. We used these altitude profiles to make estimates of sodium density and temperature. The bulk of the exosphere, at about 1200 K, is much warmer than Mercury's surface. This value is consistent with some ground-based measurements and suggests that photon-stimulated desorption is the primary ejection process. We also observe a tenuous energetic component but do not see evidence of the predicted thermalized (or partially thermalized) sodium near Mercury's surface temperature. Overall we do not see the variable mixture of temperatures predicted by most Monte Carlo models of the exosphere.

The Mercury Atmospheric and Surface Composition Spectrometer on the MESSENGER spacecraft has observed calcium emission in Mercury's exosphere on a near-daily basis since March 2011. During MESSENGER's primary and first extended missions (March 2011 - March 2013) the dayside calcium exosphere was measured over eight Mercury years. We have simulated these data with a Monte Carlo model of exospheric source processes to show that (a) there is a persistent source of energetic calcium located in the dawn equatorial region, (b) there is a seasonal dependence in the calcium source rate, and (c) there are no obvious year-to-year variations in the near-surface dayside calcium exosphere. Although the precise mechanism responsible for ejecting the calcium has not yet been determined, the most likely process is the dissociation of Ca-bearing molecules produced in micrometeoroid impact plumes to form energetic, escaping calcium atoms.

We present observations of the binary near-Earth Asteroid (175706) 1996 FG3 in the 2–4 μm spectral region, obtained on multiple nights using the SpeX instrument on the NASA Infrared Telescope Facility (IRTF). We find evidence for an absorption band of ∼5–10% (depending on the night and details of continuum choice), diagnostic for hydrated/hydroxylated minerals on the asteroid surface. This demonstrates that the optical surface of 1996 FG3 has not been heated to dehydrating temperatures. Sample-return missions targeting 1996 FG3 will likely find relatively fresh areas for sampling.

We observed near-Earth Asteroid (8567) 1996 HW1 at the Arecibo Observatory on six dates in September 2008, obtaining radar images and spectra. By combining these data with an extensive set of new lightcurves taken during 2008–2009 and with previously published lightcurves from 2005, we were able to reconstruct the object’s shape and spin state. 1996 HW1 is an elongated, bifurcated object with maximum diameters of 3.8 × 1.6 × 1.5 km and a contact-binary shape. It is the most bifurcated near-Earth asteroid yet studied and one of the most elongated as well. The sidereal rotation period is 8.76243 ± 0.00004 h and the pole direction is within 5° of ecliptic longitude and latitude (281°, −31°). Radar astrometry has reduced the orbital element uncertainties by 27% relative to the a priori orbit solution that was based on a half-century of optical data. Simple dynamical arguments are used to demonstrate that this asteroid could have originated as a binary system that tidally decayed and merged.

We have used observations of sodium emission obtained with the McMath-Pierce solar telescope and MESSENGER's Mercury Atmospheric and Surface Composition Spectrometer (MASCS) to constrain models of Mercury's sodium exosphere, The distribution of sodium in Mercury's exosphere during the period January 12-15. 2008. was mapped using the McMath-Pierce solar telescope with the 5" X 5" image slicer to observe the D-line emission. On January 14, 2008, the Ultraviolet and Visible Spectrometer (UVVS) channel on MASCS sampled the sodium in Mercury's anti-sunward tail region. We find that the bound exosphere has an equivalent temperature of 900-1200 K, and that this temperature can be achieved if the sodium is ejected either by photon-stimulated desorption (PSD) with a 1200 K Maxwellian velocity distribution, or by thermal accommodation of a hotter source. We were not able to discriminate between the two assumed velocity distributions of the ejected particles for the PSD. but the velocity distributions require different values of the thermal accommodation coefficient and result in different upper limits on impact vaporization, We were able to place a strong constraint on the impact vaporization rate that results in the release of neutral Na atoms with an upper limit of 2.1 x 10(exp 6) sq cm/s, The variability of the week-long ground-based observations can be explained by variations in the sources, including both PSD and ion-enhanced PSD, as well as possible temporal enhancements in meteoroid vaporization. Knowledge of both dayside and anti-sunward tail morphologies and radiances are necessary to correctly deduce the exospheric source rates, processes, velocity distribution, and surface interaction.

From observations of the metallic species sodium (Na), potassium (K), and magnesium (Mg) in Mercury's exosphere, we derive implications for source and loss processes. All metallic species observed exhibit a distribution and/or line width characteristic of high to extreme temperature - tens of thousands of degrees K. The temperatures of refractory species, including magnesium and calcium, indicate that the source process for the atoms observed in the tail and near-planet exosphere are consistent with ion sputtering and/or impact vaporization of a molecule with subsequent dissociation into the atomic form. The extended Mg tail is consistent with a surface abundance of 5-8% Mg by number, if 30% of impact-vaporized Mg remains as MgO and half of the impact vapor condenses. Globally, ion sputtering is not a major source of Mg, but locally the sputtered source can be larger than the impact vapor source. We conclude that the Na and K in Mercury's exosphere can be derived from a regolith composition similar to that of Luna 16 soil (or Apollo 17 orange glass), in which the abundance by number is 0.0027 (0.0028) for Na and 0.0006 (0.0045) for K.

We present a Monte Carlo model of the distribution of neutral sodium in Mercury's exosphere and tail using data from the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during the first two flybys of the planet in January and September 2008. We show that the dominant source mechanism for ejecting sodium from the surface is photon-stimulated desorption (PSD) and that the desorption rate is limited by the diffusion rate of sodium from the interior of grains in the regolith to the topmost few monolayers where PSD is effective. In the absence of ion precipitation, we find that the sodium source rate is limited to approximately 10(exp 6) - 10(exp 7) per square centimeter per second, depending on the sticking efficiency of exospheric sodium that returns to the surface. The diffusion rate must be at least a factor of 5 higher in regions of ion precipitation to explain the MASCS observations during the second MESSENGER f1yby. We estimate that impact vaporization of micrometeoroids may provide up to 15% of the total sodium source rate in the regions observed. Although sputtering by precipitating ions was found not to be a significant source of sodium during the MESSENGER flybys, ion precipitation is responsible for increasing the source rate at high latitudes through ion-enhanced diffusion.

The Alice ultraviolet spectrograph onboard the New Horizons spacecraft observed two occultations of the bright star χ Ophiucus by Jupiter’s atmosphere on February 22 and 23, 2007 during the approach phase of the Jupiter flyby. The ingress occultation probed the atmosphere at 32°N latitude near the dawn terminator, while egress probed 18°N latitude near the dusk terminator. A detailed analysis of both the ingress and egress occultations, including the effects of molecular hydrogen, methane, acetylene, ethylene, and ethane absorptions in the far ultraviolet (FUV), constrains the eddy diffusion coefficient at the homopause level to be 3.4 - 2.8 + 9.0 × 10 6 cm2 s−1, consistent with Voyager measurements and other analyses (Festou, M.C., Atreya, S.K., Donahue, T.M., Sandel, B.R., Shemansky, D.E., Broadfoot, A.L. [1981]. J. Geophys. Res. 86, 5717–5725; Vervack Jr., R.J., Sandel, B.R., Gladstone, G.R., McConnell, J.C., Parkinson, C.D. [1995]. Icarus 114, 163–173; Yelle, R.V., Young, L.A., Vervack Jr., R.J., Young, R., Pfister, L., Sandel, B.R. [1996]. J. Geophys. Res. 101 (E1), 2149–2162). However, the actual derived pressure level of the methane homopause for both occultations differs from that derived by Festou et al., 1981 , Yelle et al., 1996 from the Voyager ultraviolet occultations, suggesting possible changes in the strength of atmospheric mixing with time. We find that at 32°N latitude, the methane concentration is 3.1 - 0.5 + 0.5 × 10 8 cm−3 at 70,397 km, the methane concentration is 1.2 - 0.3 + 0.3 × 10 9 cm−3 at 70,383 km, the acetylene concentration is 1.4 - 0.2 + 0.4 × 10 8 cm−3 at 70,364 km, and the ethane concentration is 6.8 - 0.8 + 1.1 × 10 8 cm−3 at 70,360 km. At 18°N latitude, the methane concentration is 3.2 - 0.7 + 0.7 × 10 8 cm−3 at 71,345 km, the methane concentration is 1.2 - 0.2 + 0.6 × 10 9 cm−3 at 71,332 km, the acetylene concentration is 1.6 - 0.6 + 0.3 × 10 8 cm−3 at 71,318 km, and the ethane concentration is 7.0 - 2.5 + 2.4 × 10 8 cm−3 at 71,315 km. We also find that the H2 occultation light curve is best reproduced if the atmosphere remains cold in the microbar region such that the base of the thermosphere is located at a lower pressure level than that determined by in situ instruments aboard the Galileo probe (Seiff, A., Kirk, D.B., Knight, T.C.D., Young, R.E., Mihalov, J.D., Young, L.A., Milos, F.S., Schubert, G., Blanchard, R.C., Atkinson, D. [1998]. J. Geophys. Res. 103 (E10), 22857–22889) – the Sieff et al. temperature profile leads to too much absorption from H2 at high altitudes. However, this result is highly model dependent and non-unique. The observations and analysis help constrain photochemical models of Jupiter’s atmosphere.

C/2006 P1 McNaught is a dynamically new comet from the Oort cloud that passed very close to the Sun, driving overall volatile production rates up to about 10 31 molecules s −1 . Post-perihelion observations were obtained in a target-of-opportunity campaign using the CSHELL instrument at the NASA Infrared Telescope Facility atop Mauna Kea, Hawaii, on UT 2007 January 27 and 28. Eight parent volatiles (H 2 O, CH 4 , C 2 H 2 , C 2 H 6 , HCN, CO, NH 3 , H 2 CO) and two daughter fragments (OH and NH 2 ) were detected, enabling the determination of a rotational temperature and production rate for H 2 O on UT January 27 and absolute and relative production rates for all the detected parent species on UT January 28. The chemical composition measured in the coma suggests that this close perihelion passage stripped off processed outer surface layers, likely exposing relatively fresh primordial material during these observations. The post-perihelion abundances we measure for CO and CH 4 (relative to H 2 O) are slightly depleted while C 2 H 2 , NH 2 and possibly NH 3 are enhanced when compared to the overall comet population. Measured abundances for other detected molecular species were within the range typically observed in comets.

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 N2 densities that are 25–60% higher, CH4 densities that are smaller by a factor of 3–7, and C2H2 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 for C2H4, HCN, and HC3N. 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.

Recent studies of the upper atmosphere of Jupiter and the H Lyman α bulge in particular have prompted us to reevaluate Voyager UVS measurements of Jupiter's He 584 A dayglow. We employ more sophisticated data reduction techniques than for earlier analyses to better quantify the UVS observations. We model Jupiter's He 584 A intensity using partial frequency redistribution and inhomogeneous atmospheric models that are more realistic than the isothermal atmospheric models used in the earlier analyses. These calculations show that the eddy diffusion coefficient at the homopause Kh is roughly 2+2-1 × 106 cm2 sec-1. One question we address is whether Jupiter's He 584 A dayglow shows a longitudinal asymmetry in intensity. The present work suggests that the evidence for such an asymmetry is not conclusive but is consistent with a variation anticorrelated with H Lyman α.

The distribution of hydrocarbons in the upper atmosphere of Neptune is determined on the basis of data from the Voyager UVS solar occultation experiment. Densities are inferred from the transmission properties of the atmosphere measured by the UVS. The CH4 mole fraction in the lower stratosphere is between 0.0006 and 0.005. It is inferred that CH4 in Neptune's lower stratosphere is oversaturated by at least a factor of 10, and possibly by as much as a factor of 100. The density of C2H6 reaches values of 3 x 10 exp 9/cu cm near 100 microbar; a C2H6 production efficiency of 35 +15/-5 percent is derived. The eddy diffusion coefficient is approximately constant at a value of 10,000 sq cm/s at altitudes below 300 km and increases at higher altitudes as the pressure to the 0.75 power. A mole fraction of 0.001 is found to be consistent with the CH4 vapor pressure at 3.4 mbar; consequently, CH4 ice particles must reach this level to produce the inferred oversaturation.