Saturn upper atmospheric structure from Cassini EUV and FUV occultations 1

817 Saturn upper atmospheric structure from Cassini EUV and FUV occultations 1 D.E. Shemansky and X. Liu Abstract: Stellar occultations of the Saturn atmosphere using the Cassini ultraviolet imaging spectrograph (UVIS) experiment have provided vertical structure at a range of latitudes. The transmission spectra in the extreme far ultraviolet (EUV FUV) range allow extraction of vertical profiles of H 2 and hydrocarbon abundances from the top of the atmosphere to about 300 km above the 1 bar (1 bar D 100 kpa) pressure level. A reanalysis of the Voyager 2 Sco occultation in 1981 is consistent with the original report. The hydrocarbon homopause is near a pressure of 0.2 bar in the UVIS analysis, compared to 0.01 bar obtained from the Voyager occultation. Measured hydrocarbon abundances are obtained in the pressure range 600 0.1 bar in the Cassini UVIS experiment. The combined UVIS results provide evidence for significant latitudinal dependence of vertical temperature profile. The confinement of the hydrocarbons in the current observations compared to published models and the Voyager ultraviolet spectrograph (UVS) results at solar maximum, infer smaller eddy diffusion coefficients in this epoch. Model calculations indicate that the latitudinal dependence of H 2 vertical displacements is caused primarily by the combined effects of gravitational potential and evident differences in electron energy deposition at the top of the atmosphere affecting the temperature profile. The derived H 2 density profiles from 40 ı latitude and others close to the equator, are found to be nearly identical on a pressure scale below the exobase. The inference is that that the pressure profile of H 2 density at Saturn is unchanged over a broad range of latitudes. PACS Nos.: 96., 96.12.jt, 96.12.Ma, 96.30.Mh Résumé : En utilisant le spectrographe ultraviolet à image Cassini (UVIS), les occultations solaires de l atmosphère de Saturne ont fourni la structure verticale pour une gamme de latitude. Les spectres en transmission dans le domaine EUV FUV permettent d extraire les profiles verticaux d abondance en H 2 et en hydrocarbures à partir de la partie supérieure de l atmosphère jusqu à environ 300 km du niveau de pression de 1 bar (1 bar D 100 kpa). Une réanalyse des occultations Sco de Voyager 2 en 1981 est cohérente avec le rapport initial. L homopause des hydrocarbures est proche du niveau de 0.2 bar dans l analyse UVIS, mais plus près du niveau de 0.01 bar à dans l occultation de Voyager. Cassini mesure les abondances d hydrocarbures dans une gamme allant de 600 à 0.1 bar. Les résultats combinés de UVIS indiquent une nette dépendance en latitude du profile vertical de température. Le confinement des hydrocarbures dans les mesures récentes, comparées à celles de modèles récents et à celles de Voyager prises lors d un maximum solaire, suggèrent un plus faible coefficient de turbulence à notre époque. Des modélisations numériques indiquent que la dépendance en latitude des déplacements verticaux de H 2 est causée surtout par l effet combiné du potentiel gravitationnel et une différence évidente de la déposition en énergie par les électrons à la partie supérieure de l atmosphère, ce qui affecte le profile de température. Les profiles de densité de H 2 obtenus entre 40 ı de latitude et d autres près de l équateur sont pratiquement identiques à une échelle de pression sous l exobase. Nous en concluons que le profile en pression de la densité de H 2 dans l atmosphère de Saturne reste inchangé sur un large domaine de latitudes. [Traduit par la Rédaction] 1. Introduction Results from the Cassini UVIS experiment occultation probes of the vertical structure of the Saturn atmosphere are described in this work. The analysis of stellar occultations are targeted. The highest quality transmission data has been obtained from the occultation of Delta Orionis ( Ori) obtained on 2005 DOY 103 at planetocentric latitude 42:7 ı (Note: All latitudes quoted in this paper are planetocentric). The analysis of this occultation and the occultation of Zeta Orionis ( Ori) obtained 2006 DOY 142 at latitude 15.2 ı is described in detail in this paper. A reduction in H 2 structure of the Beta Crucis ( Cru) occultation on 2009 DOY003 latitude 3.6 ı is included. Essential details of the occultations analyzed here are given in Table 1 for two selected radial distances of the impact parameter (IP). Data records are established using fixed time interval integration periods throughout an occultation sequence; the vertical intervals given in Table 1 define the intrinsic radial resolution element in the data profile. The spectral Received 19 November 2011. Accepted 16 March 2012. Published at www.nrcresearchpress.com/cjp on 14 August 2012. D.E. Shemansky. and X. Liu. Planetary and Space Science Division, Space Environment Technologies, Altadena, CA 91001, USA. 1 This article is part of a Special Issue that honours the work of Dr. Donald M. Hunten FRSC who passed away in December 2010 after a very illustrious career. Corresponding author: D.E. Shemansky (e-mail: dshemansky@spacenvironment.net). Can. J. Phys. 90: 817 831 (2012) doi: 10.1139/p2012-036

818 Can. J. Phys. Vol. 90, 2012 Table 1. Occultation properties. Source a r b (km) SSlat c lat d long e r (FUV) f r (EUV) g Cru 60849 3.8 106.4 6.3 17.8 09 003:04:19:58 62252 3.1 3.4 108.2 6.5 18.0 Ori 60411 15.3 332.9 17.6 50.3 06 142:01:41:43 61831 18.2 14.8 334.2 17.6 50.4 Ori 57898 43.0 152.3 2.3 4.0 05 103:16:34:24 59459 23.9 41.4 159.0 2.3 4.2 a Star source time; read time (09 003:04:19:58) as UT spacecraft event time 2009 DOY 103:04:19:58. b Impact parameter radial distance to planet center. c Subsolar latitude ( ı ). d Planetocentric latitude ( ı ). e Longitude ( ı ). f Vertical interval (km) per integral record FUV channel. g Vertical interval (km) per integral record EUV channel. resolution of the UVIS spectrographs is high enough to allow the analysis of H 2 discrete absorption in rotational structure, and for the first time kinetic temperatures are constrained by measurement of rotational temperature over a range of altitudes, as well as through modeling of the vertical abundance profile. The vibrational populations of the H 2 X state are also constrained by a non local thermal equilibrium (LTE) approach to the data reduction, although the heavy spectral overlap of the vibrational vectors does not allow a strong constraint. The present work benefits from the application of a highly accurate H 2 model structure that has been developed over several years [1]. Simultaneously with the H 2 spectral transmission spectra obtained with the EUV UVIS spectrograph [2], hydrocarbon absorption spectra are obtained with the FUV spectrograph where H 2 extinction is negligible. The vertical structures of CH 4, C 2 H 2, and C 2 H 4 have been extracted through spectral analysis of the Ori and Ori occultations. No other hydrocarbon species were identified in the spectra, compared to 10 species at Titan using the same instrument. The temperature sensitivity of the hydrocarbon cross sections have been utilized in the analysis to the extent of available laboratory data. 2. The Cassini UVIS experiment The Cassini UVIS experiment is fully described by [2]. The observations utilized in the work referenced here are obtained using the EUV and FUV spectrograph units identified as channels [2]. The EUV and FUV channels in a single exposure produce 64 spectral vectors of maximum length 1024 pixels (pxs). The sky projection of pixel spatial size is (pxs pxw) D (0:25 1:0) mrad, where pxw is a pixel in the spatial dimension. Each pxw contains a vector of 1024 pxs. Spectral pixel size is pxs D 0.609 652 4 Å (EUV) and pxs D 0.779 589 Å (FUV). The spectral range of the UVIS is 563. 1182. Å (EUV) and 1115. 1912. Å (FUV). The airglow ports of these instruments are used in stellar occultation observations in addition to spatially extended airglow measurements. The FUV spectral region shows no measurable H 2 discrete absorption, allowing unobstructed measurement of hydrocarbon extinction. 3. Stellar occultations The H 2 vertical profile for both Ori and Ori has been modeled to obtain a first measure of latitude dependence of the vertical abundance and temperature profiles above 600 km. Other stellar occultations have been reduced photometrically to provide preliminary data on latitudinal dependence of vertical structure. Photometric analysis is currently restricted to H 2 and CH 4. Instrument pointing for the events was stabilized by the spacecraft reaction wheels, and in each case the star image motion was constrained to less than a spectral pixel width. The latitude of the Ori egress occultation varied moderately ( 43:4 ı to 41:4 ı ) on the sunlit southern hemisphere, but the critical region of the occultation was located at 42:7 ı, and Ori varied from 15.4 ı to 15.0 ı in the dark atmosphere. Measurements of H 2 atmospheric absorption were possible over the altitude range of 831 1689 km for Ori, and 949 1807 km for Ori, above the 1 bar (1 bar D 100 kpa) radius after correction of the Cassini navigation package (see Sect. 3.1.2) value using privately communicated results from the Cassini Radio Science experiment (RSS) radio occultations 2. The analysis of the Cru occultation is currently limited to the region above 1250 km. The analysis of H 2 absorption at altitudes greater than 1700 km is limited by signal noise. The spectra are forward synthesized using a rotational-level H 2 absorption model (see Sect. 3.2) to produce the H 2 line-of-sight (LOS) abundance and temperature profiles. The data profiles constrain the derivation of the density profile through application of a hydrostatic model. The measurement of hydrocarbons begins at about 800 km for Ori and 1000 km for Ori, and both extend downward to a limit of about 300 km. The larger limiting altitude range for Ori is indicative of the more extensive vertical profile near the equator. The hydrocarbon vertical profiles obtained from the FUV channel measurements are spectrally analyzed using temperature-dependent cross sections. These reduction processes are necessarily iterative. The occultations in the EUV FUV photon spectrum provide measurements of atmospheric structure and composition from the exobase to 300 km altitude above the nominal 1 bar pressure level. The Cassini UVIS experiment provides the most accu- 2 P. Schinder. Goddard Space Flight Center. 2009.

Shemansky and Liu 819 rate platform to date in EUV FUV transmission, for extracting atmospheric structure in outer planet research primarily through higher spectral resolution, signal rates, and dynamic range. 3.1. Data reduction methodology The data constitute LOS transmission spectra of the source through the atmosphere integrated through time intervals selected on the basis of optimizing the combined needs of vertical altitude resolution and signal statistics. A reference source spectrum is obtained for each occultation either immediately before or following passage of the LOS through measurable atmosphere. The observed spectra can be described by IŒ;.; T; r 0 / D exp Œ.; T; r 0 / (1) I 0./ Z.; T; r 0 / D.; T /n.`; r 0 /d` (2) Z.r 0 / D n.`; r 0 /d` (3) where IŒ;.; T; r 0 / is the transmitted photon flux spectrum, which is a function of wavelength (), and optical depth Œ.; T; r 0 /..; T; r 0 / is dependent on temperature (T ) of the absorber and r 0, the IP. The IP, r 0, is defined as the magnitude of the vector from planet center perpendicular to the LOS, `. The abundance.r 0 / (particles cm 2 ) is extracted directly from the occultation data reduction through iterative simulation of the extinction spectrum. Forward modeling extracts the species number density, n.h eff /, in which the effective altitude above the 1 bar pressure level, h eff, is determined following the complete vertical reduction process, accounting for the finite integration interval of the occultation timeline in the numerical inversion of 3 and 2 (see Table 1). See the extensive review of methodology in ref. 3. The hydrocarbon vertical structure deviates significantly from a hydrostatic distribution. For this reason the densities are derived from incremental iterative LOS simulated atmospheric extinction using mixtures of the measurable absorbing species. The low latitude occultation of Cru (IP latitude 3:6 ı ) on 2009 DOY 003 in the subsolar atmosphere along with the other UVIS occultations shows deviation from a hydrostatic profile at high altitude, inferred to be caused by dissociative loss of H 2 (Sect. 3.2). Data reduction is carried out through the simulation of the instrument response function. Details of instrument function can be obtained from the UVIS calibration reports and the UVIS Users Guide, to be found in the NASA planetary data system. The detail of iterative fitting of the observed spectrum depends on vertical structure. In the case of the hydrocarbon spectra in the UVIS FUV spectrograph, the complexity of the vertical profile requires top-down iterative simulation, carrying out (2) in detail along the LOS, assuming symmetry surrounding the IP LOS vector intercept. The analysis of H 2 electronic system extinction in the UVIS FUV spectrograph is mainly carried out by assuming that the extinction is dominated inside one scale height at the location of the IP, to minimize computation time for the H 2 absorption probability vectors. The accuracy of this methodology for the H 2 spectra has been verified at selected altitudes by carrying out the full numerical integration of (2). The stellar spectra, which contain discrete spectral structure caused by blanketing, are modeled at an absolute resolution of 2. må to retain accuracy in the calculated effects of structure in the species cross sections. H 2 rotational structure is assumed to be in LTE, although this is not the case at high altitudes. The H 2 absorption probability vector models are limited to rotational levels, J 12. The data in the form of extinction spectra (1) are compared to model calculations by simulation of instrument response to the modeled transmitted stellar flux. The instrument point spread function (PSF) is simulated by combining laboratory calibration with calculated properties near the core. In general the PSF is wavelength dependent. The core of the PSF has absolute full widths at half maxima of 0.91 and 1.16 Å in the EUV and FUV spectrographs, respectively. The wings of the PSF extend more than 400 Å on either side of line center. 3.1.1. Data processing The first-order processing of the data from the UVIS archive is the calculation of the transmission spectral vectors. The reference spectral vector (I 0 ()), (1), obtained from the period before or after atmospheric entry of the LOS, is divided into the integrated signal during passage of the LOS through the atmosphere to produce the quantities represented by (1). A mathematical filter is applied, slightly reducing the intrinsic instrumental spectral resolution. Model calculations simulating the observations include all factors affecting the processed data (see preceding section). The interval of integration is designed to prevent signal statistical errors from impacting the accuracy of the analysis. Statistical errors in the data analysis here appear at an optical depth of about six where data analysis is terminated. There are occasions in which results are affected by minor motion of the spacecraft pointing system. Figure 1 shows deviation from the model at the ends of the spectrum. This is caused primarily by about 0.02 mrad of pointing motion in the spatial dimension where skew in the FUV slit image causes the ends of the slit to expose adjacent spatial pixels. These deviations are ignored in the model fitting of the data. Given these facts, there are no discernable systematic deviations in the model spectrum from the data in this figure. Figure 2 shows a case where the slit image is fully contained and no artifacts are identifiable in either of the occultation extinction spectra shown. 3.1.2. Atmospheric altitude reference The IP is determined from the Cassini navigation package data, which provide the spacecraft post-event reconstructed trajectory. Corrections to the data altitude scale have been made following recently discovered record dropouts caused by the data acquisition program code. The 1 bar radius for the occultations in the present analysis is obtained from Cassini RSS radio occultation results provided by the RSS experiment 3. Without these corrections to the 1 bar reference radius, it has been found that vertical structure models based on the UVIS results would be incompatible with the Cassini CIRS profiles [4 6] in 3 P. Schinder. Private communication. 2009.

820 Can. J. Phys. Vol. 90, 2012 Fig. 1. UVIS EUV stellar occultation transmission spectrum obtained 2005 DOY 103 at effective IP 1031 km. The rotational temperature is iteratively determined assuming LTE. The vibrational population distribution is non-lte determined iteratively by fitting separate vibrational vectors into the model for optimal match to the spectrum. Fig. 2. Comparison of extinction spectra from the UVIS EUV experiment at the Ori (lat 42.7 ı ) and Ori (lat 15.2 ı ) occultations selected for equality in equivalent width. The Ori spectrum was obtained at IP 858.5 km and the Ori spectrum at 1050.8 km above the 1 bar pressure level. Differences in optical depth in the narrow features in the spectra are attributed to the 14 K temperature difference in the derived profiles at these particular altitudes (see plot legend). the lower atmosphere. The 1 bar radius for the Voyager occultation at latitude 3.8 ı used here and by [7] needs no adjustment from the Cassini NAIF nominal radius. 3.2. H 2 vertical profile The H 2 component is obtained in the EUV channel. An example of the transmission spectrum for Ori compared to the model fit is shown in Fig. 1. The transmission spectrum of a model atmosphere using a modeled stellar source spectrum is applied iteratively in simulated instrument output to obtain an optimal fit to the spectra. The EUV transmission spectrum at Saturn is uncontaminated hydrogen over the entire altitude range to the point of complete extinction (optical depths D 6 7). The H 2 model used in the analysis was developed over the past several years successively at the University of Southern California and Space Environment Technologies [8,9]. The temperature-dependent absorption cross section spectra for the analysis are calculated at a resolution = 500 000 to an accuracy of 5% for those transitions that contribute measurably to the absorption spectrum. The transmission spectra are fitted using separate vibrational vectors of the ground state H 2 X.v:J / structure. Details of H 2 physical properties are described in refs. 8 and 9. Strong non-lte structure in H 2 is observed in the excited atmosphere [10] at high altitude. The fundamental physical quantities (coupled state structure, absorption probabilities, and predissociation probabilities) used in these calculations are referenced in refs. 1, 8, and 9. The calculation methodology assumes that rotational structure is in thermal equilibrium at the gas kinetic temperature, but vibrational population is non-lte [8]. Detailed model calcula-

Shemansky and Liu 821 Fig. 3. Model calculations of H 2 absorption spectra at thermal rotational temperatures of 236 and 250 K, showing shape differences caused by rotational population distribution. The spectra in the figure are calculated simulations of the UVIS spectrograph function. In this case H 2 abundance is 1:3 10 20 cm 2, which produces deeply saturated line cores. At this optical depth temperature differences manifest as changes in the minima of the structure where the wings of the line profiles tend to dominate. The temperatures of the two curves are indicated on the plot. The vibrational population distribution is non-lte determined iteratively by fitting separate vibrational vectors into the model for optimal match to the spectrum. The spectral dependence on rotational temperature manifests differently at lower abundances. The spectrum consists of several hundred thousand discrete lines. tions [8] show that in actuality there is deviation from thermal populations in rotation, but evidently not enough to significantly affect the absorption spectral properties except at high altitude. H 2 X vibrational populations, however, deviate significantly from thermal and critically affect the state of the ionospheric plasma [8,9]. It has been found that the spectral fitting process is more sensitive to rotational temperature than vibrational population distribution. Figure 3 shows calculated spectra for rotational temperatures separated by 14 K. In this case a lower temperature manifests as weaker minima on the absorption profile. In the Cassini UVIS observations, therefore, the atmospheric temperature is derived through both iterative determination of rotational temperature, and the shape of the vertical abundance distribution in the hydrostatic model calculations [11]. At lower altitudes the kinetic temperature is also constrained by the measurements of the absorption structure of the C 2 H 2 diffuse temperature sensitive (zc zx) bands [12], although the laboratory data require corrections for saturation effects. Below 550 km the temperature profile is constrained by the Cassini CIRS published and communicated results as discussed in Sect. 3.6. Figure 1 shows the observed extinction spectrum at IP 1031 km against the modeled non-lte calculation. Derived rotational temperature and H 2 abundance are given in the figure. The 2005 DOY 103 occultation was a dayside observation. Figure 2 shows a comparison of the optical depth spectra from the UVIS EUV experiment at the Ori (lat 42:7 ı ) and Ori (lat 15.2 ı ) at altitudes 858.5 and 1050.8 km selected for equality in equivalent width, giving a direct comparison of data quality in the two occultations. Differences in shape with the Ori spectrum showing deeper absorption in narrow features in the spectrum are attributed to the 14 K difference in derived temperature as in Fig. 3. Figure 4 shows the reduced H 2 abundances (, (3)) obtained from the Cassini UVIS and Voyager UVS stellar occultations against altitude compared to a hydrostatic model fitting the data. The model profiles below the lower limit of measured H 2 data indicated in Fig. 4 are constrained by results obtained from the Cassini CIRS experiment 4 [4 6] and Voyager radio occultation results [13]. Error bars for the data in Fig. 4 are described in the figure caption. The UVIS occultations in H 2 extinction shown in the present work have a common characteristic at high altitude, in which the scale height inflects to lower values above about 1400 km (Sect. 3.2.1). This property in a hydrostatic environment indicates a decrease in temperature, but the inference here is that loss of H 2 population is indicated (Sect. 3.2.1). The H 2 profile of the latitude 15.2 ı occultation differs significantly from the other occultations shown in Fig. 4 in the altitude range 800 1200 km, while as discussed later, this deviation on an altitude scale vanishes on a pressure scale. Figure 5 shows the forward modeled hydrostatic density distributions, derived from the H 2 abundances shown in Fig. 4. The Voyager 2 (V2) UVS stellar occultation has been reanalyzed using the current H 2 model structure. The V2 occultation was on the darkside [7] at a latitude of 3.8 ı. The differences in density at a given altitude are evident in Fig. 4. Figure 5 on a density scale also shows the modeled Helium distribution anchored at a [He]/[H 2 ] D 0:12 mixing ratio at 1 bar. The [He]/[H 2 ] mixing ratio affects the modeled temperature structure vertically through the mesopause, and the applied ratio is limited by the temperature dependence of the C 2 H 2 (zc zx) band cross section. Detailed analysis will be carried out later using recently acquired 5 low temperature C 2 H 2 cross sections. The vertical temperature profile is discussed in Sect. 3.6. Figure 6 shows the 4 L. Fletcher and S. Guerlet. Private communication. 2009. 5 C-Y. Wu. Private communication. 2009.

822 Can. J. Phys. Vol. 90, 2012 Fig. 4. Altitude abundance profiles of H 2 obtained from the Cassini UVIS stellar occultations Ori on 2005 DOY 103, Ori on 2006 DOY 142, Cru on 2009 DOY 003, and from the Voyager 2 UVS stellar occultation Sco on 1981 DOY 238. The plotted points are the locations at which data are obtained, and the lines passing through the points are hydrostatic model fits to the data. Error bars are indicated for Ori. Error bars for Ori are less than the size of the plotted points, and for Sco the statistical uncertainty can be assessed from the scatter of the points relative to the model. The occultations are identified on the plot legend. Fig. 5. Altitude density profiles of H 2 and He obtained from the stellar occultation models shown in Fig. 4. The occultations and latitudes are identified in the plot legend. Superimposed plotted points on the H 2 profiles show the range of the measured UVIS and CIRS data constraining the models. modeled H 2 density profiles for the Ori and Ori occultations against pressure. The curves are coincident at the 10% level over the plotted range. The Sco Voyager profile is not shown because it is also nearly coincident with the other curves. The derived temperature profiles evidently compensate for the differences in Fig. 4 to create a common pressure profile in H 2 density. The currently limited Cru profile is also not shown in Fig. 6. 3.2.1. High altitude H 2 profile properties Figure 7 shows the vertical density profile for the Cru occultation at latitude 3:6 ı compared to Ori at latitude 15.2 ı. The most pronounced decrease in scale height occurs in the Cru occultation. This phenomenon is consistent with observations of significant high altitude electron excitation of H 2 electronic bands, colocated with atomic hydrogen escaping from the top of the atmosphere at low latitude [10]. The implication of this behavior is that hydrostatically derived temperatures at the top of the atmosphere are underestimates and not a valid measure of the kinetic state. Using a polynomic fit to the scale height in the Cru occultation, a temperature of 612 K is obtained at 1530 km, the location of the maximum. Festou and Atreya [14] reported a temperature in the range 780 950 K at the same altitude from analysis of the Voyager Sco occulta-

Shemansky and Liu 823 Fig. 6. Pressure scale density profiles of H 2 obtained from the stellar occultation models for Ori and Ori. The occultations and latitudes are identified in the plot legend. Superimposed plotted points on the H 2 profiles show the range of the measured UVIS and CIRS data constraining the models as in Fig. 5. The remaining model calculations shown in Fig. 5 are not included because the results, including the Voyager model, are nearly conincident on this scale. Moderate differences are apparent only at the top and bottom of the atmosphere. Fig. 7. Vertical density profiles of H 2 from the Cru 2009 DOY 003 and Ori occultations at latitudes 3:6 ı and 15.2 ı. The plot includes an HI profile above 1400 km derived from the deviation of the Cru H 2 density profile from a hydrostatic model. tion. Using the 612 K value derived here as the terminal temperature, a crude estimate of atomic hydrogen (H I) densities can be obtained. This is shown in Fig. 7. The uncertainty in this H I density profile is difficult to assess, of the order factor of two. The H I profile derived in ref. 7 in the same low latitude region is a factor of 20 below the values shown in Fig. 7. The H I profile shown here is consistent with downward diffusion of a source at high altitude. The H 2 profile for the Voyager occultation, shown in Fig. 5, is in good agreement with the [7] result. The Voyager Sco H 2 profile follows the UVIS Ori profile above 1400 km as shown in Fig. 5. 3.3. Hydrocarbon vertical profile The analysis of the UVIS FUV spectrograph stellar occultation data yields the hydrocarbon abundances shown in Fig. 8. The analysis methodology is discussed in Sect. 3.1. Although there is an indication of the presence of other species in the Cassini transmission spectra, the species for which vertical distributions are obtained are CH 4, C 2 H 2, and C 2 H 4. The FUV EUV hydrocarbon extinction spectrum is much simpler at Saturn than for Titan [11]. The evidence for other species will be discussed in later works. An example of the stellar extinction spectrum in the FUV is shown in Fig. 9. The only species profile that can be reliably extracted from the V2 stellar occultation is CH 4. The hydrocarbon homopause is just above 750 km in the UVIS Ori occultation, 950 km for Ori and the V2 Sco occultations. The 1 bar radius [7] is consistent with the Cassini NAIF nominal model (Sect. 3.1.2). The vertical displacement of the hydrocarbon homopause levels in the two cases is consistent with the vertical displacement of the H 2 densities (Fig. 10). The CH 4 distributions are anchored at the 1 bar level with the value ([CH 4 ]/[H 2 ] D 5:1 10 3 ) estab-

824 Can. J. Phys. Vol. 90, 2012 Fig. 8. Extracted abundances of CH 4, C 2 H 2, and C 2 H 4 from the UVIS Ori (2005 DOY 103), Ori (2006 DOY 142), and CH 4 for Voyager UVS Sco (1981 DOY 238) occultations with model fits plotted through the data points. The Voyager data is heavily smoothed. Fig. 9. Cassini UVIS FUV Ori extinction spectrum against a model fit containing CH 4, C 2 H 2, C 2 H 4, and upper limits to C 2 H 6 and several other species. The temperature of the diffuse C 2 H 2 (zc zx) band cross section in this calculation is 150 K. Weak contributions from dayside airglow appear in the short wavelength deep optical depth region of the spectrum, and in the 1500 1600 Å region. The effective IP of this spectrum is 695 km. lished in ref. 15. Structure in the Cassini vertical distributions is evident in the Fig. 8 data, and not correlated between species or for the same species at different latitudes except for CH 4 and C 2 H 2 at 880 and 720 km at latitude 15.2 ı. Figure 10 shows the altitude density profiles derived from the modeled abundances shown in Fig. 8. The much larger vertical span of the hydrocarbons in the Ori occultation is evident in Fig. 10. The hydrocarbon mixing ratio relative to H 2 is shown in Fig. 11. Although there are significant H 2 density differences at different latitudes for given altitudes as shown in Figs. 4 and 5, this does not effectively compensate for hydrocarbon vertical separations and altitude scale differences remain significant in mixing ratio. Figure 12 shows the dependence of hydrocarbon mixing ratio on pressure. On a pressure scale the hydrocarbons now show a common homopause in the Cassini occultations, appearing near 0.2 bar, and at 0.1 bar the mixing ratios of the hydrocarbons are reduced by approximately an order of magnitude from the peak values. Figure 12 shows significant anticorrelations within the same species in vertical structure for C 2 H 2 and C 2 H 4 at 0.7, 1.7, and 50 bar. This figure includes plots of derived mixing ratios from the Cassini CIRS experiment for C 2 H 2 [5, 16]. The dash dot line in Fig. 12 is the profile from [5] at planetocentric latitude 16:7 ı. At the minimum pressure measurements found in ref. 5, 0.1 bar, the UVIS mixing ratio falls an order of magnitude below the CIRS value. Two points from ref. 16 at higher pressures than the upper range of the UVIS measurements are plotted in Fig. 12. Figure 12 also shows the distributions for CH 4, C 2 H 2, and C 4 H 2 from the ref. 17 global model A. There is general agreement of the UVIS profiles for CH 4, C 2 H 2, and C 2 H 4 with the ref. 17 model in the range 1 100 bar, but at 0.1 bar

Shemansky and Liu 825 Fig. 10. Densities of CH 4, C 2 H 2, and C 2 H 4 obtained in the model fits shown in Fig. 8. The vertical range of abundances for the Ori occultation at planetocentric latitude 15.2 ı are measurable in the extinction spectra over a range of altitude as much as 300 km larger than for the Ori occultation. The limited dynamic range of the Voyager CH 4 does not allow determination of the possible presence of structure in the vertical distribution as indicated in the Cassini results, and the model extension to lower altitude for the Voyager occultation contains significant uncertainty between 1000 and 300 km. Fig. 11. Mixing ratios [N]/[H 2 ] as a function of altitude for the hydrocarbons shown in Fig. 10. The differences in H 2 density scales between the occultations evidently do not effectively compensate for the hydrocarbon scale differences. the UVIS mixing ratios are an order of magnitude below the model. The UVIS mixing ratios for C 2 H 2 and C 2 H 4 fall below the ref. 17 model and CIRS results at the high pressure end of the range of measurement by factors of roughly three. Model calculations in ref. 18 for Saturn summer southern solstice for C 2 H 2 at latitude 15 ı, not included in Fig. 12, show mixing ratios moderately higher than those in ref. 17. Figure 12 includes the ref. 17 model calculations for C 4 H 2 with a compatible single upper limit value obtained from the UVIS Ori occultation at 40 bar. The modeled Voyager CH 4 mixing ratio included in Fig. 12 is significantly different from the UVIS distribution at pressures below 10 bar. Although on an altitude scale (see Fig. 10) the asymptotic distribution above the homopause for the Voyager result at latitude 3.8 ı is close to that of the UVIS occultation at latitude 15.2 ı, the pressure scale comparison shows an implied much larger eddy diffusion coefficient in the 1981 epoch, coinciding with the ref. 17 model (Fig. 12) at 0.01 bar (see Sect. 3.4). The model of the Voyager profile is uncertain at pressures higher than 0.03 bar because there is no data constraint below the homopause, and the profile is assumed to be structure free. The hydrocarbon homopause in the UVIS data is at 0.2 bar, compared to 0.01 bar for the Voyager measurements and the ref. 17 model (Fig. 12). Significant structure in the pressure profiles of the UVIS C 2 H 4 and C 2 H 2 results are evident in Fig. 12, showing differences in mixing ratio in confined pressure regions at 0.7, 1.7, and 50 bar between the two latitudes. Guerlet et al. [6] report strong depletions in C 2 H 2 and C 2 H 6 mixing ratios at south latitudes compared to north (. 17 ı 35 ı /=.17 ı 25 ı /) at pressures below 1 mbar. Using the ref. 6 measurements for C 2 H 2 at latitudes 42:7 ı and 15:2 ı for direct comparison to the UVIS measurements, the values CIRS (C 2 H 2 )Œ. 42:7 ı /=.15:2 ı / D 0:6 and 0.18 are obtained at pressures of 100 and 10 bar. The Cassini UVIS latitude dependence for C 2 H 2 follows the trend found in the CIRS results in the 100 30 bar range, but at lower pressures beyond the CIRS measurement capability the

826 Can. J. Phys. Vol. 90, 2012 Fig. 12. Mixing ratios [N]/[H 2 ] as a function of pressure for the hydrocarbons shown in Fig. 10, compared to the V2 ısco latitude 3.8 ı CH 4 reduced data, Cassini CIRS analyses (see text), and model calculations by [17]. The ref. 5 profile for C 2 H 2 shown as the dash dot curve is for latitude 16.7 ı. The ref. 17 model A profiles are global. The plotted points (C 2 H 2 ) for ref. 16 are for planetocentric latitude 42 ı. The Cassini UVIS occultations at latitudes 42:7 ı and 15.2 ı show a common homopause on a pressure scale. The Voyager CH 4 profile shows a homopause at substantially lower pressure, comparable to the ref. 17 model. Significant structure appears in the Cassini UVIS profiles (see text). The plotted point for UVIS Ori C 4 H 2 is an upper limit. Error bars in the Cassini UVIS mixing ratios are smaller than the size of the plotted points, except at the ends of the pressure range. Fig. 13. Mixing ratios [C 2 H 2 ]/[CH 4 ] and [C 2 H 4 ]/[CH 4 ]as a function of altitude, for latitudes vertical displacements of 200 km and different vertical extents at the two latitudes. 42:7 ı and 15.2 ı. The profiles show UVIS results do not show significant C 2 H 2 depletions at the south latitude. The ratios UVIS (C 2 H 2 )Œ. 42:7 ı /=.15:2 ı / D 0:36 and 6.5 at 50 and 1.7 bar indicate the south latitude C 2 H 2 mixing ratio has a strong low pressure peak at 1.7 bar. The ratio UVIS (C 2 H 4 )Œ. 42:7 ı /=.15:2 ı / D 21 at 0.7 bar reflects an apparent enhancement in the south and a corresponding depletion in the north for C 2 H 4. The width of the UVIS features are 50 80 km. The depletion at midsouth latitudes discussed in ref. 6 is therefore consistent with the UVIS results over the limited range 100 to 30 bar, but the measurements in the latter case going to pressures below 30 bar do not follow the CIRS trend, and in fact show enhancement of C 2 H 2 and C 2 H 4 at south latitude in the vicinity of 1 bar. Figures 13, 14, and 15 show the C 2 H 2 and C 2 H 4 mixing ratios to CH 4 on altitude and pressure scales. The solid circles in the figures indicate Ori data at latitude 42:7 ı and open circles refer to

Shemansky and Liu 827 Fig. 14. Measured mixing ratio [C 2 H 2 ]/[CH 4 ] as a function of pressure, for latitudes by ref. 18 at 40 ı. and 8 ı. See text. 42:7 ı and 15.2 ı, compared to model calculations Fig. 15. Measured mixing ratio [C 2 H 4 ]/[CH 4 ]as a function of pressure, for latitudes by ref. 18 at 40 ı and 8 ı. See text. Ori data at 15.2 ı. In Fig. 13 the curves of the two species are vertically separated by about 200 km. The span of the measurements is roughly 800 km at 15.2 ı latitude, compared to 500 km at 42:7 ı, reflecting the expanded low latitude atmosphere. The plots of these data on a pressure scale show the distinct transformation to alignment of structure. Figure 14 shows the pressure scale distribution for C 2 H 2 compared to the Moses and Greathouse [18] model. The mean location of the present results is 0.2 b compared to 0.08 b for ref. 18. The main structural feature peaks in the measured profiles at the two latitudes in Fig. 14 are aligned near 0.4 and 0.1 b. The model ref. 18 is much more broadly distributed in the pressure scale than the measurement, as readily observed in Fig. 14. The two latitudes shown for the model calculations [18] in Fig. 14 are 42:7 ı and 15.2 ı, compared to model calculations closely aligned on the pressure scale. Figure 15 shows the results for C 2 H 4, in which the pressure scale distribution is narrower for both the observations and the model. The measured low pressure end at 15.2 ı latitude may be caused by a low level bias in the data. 3.4. The impact of helium on the atmospheric model Establishing an atmospheric model at Saturn starting at 1 bar requires the inclusion of helium because of the significant impact on the mean mass of the fully mixed gas. At this time the [He]/[H 2 ] ratio is not definitively established. Lindal et al. [13] in the analysis of RSS occultations, used the mixing ratio established by Conrath et al. [19], [He]/[H 2 ] D 0.034. Conrath et al. [19] obtained [He]/[H 2 ] by analyzing the RSS occultation

828 Can. J. Phys. Vol. 90, 2012 Fig. 16. Saturn temperature profiles derived from the UVIS ( Ori), Ori, V2 Sco occultations, and the Cassini CIRS results found in ref. 4 6 (see text). The [21] multiple ground based stellar occultations are included. The UVIS profiles are forced to conform to the CIRS results from 1 bar to 0.04 mbar at 15.2 ı and 0.01 mbar at 42.7 ı. The plot legend identifies the profiles and latitudes. Fig. 17. Projected photoabsorption cross section (Mb) of the C 2 H 2 zc 1 u.0; 1; 0; 0; 0/ zx 1 g.0; 0; 0; 0; 0/ band for selected temperatures. The plot legend identifies the plot temperatures, and the source. The measured cross sections from ref. 12 are shown against the model fits. results and infrared thermal emission. Sixteen years later, following the Galileo probe results at Jupiter, Conrath and Gautier [20] found a systematic divergence from their approach and on reanalysis obtained [He]/[H 2 ] D 0:11 0:16. The most direct determination would be obtained by utilizing an accurately measured scale height, and an independent measurement of temperature to establish the mean mass of the gas. The UVIS occultation measurements provide the scale height structure in H 2 with an independent rotational temperature measurement, but the current data reduction reaches only down to 800 km above 1 bar for the Ori occultation. The hydrocarbon data are analyzed down to 300 km, and therefore overlap the RSS and CIRS experiment results (Figs. 10 and 16). The absorption spectrum of C 2 H 2 contains the strong temperature dependent.zc zx/ bands showing peaks at 1477.91 Å (zc 1 u.0; 1; 0; 0; 0/ zx 1 g.0; 0; 0; 0; 0/) and 1519.43 Å (zc 1 u.0; 0; 0; 0; 0/ zx 1 g.0; 0; 0; 0; 0/) (see Fig. 9). The experimental measurements need further refinement to correct for saturation effects at the 1519.43 Å resonance feature. New results obtained by Wu now extend to lower temperature, but saturation effects have not been corrected to date. The upper state is heavily coupled and the diffuse rotational structure is naturally blended as shown in Fig. 17. The width and magnitudes of the band heads change with temperature, and the blended diffuse p-branch lines in the 1485 1510 Å region are also temperature dependent. Temperature analysis on the C 2 H 2 extinction has been deferred until the laboratory measurements have been refined. The model calculations shown in Figs. 5 and 16 are based on [He]/[H 2 ] D 0.12 at 1 bar. The temperature profile shown in Fig. 16 is forced by continuity with the H 2 density and scale height at 800 km at latitude 42:7 ı and 950 km at latitude 15.2 ı and using the CIRS profiles from 1 bar

Shemansky and Liu 829 Fig. 18. Pressure scale helium mole fraction in the present model compared to the Moses and Greathouse [18] model. The helium distribution is not measured in this work. Scaling is based on order of magnitude lower diffusion coefficients indicated in comparison with Voyager based ref. 18 results. As indicated on the plot the mole fraction is at latitude 40 ı for ref. 18 and 42:7 ı for the present calculation corresponding to the Ori occultation. up to 550 km. A weak mesopause established in the CIRS results appears at 357 km (0.47 mbar) at latitude 15.2 ı. Fitting the reanalyzed Voyager data, also shown in Figs. 5 and 16, with the same [He]/[H 2 ] constraint produces a temperature profile without a distinct mesopause, similar to the ref. 21 profile, also shown in Fig. 16. The [He]/[H 2 ] ratio in Fig. 10 is the fully mixed value through the mesopause in the latitude 15.2 ı profile. The eddy diffusion coefficient applied here is small, consistent with the limited hydrocarbon distribution. In the pressure range 7 0.4 bar at latitude 15.2 ı the eddy diffusion coefficient is (K.15:2/ D) 10 3 cm 2 s 1, with He H 2 diffusion coefficients in the range (D HeH2 D) 10 4 10 5 cm 2 s 1, calculated using Lennard Jones potentials. At latitude 42:7 ı in the range 3 0.06 bar K. 42:7/ D 4 10 3 4 10 4 and D HeH2 D 2:5 10 5 10 6 cm 2 s 1. Comparing this atmospheric state with the ref. 17 model, the hydrocarbon homopause was at 10 2 bar, while in the present observations the homopause location is 0.2 bar (Fig. 12). At 10 2 bar, the hydrocarbons are not measurable in the UVIS observations with mixing ratios <10 11 (Fig. 12). The ref. 17 model A eddy diffusion coefficient is 2: 10 7 cm 2 s 1 at the homopause, while the He forced coefficient in the current model is latitude dependent with K.15:2/ D 4: 10 5 and K. 42:7/ D 3: 10 6 at a pressure of 10 2 bar. Figure 18 shows a comparision of helium mole fraction with the ref. 18 model. This comparison is not very meaningful given the current derived state of the atmosphere, but it serves to illustrate the inferred change from the Voyager observation in 1981. 3.5. Latitudinal dependencies Figure 10, comparing derived hydrocarbon densities for the UVIS Ori occultation at latitude 42:7 ı and the Voyager UVS occultation at 3.8 ı, shows vertical separations of 200, and 350 km for CH 4 at given densities. The H 2 vertical profile model calculations for these two cases, indicate that the difference is primarily caused by the latitudinal dependence in gravitational potential. Within the UVIS occultation events, a comparision of the extinction spectrum of Ori at 42:7 ı with the Ori spectrum of nearly equal magnitude at 15.2 ı in Fig. 2 shows a vertical separation of 200 km, the same magnitude as the separation with the Voyager UVS low latitude result. H 2 abundances as a function of pressure for the three occultations examined here differ by less than 15% at any given point in the pressure range 1 bar to 0.01 bar as stated in Sect. 3.2. Both abundance profiles in Fig. 4 show distinct changes in the log scale slope, at 1000 km for Ori and at 1300 km for Ori, signaling the impact of rising upper thermospheric temperatures. There are also distinct differences in abundance distribution above the slope transitions in the two occultations, with a curvature in the Ori data to smaller scale height toward higher altitudes forcing a distinct peak in temperature (Fig. 16), and a significantly larger scale height developing in the Ori data above 1250 km. As discussed in Sect. 4, the temperatures forced by the abundance profiles in the high thermosphere are based on hydrostatic modeling, and no attempt is made to correct for the deviation caused by the loss of H 2 through dissociation. The Fig. 8 shows vertical separations of 200 300 km in CH 4 abundance for the Ori and Ori occultations. The latitudinal effects internal to the UVIS occultations are therefore similar to the comparison with the Voyager UVS outcome. Figure 19 shows a plot of the IP above the 1 bar radius at the point of transmission extinction for H 2 and CH 4 in the data set examined here, as well as the Voyager UVS event, as a function of latitude. With the exception of Voyager UVS, the CH 4 extinction altitude falls below the H 2 locations. 3.6. Vertical kinetic temperature profiles The derived temperature profiles from UVIS 2005 DOY 103, 2006 DOY 142, V2 1981 DOY 238, the ref. 21 multiple Earth based stellar occultations, and the CIRS profile at latitude 40 ı [4, 6] as discussed earlier are shown in Fig. 16. The CIRS results constrain the lower altitude profiles in this model. Thermal infrared measurements 43 ı at 3 and 100 mbar by ref. 22 using the Keck I telescope, are 2.8 and 1.3 K, respectively, higher than the profiles shown in Fig. 16, but System-III longitude variations in those observations have an envelope larger

830 Can. J. Phys. Vol. 90, 2012 Fig. 19. Extinction ( D 6) IPs above the 1 bar pressure level for H 2 and CH 4 for 12 UVIS stellar occultations and the V2 UVS Sco occultation as a function of latitude. The 1 bar radius is obtained from the privately communicated Cassini RSS experiment analysis (P. Schinder, private communication, 2009). The point of extinction for the Voyager occultation is coincident for H 2 and CH 4. than these differences. At higher altitudes the temperature profiles are determined by the H 2 extinction spectra as described earlier. In the region up to the exobase the temperature is also limited by the rotational temperature as well as the vertical abundance profile. The two constraints, rotational structure and vertical profile, are in good agreement but a more thorough analysis with a broader range of rotational temperature vectors needs to be carried out to set definitive limits on the temperature uncertainty using the combined methodologies. The UVIS Ori occultation analysis is the only existing derivation from the sunlit atmosphere in the UVIS accumulated results. The T 1 D 460 K thermosphere obtained from the 1981 occultation in the present analysis is 40 K higher than the original analysis by ref. 7, which is presumed to be caused mainly by the use of a much more accurate H 2 model in the present case. It is evident that substantial differences arise as an apparent function of latitude. The Voyager Sco profile (latitude 3.8 ı ) is more similar to the UVIS result for Ori (latitude 15.2 ı ), than to the UVIS result at Ori (latitude 42:7 ı ). The value of T 1 D 460 30 K from the Voyager Sco occultation, compared to UVIS Ori 388 15 K, and UVIS Ori, 318 5 K, indicate latitudinal differences within the UVIS results and probable temporal variation in the relationship to the Voyager outcome at solar maximum. Given the scale height properties at the top of the atmosphere discussed in Sect. 3.2.1 the top of atmosphere temperatures from the hydrostatic analysis are considered lower limits to actuality. The estimated temperature for the Cru occultation at latitude 3:6 ı, two to three scale heights below the exobase is estimated at 612 K. At low south latitude the temperature at the exobase could be substantially larger. The gap between the CIRS temperature profiles and the UVIS occultation measurements is filled by model calculations constrained by scale height continuity and limited by the atmospheric scale height. This leaves some uncertainty in the profile structure in the gap which shows temperature minima at high altitude at 0.1 and 0.01 bar. Given that these minima are real may require a physical explanation in radiative loss by the increasing mixing ratio of ionospheric H C 3 with increasing altitude in this region. Further work in iteration of results with the CIRS researchers, who depend on an a priori CH 4 profile, may allow refinement of the temperature profile in the data gap (see Sect. 4). 4. Discussion and conclusions 4.1. H 2 vertical structure The Cassini UVIS occultation measurements of H 2 abundance on an altitude scale reveal a significant dependence of atmospheric structure on latitude extending to the top of the thermosphere. Abundances at given pressures, however, including the reanalyzed V2 results, differ by less than 15% over the modeled pressure range, 1 bar to 10 4 bar. The inference is that over this range the H 2 density on a pressure scale from at least 40 ı to the equator is invarient. Significant temperature differences are found at the top of the thermosphere. At the low south latitude of the projected Cassini UVIS proximal orbits the temperature is estimated to be 612 K two to three scale heights below the exobase. The hydrostatic analysis at high altitude predict a temperature deviating from reality on the low side because of high dissociation rates [10]. The measured temperatures at the top of the thermosphere at latitudes 42:7 ı and 15.2 ı in the Cassini UVIS observations are significantly different (Fig. 16), and can be explained by the observation of confined high heating rates inferred from emissions observed in the vicinity of the exobase at low latitude [10]. The reanalysis of the Voyager UVS low latitude occultation giving a relatively high hydrostatic temperature and apparent inferred larger eddy diffusion coefficient in 1981 suggests that the major solar cycle variation in deposition has a measurable impact on the state of the atmosphere. 4.2. Hydrocarbon vertical structure On a pressure scale hydrocarbon density profiles from the UVIS occultations at latitude 15.2 ı and 42.7 ı show a common homopause near 0.2 bar, while the V2 occultation, and models ref. 17 indicate a homopause near 10 2 bar, inferring a substantial difference in the eddy diffusion coefficient (see Fig. 12). The current atmospheric model for this reason