saturn upper atmospheric structure from cassini...

817

Saturn upper atmospheric structure from

Cassini EUV and FUV occultations1

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 H2 and hydrocarbon abundances from the top of the atmosphereto about 300 km above the 1 bar (1 bar D 100 kPa) pressure level. A reanalysis of the Voyager 2 •Sco occultation in 1981is 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 thepressure range 600–0.1 �bar in the Cassini UVIS experiment. The combined UVIS results provide evidence for significantlatitudinal dependence of vertical temperature profile. The confinement of the hydrocarbons in the current observationscompared to published models and the Voyager ultraviolet spectrograph (UVS) results at solar maximum, infer smallereddy diffusion coefficients in this epoch. Model calculations indicate that the latitudinal dependence of H2 verticaldisplacements is caused primarily by the combined effects of gravitational potential and evident differences in electronenergy deposition at the top of the atmosphere affecting the temperature profile. The derived H2 density profiles from�–40ı latitude and others close to the equator, are found to be nearly identical on a pressure scale below the exobase. Theinference is that that the pressure profile of H2 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 deSaturne 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 H2 et en hydrocarbures à partir de la partie supérieure del’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 de0.2 �bar dans l’analyse UVIS, mais plus près du niveau de �0.01 �bar à dans l’occultation de Voyager. Cassini mesureles abondances d’hydrocarbures dans une gamme allant de 600 à 0.1 �bar. Les résultats combinés de UVIS indiquentune nette dépendance en latitude du profile vertical de température. Le confinement des hydrocarbures dans les mesuresrécentes, comparées à celles de modèles récents et à celles de Voyager prises lors d’un maximum solaire, suggèrentun plus faible coefficient de turbulence à notre époque. Des modélisations numériques indiquent que la dépendanceen latitude des déplacements verticaux de H2 est causée surtout par l’effet combiné du potentiel gravitationnel et unedifférence évidente de la déposition en énergie par les électrons à la partie supérieure de l’atmosphère, ce qui affecte leprofile de température. Les profiles de densité de H2 obtenus entre �–40ı de latitude et d’autres près de l’équateur sontpratiquement identiques à une échelle de pression sous l’exobase. Nous en concluons que le profile en pression de ladensité de H2 dans l’atmosphère de Saturne reste inchangé sur un large domaine de latitudes.

[Traduit par la Rédaction]

1. IntroductionResults from the Cassini UVIS experiment occultation

probes of the vertical structure of the Saturn atmosphere aredescribed in this work. The analysis of stellar occultations aretargeted. The highest quality transmission data has been ob-tained from the occultation of Delta Orionis (•Ori) obtainedon 2005 DOY 103 at planetocentric latitude �42:7ı (Note: Alllatitudes quoted in this paper are planetocentric). The analysisof this occultation and the occultation of Zeta Orionis (—Ori)

obtained 2006 DOY 142 at latitude 15.2ı is described in de-tail in this paper. A reduction in H2 structure of the Beta Cru-cis (“Cru) occultation on 2009 DOY003 latitude �–3.6ı is in-cluded. Essential details of the occultations analyzed here aregiven in Table 1 for two selected radial distances of the im-pact parameter (IP). Data records are established using fixedtime interval integration periods throughout an occultation se-quence; the vertical intervals given in Table 1 define the intrin-sic 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 avery illustrious career.Corresponding author: D.E. Shemansky (e-mail: [email protected]).

Can. J. Phys. 90: 817–831 (2012) doi: 10.1139/p2012-036 Published by NRC Research Press

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Table 1. Occultation properties.

Sourcea rb(km) SSlatc latd longe �r (FUV)f �r (EUV)g

“Cru 60849 — –3.8 106.4 6.3 17.809–003:04:19:58 62252 –3.1 –3.4 108.2 6.5 18.0—Ori 60411 — 15.3 332.9 17.6 50.306–142:01:41:43 61831 –18.2 14.8 334.2 17.6 50.4•Ori 57898 — –43.0 152.3 2.3 4.005–103:16:34:24 59459 –23.9 –41.4 159.0 2.3 4.2

aStar source – time; read time (09–003:04:19:58) as UT spacecraft event time 2009 DOY103:04:19:58.

bImpact parameter radial distance to planet center.cSubsolar latitude (ı).dPlanetocentric latitude (ı).eLongitude (ı).fVertical interval (km) per integral record FUV channel.gVertical interval (km) per integral record EUV channel.

resolution of the UVIS spectrographs is high enough to allowthe analysis of H2 discrete absorption in rotational structure,and for the first time kinetic temperatures are constrained bymeasurement of rotational temperature over a range of alti-tudes, as well as through modeling of the vertical abundanceprofile. The vibrational populations of the H2 X state are alsoconstrained by a non local thermal equilibrium (LTE) approachto the data reduction, although the heavy spectral overlap ofthe vibrational vectors does not allow a strong constraint. Thepresent work benefits from the application of a highly accu-rate H2 model structure that has been developed over severalyears [1].

Simultaneously with the H2 spectral transmission spectraobtained with the EUV UVIS spectrograph [2], hydrocarbonabsorption spectra are obtained with the FUV spectrographwhere H2 extinction is negligible. The vertical structures ofCH4, C2H2, and C2H4 have been extracted through spectralanalysis of the •Ori and —Ori occultations. No other hydro-carbon species were identified in the spectra, compared to 10species at Titan using the same instrument. The temperaturesensitivity of the hydrocarbon cross sections have been utilizedin the analysis to the extent of available laboratory data.

2. The Cassini UVIS experiment

The Cassini UVIS experiment is fully described by [2]. Theobservations utilized in the work referenced here are obtainedusing the EUV and FUV spectrograph units identified as chan-nels [2]. The EUV and FUV channels in a single exposureproduce 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 dimen-sion. Each pxw contains a vector of 1024 pxs. Spectral pixelsize 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 instru-ments are used in stellar occultation observations in additionto spatially extended airglow measurements. The FUV spectralregion shows no measurable H2 discrete absorption, allowingunobstructed measurement of hydrocarbon extinction.

3. Stellar occultations

The H2 vertical profile for both •Ori and —Ori has been mod-eled to obtain a first measure of latitude dependence of thevertical abundance and temperature profiles above 600 km.Other stellar occultations have been reduced photometricallyto provide preliminary data on latitudinal dependence of verti-cal structure. Photometric analysis is currently restricted to H2

and CH4. Instrument pointing for the events was stabilized bythe spacecraft reaction wheels, and in each case the star im-age motion was constrained to less than a spectral pixel width.The latitude of the •Ori egress occultation varied moderately(�43:4ı to �41:4ı) on the sunlit southern hemisphere, but thecritical region of the occultation was located at �42:7ı, and—Ori varied from 15.4ı to 15.0ı in the dark atmosphere. Mea-surements of H2 atmospheric absorption were possible overthe altitude range of 831–1689 km for •Ori, and 949–1807 kmfor —Ori, above the 1 bar (1 bar D 100 kPa) radius after correc-tion of the Cassini navigation package (see Sect. 3.1.2) valueusing privately communicated results from the Cassini RadioScience experiment (RSS) radio occultations2. The analysis ofthe “Cru occultation is currently limited to the region above1250 km. The analysis of H2 absorption at altitudes greaterthan 1700 km is limited by signal noise. The spectra are for-ward synthesized using a rotational-level H2 absorption model(see Sect. 3.2) to produce the H2 line-of-sight (LOS) abun-dance and temperature profiles. The data profiles constrain thederivation of the density profile through application of a hy-drostatic model. The measurement of hydrocarbons begins atabout 800 km for •Ori and 1000 km for —Ori, and both ex-tend downward to a limit of about 300 km. The larger lim-iting altitude range for —Ori is indicative of the more exten-sive vertical profile near the equator. The hydrocarbon ver-tical profiles obtained from the FUV channel measurementsare spectrally analyzed using temperature-dependent cross sec-tions. These reduction processes are necessarily iterative. Theoccultations in the EUV–FUV photon spectrum provide mea-surements of atmospheric structure and composition from theexobase to 300 km altitude above the nominal 1 bar pressurelevel. The Cassini UVIS experiment provides the most accu-

2P. Schinder. Goddard Space Flight Center. 2009.

Published by NRC Research Press

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rate platform to date in EUV–FUV transmission, for extract-ing atmospheric structure in outer planet research primarilythrough higher spectral resolution, signal rates, and dynamicrange.

3.1. Data reduction methodologyThe data constitute LOS transmission spectra of the source

through the atmosphere integrated through time intervals se-lected on the basis of optimizing the combined needs of verti-cal altitude resolution and signal statistics. A reference sourcespectrum is obtained for each occultation either immediatelybefore or following passage of the LOS through measurableatmosphere. The observed spectra can be described by

I Œ�; �.�; T; r0/�

I0.�/D exp Œ��.�; T; r0/� (1)

�.�; T; r0/ D

Z�.�; T /n.`; r0/d` (2)

�.r0/ D

Zn.`; r0/d` (3)

where I Œ�; �.�; T; r0/� is the transmitted photon flux spec-trum, which is a function of wavelength (�), and optical depthŒ�.�; T; r0/�. �.�; T; r0/ is dependent on temperature (T ) ofthe absorber and r0, the IP. The IP, r0, is defined as the magni-tude of the vector from planet center perpendicular to the LOS,`. The abundance �.r0/ (particles cm�2) is extracted directlyfrom the occultation data reduction through iterative simula-tion of the extinction spectrum. Forward modeling extracts thespecies number density, n.heff/, in which the effective altitudeabove the 1 bar pressure level, heff, is determined following thecomplete vertical reduction process, accounting for the finiteintegration interval of the occultation timeline in the numericalinversion of 3 and 2 (see Table 1). See the extensive review ofmethodology in ref. 3.

The hydrocarbon vertical structure deviates significantlyfrom a hydrostatic distribution. For this reason the densities arederived from incremental iterative LOS simulated atmosphericextinction using mixtures of the measurable absorbing species.The low latitude occultation of “Cru (IP latitude ��3:6ı) on2009 DOY 003 in the subsolar atmosphere along with the otherUVIS occultations shows deviation from a hydrostatic profileat high altitude, inferred to be caused by dissociative loss ofH2 (Sect. 3.2).

Data reduction is carried out through the simulation of theinstrument response function. Details of instrument functioncan be obtained from the UVIS calibration reports and theUVIS Users’ Guide, to be found in the NASA planetary datasystem. The detail of iterative fitting of the observed spectrumdepends on vertical structure. In the case of the hydrocarbonspectra in the UVIS FUV spectrograph, the complexity of thevertical profile requires top-down iterative simulation, carryingout (2) in detail along the LOS, assuming symmetry surround-ing the IP–LOS vector intercept. The analysis of H2 electronicsystem extinction in the UVIS FUV spectrograph is mainlycarried out by assuming that the extinction is dominated insideone scale height at the location of the IP, to minimize com-putation time for the H2 absorption probability vectors. The

accuracy of this methodology for the H2 spectra has been ver-ified at selected altitudes by carrying out the full numericalintegration of (2). The stellar spectra, which contain discretespectral structure caused by blanketing, are modeled at an ab-solute resolution of 2. mÅ to retain accuracy in the calculatedeffects of structure in the species cross sections. H2 rotationalstructure is assumed to be in LTE, although this is not the caseat high altitudes. The H2 absorption probability vector modelsare limited to rotational levels, J � 12.

The data in the form of extinction spectra (1) are comparedto model calculations by simulation of instrument responseto the modeled transmitted stellar flux. The instrument pointspread function (PSF) is simulated by combining laboratorycalibration with calculated properties near the core. In generalthe PSF is wavelength dependent. The core of the PSF has ab-solute full widths at half maxima of 0.91 and 1.16 Å in theEUV and FUV spectrographs, respectively. The wings of thePSF 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 archiveis the calculation of the transmission spectral vectors. The ref-erence spectral vector (I0(�)), (1), obtained from the periodbefore or after atmospheric entry of the LOS, is divided intothe integrated signal during passage of the LOS through theatmosphere to produce the quantities represented by (1). Amathematical filter is applied, slightly reducing the intrinsicinstrumental spectral resolution. Model calculations simulat-ing the observations include all factors affecting the processeddata (see preceding section). The interval of integration is de-signed to prevent signal statistical errors from impacting theaccuracy of the analysis. Statistical errors in the data analysishere appear at an optical depth of about six where data analysisis terminated. There are occasions in which results are affectedby minor motion of the spacecraft pointing system. Figure 1shows deviation from the model at the ends of the spectrum.This is caused primarily by about 0.02 mrad of pointing mo-tion in the spatial dimension where skew in the FUV slit imagecauses 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 devia-tions in the model spectrum from the data in this figure. Fig-ure 2 shows a case where the slit image is fully contained andno artifacts are identifiable in either of the occultation extinc-tion spectra shown.

3.1.2. Atmospheric altitude reference

The IP is determined from the Cassini navigation packagedata, which provide the spacecraft post-event reconstructed tra-jectory. Corrections to the data altitude scale have been madefollowing recently discovered record dropouts caused by thedata acquisition program code. The 1 bar radius for the occul-tations in the present analysis is obtained from Cassini RSS ra-dio occultation results provided by the RSS experiment3. With-out these corrections to the 1 bar reference radius, it has beenfound that vertical structure models based on the UVIS resultswould be incompatible with the Cassini CIRS profiles [4–6] in

3P. Schinder. Private communication. 2009.


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Fig. 1. UVIS EUV stellar occultation transmission spectrum obtained 2005 DOY 103 at effective IP 1031 km. The rotationaltemperature is iteratively determined assuming LTE. The vibrational population distribution is non-LTE determined iteratively by fittingseparate 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 �42.7ı) and —Ori (lat 15.2ı) occultationsselected for equality in equivalent width. The •Ori spectrum was obtained at IP 858.5 km and the —Ori spectrum at 1050.8 km above the1 bar pressure level. Differences in optical depth in the narrow features in the spectra are attributed to the 14 K temperature difference inthe derived profiles at these particular altitudes (see plot legend).

the lower atmosphere. The 1 bar radius for the Voyager occul-tation at latitude 3.8ı used here and by [7] needs no adjustmentfrom the Cassini NAIF nominal radius.

3.2. H2 vertical profileThe H2 component is obtained in the EUV channel. An ex-

ample of the transmission spectrum for •Ori compared to themodel fit is shown in Fig. 1. The transmission spectrum ofa model atmosphere using a modeled stellar source spectrumis applied iteratively in simulated instrument output to obtainan optimal fit to the spectra. The EUV transmission spectrumat Saturn is uncontaminated hydrogen over the entire altituderange to the point of complete extinction (optical depths � D

6–7). The H2 model used in the analysis was developed overthe past several years successively at the University of South-

ern California and Space Environment Technologies [8,9]. Thetemperature-dependent absorption cross section spectra for theanalysis are calculated at a resolution �=•� � 500 000 to anaccuracy of �5% for those transitions that contribute mea-surably to the absorption spectrum. The transmission spectraare fitted using separate vibrational vectors of the ground stateH2X.v:J / structure. Details of H2 physical properties are de-scribed in refs. 8 and 9. Strong non-LTE structure in H2 isobserved in the excited atmosphere [10] at high altitude. Thefundamental physical quantities (coupled state structure, ab-sorption probabilities, and predissociation probabilities) usedin these calculations are referenced in refs. 1, 8, and 9. Thecalculation methodology assumes that rotational structure isin thermal equilibrium at the gas kinetic temperature, but vi-brational population is non-LTE [8]. Detailed model calcula-


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Fig. 3. Model calculations of H2 absorption spectra at thermal rotational temperatures of 236 and 250 K, showing shape differencescaused by rotational population distribution. The spectra in the figure are calculated simulations of the UVIS spectrograph function. Inthis case H2 abundance is 1:3 � 1020 cm�2, which produces deeply saturated line cores. At this optical depth temperature differencesmanifest as changes in the minima of the structure where the wings of the line profiles tend to dominate. The temperatures of the twocurves are indicated on the plot. The vibrational population distribution is non-LTE determined iteratively by fitting separate vibrationalvectors into the model for optimal match to the spectrum. The spectral dependence on rotational temperature manifests differently atlower abundances. The spectrum consists of several hundred thousand discrete lines.

tions [8] show that in actuality there is deviation from ther-mal populations in rotation, but evidently not enough to signif-icantly affect the absorption spectral properties except at highaltitude. H2X vibrational populations, however, deviate signif-icantly from thermal and critically affect the state of the iono-spheric plasma [8,9]. It has been found that the spectral fittingprocess is more sensitive to rotational temperature than vibra-tional population distribution. Figure 3 shows calculated spec-tra for rotational temperatures separated by 14 K. In this casea lower temperature manifests as weaker minima on the ab-sorption profile. In the Cassini UVIS observations, therefore,the atmospheric temperature is derived through both iterativedetermination of rotational temperature, and the shape of thevertical abundance distribution in the hydrostatic model calcu-lations [11]. At lower altitudes the kinetic temperature is alsoconstrained by the measurements of the absorption structureof the C2H2 diffuse temperature sensitive (zC–zX) bands [12],although the laboratory data require corrections for saturationeffects. Below 550 km the temperature profile is constrainedby the Cassini CIRS published and communicated results asdiscussed in Sect. 3.6.

Figure 1 shows the observed extinction spectrum at IP1031 km against the modeled non-LTE calculation. Derivedrotational temperature and H2 abundance are given in the fig-ure. The 2005 DOY 103 occultation was a dayside observa-tion. Figure 2 shows a comparison of the optical depth spec-tra from the UVIS EUV experiment at the •Ori (lat �42:7ı)and —Ori (lat 15.2ı) at altitudes 858.5 and 1050.8 km selectedfor equality in equivalent width, giving a direct comparison ofdata quality in the two occultations. Differences in shape withthe •Ori spectrum showing deeper absorption in narrow fea-tures in the spectrum are attributed to the 14 K difference inderived temperature as in Fig. 3. Figure 4 shows the reducedH2 abundances (�, (3)) obtained from the Cassini UVIS and

Voyager UVS stellar occultations against altitude compared toa hydrostatic model fitting the data. The model profiles belowthe lower limit of measured H2 data indicated in Fig. 4 areconstrained by results obtained from the Cassini CIRS exper-iment4 [4–6] and Voyager radio occultation results [13]. Er-ror bars for the data in Fig. 4 are described in the figure cap-tion. The UVIS occultations in H2 extinction shown in thepresent work have a common characteristic at high altitude,in which the scale height inflects to lower values above about1400 km (Sect. 3.2.1). This property in a hydrostatic environ-ment indicates a decrease in temperature, but the inferencehere is that loss of H2 population is indicated (Sect. 3.2.1).The H2 profile of the latitude 15.2ı occultation differs signif-icantly from the other occultations shown in Fig. 4 in the alti-tude range 800–1200 km, while as discussed later, this devia-tion on an altitude scale vanishes on a pressure scale. Figure 5shows the forward modeled hydrostatic density distributions,derived from the H2 abundances shown in Fig. 4. The Voy-ager 2 (V2) UVS stellar occultation has been reanalyzed usingthe current H2 model structure. The V2 occultation was on thedarkside [7] at a latitude of 3.8ı. The differences in densityat a given altitude are evident in Fig. 4. Figure 5 on a densityscale also shows the modeled Helium distribution anchored ata [He]/[H2] D 0:12 mixing ratio at 1 bar. The [He]/[H2] mix-ing ratio affects the modeled temperature structure verticallythrough the mesopause, and the applied ratio is limited by thetemperature dependence of the C2H2 (zC–zX) band cross sec-tion. Detailed analysis will be carried out later using recentlyacquired5 low temperature C2H2 cross sections. The verticaltemperature profile is discussed in Sect. 3.6. Figure 6 shows the

4L. Fletcher and S. Guerlet. Private communication. 2009.5C-Y. Wu. Private communication. 2009.


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Fig. 4. Altitude abundance profiles of H2 obtained from the Cassini UVIS stellar occultations •Ori on 2005 DOY 103, —Ori on 2006DOY 142, “Cru on 2009 DOY 003, and from the Voyager 2 UVS stellar occultation •Sco on 1981 DOY 238. The plotted points arethe locations at which data are obtained, and the lines passing through the points are hydrostatic model fits to the data. Error bars areindicated for —Ori. Error bars for •Ori are less than the size of the plotted points, and for •Sco the statistical uncertainty can be assessedfrom the scatter of the points relative to the model. The occultations are identified on the plot legend.

Fig. 5. Altitude density profiles of H2 and He obtained from the stellar occultation models shown in Fig. 4. The occultations andlatitudes are identified in the plot legend. Superimposed plotted points on the H2 profiles show the range of the measured UVIS andCIRS data constraining the models.

modeled H2 density profiles for the •Ori and —Ori occultationsagainst pressure. The curves are coincident at the 10% levelover the plotted range. The •Sco Voyager profile is not shownbecause it is also nearly coincident with the other curves. Thederived temperature profiles evidently compensate for the dif-ferences in Fig. 4 to create a common pressure profile in H2

density. The currently limited “Cru profile is also not shown inFig. 6.

3.2.1. High altitude H2 profile properties

Figure 7 shows the vertical density profile for the “Cru oc-cultation at latitude ��3:6ı compared to —Ori at latitude 15.2ı.

The most pronounced decrease in scale height occurs in the“Cru occultation. This phenomenon is consistent with observa-tions of significant high altitude electron excitation of H2 elec-tronic bands, colocated with atomic hydrogen escaping fromthe top of the atmosphere at low latitude [10]. The implicationof this behavior is that hydrostatically derived temperatures atthe top of the atmosphere are underestimates and not a validmeasure of the kinetic state. Using a polynomic fit to the scaleheight in the “Cru occultation, a temperature of 612 K is ob-tained at 1530 km, the location of the maximum. Festou andAtreya [14] reported a temperature in the range 780–950 K atthe same altitude from analysis of the Voyager •Sco occulta-


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Fig. 6. Pressure scale density profiles of H2 obtained from the stellar occultation models for •Ori and —Ori. The occultations andlatitudes are identified in the plot legend. Superimposed plotted points on the H2 profiles show the range of the measured UVIS andCIRS 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 theatmosphere.

Fig. 7. Vertical density profiles of H2 from the “Cru 2009 DOY 003 and —Ori occultations at latitudes �3:6ı and 15.2ı. The plotincludes an HI profile above 1400 km derived from the deviation of the “Cru H2 density profile from a hydrostatic model.

tion. Using the 612 K value derived here as the terminal tem-perature, a crude estimate of atomic hydrogen (H I) densitiescan be obtained. This is shown in Fig. 7. The uncertainty inthis H I density profile is difficult to assess, of the order fac-tor of two. The H I profile derived in ref. 7 in the same lowlatitude region is a factor of �20 below the values shown inFig. 7. The H I profile shown here is consistent with down-ward diffusion of a source at high altitude. The H2 profile forthe Voyager occultation, shown in Fig. 5, is in good agreementwith the [7] result. The Voyager •Sco H2 profile follows theUVIS —Ori profile above 1400 km as shown in Fig. 5.

3.3. Hydrocarbon vertical profileThe analysis of the UVIS FUV spectrograph stellar occulta-

tion 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 theCassini transmission spectra, the species for which vertical dis-tributions are obtained are CH4, C2H2, and C2H4. The FUV–EUV hydrocarbon extinction spectrum is much simpler at Sat-urn than for Titan [11]. The evidence for other species willbe discussed in later works. An example of the stellar extinc-tion spectrum in the FUV is shown in Fig. 9. The only speciesprofile that can be reliably extracted from the V2 stellar oc-cultation is CH4. The hydrocarbon homopause is just above750 km in the UVIS •Ori occultation, 950 km for —Ori andthe V2 •Sco occultations. The 1 bar radius [7] is consistentwith the Cassini NAIF nominal model (Sect. 3.1.2). The verti-cal displacement of the hydrocarbon homopause levels in thetwo cases is consistent with the vertical displacement of the H2

densities (Fig. 10). The CH4 distributions are anchored at the1 bar level with the value ([CH4]/[H2] D 5:1 � 10�3) estab-


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Fig. 8. Extracted abundances of CH4, C2H2, and C2H4 from the UVIS •Ori (2005 DOY 103), —Ori (2006 DOY 142), and CH4 forVoyager 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 CH4, C2H2, C2H4, and upper limits to C2H6 andseveral other species. The temperature of the diffuse C2H2 (zC–zX) band cross section in this calculation is 150 K. Weak contributionsfrom dayside airglow appear in the short wavelength deep optical depth region of the spectrum, and in the 1500–1600 Å region. Theeffective IP of this spectrum is 695 km.

lished in ref. 15. Structure in the Cassini vertical distributionsis evident in the Fig. 8 data, and not correlated between speciesor for the same species at different latitudes except for CH4 andC2H2 at 880 and 720 km at latitude 15.2ı. Figure 10 showsthe altitude density profiles derived from the modeled abun-dances shown in Fig. 8. The much larger vertical span of thehydrocarbons in the —Ori occultation is evident in Fig. 10. Thehydrocarbon mixing ratio relative to H2 is shown in Fig. 11.Although there are significant H2 density differences at dif-ferent latitudes for given altitudes as shown in Figs. 4 and 5,this does not effectively compensate for hydrocarbon verticalseparations and altitude scale differences remain significant inmixing ratio.

Figure 12 shows the dependence of hydrocarbon mixing ra-tio on pressure. On a pressure scale the hydrocarbons nowshow a common homopause in the Cassini occultations, ap-

pearing near 0.2 �bar, and at 0.1 �bar the mixing ratios ofthe hydrocarbons are reduced by approximately an order ofmagnitude from the peak values. Figure 12 shows significantanticorrelations within the same species in vertical structurefor C2H2 and C2H4 at 0.7, 1.7, and 50 �bar. This figure in-cludes plots of derived mixing ratios from the Cassini CIRSexperiment for C2H2 [5, 16]. The dash–dot line in Fig. 12 isthe profile from [5] at planetocentric latitude �16:7ı. At theminimum pressure measurements found in ref. 5, 0.1 �bar, theUVIS mixing ratio falls an order of magnitude below the CIRSvalue. Two points from ref. 16 at higher pressures than the up-per range of the UVIS measurements are plotted in Fig. 12.Figure 12 also shows the distributions for CH4, C2H2, andC4H2 from the ref. 17 global model A. There is general agree-ment of the UVIS profiles for CH4, C2H2, and C2H4 withthe ref. 17 model in the range 1–100 �bar, but at 0.1 �bar


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Fig. 10. Densities of CH4, C2H2, and C2H4 obtained in the model fits shown in Fig. 8. The vertical range of abundances for the —Orioccultation at planetocentric latitude 15.2ı are measurable in the extinction spectra over a range of altitude as much as 300 km largerthan for the •Ori occultation. The limited dynamic range of the Voyager CH4 does not allow determination of the possible presenceof structure in the vertical distribution as indicated in the Cassini results, and the model extension to lower altitude for the Voyageroccultation contains significant uncertainty between 1000 and 300 km.

Fig. 11. Mixing ratios [N]/[H2] as a function of altitude for the hydrocarbons shown in Fig. 10. The differences in H2 density scalesbetween the occultations evidently do not effectively compensate for the hydrocarbon scale differences.

the UVIS mixing ratios are an order of magnitude below themodel. The UVIS mixing ratios for C2H2 and C2H4 fall belowthe ref. 17 model and CIRS results at the high pressure end ofthe range of measurement by factors of roughly three. Modelcalculations in ref. 18 for Saturn summer southern solstice forC2H2 at latitude 15ı, not included in Fig. 12, show mixing ra-tios moderately higher than those in ref. 17. Figure 12 includesthe ref. 17 model calculations for C4H2 with a compatible sin-gle upper limit value obtained from the UVIS —Ori occultationat 40 �bar. The modeled Voyager CH4 mixing ratio includedin Fig. 12 is significantly different from the UVIS distribu-tion at pressures below 10 �bar. Although on an altitude scale(see Fig. 10) the asymptotic distribution above the homopausefor the Voyager result at latitude 3.8ı is close to that of theUVIS occultation at latitude 15.2ı, the pressure scale compar-ison shows an implied much larger eddy diffusion coefficientin 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 isno data constraint below the homopause, and the profile is as-sumed to be structure free. The hydrocarbon homopause in theUVIS data is at �0.2 �bar, compared to �0.01 �bar for theVoyager measurements and the ref. 17 model (Fig. 12). Sig-nificant structure in the pressure profiles of the UVIS C2H4

and C2H2 results are evident in Fig. 12, showing differencesin mixing ratio in confined pressure regions at 0.7, 1.7, and50 �bar between the two latitudes. Guerlet et al. [6] reportstrong depletions in C2H2 and C2H6 mixing ratios at south lat-itudes compared to north (.�17ı� �35ı/=.17ı� 25ı/) at pres-sures below 1 mbar. Using the ref. 6 measurements for C2H2 atlatitudes �42:7ı and 15:2ı for direct comparison to the UVISmeasurements, the values CIRS (C2H2)Œ.�42:7ı/=.15:2ı/� D

0:6 and 0.18 are obtained at pressures of 100 and 10 �bar. TheCassini UVIS latitude dependence for C2H2 follows the trendfound in the CIRS results in the 100–30 �bar range, but atlower pressures beyond the CIRS measurement capability the


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826 Can. J. Phys. Vol. 90, 2012

Fig. 12. Mixing ratios [N]/[H2] as a function of pressure for the hydrocarbons shown in Fig. 10, compared to the V2 ıSco latitude 3.8ı

CH4 reduced data, Cassini CIRS analyses (see text), and model calculations by [17]. The ref. 5 profile for C2H2 shown as the dash–dotcurve is for latitude 16.7ı. The ref. 17 model A profiles are global. The plotted points (C2H2) for ref. 16 are for planetocentric latitude�42ı. The Cassini UVIS occultations at latitudes �42:7ı and 15.2ı show a common homopause on a pressure scale. The Voyager CH4

profile shows a homopause at substantially lower pressure, comparable to the ref. 17 model. Significant structure appears in the CassiniUVIS profiles (see text). The plotted point for UVIS —Ori C4H2 is an upper limit. Error bars in the Cassini UVIS mixing ratios aresmaller than the size of the plotted points, except at the ends of the pressure range.

Fig. 13. Mixing ratios [C2H2]/[CH4] and [C2H4]/[CH4]as a function of altitude, for latitudes �42:7ı and 15.2ı. The profiles showvertical displacements of �200 km and different vertical extents at the two latitudes.

UVIS results do not show significant C2H2 depletions at thesouth latitude. The ratios UVIS (C2H2)Œ.�42:7ı/=.15:2ı/� D

0:36 and 6.5 at 50 and 1.7 �bar indicate the south latitudeC2H2 mixing ratio has a strong low pressure peak at 1.7 �bar.The ratio UVIS (C2H4)Œ.�42:7ı/=.15:2ı/� D 21 at 0.7 �barreflects an apparent enhancement in the south and a corre-sponding depletion in the north for C2H4. The width of theUVIS features are 50–80 km. The depletion at midsouth lati-

tudes discussed in ref. 6 is therefore consistent with the UVISresults over the limited range 100 to 30 �bar, but the measure-ments in the latter case going to pressures below 30 �bar do notfollow the CIRS trend, and in fact show enhancement of C2H2

and C2H4 at south latitude in the vicinity of 1 �bar. Figures 13,14, and 15 show the C2H2 and C2H4 mixing ratios to CH4 onaltitude and pressure scales. The solid circles in the figures in-dicate •Ori data at latitude �42:7ı and open circles refer to


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Fig. 14. Measured mixing ratio [C2H2]/[CH4] as a function of pressure, for latitudes �42:7ı and 15.2ı, compared to model calculationsby ref. 18 at �40ı. and 8ı. See text.

Fig. 15. Measured mixing ratio [C2H4]/[CH4]as a function of pressure, for latitudes �42:7ı and 15.2ı, compared to model calculationsby ref. 18 at �40ı and 8ı. See text.

—Ori data at 15.2ı. In Fig. 13 the curves of the two species arevertically separated by about 200 km. The span of the measure-ments is roughly 800 km at 15.2ı latitude, compared to 500 kmat �42:7ı, reflecting the expanded low latitude atmosphere.The plots of these data on a pressure scale show the distincttransformation to alignment of structure. Figure 14 shows thepressure scale distribution for C2H2 compared to the Mosesand Greathouse [18] model. The mean location of the presentresults is 0.2 �b compared to 0.08 �b for ref. 18. The mainstructural feature peaks in the measured profiles at the two lat-itudes in Fig. 14 are aligned near 0.4 and 0.1 �b. The modelref. 18 is much more broadly distributed in the pressure scalethan the measurement, as readily observed in Fig. 14. The twolatitudes shown for the model calculations [18] in Fig. 14 are

closely aligned on the pressure scale. Figure 15 shows the re-sults for C2H4, in which the pressure scale distribution is nar-rower for both the observations and the model. The measuredlow pressure end at 15.2ı latitude may be caused by a low levelbias in the data.

3.4. The impact of helium on the atmospheric modelEstablishing an atmospheric model at Saturn starting at 1 bar

requires the inclusion of helium because of the significant im-pact on the mean mass of the fully mixed gas. At this time the[He]/[H2] ratio is not definitively established. Lindal et al. [13]in the analysis of RSS occultations, used the mixing ratio es-tablished by Conrath et al. [19], [He]/[H2] D 0.034. Conrathet al. [19] obtained [He]/[H2] by analyzing the RSS occultation


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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 inref. 4–6 (see text). The [21] multiple ground based stellar occultations are included. The UVIS profiles are forced to conform to theCIRS results from 1 bar to 0.04 mbar at 15.2ı and 0.01 mbar at �42.7ı. The plot legend identifies the profiles and latitudes.

Fig. 17. Projected photoabsorption cross section (Mb) of the C2H2zC1…u.0; 1; 0; 0; 0/ � zX1†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 shownagainst the model fits.

results and infrared thermal emission. Sixteen years later, fol-lowing the Galileo probe results at Jupiter, Conrath and Gau-tier [20] found a systematic divergence from their approachand on reanalysis obtained [He]/[H2] D 0:11 � 0:16. The mostdirect determination would be obtained by utilizing an accu-rately measured scale height, and an independent measurementof temperature to establish the mean mass of the gas. The UVISoccultation measurements provide the scale height structurein H2 with an independent rotational temperature measure-ment, but the current data reduction reaches only down to800 km above 1 bar for the •Ori occultation. The hydrocar-bon data are analyzed down to 300 km, and therefore over-lap the RSS and CIRS experiment results (Figs. 10 and 16).The absorption spectrum of C2H2 contains the strong tem-perature dependent .zC–zX/ bands showing peaks at 1477.91 Å(zC1…u.0; 1; 0; 0; 0/ � zX1†g.0; 0; 0; 0; 0/) and 1519.43 Å

(zC1…u.0; 0; 0; 0; 0/� zX1†g.0; 0; 0; 0; 0/) (see Fig. 9). The ex-perimental measurements need further refinement to correctfor saturation effects at the 1519.43 Å resonance feature. Newresults obtained by Wu now extend to lower temperature, butsaturation effects have not been corrected to date. The upperstate is heavily coupled and the diffuse rotational structure isnaturally blended as shown in Fig. 17. The width and mag-nitudes of the band heads change with temperature, and theblended diffuse p-branch lines in the 1485–1510 Å region arealso temperature dependent. Temperature analysis on the C2H2

extinction has been deferred until the laboratory measurementshave been refined. The model calculations shown in Figs. 5and 16 are based on [He]/[H2] D 0.12 at 1 bar. The tempera-ture profile shown in Fig. 16 is forced by continuity with theH2 density and scale height at 800 km at latitude �42:7ı and950 km at latitude 15.2ı and using the CIRS profiles from 1 bar


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Fig. 18. Pressure scale helium mole fraction in the present model compared to the Moses and Greathouse [18] model. The heliumdistribution is not measured in this work. Scaling is based on order of magnitude lower diffusion coefficients indicated in comparisonwith Voyager based ref. 18 results. As indicated on the plot the mole fraction is at latitude �40ı for ref. 18 and �42:7ı for the presentcalculation corresponding to the •Ori occultation.

up to 550 km. A weak mesopause established in the CIRSresults appears at 357 km (0.47 mbar) at latitude 15.2ı. Fit-ting the reanalyzed Voyager data, also shown in Figs. 5 and16, with the same [He]/[H2] constraint produces a temperatureprofile without a distinct mesopause, similar to the ref. 21 pro-file, also shown in Fig. 16. The [He]/[H2] ratio in Fig. 10 isthe fully mixed value through the mesopause in the latitude15.2ı profile. The eddy diffusion coefficient applied here issmall, consistent with the limited hydrocarbon distribution. Inthe pressure range 7–0.4 �bar at latitude 15.2ı the eddy diffu-sion coefficient is (K.15:2/ D) �103 cm2s�1, with He–H2 dif-fusion coefficients in the range (DHe�H2 D) 104–105 cm2s�1,calculated using Lennard–Jones potentials. At latitude �42:7ı

in the range 3–0.06 �bar K.�42:7/ D 4 � 103–4 � 104 andDHe�H2 D 2:5 � 105–106 cm2s�1. Comparing this atmosphericstate with the ref. 17 model, the hydrocarbon homopause wasat 10�2 �bar, while in the present observations the homopauselocation is 0.2 �bar (Fig. 12). At 10�2 �bar, the hydrocarbonsare not measurable in the UVIS observations with mixing ra-tios <10�11 (Fig. 12). The ref. 17 model A eddy diffusion co-efficient is 2: � 107 cm2s�1 at the homopause, while the Heforced coefficient in the current model is latitude dependentwith K.15:2/ D 4: � 105 and K.�42:7/ D 3: � 106 at a pressureof 10�2 �bar. Figure 18 shows a comparision of helium molefraction with the ref. 18 model. This comparison is not verymeaningful given the current derived state of the atmosphere,but it serves to illustrate the inferred change from the Voyagerobservation in 1981.

3.5. Latitudinal dependenciesFigure 10, comparing derived hydrocarbon densities for the

UVIS •Ori occultation at latitude �42:7ı and the Voyager UVSoccultation at 3.8ı, shows vertical separations of �200, and�350 km for CH4 at given densities. The H2 vertical profilemodel calculations for these two cases, indicate that the dif-ference is primarily caused by the latitudinal dependence ingravitational potential. Within the UVIS occultation events, acomparision of the extinction spectrum of •Ori at �42:7ı with

the —Ori spectrum of nearly equal magnitude at 15.2ı in Fig. 2shows a vertical separation of �200 km, the same magnitudeas the separation with the Voyager UVS low latitude result. H2

abundances as a function of pressure for the three occultationsexamined here differ by less than 15% at any given point inthe pressure range 1 bar to 0.01 �bar as stated in Sect. 3.2.Both abundance profiles in Fig. 4 show distinct changes in thelog scale slope, at �1000 km for •Ori and at �1300 km for—Ori, signaling the impact of rising upper thermospheric tem-peratures. There are also distinct differences in abundance dis-tribution above the slope transitions in the two occultations,with a curvature in the •Ori data to smaller scale height to-ward higher altitudes forcing a distinct peak in temperature(Fig. 16), and a significantly larger scale height developingin the —Ori data above 1250 km. As discussed in Sect. 4, thetemperatures forced by the abundance profiles in the high ther-mosphere are based on hydrostatic modeling, and no attemptis made to correct for the deviation caused by the loss of H2

through dissociation. The Fig. 8 shows vertical separations of200–300 km in CH4 abundance for the •Ori and —Ori occulta-tions. The latitudinal effects internal to the UVIS occultationsare therefore similar to the comparison with the Voyager UVSoutcome. Figure 19 shows a plot of the IP above the 1 bar ra-dius at the point of transmission extinction for H2 and CH4 inthe data set examined here, as well as the Voyager UVS event,as a function of latitude. With the exception of Voyager UVS,the CH4 extinction altitude falls below the H2 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 Earthbased stellar occultations, and the CIRS profile at latitude �40ı

[4, 6] as discussed earlier are shown in Fig. 16. The CIRS re-sults constrain the lower altitude profiles in this model. Ther-mal infrared measurements �43ı at 3 and 100 mbar by ref. 22using the Keck I telescope, are 2.8 and 1.3 K, respectively,higher than the profiles shown in Fig. 16, but System-III lon-gitude variations in those observations have an envelope larger


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830 Can. J. Phys. Vol. 90, 2012

Fig. 19. Extinction (� D 6) IPs above the 1 bar pressure level for H2 and CH4 for 12 UVIS stellar occultations and the V2 UVS •Scooccultation 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 H2 and CH4.

than these differences. At higher altitudes the temperature pro-files are determined by the H2 extinction spectra as describedearlier. In the region up to the exobase the temperature is alsolimited by the rotational temperature as well as the verticalabundance profile. The two constraints, rotational structure andvertical profile, are in good agreement but a more thoroughanalysis with a broader range of rotational temperature vectorsneeds to be carried out to set definitive limits on the tempera-ture uncertainty using the combined methodologies. The UVIS•Ori occultation analysis is the only existing derivation fromthe sunlit atmosphere in the UVIS accumulated results. TheT1 D 460 K thermosphere obtained from the 1981 occultationin the present analysis is 40 K higher than the original analysisby ref. 7, which is presumed to be caused mainly by the use of amuch more accurate H2 model in the present case. It is evidentthat substantial differences arise as an apparent function of lat-itude. The Voyager •Sco profile (latitude 3.8ı) is more similarto the UVIS result for —Ori (latitude 15.2ı), than to the UVISresult at •Ori (latitude �42:7ı). The value of T1 D 460˙30 Kfrom the Voyager •Sco occultation, compared to UVIS —Ori388 ˙ 15 K, and UVIS •Ori, 318 ˙ 5 K, indicate latitudi-nal differences within the UVIS results and probable temporalvariation in the relationship to the Voyager outcome at solarmaximum. Given the scale height properties at the top of theatmosphere discussed in Sect. 3.2.1 the top of atmosphere tem-peratures from the hydrostatic analysis are considered lowerlimits to actuality. The estimated temperature for the “Cru oc-cultation at latitude ��3:6ı, two to three scale heights belowthe exobase is estimated at 612 K. At low south latitude thetemperature at the exobase could be substantially larger.

The gap between the CIRS temperature profiles and theUVIS occultation measurements is filled by model calculationsconstrained by scale height continuity and limited by the atmo-spheric scale height. This leaves some uncertainty in the pro-file structure in the gap which shows temperature minima athigh altitude at 0.1 and 0.01 �bar. Given that these minima arereal may require a physical explanation in radiative loss by theincreasing mixing ratio of ionospheric HC

3 with increasing alti-tude in this region. Further work in iteration of results with the

CIRS researchers, who depend on an a priori CH4 profile, mayallow refinement of the temperature profile in the data gap (seeSect. 4).

4. Discussion and conclusions

4.1. H2 vertical structureThe Cassini UVIS occultation measurements of H2 abun-

dance on an altitude scale reveal a significant dependence of at-mospheric structure on latitude extending to the top of the ther-mosphere. Abundances at given pressures, however, includingthe reanalyzed V2 results, differ by less than 15% over themodeled pressure range, 1 bar to 10�4 �bar. The inference isthat over this range the H2 density on a pressure scale from atleast �40ı to the equator is invarient. Significant temperaturedifferences are found at the top of the thermosphere. At thelow south latitude of the projected Cassini UVIS proximal or-bits the temperature is estimated to be 612 K two to three scaleheights below the exobase. The hydrostatic analysis at high al-titude predict a temperature deviating from reality on the lowside because of high dissociation rates [10].

The measured temperatures at the top of the thermosphereat latitudes �42:7ı and 15.2ı in the Cassini UVIS observa-tions are significantly different (Fig. 16), and can be explainedby the observation of confined high heating rates inferred fromemissions observed in the vicinity of the exobase at low lati-tude [10]. The reanalysis of the Voyager UVS low latitude oc-cultation giving a relatively high hydrostatic temperature andapparent inferred larger eddy diffusion coefficient in 1981 sug-gests that the major solar cycle variation in deposition has ameasurable impact on the state of the atmosphere.

4.2. Hydrocarbon vertical structureOn a pressure scale hydrocarbon density profiles from the

UVIS occultations at latitude 15.2ı and �42.7ı show a com-mon homopause near 0.2 �bar, while the V2 occultation, andmodels ref. 17 indicate a homopause near �10�2 �bar, infer-ring a substantial difference in the eddy diffusion coefficient(see Fig. 12). The current atmospheric model for this reason


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contains a helium profile with diffusive separation characteris-tic of eddy diffusion coefficients in the range 103–105 cm2s�1

(Sect. 3.4). The Voyager •Sco occultation results show mea-sured abundances of CH4 to a much lower pressure than thecurrent Cassini UVIS observations (Fig. 12). The Voyager oc-cultation measurement shows a remarkable difference with theUVIS results in that the points of extinction of CH4 and H2 arecoincident, while the UVIS data shows H2 going into extinc-tion at higher altitudes (Fig. 19).

Significant vertical structure in the hydrocarbons on a scalecomparable to the H2 scale height are observed in the CassiniUVIS data. Guerlet et al. [6] report C2H2 and C2H6 mixingratios significantly depleted at midsouth latitudes relative tomodel calculations over the 100–10 �bar range. The CassiniUVIS measurements show a similar effect for C2H2 in the100–30 �bar pressure range, but at pressures below 30 �barthe UVIS results show no significant latitudinal deviations inmixing ratio to 3 �bar, where C2H2 is enhanced at south lati-tude, and depressed at north latitude over the range down to apressure of 1 �bar. The UVIS results show a similar very largeeffect for C2H4 over the range 2–0.3 �bar (Sect. 3.3).

4.3. Kinetic temperature profiles

Vertical temperature profiles have been established at lati-tudes �42:7ı and 15.2ı by forcing the model profiles to con-form to both the Cassini CIRS results (Sect. 3.6) at pressuresabove 20 and 40 �bar, and the H2 extinction measurementswith high pressure limits of 0.1 and 0.14 �bar. The resultingprofiles (Fig. 16) show temperature peaks near 1 �bar and min-ima at 7: � 10�2 and 9: � 10�3 �bar. The degree to whichthese profiles are in conformity with the CIRS data will bedetermined following the application of the CH4 mixing ra-tio measured over the pressure range 30–0.1 �bar combinedwith temperature profiles from the UVIS results, as an a priorireference for the CIRS data reduction process. The iterativecombination of the CIRS and UVIS results with the eventualradio occultation results in the lower atmosphere is expectedto provide a much improved model. The temperature profilesestablished here have produced a common pressure scale H2

density profile independent of the latitudes of the data (Fig. 6).This appears to confirm the accuracy of the temperature pro-files below the exobase to 10% or better.

In principal, kinetic temperature can be measured indepen-dently of the species vertical profile using the absorption spec-tral properties of the H2 at high altitudes and C2H2 at loweraltitudes where helium remains fully mixed. At this time, how-ever, a large enough database of absorption probabilities forH2 has not been established to confine the temperature deter-mination to the required accuracy, because of the large compu-tation overhead. It has also been found that one of the C2H2

absorption features with strong temperature dependence wassaturated in the laboratory measurements, and this will have tobe corrected in the future to allow sufficiently accurate mea-surements to further limit the [He]/[H2] ratio.

4.4. Atmospheric model profiles

The numerical quantities from the model calculations in thispaper will be made available on request.

Acknowledgements

Supporting data provided by and discussion with the CassiniCIRS researchers, L. Fletcher, and S. Guerlet were essential inconfining the model calculations in this work. J. Moses hasalso provided model results and valuable advice used in theproduction of this paper. The authors wish to thank J. Yoshii forassistance in data accumulation. This research was supportedby the University of Colorado Cassini UVIS Program contract1531660 to Space Environment Technologies.

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