2
Saturn upper atmospheric structure from Cassini EUV/FUV
occultations
3
D. E. Shemansky
4
1
5
and X. Liu
6
1
7
Planetary and Space Science Div., Space Environment Technologies, Pasadena, CA, USA
8
dshemansky@spacenvironment.net
1
9
Received
;
accepted
–2–
10
11
ABSTRACT
The Cassini UVIS experiment provides data on Saturn atmospheric physical properties through solar and stellar occultations. Transmission spectra in
the EUV/FUV range allow extraction of vertical profiles of H2 and hydrocarbon
abundances from the top of the atmosphere to about 300 km above the 1 bar
pressure level. The stellar occultation (δOri egress, 2005 DOY 103) has allowed
the most accurate data reduction from 300 km above the 1 bar level through the
exobase for latitude -42.7◦ in the sunlit atmosphere. The hydrocarbon homopause
is near the mesopause at 560 km ( 1. µb), based on measurements of CH4 , C2 H2 ,
C2 H4 . Other stellar occultations although not completely reduced, provide critical measurements of the latitudinal dependence of vertical atmospheric structure.
Analysis of a stellar occultation at latitude 15.2◦ shows vertical abundance displacements of 100 – 200 km above the measurements at −42.7◦ , and a significantly
different temperature profile. This work constrains the volumetric He/H2 ratio
to 0.12, the low end of the range of values derived by Conrath & Gautier. The
derived temperature profile below 700 km is sensitive to the He/H2 mixing ratio,
moderately affecting the extracted hydrocarbon densities. The thermospheric
temperatures derived from the H2 vertical profile are based on the assumption
of a hydrostatic distribution. This assumption introduces inaccuracy in the high
thermosphere because of a substantial rate of conversion of H2 to atomic hydrogen. Details of the atomic hydrogen vertical structure have not been extracted
in the current work. The Voyager 2 stellar occultation at 3.8◦ latitude has been
reanalyzed using a current H2 physical model in a photometric analysis, and
shows vertical displacements similar to the UVIS results at 15.2◦ . Preliminary
analysis of latitude dependence indicates the presence of significant differences in
CH4 vertical structure. Model calculations indicate that the latitudinal depen-
–3–
dence of H2 vertical displacements is caused primarily by the combined effects of
gravitation and temperature profile.
12
Subject headings: Outer planets: general — thermosphere: individual(vertical
13
structure
–4–
14
15
1.
Introduction
Results from the Cassini UVIS experiment occultation probes of the vertical structure
16
of the Saturn atmosphere are described in this work. The analysis of stellar occultations
17
are targeted in this preliminary analysis. Solar occultations have also been obtained but
18
data reductions are not available at this time. The highest quality transmission data has
19
been obtained from the occultation of Delta Orionis (δOri) obtained on 2005 DOY 103 at
20
latitude −42.7◦ . The analysis of this occultation is described in detail in this paper. Figure
21
1 shows a plot of latitude location and statistical quality of 14 stellar occultations examined
22
in varying degrees of detail in the present work. The spectral resolution of the UVIS
23
spectrographs is high enough to allow the analysis of H2 discrete absorption in rotational
24
structure, and for the first time kinetic temperatures are constrained by measurement of
25
rotational temperature over a range of altitudes, as well as through modeling of the vertical
26
abundance profile. The vibrational populations of the H2 X state are also constrained by
27
a non-LTE approach to the data reduction, although the heavy spectral overlap of the
28
vibrational vectors does not allow a strong constraint. Figure 1 indicates that the δOri
29
occultation has a far higher quality than any of the other 13 events. The horizontal line in
30
Figure 1 partitions the occultations into spectrally analyzable events as opposed to those
31
limited to photometric analysis. The present work also benefits from the application of a
32
highly accurate H2 model structure that has been developed over several years in the recent
33
past (see Glass-Maujean et al. 2009, and references therein).
34
Simultaneously with the H2 spectral transmission spectra obtained with one of
35
the UVIS spectrographs, hydrocarbon absorption spectra are obtained with a second
36
spectrograph. The vertical structure of CH4 , C2 H2 , and C2 H4 has been extracted through
37
spectral analysis of the δOri occultation. Research on the temperature sensitivity of C2 H2
38
has been utilized here to constrain the kinetic temperature in the vicinity of the mesopause
–5–
39
at −42.7◦ latitude. This temperature constraint then imposes a limit on the He/H2 mixing
40
ratio in the hydrostatic atmospheric modeling process.
41
2.
42
The Cassini UVIS experiment
The Cassini UVIS (Ultraviolet Imaging Spectrograph) experiment is fully described by
43
Esposito et al. (2004). The observations utilized in the work referenced here are obtained
44
using the EUV and FUV spectrograph units identified as Channels (Esposito et al. 2004).
45
The EUV and FUV Channels in a single exposure produce 64 spectral vectors of maximum
46
length 1024 pixels (pxs). The sky projection of spatial pixel size is (pxs X pxw) = (0.25
47
X 1.0) mr, spectral pixel size is pxs = 0.6096524 Å (EUV) and pxs = 0.779589 Å (FUV).
48
The spectral range of the UVIS is 563. – 1182 Å (EUV) and 1115. - 1912. Å (FUV). The
49
airglow ports of these instruments are used in stellar occultation observations in addition to
50
spatially extended airglow measurements. Unlike the Voyager UVS experiment, the UVIS
51
contains a solar occultation port only in the EUV Channel. UVIS solar occultations cannot
52
provide information on the hydrocarbon structure because spectral signal in the EUV is in
53
complete H2 extinction before the altitude region where measurable hydrocarbon absorption
54
is reached. The FUV spectral region shows no measurable H2 discrete absorption, allowing
55
unobstructed measurement of hydrocarbon extinction.
56
57
3.
Stellar Occultations
The δOri occultation, which provides the most accurate available data, has been
58
analyzed spectrally in detail. The ζOri occultation on 2006 DOY 141 has been reduced
59
spectrally for H2 . The H2 vertical profile for both δOri and ζOri has been modeled to
60
obtain a first measure of latitude dependence of the vertical abundance and temperature
–6–
61
profiles above 600 km. The remaining stellar occultations indicated in Figure 1 have been
62
reduced photometrically to provide preliminary data on latitudinal dependence of vertical
63
structure. The spectral analysis of the δOri event provides data on H2 , CH4 , C2 H2 , and
64
C2 H4 , but photometric analysis is restricted to H2 and CH4 . Instrument pointing for the
65
events shown in Figure 1 was stabilized by the spacecraft reaction wheels, and in each
66
case the star image motion was constrained to less than a spectral pixel width. The
67
latitude of the δOri egress occultation varied moderately on the sunlit side of the southern
68
hemisphere, but the critical region of the occultation was located at −42.7◦ . Measurements
69
of H2 atmospheric absorption were possible over the altitude range of 719 km to 1577
70
km above the nominal 1 bar level as determined by the Cassini navigation package. The
71
analysis of H2 absorption at altitudes greater than 1600 km is limited by signal noise.
72
Below 670 km, CH4 begins to contribute to the stellar absorption. The spectra are forward
73
synthesized using a rotational-level H2 absorption model (see Section 3.1) to produce the H2
74
line-of-sight abundance and temperature profiles. The data profiles constrain the derivation
75
of the density profile through application of a hydrostatic model. The hydrocarbon vertical
76
profiles obtained from the FUV channel measurements are spectrally analyzed using the
77
most recent temperature dependent cross sections. These reduction processes are necessarily
78
iterative.
79
The occultations in the EUV/FUV photon spectrum provide measurements of
80
atmospheric structure and composition from the exobase to 300 km altitude above the 1 bar
81
pressure level. The Cassini UVIS experiment provides the most accurate platform to date
82
in EUV/FUV transmission, for extracting atmospheric structure in outer planet research
83
primarily through higher spectral resolution, signal rates, and dynamic range.
–7–
84
85
3.1.
Vertical profile of H2 and hydrocarbons
The H2 component is obtained in the EUV Channel. An example of the transmission
86
spectrum is shown in Figure 2. The analysis of the transmission spectra is carried out
87
through forward modeling, simulating the instrument response in detail. This process
88
is required in order to account for the finite point spread function (psf) of the UVIS
89
spectrographs. The transmission spectrum of a model atmosphere using a modeled stellar
90
source spectrum is applied iteratively in simulated instrument output to obtain an optimal
91
fit to the observations. The EUV spectral resolution in the core of the psf is δλ = 0.9
92
Å FWHM. The EUV transmission spectrum at Saturn is uncontaminated hydrogen over
93
the entire altitude range to the point of complete extinction (optical depths τ = 6 - 7).
94
The H2 model used in the analysis was developed over the past several years successively
95
at the University of Southern California (USC) and Space Environment Technologies
96
(SET)(Shemansky et al. 2009; Hallett et al. 2005a,b). The temperature dependent
97
absorption cross section spectra for the analysis are calculated at a resolution
98
to an accuracy of ∼ 5% for those transitions that contribute measurably to the absorption
99
spectrum. The transmission spectra are fitted using separate vibrational vectors of the
λ
δλ
∼ 500000
100
ground state H2 X(v:J) structure. Details of H2 physical properties are described by Hallett
101
et al. (2005a,b) for the non-LTE environment that develops in the excited atmosphere.
102
The fundamental physical quantities (coupled state structure, absorption probabilities,
103
predissociation probabilities) used in these calculations are referenced by Shemansky et al.
104
(2009); Hallett et al. (2005a,b); Glass-Maujean et al. (2009). The calculation methodology
105
assumes that rotational structure is in thermal equilibrium at the gas kinetic temperature,
106
but vibrational population is non-LTE (Hallett et al. 2005a). Detailed model calculations
107
(Shemansky et al. 2009; Hallett et al. 2005a) show that in actuality there is some deviation
108
from thermal populations in rotation, but evidently not enough to significantly affect the
109
absorption spectral properties. H2 X vibrational populations, however, deviate significantly
–8–
110
from thermal and critically affect the state of the ionospheric plasma (Shemansky et
111
al. 2009; Hallett et al. 2005a,b). It has been found that the spectral fitting process is
112
more sensitive to rotational temperature than vibrational population distribution. In the
113
Cassini UVIS observations, therefore, the atmospheric temperature is derived through both
114
iterative determination of rotational temperature, and the shape of the vertical abundance
115
distribution in the hydrostatic model calculations (Shemansky et al. 2005). At lower
116
altitudes the kinetic temperature is also constrained by the measurements of the absorption
117
structure of the C2 H2 diffuse temperature sensitive (C̃ – X̃) bands (Wu et al. 2001).
118
Figure 2 shows the observed extinction spectrum at impact parameter 929 km against
119
the modeled non-LTE calculation. Derived rotational temperature and H2 abundance are
120
given in the figure. The 2005 DOY 103 occultation was a dayside observation at solar
121
phase ∼38◦ , with subsolar latitude −18.1◦ . Figure 3 shows the forward modeled hydrostatic
122
density distribution anchored at the 1 bar level to 400 km using the Lindal, et al. (1985)
123
results. The Voyager 2 (V2) UVS stellar occultation has been reanalyzed using the current
124
H2 model structure, and the resulting vertical H2 profile is also shown in Figure 3. The V2
125
occultation was on the darkside (Smith et al. 1983) at a latitude of 3.8◦ . The differences
126
in density at a given altitude (above the nominal 1 bar level) evident in the figure are
127
the combined consequence of the gravitation scale at the different latitudes of the two
128
occultations and different temperature profiles. Figure 3 also shows the modeled Helium
129
distribution anchored at a [He]/[H2 ] = 0.12 mixing ratio at 1 bar (Shemansky & Liu 2008).
130
The [He]/[H2 ] mixing ratio affects the modeled temperature structure in the vicinity of the
131
mesopause, and the applied ratio is limited by the temperature dependence of the C2 H2 (C̃
132
– X̃) band cross section. In the upper thermosphere, density differences are extended by
133
the fact that the top of thermosphere temperatures at V2 and Cassini differ by more than
134
100K. The vertical partial pressure profiles from Voyager UVS and Cassini UVIS compared
135
to the total pressure derived from CIRS at latitude -20◦ (Fletcher et al. 2007) are shown in
–9–
136
Figure 4. The vertical temperature profile is discussed in Section 3.4.
137
The analysis of the UVIS FUV spectrograph stellar occultation data yields the
138
hydrocarbon densities shown in Figure 3. Although there is an indication of the presence
139
of other species in the transmission spectra, the species for which vertical distributions are
140
obtained, CH4 , C2 H2 , and C2 H4 , are shown in Figure 3. The evidence for other species will
141
be discussed in later work. The only species profile that can be reliably extracted from the
142
V2 stellar occultation is CH4 (see Figure 3). The hydrocarbon homopause is just above
143
600 km in the UVIS occultation and at ∼ 900 km in the V2 occultation. A similar large
144
difference in homopause altitude is obtained in the UVIS ζOri occultation on 2006 DOY
145
141 at latitude 15.2◦ . The 1 bar radius in the Smith et al. (1983) work is consistent with
146
the Cassini results (Section 3.3). The vertical displacement of the hydrocarbon homopause
147
levels in the two cases is consistent with the vertical displacement of the H2 densities (see
148
Figure 3). The CH4 distributions are anchored at the 1 bar level with the value ([CH4 ]/[H2 ]
149
= 5.1 × 10−3 ) established by Flasar et al. (2005). Figure 5 shows the reduced abundance
150
data values for H2 and the hydrocarbons from the δOri occultation, compared to the model
151
calculations that allow the determination of the density profiles.
152
3.2.
The impact of helium on the atmospheric model
153
Establishing an atmospheric model at Saturn starting at 1 bar requires the inclusion
154
of helium because of the significant impact on the mean mass of the fully mixed gas. At
155
this time the [He]/[H2 ] ratio is not definitively established. Lindal, et al. (1985) in the
156
analysis of Voyager radio experiment (RSS) occultations, used the mixing ratio established
157
by Conrath et al. (1984), [He]/[H2 ] = 0.034. Conrath et al. (1984) obtained [He]/[H2 ]
158
by analyzing the RSS occultation results and infrared thermal emission. Sixteen years
159
later, following the Galileo probe results at Jupiter, Conrath & Gautier (2000) found a
– 10 –
160
systematic divergence from their approach and on reanalysis obtained [He]/[H2 ] = 0.11
161
– .16. The most direct determination would be obtained by utilizing a measured scale
162
height, and an independent measurement of temperature to establish the mean mass of the
163
gas. The UVIS occultation measurements provide the scale height structure in H2 with an
164
independent rotational temperature measurement, but the current data reduction reaches
165
only down to 700 km above 1 bar. The hydrocarbon data are analyzed down to 300 km,
166
and therefore overlap the RSS and CIRS experiment results (Figures 3, 6). The absorption
167
spectrum of C2 H2 contains the strong temperature dependent (C̃ – X̃) bands showing peaks
168
at 1477.91 Å (C̃ 1 Πu (0,1,0,0,0) – X̃ 1 Σg (0,0,0,0,0)) and 1519.43 Å (C̃ 1 Πu (0,0,0,0,0) – X̃
169
1
Σg (0,0,0,0,0)) (see Figure 7). Figure 8 shows part of the C2 H2 absorption cross section
170
temperature dependence projected from the Wu et al. (2001) measurements. The upper
171
state is heavily coupled and the diffuse rotational structure is naturally blended in Figure
172
8. The width and magnitudes of the band heads change with temperature, and the blended
173
diffuse p-branch lines in the 1485 Å – 1510 Å region are also temperature dependent. Figure
174
7 shows the observed transmission spectrum at 583 km, compared to a simulation of the
175
instrument response to transmission through the modeled atmosphere at T = 120 K. The
176
model calculation shown in Figures 3 and 6 are based on [He]/[H2 ] = 0.12 at 1 bar. The
177
temperature profile shown in Figure 6 is forced by continuity with the H2 density and scale
178
height at 700 km with the RSS and CIRS profiles from 1 bar up to 300 km. The result
179
shows a distinct mesopause at 550 km. Fitting the reanalyzed Voyager results, also shown
180
in Figures 3 and 6, with the same [He]/[H2 ] constraint produces a different temperature
181
profile without a distinctly defined mesopause, very similar in shape to the Hubbard, et al.
182
(1997) profile between 300 km and 800 km, also shown in Figure 6. Other models with
183
lower values of [He]/[H2 ] can satisfy the observational constaints, but higher ratios force the
184
mesopause temperature too high for compatibility with the C2 H2 absorption spectrum and
185
continuity with the observed structure at 700 km for the profile in the δOri occultation.
– 11 –
186
This analysis includes the basic assumption the vertical period in density and temperature
187
variance is limited to 1 or more atmospheric scale heights. At this time a lower limit on
188
the [He]/[H2 ] value has not been explored. A projection of the C2 H2 cross section has not
189
been obtained for temperatures below those shown in Figure 8, and a spectrum of C2 H2
190
dimer or the solid state is not available to establish a limit to the C̃ – X̃ low temperature
191
behavior. The modeled [He]/[H2 ] in Figure 3 remains fully mixed to the mesopause. The
192
calculation of diffusivity at that altitude using the Lennard-Jones potentials for He–He and
193
H2 –H2 , indicate a value of DHe,H2 (555km) = 1.6 × 104 cm2 s−1 . Diffusive separation for He
194
in the upper thermosphere is crudely estimated and will be refined in following work.
195
196
3.3.
Latitudinal dependencies
Figure 3 comparing derived densities for the UVIS δOri occultation at latitude −42.7◦
197
and the Voyager UVS occultation at 3.8◦ shows vertical separations of equal densities of
198
∼200 km, and ∼350 km for CH4 . The H2 vertical profile model calculations for these
199
two cases, starting with the same conditions at 1 bar, indicate that the difference is the
200
combined effect of latitude dependent temperature and gravitation. The assumption in
201
this case is that the 1 bar level is adequately determined by the ellipsoid installed in the
202
Cassini navigation package. The equatorial bulge is not included in these considerations.
203
Within the UVIS occultation events, a comparision of the optical depth spectrum of δOri
204
at −42.7◦ with the ζOri spectrum of nearly equal magnitude at 15.2◦ in Figure 9 shows a
205
vertical separation of ∼250 km. Figure 10 shows the abundance profiles of the δOri and
206
ζOri occultations compared to the current derivation from the Voyager δSco occultaion.
207
Data points with model calculations are shown in Figure 10, with error bars included on
208
the ζOri abundance values, which are less accurate than the δOri result (see Figure 1).
209
Both abundance profiles in Figure 10 show distinct changes in the log scale slope, at ∼900
– 12 –
210
km for δOri and at ∼1250 km for ζOri, signaling the impact of rising upper thermospheric
211
temperatures. There are also distinct differences in abundance distribution above the slope
212
transitions in the two occultations, with a curvature in the δOri data to smaller scale height
213
toward higher altitudes forcing a peak in temperature (Figure 6), and a significantly larger
214
scale height developing in the ζOri data above 1250 km. The ζOri abundance profile in
215
Figure 10 is close to linear on a log scale in the 700 km – 1200 km region, forcing the
216
temperature profile in this region to near constancy in the modeling process (Section 3.4).
217
As discussed in Section 4, the temperatures forced by the abundance profiles in the high
218
thermosphere are based on hydrostatic modeling, and the deviation caused by the loss of H2
219
through dissociation is not considered at this time. The Figure 11 shows vertical separations
220
of 200 – 300 km in CH4 abundance for the δOri and ζOri occultations. The latitudinal
221
effects internal to the UVIS occultations are therefore apparently similar to the comparison
222
with the Voyager UVS outcome. Figure 12 shows a plot of the impact parameter above
223
the assumed 1 bar oblate radius of transmission extinction for H2 and CH4 in the data
224
set examined here, as well as the Voyager UVS event, as a function of latitude. With the
225
exception of Voyager UVS, the CH4 extinction altitude falls below the H2 locations, but the
226
two species show a similar general trend.
227
228
3.4.
Vertical kinetic temperature profiles
The derived temperature profiles from UVIS 2005 DOY 103, V2 1981 DOY 238, the
229
Hubbard, et al. (1997) multiple Earth based stellar occultations, and the CIRS profile
230
at latitude −40◦ (Fletcher et al. 2007) are shown in Figure 6. The UVIS result at lat
231
−42.7◦ shows a distinct mesopause at 545 km at a temperature of 121 K. The mesopause
232
temperature is limited by the measured structure of the C2 H2 (C̃ – X̃) bands. The
233
uncertainty in temperature above 300 km is estimated to be ± 10 K for the UVIS result.
– 13 –
234
The hydrostatic model calculation of the structure confined by the measured H2 profile at
235
higher altitudes, and the radio occultation results at altitudes below 400 km, is dependent
236
on the [He]/[H2 ] mixing ratio. In the region up to 1450 km in the δOri occultation the
237
temperature is also limited by the rotational temperature in the model analysis. The
238
two constraints, rotational structure and vertical profile are in good agreement, but a
239
more thorough analysis with a broader range of rotational temperature vectors needs to
240
be carried out to set definitive limits on the temperature uncertainty using the combined
241
methodologies. The UVIS δOri occultation analysis is the only existing derivation from
242
the sunlit atmosphere. The T∞ = 460 K thermosphere obtained from the 1981 occultation
243
in the present analysis is 40 K higher than the original analysis by Smith et al. (1983),
244
which is presumed to be caused mainly by the use of a much more accurate H2 model
245
in the present case. The temperature profiles derived from the spacecraft data using the
246
present H2 physical model are shown in Figure 13. It is evident that substantial differences
247
arise as an apparent function of latitude. The Voyager δSco profile (latitude 3.8◦ ) is much
248
more similar to the UVIS result for ζOri (latitude 15.2◦ ), than to the UVIS result at δOri
249
(latitude −42.7◦ ). The value of T∞ = 460 K from the Voyager δSco occultation is within
250
error estimates of the value, 407 K, from the derived result at UVIS ζOri, and both are
251
∼100 K higher than from UVIS δOri at T∞ = 320 K.
252
253
4.
Discussion and conclusions
The Cassini UVIS occultation measurements reveal a significant dependence of
254
atmospheric vertical structure on latitude extending to the top of the thermosphere.
255
Although data analysis is not complete, selected occultations at latitudes −42.7◦ and
256
15.2◦ indicate significant differences in abundance and temperature vertical profiles. A
257
reanalysis in which the Voyager results were established on the same model basis as those
– 14 –
258
of Cassini UVIS has verified the original work, and shows similarity to the low latitude
259
UVIS ζOri 15.2◦ occultation in derived H2 abundance and temperature (Figures 10 and
260
13). The Cassini UVIS temperature profile at latitude −42.7◦ shows two distinctive
261
properties: 1) A distinctive mesosphere with a temperature near 120 K (Figure 6), 2) An
262
apparent temperature peak near 1200 km at latitude −42.7◦ that may indicate localized
263
heat deposition in the sunlit atmosphere over a range of 1 to 2 scale heights. This feature,
264
however, does not have a clear explanation because of the fact that the model calculation is
265
hydrostatic. It is known that the upper thermosphere is subject to a substantial conversion
266
of H2 to the atomic state (Shemansky & Liu 2008). The temperature derived in the
267
hydrostatic calculation is therefore affected by the loss of H2 in the upper thermosphere and
268
on this basis may be too low by some uncertain factor. All of the hydrostatic models will
269
be affected by this process in the high thermosphere region. The reanalysis of the Voyager
270
results and the UVIS ζOri data at low latitude show a much less distinctive mesosphere
271
(Figures 6 and 13) similar to that derived by Hubbard, et al. (1997) (Figure 6). The
272
strong differences in the temperature vertical profiles at high and low latitudes indicate
273
that latitudinal mixing is a weak process at Saturn. The distributions shown in Figure 13
274
suggest that hydrocarbon radiative cooling at high latitude is vertically much more confined
275
than at low latitude. The profile at latitude −42.7◦ shows a distinct cold mesosphere at 545
276
km, while the low latitude profiles show broad vertical ranges extending over several scale
277
heights that are low temperature and nearly isothermal suggesting extensive vertical mixing
278
of the hydrocarbons. The vertical profiles of the higher order hydrocarbons at low latitude
279
have not been established. The UVIS ζOri occultation contains sufficient data quality to
280
allow determination of C2 H2 and C2 H4 vertical profiles, potentially allowing assessment of
281
the apparent vertical extent of the cooling process. The higher T∞ values at low latitude
282
may be related to the apparent concentration of heating by the dissociation of H2 at low
283
latitudes (Shemansky & Liu 2008).
– 15 –
284
The vertical profiles of CH4 , C2 H2 , and C2 H4 have been obtained at latitude −42.7◦ .
285
Peak densities C2 H2 and C2 H4 are indicated at 400 km and 600 km, inferring that the main
286
region of production of higher order hydrocarbons is just above 600 km at −42.7◦ . The
287
equivalent region at low latitude is approximately 300 km higher, using the nominal 1 bar
288
oblate shape function.
289
Using the temperature dependence of the C2 H2 absorption cross section to limit the
290
temperature near the mesopause at latitude −42.7◦ , an upper limit of the He/H2 mixing
291
ratio has been determined at [He]/[H2 ] = 0.12 at 1 bar. This falls in the mid range of the
292
Conrath & Gautier (2000) estimate.
– 16 –
293
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Shemansky, D. E., and X. Liu 2008, The Saturn hydrogen plume, Planet. Space Sci.,
321
submitted
322
Smith, G. R., D. E. Shemansky, J. B. Holberg, A. L. Broadfoot, B. R. Sandal, & J. C.
323
McConnell 1983, Saturns upper atmosphere from the Voyager 2 solar and stellar
324
occultations, J. Geophys. Res., 88, 8667 - 8678
325
Wu, C. Y. R., F. Z. Chen, D. L. Judge 2001 , Measurement of temperature-dependent
326
absorption cross sections of C2 H2 in the VUV-UV region, J. Geophys. Res , 106,
327
7629
328
The authors wish to thank J. Yoshii for assistance in data accumulation. This research
329
was supported by the University of Colorado Cassini UVIS Program contract 1531660 to
330
Space Environment Technologies.
This manuscript was prepared with the AAS LATEX macros v5.2.
– 18 –
Data statistical quality
Saturn UVIS star occultations to date
12
11
10
9
8
7
6
5
4
3
2
1
0
signal rate quality
photometric cutoff
-40
-20
-0
Latitude
20
40
Fig. 1.— UVIS star occultation statistical quality for reduced data presented in the current
work, as a function of latitude. The main target of the present work is δOri at latitude
−42.7◦ showing a quality value of 12 on the plot. The horizontal line marks the level below
which data must be analyzed photometrically in order to obtain satisfactory signal levels.
– 19 –
1.2
1.0
Alt: 929 km
T = 240 K; C (H2) = 1.3x1020 cm-2
Vib. Distribution: F13
I/I0
0.8
0.6
0.4
0.2
0.0
900
950
1000
1050
1100
1150
λ (A)
Fig. 2.— UVIS EUV stellar occultation transmission spectrum obtained 2005 DOY 103 at
effective impact parameter 929 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.
– 20 –
2000
H2 UVIS lat -40o
He UVIS Lat -40o
CH4 UVIS Lat -40o
C2H2 UVIS Lat -40o
C2H4 UVIS Lat -40o
H2 V2 UVS Lat 3.8o
He V2 UVS Lat 3.8o
CH4 V2 UVS Lat 3.8o
[N] CIRS Lat -40o
1800
1600
h (km)
1400
1200
1000
800
600
400
V2 UVS 1981 DOY 238 δδ-Sco Lat 3.8o
200
UVIS 2005 DOY 103 εε-Ori
Lat -40o
ε
0
0
2
4
6
8
10
12
14
16
18
20
-3
Log{[N] (cm )}
Fig. 3.— Derived densities of H2 , He, CH4 , C2 H2 , and C2 H4 from UVIS 2005 DOY 103 (Lat
−40◦ ) and H2 , He, CH4 from the V2 Voyager 1981 DOY 237 (Lat 3.8◦ ) events. The CIRS
results giving total density at latitude −40◦ (Fletcher et al. 2007) are included. The Voyager
results are green in the plot with superposed light green points indicating the range of the
measured data. The UVIS results are blue with superposed cyan points showing the range
of measured data.
– 21 –
CH4 UVIS Lat -40o
H2 UVIS Lat -40o
H2 V2 UVS Lat 3.4o
CH4 V2 UVS Lat 3.8o
C2H2 UVIS Lat -40o
C2H4 UVIS Lat -40o
He V2 UVS Lat 3.8o
He UVIS Lat -40o
CIRS_Fletcher 2007
2000
1800
1600
h (km)
1400
1200
1000
CIRS P total Lat -40o
800
600
400
200
0
V2 UVS 1981 DOY 238 δδ-Sco Lat 3.8o
UVIS 2005 DOY 103 εε-Ori
Lat -40o
ε
10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100
101
102
103
P (mb)
Fig. 4.— The equivalent of Figure 3 on a pressure scale. The CIRS results giving total
pressure at latitude -40◦ (Fletcher et al. 2007) are included.
– 22 –
H2 08_13 model
H2 Saturn rev6 data
CH4 08_13 model
CH4 Saturn rev6 data
C2H2 08_13 model
C2H2 Saturn rev6 data
C2H4 08_13 model
C2H4 Saturn rev6 data
2000
1800
1600
h (km)
1400
1200
1000
800
600
400
200
0
0
10
20
30
log([N] cm-2)
Fig. 5.— Extracted abundances of H2 , CH4 , C2 H2 , and C2 H4 from the UVIS δOri occultation
on 2005 DOY 103 with model fits plotted through the data.
– 23 –
V2_lat4_00_reanal
Cassini UVIS_rev6 _08_13_1
Hubbard et al. 1997
CIRS Fletcher 2007
2000
1800
V2 UVS 1981 DOY 238 δδ-Sco Lat 3.8o
1600
UVIS 2005 DOY 103 εε-Ori
Lat -40o
ε
Hubbard et al. (1997) Earth based stellar:
equator model
h (km)
1400
1200
1000
800
600
400
Fletcher et al.(2007) CIRS Lat -40o
200
0
0
100
200
T (K)
300
400
500
Fig. 6.— Saturn temperature profiles derived from the UVIS (δOri), V2, Hubbard, et al.
(1997), occultations, and the Cassini CIRS result (Fletcher et al. 2007) at latitude -40◦ .
– 24 –
0
-1
UVIS FUV 2005 DOY 103 583 km
model _042e_02
-2
-ττ
-3
-4
-5
-6
-7
1300
1400
1500
1600
1700
1800
1900
λ (A)
Fig. 7.— Cassini UVIS FUV stellar occultation optical depth spectrum against a model
fit containing CH4 , C2 H2 , C2 H4 , and upper limits to C2 H6 and several other species. The
temperature of the diffuse C2 H2 (C̃ – X̃) band cross section in this calculation is empirically
determined at 120 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 impact parameter of this spectrum is 583 km.
– 25 –
100
90
c2h2_295K_wu_Mb
c2h2_150K_Wu_Mb
xsec_T150_148_sim
xsec_T295_148_sim
xsec_T195_148_sim
xsec_T120_148_sim
xsec_T110_148_sim
C Π
Πu (0,1,0,0,0) -- X Σg (0,0,0,0,0)
80
1
1
λ00 = 1477.91 Å
70
σ (Mb)
60
50
40
30
20
10
0
1460
1470
1480
1490
1500
1510
λ (A)
Fig. 8.— Projected photoabsorption cross section (Mb) of the C2 H2 C̃ 1 Πu (0,1,0,0,0) – X̃
1
Σg (0,0,0,0,0) band for selected temperatures. The measured cross sections from Wu et al.
(2001) are shown against the model fits.
– 26 –
0
-1
-ττ
-2
ζOri egr rec 073
δOri egr rec 045
-3
-4
ζOri = 997.77 km
δOri= 722.1 km
-5
-6
950
970
990
1010 1030 1050 1070 1090 1110 1130 1150
λ (A)
Fig. 9.— Comparison of optical depth spectra from the UVIS EUV experiment at the δOri
(lat −42.7◦ ) and ζOri (lat 15.2◦ ) occultations selected for equality in optical depth. The ζOri
spectrum was obtained at impact parameter 997.8 km and the δOri spectrum was obtained
at 722.1 km, above the nominal 1 bar pressure level.
– 27 –
2000
H2 _06_141 lat 15.2o
H2 _05_103 lat -42.7o
model lat -42.7o
model lat 15.2o
H2 V2 81_237 model lat 3.8o
1800
h (km)
1600
1400
1200
1000
800
600
17
18
19
20
21
22
23
24
Log([H2] (cm-2))
Fig. 10.— Derived and modeled abundance of H2 from the stellar occultations using the
Cassini UVIS experiment at δOri (lat −42.7◦ , 2005 DOY 103 ) and ζOri (lat 15.2◦ ,2006
DOY 141) and from the Voyager experiment at δSco (lat 3.8◦ , 1981 DOY 237).
– 28 –
1000
CH4 ζOri _06_141 lat 15.2o
CH4 δOri _05_103 lat -42.7o
900
h (km)
800
700
600
500
400
15
16
17
18
19
20
21
22
Log([CH4] (cm-2))
Fig. 11.— Extracted abundances from the UVIS FUV experiment at the δOri (lat −42.7◦ )
and ζOri (lat 15.2◦ ) occultations, as a function of altitude above the nominal 1 bar pressure
level.
kc_i_07-002
800
700
600
500
do_e_05-103
h (km)
900
-40
-20
-0
Lat
20
eo_i_06-079
UVIS H2 2005 -- 2007
V2 H2, CH4 1981
UVIS CH4 2005 --2007
zo_e_06-141
zo_e_07-202
eo_e_06-141
zo_e_06-079
eo_i_06-141
eo_e_07-202
do_i_07-202
eo_e_06-079
eo_e_06-118
do_e_07-202
1000
dsc_e_81-237
– 29 –
40
Fig. 12.— Extinction (τ = 6) impact parameters above the nominal 1 bar pressure level for
H2 and CH4 for 12 UVIS stellar occultations as a function of latitude.
– 30 –
500
T δOri 05_103 lat -42.7o
T δSco 81_237 lat 3.8o
T ζOri 06_141 lat 15.2o
400
Tg (K)
300
200
100
0
0
200
400
600
800
1000 1200 1400 1600 1800 2000
h (km)
Fig. 13.— Derived vertical temperature profiles at Saturn from the Cassini UVIS occultations
of δOri (lat −42.7◦ , 05 DOY 103 ) and ζOri (lat 15.2◦ , 06 DOY 141) and from the Voyager
experiment at δSco (lat 3.8◦ , 81 DOY 237). These results are obtained using a common H2
physical model.