New laboratory observations of the VUV absorption cross-section of C2H2, obtained under physical conditions approximating stratospheres of the giant planets, were combined with IUE observations of the albedo of Saturn, for which improved data reduction techniques have been used, to produce new models for that atmosphere. When the effects of C2H2 absorption are accounted for, additional absorption by other molecules is required. The best-fitting model also includes absorption by PH3, H2O, C2H6 and CH4. A small residual disagreement near 1600 A suggests that an additional trace species may be required to complete the model

Caldwell, John

Judge, D. L.

Wagener, R.

Wu, C. Y. R.

Xia, T. J.

English

NASA Technical Reports Server

M90-26757
HIGH RESOLUTION, LOW TEMPERATURE PHOTOABSORPTION
CROSS-SECTION OF C2H2, WITH APPLICATION TO SATURN'S ATMOSPHERE
JOHN CALDWELL*, C.Y.R. WU**, T.J. XIA**, D.L. JUDGE** AND R. WAGENER*
* Space Astrophysics Laboratory, Institute for Space and Terrestrial Science, 2700 Steeles
Ave. West, Concord, Ontario L4K 3C8, Canada
** Space Science Center and Dept. of Physics, University of Southern California, Los
Angeles, California 90089-1341
ABSTRACT
New laboratory observations of the VUV absorption cross-section of C2H2, obtained
under physical conditions approximating stratospheres of the giant planets, have been com-
bined with IUE observations of the albedo of Saturn, for which improved data reduction
techniques have been used, to produce new models for that atmosphere. When the effects
of C2H2 absorption are accounted for, additonal absorption by other molecules is required.
The best-fitting model also includes absorption by PH3, H20, C2H6 and CH4. A small
residual disagreement near 1600/_ suggests that an additional trace species may be required
to complete the model.
INTRODUCTION
Extensive solar system observations of absorption and emission spectra have been ob-
tained from a diverse collection of Earth-orbital satellites, interplanetary probes and ground-
based telescopes. The pressures and temperatures of the various planetary environments
being observed are generally very different from the usual laboratory conditions. There has
been little incentive for the laboratory workers to go to the extra trouble of emulating the
planetary conditions , and this has led to a situation where much of the laboratory spectral
data which are required to interpret the planetary observations are obtained under conditions
that are inappropriate for direct application to planetary modelling.
In the ultraviolet, planetary observations have been obtained for more than a decade
by the NASA/ESA International Ultraviolet Explorer satellite. In the near future, it is ex-
pected that data from the Hubble Space Telescope will be available. The IUE data have
provided significant new insights to the composition and auroral processes in planetary at-
mospheres. However, it is universally recognized that the advent of the HST will produce
spectra that are greatly superior to the IUE data with respect to spatial resolution, spec-
tral resolution, dynamic range and signal:to-noise. Whereas existing laboratory data are
marginally acceptable for interpreting data of the quality produced by the IUE, it is clear
that they are absolutely inadequate for modelling HST data.
Motivated by this mismatch between laboratory and observatory capabilities, we have
implemented an experimental program to investigate systematically the temperature- and
pressure-dependent absorption cross-sections of gases of interest for studying planetary at-
mospheres. Our first species is C2H2, chosen because it is one of the most important UV
absorbers in the stratospheres of the giant planets. We have applied the new data to previous
142
https://ntrs.nasa.gov/search.jsp?R=19900017441 2020-03-24T03:05:10+00:00Z
IUE observations of Saturn, where C2H2 is clearly the dominant absorber. The reduction of
the IUE spectra has been improved over previously published versions, as discussed below.
After the effects of C2H2 absorption have been accounted for, other gases have been added
to the model to optimize agreement with the planetary spectra.
7.0 " ' ' '
8.0--
, l,ll5.5-
5.0 . , ilL
1600 1800 2000
Wavelength (A)
Fig. 1: New absorption coefficients, obtained by Wu et aL 1 at
two temperatures.
LABORATORY RESULTS
The VUV photoabsorption cross-section measurements of C2H 2 were performed at
the Synchrotron Radiation Center at the University of Wisconsin:Madison. The data were
obtained at a spectral resolution of 0.07/_ FWHM. The absorption coefficient spectra shown
in Fig. 1 were smoothed by a 5 point gaussian filter to reduce high frequency noise. In
the region between 1530 and 1800/_ the absorptions at low temperature (155K) compared
to the room temperature data show stronger peaks (10 to 50%) and some enhancement in
the valleys (up to 20%). In the region between 1900 and 2100 /_ hot bands due to the
_41Au - X 1E+ transition are observed in the room temperature data. Further details are
given in Ref. 1. The lower temperature absorption coefficients are more applicable to outer
planet stratospheres and are used for the first time in the following reanalysis of the UV
spectrum of Saturn.
143
IMPROVEMENTS IN IUE DATA ANALYSIS
Since the study of Saturn's UV spectrum by Winkelstein et al. 2 a number of improve-
ments have been made in the IUE data reduction. All of the data taken between 1978 and
1980 was reprocessed by the IUE Standard Image Processing System at Goddard Space
Flight Center using the latest software and the new Intensity Transfer Functions (ITF), in
particular correcting for a known error in the ITF of the short wavelength prime camera
(SWP, 1100 to 1980/_) at low exposure levels.
Because of the very limited dynamic range of the IUE cameras and because of the
rapidly decreasing planetary flux from long to short wavelengths, it is necessary to combine
a large number of individual spectra, having a wide range in exposure times, from seconds
to hours, to cover the useful range of the IUE. The individual spectra are normalized in the
overlapping region and averaged together.
O
<
.9
L.
E
$
O
0.50
0.40
0.30
0.20
0.10
0.00
I I " "
• I .... I .... I ' 1
1
Old reduction
1500
I T , , , I , , , , t
2000 2500 3000
Wavelength (A)
Fig. 2: The composite Saturn spectrum obtained by the IUE.
The older extraction process is shown by the lighter line, the newer,
improved one by the darker line.
Since the SWP camera is sensitive to longer wavelength photons, some contribution
due to long wavelength light scattered within the spectrograph has to be corrected for in
solar type spectra. An improved algorithm was used in this analysis that explicitly included
the contribution of H2 emission. The newly derived albedo spectrum is shown in Fig. 2
together with the previous version of Winkelstein et al..
144.
O.o
E
19
0.40
0.30
0.20
0.10
0.00
I .... I I .... I '
I| Cloud \"
. C2Hs CsHe_
C
Pl I , , _ , I , _ , , I _ , i i l ,
1500 2000 2500 3000
Wavelength (A)
Fig. 3: The Saturn albedo spectrum (thin line) with the best-
fitting model obtained to date (thick line), showing the regions where
various absorbers are thought to be important.
MODELS OF THE ULTRAVIOLET ALBEDO OF SATURN
The new extraction process has introduced a relative albedo maximum near 2500/_, in
good agreement with OAO albedos obtained in the early 1970s 3. The low albedo at 3000
must be due to some absorbing cloud at a pressure level of about 170 mbar in the Saturnian
atmosphere (Fig. 3). The rise in reflectivity at shorter wavelengths is due to Rayleigh
scattering from overlying H2. The decreasing albedo below 2500/_ can be explained by PH3
absorption. The best fitting model in Fig. 3 had no PH 3 at pressure levels p < 60 mbar,
a mixing ratio of 5 x 10 -7 between 60 and 120 mbar and 1.5 x 10 -6 for p > 120 mbar,
consistent with the distribution of Tokunaga et al. 4, but incompatible with the distribution
of Courtin et al. 5.
Below 2000 /_, the spectrum (Fig. 4) is dominated by C2H2 absorption. Our best
fitting model has C2H2 distributed between 2 and 20 mbar at a mixing ratio of 1.2 x 10 -7.
A model with C2H2 alone is unsatisfactory, an additional continuum absorber in the top
2 mbar is required. We have examined the following candidate species C2H4, C3H4, C4H2,
and H20 and find that H20 at a mixing ratio of 1.5 x 10 -7 provides the best fit, although still
unsatisfactory between 1550 and 1600/_. The same conclusion was reached by Winkelstein
et al., but the abundance of H20 is reduced by a factor of 2 as a direct consequence of the
new low temperature absorption coefficients of C2H2.
Since we restricted the C2H6 distribution to follow the C2H2, i.e. uniform between 2
and 20 mbar, the influence of C2H6 is felt only weakly near 1500/_. CH4 causes the drop to
essentially zero albedo near 1450/_.
145
0.40
0.30
o
0.20
0.10
0.00
1400
I ' I
CH
CH 2 8
1600 1800 2000
Wavelength (A)
Fig. 4: Details of the best-fitting model below 2000/_, with the
regions where several absorbers may be important.
To conclude, we speculate on the possible cause of the remaining discrepancy between
the model and the observations, near 1600 /_, and indicate where future laboratory effort
may be required. The discrepancy suggests that another gas may be present on Saturn but
not yet recognized in the modelling. One possibility for the missing gas is arsine, recently
detected in Saturn's mid-infrared spectrum by B6zard et al. 6. The observed mole fraction is
of order several parts per billion, with the clear indication that the abundance is decreasing
with increasing altitude, '% possible consequence of UV photolysis". If that is indeed the
explanation for the vertical distribution of AsH3 on Saturn, then it is reasonable to expect
that some effect of that absorption may be visible in the UV spectrum. Unfortunately, no
relevant laboratory observations of AsH 3 have been made, and it will be a prime candidate
for future work in our program.
1. C.Y.R. Wu, T.S. Chien, G.S. Liu, D.L. Judge and J.J. Caldwell, J. Chem. Phys. 91,
272( 9s9).
2. P. Winkelstein, J. Caldwell, T. Owen, M. Combes, Th. Encrenaz, G. Hunt and
V. Moore, Icarus 54, 309 (1983).
3. L. Wallace, J. Caldwell and B.D. Savage, Astrophys. J. 172, 755 (1972).
4. A.T. Tokunaga, H.L. Dinerstein, D.F. Lester and D.M. Rank, Icarus 42, 79 (1980);
Icarus 48, 540 (1981).
5. R. Courtin, J.P. Baluteau, D. Gautier, A. Marten and W. Maguire, Saturn Conference,
Tucson, AZ, May 1982.
6. B. B6zard, P. Drossart, E. Lellouch, G. Tarrago and J.-P. Maillard, Astrophys. J. 346,
509 (1989).
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High resolution, low temperature photoabsorption cross-section of C2H2 with application to Saturn's atmosphere

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