Ground-based spectroscopic observations of isotopes of CO in the atmospheres of Mars, Venus, and Titan have been collected over the 1982-1990 period. These observations have been analyzed to obtain information on the photochemistry, dynamics, and thermal profiles of these planetary atmospheres. In the cases of the mesosphere (80-100 km altitude) of Venus and the lower atmosphere (0-70 km altitude) of Mars, the primary conclusion of this research is that significant interannual variations in the global thermal and compositional structures of these atmospheres occur over 10 year periods. The Titan studies have focussed on pinning down the true atmospheric CO abundance. A more detailed summary of the results for each of these planetary atmospheres is provided

Clancy, R. Todd

English

NASA Technical Reports Server

NASA-CR-192165
Final Report for NASA Contract NAGW-1228
Planetary Atmospheres
Microwave Spectroscopic Studies of Planetary Atmospheres
(
P.I. - R. Todd Clancy
Laboratory for Atmospheric and Space Physics
University of Colorado
Boulder, Colorado 80309
(approximate level of support provided was 2-3 months/yr for 1987-1990)
Ground-based spectroscopic observations of isotopes of CO in the atmospheres of
Mars, Venus, and Titan have been collected over the 1982-1990 period. These obser-
vations have been analyzed to obtain information on the photochemistry, dynamics, and
thermal profiles of these planetary atmospheres. In the cases of the mesosphere (80-
100 km altitude) of Venus and the lower atmosphere (0-70 km altitude) of Mars, the pri-
mary conclusion of this research is that significant interannual variations in the global
thermal and compositional structures of these atmospheres occur over 10 year periods.
The Titan studies have focussed on pinning down the true atmospheric CO abundance.
A more detailed summary of the results for each of these planetary atmospheres is pro-
vided below.
Mars - Three publications regarding microwave studies of the Mars atmosphere have
been completed during the course of this 3 year grant (see accompanying list of publi-
cations). We have developed a technique of combining observations of the optically
thick 1200 microwave spectrum with the optically thin microwave 1300 spectrum to re-
trieve both the CO mixing ratio and the temperature profile (0-70 km altitude) of the
Mars atmosphere. Since we do not resolve the disk of Mars, these measurements refer
to low-to-mid latitude average properties of the Mars atmosphere. A comparison of such
measurements taken in 1975, 1980, 1982, 1988, 1989, and 1990 leads to two important
conclusions. To within the accuracy of the CO mixing determination (~15%), there exists
no evidence for the large CO abundance variations proposed by Hunten (1974). We
also show that the measurements of 100 % variations in Mars CO by Lellouch et al.
(1989) were based on incorrect analysis of their data. Secondly, we show that the Mars
atmosphere is much colder for the period corresponding to our microwave observations
than was observed during the dusty periods corresponding to Viking and Mariner 9
measurements of the thermal state of the Mars atmosphere. The thermal profiles we
derive from the microwave spectra are much more consistent with the radiative-convec-
tive profiles calculated for the clear, dust-free Mars atmosphere (Gierasch and Goody,
1968). We conclude that the 1980-1990 period was a period of relatively low dust load-
ing in the Mars atmosphere, which may be more representative of the Mars atmosphere
than the conditions observed during Viking and Mariner 9. A complete discussion of
(NASA-CR-192145) PLANETARY
ATHnSPHERFS: M[CRONAV c
SPECTROSCOPIC STUD[FS OF PLANETARY
ATMQSPHFRF$ Final Report (Colorado
Univ.) I_ P
N93-Z164Z
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these results and further implications is provided in Clancy et al. (1990), a much briefer
presentation (Clancy and Muhleman, 1990) is included with this report.
Venus - Studies of the Venus atmosphere are based on the same technique described
above for Mars. However the Venus CO spectra return information on temperatures and
CO abundances in the mesosphere of Venus, which is a transition region between the
massive zonally rotating lower atmosphere and the upper atmosphere, or cryosphere, in
which subsolar-to-antisolar circulation drives extreme thermal and compositional gradi-
ents. We find that both the temperatures and global distribution of CO in the Venus
mesosphere exhibited dramatic variations over the 1982-1990 period of our observa-
tions. These variations can be characterized by a global scale warming of low-to-mid
latitude temperatures in the mesosphere between 1984 and 1986, accompanied by
factors-of-ten changes in the diurnal gradient of CO mixing ratios. The accompanying
preprint (Clancy and Muhleman, 1991) presents the analysis and conclusions of this
study in great detail.
Titan - New observations of the microwave spectra of CO and HCN in the atmosphere
of Titan were obtained in 1989 and 1990. These spectra have been analyzed to provide
preliminary results. We obtain a CO abundance which agrees with our previous broad-
band result (Muhleman et al., 1984). This result is in disagreement with the microwave
observations of Marten et al. (1988), but it is in agreement with the infrared observations
of Titan CO by Lutz et al. (1983). The HCN profile we measure agrees with the upper
stratospheric determination by Tanguy et al. (1990), but our broadband, high frequency
(354 GHz) spectrum appears to require that HCN freezes out at a higher altitude in the
lower stratosphere that previously thought. These results will be developed in more
detail in a forthcoming publication.
Publications:
Clancy, R.T. and D.O. Muhleman, Corrections regarding the Lellouch et al. (1989)
analysis of Mars atmospheric 1200 and 1300 spectra, Icarus, 85, 120-129, 1990.
Clancy, R.T., D.O. Muhleman, and G.L. Berge, Global change in the 0-70 km thermal
structure of the Mars atmosphere derived from 1975 to 1989 microwave CO spectra,
J. Geophys. Res., 95, 14,543-14,554, 1990.
Clancy, R.T. and D.O. Muhleman, Measurements of a cold-clear state for the Martian
atmosphere, Ad. Space. Res., submitted, 1990.
Clancy, R.T. and D.O. Muhleman, Long term (1979-1990) changes in the thermal,
dynamical, and compositional structure of the Venus mesosphere as inferred from
microwave spectral line observations of 12OO, 1300 and C180, Icarus, in press,
1991.
Measurements of a Cold-Clear State
for the Martian Aunosphere
R. Todd Clancy
LASP
University of Colorado
Boulder, CO 80309-0392
USA
Duane O. Muhleman
Division of Geological and Planetary Sciences
California Institute of Technology
Pasadena, CA 91125
USA
COSPAR Proceedings
Paper Identification N ° S.3.1.3
ABSTRACT
An analysisof ground-basedmicrowavespectra of CO in the atmosphere of Mars yields the 0-70 km
atmospheric temperature profile for periods of observations in 1975, 1980, 1982, 1988, 1989, and 1990. The
derived atmospheric temperatures are -20K cooler than observed by Viking spacecraft observations in 1976-1977.
Seasonal (Ls) variations of 20K are apparent (maximum near Ls=270 °) for atmospheric temperatures up to an altitude
of-50 km. It is inferred that the atmosphere of Mars is often much clearer of dust and colder than indicated by the
Viking and Mariner 9 periods of observation during the 1970's.
INTRODUCTION
Ground-based microwave (1.2-2.6 ram) observations of 12CO and 13CO in the atmospheres of Mars and Venus
provide a means to monitor both the global abundance of CO and atmospheric temperatures. Such microwave spec-
tra possess several advantages in terms of their analysis. They are usually self-calibrating in that a well-defined con-
tinuum level of microwave emission, due either to surface or broadband atmospheric radiation, accompanies the spec-
tral line absorption/emission within the atmosphere. The spectral line radiative transfer is also relatively straight-
forward. Atmospheric scattering processes are negligible at microwave frequencies, LTE holds to very low pressures
(<nanobar), the source function is very nearly linear with the local temperature, and the lines are typically well sepa-
rated. Another particularly useful feature of microwave spectral lines in planetary atmospheres is that collisional
broadening exceeds thermal broadening for atmospheric pressures > 0.1 mbar. As a consequence, it is possible to re-
trieve mixing profiles of molecular species from remote, non-limbing sounding geometries.
MICROWAVE CO SPECTRA
Our recent microwave studies of CO in the Mars atmosphere are documented in Clancy, Muhleman, and Berge
(1990)/1/. Microwave opacities of the 1.3 mm transition of 12CO are sufficiently large to provide temperature
weighting functions from the surface of Mars to an altitude of 70 km (figure 1). The photochemical lifetime of CO
in the Mars atmosphere is long enough (>3 years; e.g., Hunten, 1974)/2/to ensure that it is vertically well mixed.
We measured the optically thin 1.2 mm transition of 13CO in November of 1988 to determine the CO volume
mixing ratio (6.0-L-_l.5x10-4), which was found to be consistent within measurement uncertainties with the 1967
infrared determination (8+_2x10 "4) by Kaplan et al. (1969)/3/. Figure 2 indicates the observed spectrum of 13CO
with the best-fit synthetic spectrum, corresponding to a CO mixing ratio of 6 x 10 .4 and a Mars atmospheric
temperature profile (figure 4) consistent with the observed 12CO spectrum (figure 3). Observations of the 12(70
spectraprovideanaccuratemeasureof atmospherictemperaturesbecausetheyaremoresensitiveto +_10K
perturbationsinatmospherictemperatures(figure3a)thantheyareto+50% uncertainties in the CO mixing ratio
(figure 3b). Lellouch et al. (1989)/4/performed a similar analysis of microwave 12CO and 13CO spectra observed
in 1986-1987, however their analysis was shown to be in error (Clancy and Muhleman, 1990)/5/. Lellouch,
Pauben, and Encrenaz (1990)/'7/have since analyzed very high signal-to-noise spectra of 12(20 and 13CO observed
in 1988. Their analysis of these observations indicates a CO mixing ratio of 8+2x10 -4, and that the fall-off in the
CO mixing ratio in the lower 10-20 km of the Mars atmosphere as inferred from Phobos ISM observations
(Rosenquist et al., 1990)/6/does not occur on a global scale.
MARS ATMOSPHERIC TEMPERATURES
Clancy, Muhleman, and Berge (1990) focussed on the derivation of Mars atmospheric temperature profiles (0-70
km) from microwave spectra of 12(20. The predominant source of uncertainty in such derived temperatures results
from +5K uncertainties in the solid-body continuum radiation of Mars, which is used to self-calibrate the CO line
emission/absorption. CO spectra of the whole-disk of Mars, observed in 1975, 1980, 1982, 1988, and 1989, were
analyzed to retrieve low-to-mid latitude average atmospheric temperatures for Mars (figure 4). A basic conclusion
from this work is that the Mars atmosphere is colder, and presumably clearer, during all of the periods observed
relative to the warm, dusty state of the Mars atmosphere observed by Viking in 1976-1977 and by Mariner 9 in
1971-1972. Although the 1988-1989 temperatures of figure 4 compare well with the Viking profiles, they were
measured at a solar longitude (Ls) corresponding to 40% increased solar flux relative to the L s of the Viking profiles.
The 1980 and 1982 microwave temperature profiles, which are 20-40 K cooler than the Viking profile at all
altitudes, were measured at roughly the same L s as the Viking profiles. In figure 5b, we show the L s dependence of
0.5 mbar level (-25 km altitude) atmospheric temperatures inferred from Viking descent obser,'ations (Seiff,
1978)/8/, Viking IRTM (Martin, 1981)/9/, and Mariner 9 IRIS radiances (Conrath, 1975)/10/; compared to that
exhibited by the microwave observations, including two new 1990 measurements. Both data sets indicate -20 K
variations with Ls, but the microwave temperatures are 20 K cooler overall. Figures 5a and 5c demonstrate that the
seasonal (Ls) variations and the generally cooler temperatures extend over the 3 mbar (-7-8 km) to .05 mbar (40-48
km) region. Only the Viking descent and ground-based microwave measurements are included in figures 5a and 5c,
since observations comparable to the IRTM 15 I.tm brightness temperatures are not available to document seasonal
variations at these levels.
CONCLUSION
ThecoldatmospherictemperaturesweinferforMarshaveseveralimplications.Coldertemperaturesarean
indicatorofmuchreducedustloading(Pollacketal.,1979)/11/,andcanaffecthelatitudinalandverticaldistribu-
tionsofatmosphericH20andclouds.PerturbationstoatmosphericH20andH202mayalsoaffectheverticaland
latitudinaldistributions of aunospheric 03 (Clancy, Muhleman, and Berge, 1990). Furthermore, the altitude-density
profile for the Mars atmosphere changes significantly between the warm-dusty and cold-clear states of the
atmosphere. Atmospheric densities for the same Ls change by as much as a factor-of-three at the 50 km altitude
level, between these states. We assert that the Mars atmosphere observed by the Phobos mission was much colder
and clearer than observed during the Viking mission; and that such a cold-clear state may be more representative of
average conditions within the Mars atmosphere.
REFERENCES
, Clancy, R.T., D.O. Muhleman, and G.L. Berge, Global changes in the 0-70 km thermal structure of the Mars
atmosphere derived from 1975-1989 microwave CO spectra, J. Geoohvs. Res.. in press, (1990).
2. Hunten, D.M., Aeronomy on the lower atmosphere of Mars, Rev. Geophys. Space Phys., 12,529, (1974).
. Kaplan, L.D., J. Connes, and P. Connes, Carbon monoxide in the Martian atmosphere, A,_trgphys. J., 1.57,
LI87-L192, (1969).
. Lellouch, E., M. G6rin, F. Combes, S. Atreya, and T. Encrenaz, Obse_'ations of the J=l-0 CO lines in the
Mars atmosphere: radiodetection of 13CO and monitoring of 12CO, Ices, 77, 414-438, (1989).
° Ciancy, R.T., and D.O. Muhleman, Corrections regarding the Lellouch et at. (1989) analysis of Mars
atmospheric 12CO and 13CO spectra, Icarus, 85, 120-129, (1990).
. Leliouch, E., G. Paubert, and T. Encrenaz, Mapping of CO millimeter-wave lines in Mars' atmosphere: the
spatial variability of carbon monoxide on Mars, preprint, (1990).
. Rosenquist, J., J.-P. Bibring, M. Combes, P. Drossart, T. Encrenaz, S. Erard, O. Forni, B. Gondet, Y.
Langevin, E. Lellouch, P. Masson, and A. Souffiok The vertical distribution of carbon monoxide on Mars
from the ISM-Phobos experiment, _._l,..t_D_a_. in press, (1990).
. Seiff, A., Post Viking models for the structure of the summer atmosphere of Mars, in The Mars Reference
Atmosphere, COSPAR, ed. by A. Kliore, Innsbruck, Austria, 1978.
. Martin, T.Z., Mean thermal and albedo behavior of the Mars surface and atmosphere over a Martian year,
Icarus, 45,427-446, (1981).
10. Conrath, B.M., Thermal structure of the Mars atmosphere during the dissipation of the dust storm of 1971,
Icarus, 24, 36-46, (1975).
11. Pollack, J.B., D.S. Colburn, F.M. Flaser, R. Kahn, C.E. Carlston, and D. Pidek, Properties and effects of
dust particles suspended in the Martian atmosphereL J. Geophys. Res., 84, 2929-2946, (1977).
12. Gierasch, P.J., and R.M. Goody, A study of the thermal and dynamical structure of the Martian lower
atmosphere, _, 16, 615, (1968).
FIGURE CAPTIONS
Figure 1. Nadir temperature weighting functions for the 230 GHz 12CO line. The various functions
correspond to different frequency offsets, v-v o, from the line center. Higher opacities closer to the
line center lead to weighting functions which peak at higher altitudes.
Figure 2. Measured (jagged line) and best-fit (solid line) 230 GHz 13CO spectra, observed in November of
1988. The best-fit spectrum corresponds to a CO mixing ratio of 6 x 10-4 and the November,
1988 temperature profile presented in figure 4. Dashed lines correspond to __+leterrors of _+1.5 x
10 -4 for the CO mixing ratio.
Figure3. Measured(jaggedline)andbest-fit(solidline) spectra of the J=l--_2 12CO transition, observed in
November of 1988. Dashed line synthetic spectra correspond to independent perturbations of
(a)+10K in the solution atmospheric temperature profile; and (b) +50% to the CO mixing ratio.
Figure 4. Mid-to-low latitude average temperature profiles for the Mars atmosphere, derived from 12CO
spectra observed in a) May and November of 1988 and January of 1989; and b) November of 1975,
March-April of 1980, and January of 1982. A CO mixing ratio of 6 x 10 -4, derived from
November, 1988 13CO spectra, was used to calculate all six of the microwave temperature
profiles. A Viking reference profile (Seiff, 1978)/8/and a radiative convective equilibrium profile
(Gierasch and Goody, 1968)/12/are included for comparison.
Figure 5. Atmospheric temperature, at pressure levels of a) 3.0 mbar; b) 0.5 mbar; and c) 0.05 mbar, versus
Mars season (Ls). The microwave-derived temperatures are represented by circles, the average of
the Viking 1 and 2 lander descent measurements (Seiff, 1978) are presented by squares. For the 0.5
mbar level, the Viking IRTM data (diamonds, Martin, 1981)/9/are the 15 micron brightness
temperatures, averaged over 40S-40N and 10AM-6PM. The Mariner 9 IRIS data are for 9-10AM
measurements at 20S-30S latitudes (asterisks; Conrath, 1975)/10/.
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Planetary atmospheres: Microwave spectroscopic studies of planetary atmospheres

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