Planetary Atmospheres: Probing Structure through Millimeterwave Observations of Carbon Monoxide
1996
This thesis consists of inteferometric observation of carbon monoxide from
three planetary atmospheres. The observations address specific questions about the
state and structure of each atmosphere. The analysis and results for each planetary
body are contained within individual chapters of the thesis and the abstract for each
is reproduced below. Titan: Evidence for Well-Mixed Vertical Profile We report on new millimeter heterodyne observations of the ^(12)CO J(1 - 0)
rotational transition from the stratosphere of Titan made in October 1994 with the
Owens Valley Radio Observatory Millimeter Array. The spectrum obtained clearly
exhibits a strong emission core over the ~600 MHz bandwidth of the upper sideband
spectrometer. The lineshape, referenced to the flat spectrum simultaneously observed
in the lower sideband was inverted to determine a best fit CO mixing ratio profile
consistent with the observations. The bet fit profile is a constant mixing ratio of 5 ± 1 x 10^(-5) over the altitude range of 60- 200 km. Combined with IR observation
of tropospheric CO (ƒco = 6 x 10^(-5), Lutz et al. 1983) this provides strong evidence
that CO is well mixed from the surface to at least 200 km in Titan’s atmosphere. Mars: Thermal Structure from 0-70 km Millimeter-wave heterodyne observations of the ^(12)CO J (l - 0) rotational transition
from the atmosphere of Mars were made on three dates in February 1993 with
the Owens Valley Radio Observatory Millimeter Array. These observations yielded
high-quality spectra with a spatial resolution of 4.2" on a 12.5" diameter Mars. The
spectra were numerically inverted for profiles of the local atmospheric temperature
from 0 to 70 km, assuming a constant CO mixing ratio for the atmosphere. The
derived average low latitude atmospheric temperature profile is approximately 20 K
cooler than reference temperature profiles compiled during the Viking era. This new
temperature profile is well-matched by cooler profiles determined from whole disk CO
measurements, suggesting very little dust loading of the atmosphere at the time of
the observations (Clancy et al. 1990). In addition, the revealed thermal structure
shows variation with latitude, and these temperature profiles compare well with profiles
derived from Mariner 9 IRIS observations (Leovy 1982, Santee and Crisp 1993) and calculated thermal structure from the Mars General Circulation Model (Haberle
et al.. 1993). The temperature profiles were averaged in local time and the resulting
cross-section of temperature as a function of pressure and latitude used to infer the
mean zonal circulation of the atmosphere. These wind results are somewhat compromised
by the relatively low spatial resolution of the observation but do qualitatively
match both inferred zonal winds from the Mariner 9 IRIS observations and Mars
GCM calculations. These initial observations point toward the desirability of further
interferometric measurements. Venus: Temporal Variations of the Mesophere Millimeter-wave heterodyne observations of the ^(12)CO J(1-0) rotational transition
from the mesosphere of Venus were made in early November and early December
1994 with the Owens Valley Radio Observatory Millimeter Array. The spatial resolute ion for each day was about 1000 km at the sub-earth point. The high quality CO
spectra were numerically inverted for profiles of the local CO mixing ratio from 80
to 105 km, assuming a Pioneer Venus mean temperature profile for the atmosphere.
For each day the revealed CO distribution shows a nightside maximum centered at
low latitudes and shifted from the anti-solar point toward the morning terminator.
Both clays show a clear latitudinal falloff in the CO abundance. In November the
maximum was centered at roughly 2^h local time at 100 km, while in December the
maximum was at roughly 4 – 4.5^h local time at 100 km. In addition, CO abundances
were slightly higher in November. The changes in the CO distribution are
examined in the context of the mesospheric circulation model of Clancy and Muhleman
(1985b). The increased shift away from the anti-solar point and decreased CO
abundance for the December observations both point toward increased zonal and/or
decreased sub-solar to anti-solar circulation within the mesosphere during the month
between observations.
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