Millimeter and Submillimeter Heterodyne Observations of Titan: The Vertical Profile of Carbon Monoxide in Its Stratosphere

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Millimeter and submillimeter heterodyne observations performed with the IRAM 30-m telescope (Pico Veleta, Spain) and the JCMT (Mauna Kea, Hawaii) have been used to derive the stratospheric distribution of carbon monoxide on Titan. Rotational transition lines from ^12CO J(0 --> 1), J(1 --> 2), J(2 --> 3) at 115.271, 230.538, 345.796 GHz, respectively, as well as the J(1 --> 2) and J(2 --> 3) lines of the ^13CO isotope at 220.399 and 330.588 GHz, respectively, were recorded with a spectral resolution of 1 MHz. Flux calibration uncertainties were estimated to 10% for all the data. A terrestrial value of the ^12C_solar^13C ratio has been assumed in the analysis as suggested by T. Hidayat et al. (1997, Icarus 126, 170-182) from recent observations of the H^12CN(1-0) and H^13CN(4-3) lines. The ^13CO lines sound the 60- to 180-km altitude range, while the ^12CO lines permit us to probe the atmosphere up to an altitude of about 350 km. Below 180 km, the ^13CO data impose a constant-with-height CO mixing ratio of ~2.5 x 10^-5. Extending this uniform mixing ratio profile throughout the stratosphere, all the ^12CO observations could be matched only if the systematic calibration errors were greater than our estimated value by at least a factor of 2. Uncertainties related to the temperature profile adopted in the stratosphere have been also investigated. Taking into account random and systematic uncertainties, the entire set of data indicates a CO mixing ratio equal to 2.9^+0.9_-0.5 x 10^-5 at 60 km, decreasing to 2.4 +/- 0.5 x 10^-5 at 175 km, and reaching a value of 4.8^+3.8_-1.5 x 10^-6 at 350 km.

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