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

1998; Elsevier BV; Volume: 133; Issue: 1 Linguagem: Inglês

10.1006/icar.1998.5908

1090-2643

Taufiq Hidayat, A. Marten, Bruno Bézard, D. Gautier, Tobias Owen, H. E. Matthews, G. Paubert,

Ionosphere and magnetosphere dynamics

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 from12COJ(0 → 1),J(1 → 2),J(2 → 3) at 115.271, 230.538, 345.796 GHz, respectively, as well as theJ(1 → 2) andJ(2 → 3) lines of the13CO 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 the12C/13C ratio has been assumed in the analysis as suggested by T. Hidayatet al. (1997,Icarus126, 170–182) from recent observations of the H12CN(1–0) and H13CN(4–3) lines. The13CO lines sound the 60- to 180-km altitude range, while the12CO lines permit us to probe the atmosphere up to an altitude of about 350 km. Below 180 km, the13CO data impose a constant-with-height CO mixing ratio of ∼2.5 × 10−5. Extending this uniform mixing ratio profile throughout the stratosphere, all the12CO 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× 10−5at 60 km, decreasing to 2.4 ± 0.5 × 10−5at 175 km, and reaching a value of 4.8+3.8−1.5× 10−6at 350 km.