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

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 12 CO 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 13 CO 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 12 C/ 13 C 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 12 CN(1–0) and H 13 CN(4–3) lines. The 13 CO lines sound the 60- to 180-km altitude range, while the 12 CO lines permit us to probe the atmosphere up to an altitude of about 350 km. Below 180 km, the 13 CO data impose a constant-with-height CO mixing ratio of ∼2.5 × 10 −5 . Extending this uniform mixing ratio profile throughout the stratosphere, all the 12 CO 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 −5 at 60 km, decreasing to 2.4 ± 0.5 × 10 −5 at 175 km, and reaching a value of 4.8 +3.8 −1.5 × 10 −6 at 350 km.