MILLIMETER AND SUBMILLIMETER HETERODYNE OBSERVATIONS OF TITAN, THE VERTICAL PROFILE OF CARBON-MONOXIDE IN ITS STRATOSPHERE

Millimeter and submillimeter heterodyne observations performed with th e IRAM 30-m telescope (Pico Veleta, Spain) and the JCMT (Mauna Kea, Ha waii) have been used to derive the stratospheric distribution of carbo n monoxide on Titan. Rotational transition lines from (CO)-C-12 J(0 -- > 1), J(1 --> 2), J(2 --> 3) at 115.271, 230.538, 345.796 GHz, respect ively, as well as the J(1 --> 2) and J(2 --> 3) lines of the (CO)-C-13 isotope at 220.399 and 330.588 GHz, respectively, were recorded with a spectral resolution of 1 MHz. Flux calibration uncertainties were es timated to 10% for all the data. A terrestrial value of the C-12/C-13 ratio has been assumed in the analysis as suggested by T. Hidayat et a l. (1997, Icarus 126, 170-182) from recent observations of the (HCN)-C -12(1-0) and (HCN)-C-13(4-3) lines. The (CO)-C-13 lines sound the 60- to 180-km altitude range, while the (CO)-C-12 lines permit us to probe the atmosphere up to an altitude of about 350 km. Below 180 km, the ( CO)-C-13 data impose a constant-with-height CO mixing ratio of similar to 2.5 x 10(-5). Extending this uniform mixing ratio profile througho ut the stratosphere, all the (CO)-C-12 observations could be matched o nly if the systematic calibration errors were greater than our estimat ed value by at least a factor of 2. Uncertainties related to the tempe rature 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(-0.5)(+0/9) x 10( -5) at 60 km, decreasing to 2.4 +/- 0.5 x 10(-5) at 175 km, and reachi ng a value of 4.8(-1.5)(+3.8) x 10(-6) at 350 km. (C) 1998 Academic Pr ess.