The structure of Titan's atmosphere from Cassini radio occultations

We present results from the two radio occultations of the Cassini spacecraft by Titan in 2006, which probed mid-southern latitudes. Three of the ingress and egress soundings occurred within a narrow latitude range, 31-34 degrees S near the surface, and the fourth at 52.8 degrees S. Temperature-altitude profiles for all four occultation soundings are presented, and compared with the results of the Voyager 1 radio occultation (Lindal, C.F., Wood, G.E., Hotz, I-LB., Sweetnam, ON., Eshleman, V.R., Tyler, G.L. [1983]. Icarus 53, 348-363). the HASI instrument on the Huygens descent probe (Fulchignoni, M. et al. [2005]. Nature 438, 785-791), and Cassini CIRS results (Flasar. F.M. et al. [2005]. Science 308, 975-978; Achterberg, R.K., Conrath, B.J., Gierasch, P.J., Flasar, F.M., Nixon, CA. [2008b]. Icarus 194, 263-277). Sources of error in the retrieved temperature-altitude profiles are also discussed, and a major contribution is from spacecraft velocity errors in the reconstructed ephemeris. These can be reduced by using CIRS data at 300 km to make along-track adjustments of the spacecraft timing. The occultation soundings indicate that the temperatures just above the surface at 31-34 degrees S are about 93K, while that at 53 degrees S is about 1 K colder. At the tropopause, the temperatures at the lower latitudes are all about 70 K, while the 53 degrees S profile is again 1 K colder. The temperature lapse rate in the lowest 2 km for the two ingress (dawn) profiles at 31 and 33 degrees S lie along a dry adiabat except within similar to 200 m of the surface, where a small stable inversion occurs. This could be explained by turbulent mixing with low viscosity near the surface. The egress profile near 34 degrees S shows a more complex structure in the lowest 2 km, while the egress profile at 53 S is more stable. (C) 2011 Elsevier Inc. All rights reserved.