Analysis of measurements of the H 2 density in Saturn's equatorial thermosphere indicates temperatures from 340 to 370 K. The deepest measurements, obtained during Cassini's final plunge into the atmosphere, measure the thermospheric temperature profile. The measurements are well fit by a Bates profile with an exospheric temperature of 354 K and a temperature gradient at 1.2 × 10 −4 Pa of 0.4 K/km, corresponding to a thermal conduction flux of 7.3 × 10 −5 W/m 2 . The helium profiles are consistent with diffusive equilibrium. The CH 4 profiles are not in diffusive equilibrium but instead have a roughly constant mixing ratio relative to H 2 . We interpret this as the signature of a downward external flux of ∼ 10 13 m −2 s −1 . Saturn's rings are the most likely source of this external material. Plain Language SummaryThe mass spectrometer on the Cassini spacecraft measured the densities of molecular hydrogen, helium, and methane, among other species, along the spacecraft track through the atmosphere. Analyzing these data determines the temperature structure of the upper atmosphere. We find temperatures of 340 to 370 K. We also find that the methane in the upper atmosphere comes from Saturn's rings rather than the lower atmosphere.

We report the first spectroscopic detection of ethyl cyanide (C 2 H 5 CN) in Titan's atmosphere, obtained using spectrally and spatially resolved observations of multiple emission lines with the Atacama Large Millimeter/submillimeter Array (ALMA). The presence of C 2 H 5 CN in Titan's ionosphere was previously inferred from Cassini ion mass spectrometry measurements of C 2 H 5 CNH + . Here we report the detection of 27 rotational lines from C 2 H 5 CN (in 19 separate emission features detected at > 3σ confidence), in the frequency range 222-241 GHz. Simultaneous detections of multiple emission lines from HC 3 N, CH 3 CN and CH 3 CCH were also obtained. In contrast to HC 3 N, CH 3 CN and CH 3 CCH, which peak in Titan's northern (spring) hemisphere, the emission from C 2 H 5 CN is found to be concentrated in the southern (autumn) hemisphere, suggesting a distinctly different chemistry for this species, consistent with a relatively short chemical lifetime for C 2 H 5 CN. Radiative transfer models show that C 2 H 5 CN is most concentrated at altitudes 200 km, suggesting production predominantly in the stratosphere and above. Vertical column densities are found to be in the range (1-5)×10 14 cm −2 .

General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. , and HCN/H 13 C 15 N=5800±270 (1σ errors). The carbon and nitrogen ratios are consistent with and improve on the precision of previous results, confirming a factor of ∼2.3 elevation in 14 N/ 15 N in HCN compared to N 2 and a lack of fractionation in 12 C/ 13 C from the protosolar value. This is the first published measurement of D/H in a nitrile species on Titan, and we find evidence for a factor of ∼2 deuterium enrichment in hydrogen cyanide compared to methane. The isotopic ratios we derive may be used as constraints for future models to better understand the fractionation processes occurring in Titan's atmosphere.