Meridional variations of temperature, C₂H₂ and C₂H₆ abundances in Saturn's stratosphere at southern summer solstice

Measurements of the vertical and latitudinal variations of temperature and C₂H₂ and C₂H₆ abundances in the stratosphere of Saturn can be used as stringent constraints on seasonal climate models, photochemical models, and dynamics. The summertime photochemical loss timescale for C₂H₆ in Saturn's middle and lower stratosphere (∼40 - 10,000 years, depending on altitude and latitude) is much greater than the atmospheric transport timescale; ethane observations may therefore be used to trace stratospheric dynamics. The shorter chemical lifetime for C₂ H₂ (∼1 - 7 years depending on altitude and latitude) makes the acetylene abundance less sensitive to transport effects and more sensitive to insolation and seasonal effects. To obtain information on the temperature and hydrocarbon abundance distributions in Saturn's stratosphere, high-resolution spectral observations were obtained on September 13-14, 2002 UT at NASA's IRTF using the mid-infrared TEXES grating spectrograph. At the time of the observations, Saturn was at a L_S ≈270°, corresponding to Saturn's southern summer solstice. The observed spectra exhibit a strong increase in the strength of methane emission at 1230 cm^-1 with increasing southern latitude. Line-by-line radiative transfer calculations indicate that a temperature increase in the stratosphere of ≈ 10 K from the equator to the south pole between 10 and 0.01 mbar is implied. Similar observations of acetylene and ethane were also recorded. We find the 1.16 mbar mixing ratio of C₂H₂ at -1° and -83° planetocentric latitude to be 9.2_-3.8^+6.4 × 10^-7 and 2.5_-1.0^+1.8 × 10^-7, respectively. The C₂H₂ mixing ratio at 0.12 mbar is found to be 1.0_-0.3^+0.5 × 10^-5 at 1° planetocentric latitude and 2.6_-0.9^+1.3 × 10^-6 at -83° planetocentric latitude. The 2.3 mbar mixing ratio of C₂ H₆ inferred from the data is 7.5_-1.7^+2.3 × 10 ^-6 and 1.0_-0.2^+0.3 × 10^-5 at -1° and -83° planetocentric latitude, respectively. Further observations, creating a time baseline, will be required to completely resolve the question of how much the latitudinal variations of C₂H₂ and C₂H₆ are affected by seasonal forcing and/or stratospheric circulation.