Latitudinal variation of upper tropospheric NH_3 on Saturn derived from Cassini/CIRS far-infrared measurements

摘要

Ammonia (NH_3) has been detected both on Saturn and Jupiter, and although its concentration and distribution has been well-studied on Jupiter, it has proven more difficult to do so on Saturn due to higher sensitivity requirements resulting from Saturn's lower atmospheric temperatures and the dominance of Saturn's phosphine which masks the ammonia signal. Using far-infrared measurements of Saturn taken by Cassini/CIRS between February 2005 and December 2010, the latitudinal variations of upper tropospheric ammonia on Saturn are studied. Sensitivity to NH_3 in the far-infrared is explored to provide estimates of temperature, para-H_2 and PH_3, from 2.5 cm~(-1) spectral resolution measurements alone, 0.5 cm~(-1) spectral-resolution measurements alone, and 0.5 cm~(-1) measurements degraded to 2.5 cm~(-1) spectral resolution. The estimates of NH_3 from these three different datasets largely agree, although there are notable differences using the high emission angle 0.5 cm~(-1) data, which are asserted to result from a reduction in sensitivity at higher emission angles. For low emission angles, the 0.5 cm~(-1) -retrieved values of NH_3 can be used to reproduce the 2.5 cm~(-1) spectra with similar efficacy as those derived directly from the 2.5 cm~(-1) resolution data itself, and vice versa. Using low emission angle data, NH_3 is observed to have broad peak abundances at ± 25° latitude, attributed to result from condensation and/or photolytic processes. Lack of data coverage at equatorial latitudes precludes analysis of NH_3 abundance at less than about 10° latitude. Noise levels are not sufficient to distinguish fine zonal features, although it seems that NH_3 cannot trace the zonal belt/zone structure in the upper troposphere of Saturn.