Saturn’s tropospheric composition and clouds from Cassini/VIMS 4.6–5.1 μm nightside spectroscopy

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The latitudinal variation of Saturn’s tropospheric composition (NH 3, PH 3 and AsH 3) and aerosol properties (cloud altitudes and opacities) are derived from Cassini/VIMS 4.6–5.1 μm thermal emission spectroscopy on the planet’s nightside (April 22, 2006). The gaseous and aerosol distributions are used to trace atmospheric circulation and chemistry within and below Saturn’s cloud decks (in the 1- to 4-bar region). Extensive testing of VIMS spectral models is used to assess and minimise the effects of degeneracies between retrieved variables and sensitivity to the choice of aerosol properties. Best fits indicate cloud opacity in two regimes: (a) a compact cloud deck centred in the 2.5–2.8 bar region, symmetric between the northern and southern hemispheres, with small-scale opacity variations responsible for numerous narrow light/dark axisymmetric lanes; and (b) a hemispherically asymmetric population of aerosols at pressures less than 1.4 bar (whose exact altitude and vertical structure is not constrained by nightside spectra) which is 1.5–2.0× more opaque in the summer hemisphere than in the north and shows an equatorial maximum between ±10° (planetocentric). Saturn’s NH 3 spatial variability shows significant enhancement by vertical advection within ±5° of the equator and in axisymmetric bands at 23–25°S and 42–47°N. The latter is consistent with extratropical upwelling in a dark band on the poleward side of the prograde jet at 41°N (planetocentric). PH 3 dominates the morphology of the VIMS spectrum, and high-altitude PH 3 at p < 1.3 bar has an equatorial maximum and a mid-latitude asymmetry (elevated in the summer hemisphere), whereas deep PH 3 is latitudinally-uniform with off-equatorial maxima near ±10°. The spatial distribution of AsH 3 shows similar off-equatorial maxima at ±7° with a global abundance of 2–3 ppb. VIMS appears to be sensitive to both (i) an upper tropospheric circulation (sensed by NH 3 and upper-tropospheric PH 3 and hazes) and (ii) a lower tropospheric circulation (sensed by deep PH 3, AsH 3 and the lower cloud deck).