Stratospheric benzene and hydrocarbon aerosols detected in Saturn’s auroral regions

¹ Sorbonne Universités, UPMC Paris 06, UMR 8539, LMD, 75005 Paris, France
e-mail: sandrine.guerlet@lmd.jussieu.fr
² CNRS, Laboratoire de Météorologie Dynamique, IPSL, UMR 8539, 4 place Jussieu, 75005 Paris, France
³ Sorbonne Universités, UPMC Paris 06, UMR 8109, LESIA 75005 Paris, France
⁴ LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
⁵ NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
⁶ CNRS-INSU, Institut d’Astrophysique Spatiale, UMR 8617, 91405 Orsay, France

Received: 4 August 2014
Accepted: 8 June 2015

Abstract

Context. Saturn’s polar upper atmosphere exhibits significant auroral activity; however, its impact on stratospheric chemistry (i.e. the production of benzene and heavier hydrocarbons) and thermal structure remains poorly documented.

Aims. We aim to bring new constraints on the benzene distribution in Saturn’s stratosphere, to characterize polar aerosols (their vertical distribution, composition, thermal infrared optical properties), and to quantify the aerosols’ radiative impact on the thermal structure.

Methods. Infrared spectra acquired by the Composite Infrared Spectrometer (CIRS) on board Cassini in limb viewing geometry are analysed to derive benzene column abundances and aerosol opacity profiles over the 3 to 0.1 mbar pressure range. The spectral dependency of the haze opacity is assessed in the ranges 680–900 and 1360–1440 cm^-1. Then, a radiative climate model is used to compute equilibrium temperature profiles, with and without haze, given the haze properties derived from CIRS measurements.

Results. On Saturn’s auroral region (80°S), benzene is found to be slightly enhanced compared to its equatorial and mid-latitude values. This contrasts with the Moses & Greathouse (2005, J. Geophys. Res., 110, 9007) photochemical model, which predicts a benzene abundance 50 times lower at 80°S than at the equator. This advocates for the inclusion of ion-related reactions in Saturn’s chemical models. The polar stratosphere is also enriched in aerosols, with spectral signatures consistent with vibration modes assigned to aromatic and aliphatic hydrocarbons, and presenting similarities with the signatures observed in Titan’s stratosphere. The aerosol mass loading at 80°S is estimated to be 1−4 × 10^-5 g cm^-2, an order of magnitude less than on Jupiter, which is consistent with the order of magnitude weaker auroral power at Saturn. We estimate that this polar haze warms the middle stratosphere by 6 K in summer and cools the upper stratosphere by 5 K in winter. Hence, aerosols linked with auroral activity can partly account for the warm polar hood observed in Saturn’s summer stratosphere.