Seasonal variation of Titan’s haze at low and high altitudes from HST-STIS spectroscopy

•HST-STIS image cubes in five years provide a unique probe of atmospheric variations.•The first principal component describes haze opacity variations below 80 km altitude.•The second one has similar variations above 150 km but its phase is 1–2 years ahead.•The tropics have higher (>150 km) and lower (<80 km) opacities than the global mean.•The north–south asymmetry may reverse in 2016 (>150 km) and 2017–2018 (<80 km).

The Space Telescope Imaging Spectrograph accumulated image cubes of Titan in five years between 1997 and 2004 that we calibrated and analyzed. The observations probe Titan’s early northern fall to early winter. Methane bands between 543 and 990 nm wavelength are well resolved spectrally, and Titan’s latitudinal and center-to-limb reflectivity variations are resolved spatially. A principal component analysis revealed two large components and two small components of less significance. The first principal component describes a variation of Titan’s haze below 80 ± 20 km altitude. Haze particles change their size, opacity, and/or shape of the single scattering phase function. The largest and smallest opacities occurred both in 1997 at high southern latitudes and northern latitudes, respectively. The hemispherical asymmetry switched sign in 2002 at low latitudes, in 2003 at mid latitudes, and in early 2004 at high latitudes. The seasonal amplitude increased almost linearly with distance from the Equator. Tropical latitudes had slightly lower opacities than the annual and global average if the observed variation is seasonally symmetric and shaped like a sine curve. The cause for the variation may be condensation of gases onto aerosols seasonally driven by atmospheric dynamics. The second principal component describes a variation of haze opacity at altitudes above 150 ± 50 km. Largest and smallest opacities both occurred in 2004 at northern and high southern latitudes, respectively. The asymmetry switched in late 2001. Tropical latitudes had significantly higher haze opacity than the annual and global average, opposite to the case at low altitudes. The cause for the high-altitude variation may be aerosols transported at varying speeds driven by atmospheric dynamics. We present a seasonal model that completely describes the haze parameters at each altitude, latitude, and time. It compares fairly well with Cassini results obtained since 2004. The north–south asymmetry may reverse in 2016 at high altitudes and 2017 through 2018 at low altitudes. The observed variations are significant for modeling photometric data of Titan’s surface. They describe several characteristics of Titan’s haze variations that can be compared with results from Global Circulation Models.

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