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Saturn's cloud structure and temporal evolution from ten years of Hubble Space Telescope images (1994 2003)
Pérez-Hoyos, S.; Sánchez-Lavega, A.; French, R. G.; Rojas, J. F.
AA(Departamento de Física Aplicada I, E.T.S. Ingenieros, Universidad del País Vasco, Alameda de Urquijo s/n, 48013 Bilbao, Spain), AB(Departamento de Física Aplicada I, E.T.S. Ingenieros, Universidad del País Vasco, Alameda de Urquijo s/n, 48013 Bilbao, Spain), AC(Department of Astronomy, Wellesley College, Wellesley, MA 02481, USA), AD(Departamento de Física Aplicada I, E.U.I.T.I., Universidad del País Vasco, Plaza Casilla s/n, 48013 Bilbao, Spain)
Icarus, Volume 176, Issue 1, p. 155-174. (Icarus Homepage)
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(c) 2005 Elsevier Inc.
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We present a study of the vertical structure of clouds and hazes in the upper atmosphere of Saturn's Southern Hemisphere during 1994-2003, about one third of a Saturn year, based on Hubble Space Telescope images. The photometrically calibrated WFPC2 images cover the spectral region between the near-UV (218-255 nm) and the near-IR (953-1042 nm), including the 890 nm methane band. Using a radiative transfer code, we have reproduced the observed center-to-limb variations in absolute reflectivity at selected latitudes which allowed us to characterize the vertical structure of the entire hemisphere during this period. A model atmosphere with two haze layers has been used to study the variation of hazes with latitude and to characterize their temporal changes. Both hazes are located above a thick cloud, putatively composed of ammonia ice. An upper thin haze in the stratosphere (between 1 and 10 mbar) is found to be persistent and formed by small particles (radii ˜0.2 mum). The lower thicker haze close to the tropopause level shows a strong latitudinal dependence in its optical thickness (typically tau˜20-40 at the equator but tau˜5 at the pole, at 814 nm). This tropospheric haze is blue-absorbent and extends from 50 to 100 mbar to about ˜400 mbar. Both hazes show temporal variability, but at different time-scales. First, there is a tendency for the optical thickness of the stratospheric haze to increase at all latitudes as insolation increases. Second, the tropospheric haze shows mid-term changes (over time scales from months to 1-2 years) in its optical thickness (typically by a factor of 2). Such changes always occur within a rather narrow latitude band (width ˜5-10°), affecting almost all latitudes but at different times. Third, we detected a long-term (˜10 year) decrease in the blue single-scattering albedo of the tropospheric haze particles, most intense in the equatorial and polar areas. Long-term changes follow seasonal insolation variations smoothly without any apparent delay, suggesting photochemical processes that affect the particles optical properties as well as their size. In contrast, mid-term changes are sudden and show various time-scales, pointing to a dynamical origin.
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