Solar system

Abstract:

We present results of an axisymmetric global circulation model of Titan with a simplified suite of atmospheric physics forced by seasonally varying insolation. The recent discovery of midlatitude tropospheric clouds on Titan has caused much excitement about the roles of surface sources of methane and the global circulation in forming clouds. Although localized surface sources, such as methane geysers or "cryovolcanoes," have been invoked to explain these clouds, we find in this work that clouds appear in regions of convergence by the mean meridional circulation and over the poles during solstices, where the solar forcing reaches its seasonal maximum. Other regions are inhibited from forming clouds because of dynamical transports of methane and strong subsidence. We find that for a variety of moist regimes, i.e., with the effect of methane thermodynamics included, the observed cloud features can be explained by the large-scale dynamics of the atmosphere. Clouds at the solsticial pole are found to be a robust feature of Titan's dynamics, whereas isolated midlatitude clouds are present exclusively in a variety of moist dynamical regimes. In all cases, even without including methane thermodynamics, our model ceases to produce polar clouds approximately 4-6 terrestrial years after solstices.

Abstract:

The upper jovian atmosphere is particularly transparent at wavelengths near 5 μ m. Levels well below the cloud layers, which are situated between 0.5 and 2 bar, can be sounded. Large spatial variations of the brightness are observed, which are directly related to the opacity of the overlying cloud layer. Yet, the nature of the 5- μ m absorber in the jovian atmosphere has been subject of much debate. The cloud layer has been modelled many times as a thin, non-scattering layer, the opacity adjusted to fit the overall radiance level. This has proven to work well for individual spectra. Data from the Galileo near infrared mapping spectrometer (NIMS), covering the 0.7- 5.2 μ m range, include a number of observations of the same areas, separated by several hours, at different emission angles. Should the 5 μ m absorber be a thin absorbing layer then, apart from a change in radiance level, the overall shape of the 5- μ m spectrum is also expected to change significantly with emission angle. However, comparison of the 5- μ m spectra measured by NIMS of the same location but at different viewing angles reveals that while the overall radiance level decreases with increasing emission angle, the shape of the spectra remain unchanged. In this paper we present atmospheric models that include scattering to explain this effect. We show that the 5- μ m absorbing cloud particles must be significantly scattering ( ω = 0.9 ± 0.05 ) in order to explain these observations, and find that the base of the cloud layer must reside at pressures less than 2 bar. Furthermore, we show that the scattering within this cloud has important consequences on the retrieval of gas abundances from spectra in the 5- μ m region. © 2006 Elsevier Ltd. All rights reserved.