Authors:
Terrile, Richard John
Abstract:
This study describes the design and construction of a 5 µm imaging system used at the Hale 5 m (200 inch) telescope to acquire high spatial resolution infrared images of Jupiter. These images, recorded in a spectral region clear of terrestrial and Jovian gaseous absorption, offer a unique look into the deep atmosphere and provide direct observational evidence for the existence of multiple layers of clouds in the Jovian atmosphere. Evidence of layering is provided by the observed trimodal nature and persistence of the 5 µm flux-frequency distr (...)
This study describes the design and construction of a 5 µm imaging system used at the Hale 5 m (200 inch) telescope to acquire high spatial resolution infrared images of Jupiter. These images, recorded in a spectral region clear of terrestrial and Jovian gaseous absorption, offer a unique look into the deep atmosphere and provide direct observational evidence for the existence of multiple layers of clouds in the Jovian atmosphere. Evidence of layering is provided by the observed trimodal nature and persistence of the 5 µm flux-frequency distribution of equal areas on the Jovian disk. This indicates that three distinct brightness temperatures have a higher probability of being observed than a continuum of temperatures, and that, despite significant observed variations in the lateral 5 µm cloud distribution, this phenomenon is a long term stable vertical cloud feature. Furthermore, the visible color differences correlate with areas of different 5 µm intensity, implying that the colors are due to reflection from areas of different chemistry or state at different levels in the atmosphere. Also, short time scales are observed for large 5 µm flux variations over extensive areas of the Jovian disk, supporting the concept that the redistribution of obscuring clouds accounts for the contrasts at 5 µm. Finally, the 5 µm limb-darkening and opacity models, derived from imaging and spectroscopic measurements, are consistent with multiple layering of clouds in the Jovian atmosphere. Further information about the Jovian clouds results from the combination of 5 µm spectroscopic and imaging data sets. From the shape of the 5 µm spectrum true maximum brightness temperatures are derived, corrected for the clearest regions in the Jovian atmosphere. Furthermore, from data on spectral line saturation, limits are placed on the 5 µm cloud reflectivity over the field of view of the spectrometer. With this information, combined with the knowledge of the spatial flux distribution from imaging, constraints are derived for the optical properties of the upper Jovian clouds. A three layer cloud model is developed which is consistent with all of the observational data at 5 µm. The three model cloud layers have cloud top temperatures of T_1 ≤ 190°K (presumably T_1 ≃ 140°K), T_2 = 228 ± 2°K and T_3 = 292 ± 8°K. The highest layer, found only over the white zones and red spots, has optical depth near unity and transmits radiation from deeper levels. This upper level has a mean 5 µm cloud reflectivity less than 0.4, while the whole central 25% of the disk has a mean reflectivity less than 0.1. The middle cloud deck is present under the upper level clouds and over the brown colored Jovian belts. This level is optically thick everywhere except in regions where blue-gray areas are visible. Here the middle level thins to a mean optical depth of about 2 and allows radiation from the deepest and hottest level to be detected.
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