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In radiometric mode, PPR is a single-element bolometer with a circular field of view that is 2.5 mrad in diameter, which maps thermal emission from its targets by raster scanning of the Galileo scan platform on which it is mounted [E. E. Russell et al., Space Sci. Rev. 60, 531 (1992)]. Absolute calibration is provided by an on-board blackbody reference, prelaunch calibration, and observations of deep space and has errors that are currently ≤3 K. Problems with the filter wheel have limited the choice of filters on each orbit. The maps presented in Fig. 1 were produced by median-filtering the PPR observations to reduce radiation noise and then averaging all PPR observations whose fields of view overlap each point on the map.
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-0.25, or 1.027.
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0344707656
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The open filter is sensitive to Jupiter-shine reflected from Europa's nightside in addition to thermal radiation. Calculations based on daytime observations show that the Jupiter-shine contribution to apparent brightness temperatures will be <0.3 K before dawn and <1.0 K after sunset. Because albedo on scales of 100 km or larger varies by less than a factor of 2 on Europa, apparent local postsunset temperature variations due to local albedo variations will be <0.5 K. Tests using Voyager Europa spectra show that broadband and 27.5-μm brightness temperatures generally agree to ≤1 K.
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note
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"Postsunset" observations cover a range of local time (measured in degrees of rotation after midnight) of 270° through 345°. from 0 to 18 hours after sunset. Similarly, "predawn" observations cover local times of 30° through 90°, from 14 to 0 hours before sunrise. At the equator, europan day and night each last 1.775 Earth days.
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23
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0344707655
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The Voyager images used by McEwen (19) at longitudes 0° through 75°W were taken at solar phase angles of 3° and 12° and do not show bolometric albedo, which accounts for light reflected in all directions. Variations in light-scattering properties at a high solar phase angle might produce variations in bolometric albedo that are not apparent in these images, but unrealistically large scattering variations would be required to reverse the trend of albedo with latitude, as would be required if albedo was the direct cause of the observed postsunset temperature variations.
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The resolution (12 km per pixel) of the best Galileo images of this area is sufficient to distinguish major ridge systems and show the location of chaotic terrain, which is revealed by its low albedo and mottled appearance.
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J. E. VanCleve, in preparation. For a surface heated only by sunlight, the maximum possible equatorial nighttime temperature (assuming zero albedo and infinite thermal inertia) is 130 K, and warmer nighttime temperatures would be an unambiguous sign of endogenic activity.
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39
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W. C. Tittemore and W. M. Sinton, Icarus 77, 82 (1989). This event, if real and thermal in origin, implied a region with a diameter of 176 km at 273 K or a region of 52 km at 350 K.
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0345550994
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note
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This work was supported by the Galileo project and NASA grant NAG5-6794. We thank L. Barnard and O. Liepack for assistance with design and implementation of the observation sequences and the entire Galileo team at the Jet Propulsion Laboratory for making the Galileo mission a success. The paper benefited from comments by R. Pappalardo, P. Geissler, and D. Domingue.
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