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Astron. Astrophys. 324, 357-365 (1997)

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6. Notes on absolute calibrations

Outside the deep methane bands the absolute fluxes have been calibrated using 1993 albedos by Karkoschka (1994) and solar fluxes from Neckel and Labs (1984). However, for the jovian planets, the use of disk albedos from different dates at methane bands may not be appropriate. In the case of Saturn, remarkable reflectivity changes at equatorial latitudes have been reported recently (Ortiz et al., 1995), and we have also found changes at the equatorial latitudes in Jupiter, from 1993 to 1994 (see Fig. 6). The equatorial band is broader and more reflective during 1992 and 1993 than the 1994 observation. Comparing our absolutely calibrated images from 1992 (Moreno et al., 1993) with non flux-calibrated images from 1993, and 1994, we conclude that there is a decrease in the albedo of Jupiter at 892 nm. We have estimated this change using central meridian scans from 1993 and 1994, scaled to the latitudes where the reflectivity is nearly constant during the 3 different years. We chose that region by normalizing each image (that is, dividing the data numbers at each pixel by the total counts from Jupiter) taking a Central Meridian scan from each one and comparing the plots. This, to first order, helps us identify the regions with lowest variability. By integrating the reflectivities in Fig. 7 (removing the contribution from the G impact scar), our conclusion is a 5% decrease from 1993 to 1994. This is in agreement with Moreno et al. 1995 who found a small decrease, or no change from 1993 to 1994, using the jovian satellites as a reference, but not the 10% increase suggested by West et al. (1995). We have used a lower albedo (5%) than that of Karkoschka (1994), for the 892-nm filters in 1994.

[FIGURE] Fig. 6. Images at 892 nm from three consecutive years (from left to right: 1992,1993,1994). The image from 1994 shows the D and G impact areas close to the Central Meridian. The bright area at the terminator is the H impact scar and the bright spot at the upper left is satellite Ganymede. There is a clear temporal variation in reflectivity at equatorial latitudes
[FIGURE] Fig. 7. Comparison of Central Meridian scans from 1993 and 1994
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© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998