Europa's Limb Brightening: Whole Disk Color Observations from Galileo

B.E. Clark, P. Helfenstein, J. Veverka (Cornell), C.B. Phillips, A.E. McEwen (LPL), Galileo Imaging Team

In September 1996 Galileo obtained three images (416, 559, and 989 nm) of Europa's trailing hemisphere at a resolution of 6.9 km/pixel. These data provide a sensitive test of the currently accepted models of Europa's photometric behavior derived by Domingue et al. (1991) and Domingue and Hapke (1992), from analyses of ground-based and Voyager data. We find that whereas the global model does fit the Galileo data at 989 nm, it does not reproduce the observed limb-to-terminator brightness variations at 416 and 559 nm. At these shorter wavelengths, the limbs are significantly brighter, and the center of the disk is significantly darker, than predicted. Several effects are under consideration and will be presented including; 1) albedo variations due to exogenic surface modification, shown by McEwen (1986) to be wavelength dependent, and 2) parameter modification of the Domingue et al. model.


FIGURE: Photometric equatorial scan of Galileo (GLL) images s0360063913 (VLT) and s0360063900 (GRN) compared to the Domingue et al. models (1991). The phase angle is 2.1 degrees.