An observational program designed to study the albedo and spectral reflectivity of the Galilean satellites of Jupiter was carried out during the 1969 opposition of Jupiter. A two-channel photoelectric photometer was used in conjunction with a high-speed pulse-counting data system to obtain and record the data, Narrowband interference filters (∆λ ~ 0.02µ) were used with ITT FW-118 (S-1) and FW-130 (S-20) phototubes to obtain spectral reflectivity curves from 0.3µ to 1.1µ. The 24-inch telescope on Mt. Wilson was used for most of the work but the 60-inch instrument was used for some observations, The results of the observationswere the following. 1) Spectral reflectivity curves from 0.3µ to 1.1µ for each satellite for many values of orbital phase angle and solar phase angle were obtained. 2) Spectral structure not resolved by broadband UBV work was found in J1's curve near 0.58µ and the similarity of the spectral reflectivity curves of J2, J3 and J4 was noted, 3) The very high geometric albedos of J1, J2 and J3, notedby Harris (1961), were confirmed. 4) The variation in brightness with orbital phase was confirmed for each satellite. 5) The spectral reflectivity was found tovary with the same period as the brightness, as indicated by UBV observations (Harris, 1961). 6) Variations in the spectral reflectivity of J1 and J2 beyond 0.6µ, not previously seen, were discovered. 7) The spectral form of the variation was found to be similar for each of the satellites, with the brighter side having a higherreflectivity in the blue and ultraviolet relative to 0.56µ than the darker side. 8) The eclipse brightening of J1 found by Binder and Cruikshank (1964) was confirmedat two wavelengths, 0.435µ and 0. 56fµ.
The conclusions drawn from these results and previous work are as follows. 1) J1 and J2 probably possess tenuous atmospheres while J3 and J4 probably do not. 2) All the satellites have significantly higher geometric albedos than Mercury, the moon or Mars, even allowing for large errors in the measurement of diameters. Of the satellites, J4 has a distinctly lower albedo and density than J1, J2 or J3. 3) The high geometric albedos and spectral reflectivities of the satellites can be explained by surfaces of silicate powders, possibly with considerable amounts of glassy material, having low opacities and some ingredient absorbing in the ultraviolet and blue, possibly Fe^(+++). However, the possibility of surfaces of frost or some combination of frost and rock cannot be completely evaluated without further laboratory study. 4) The similarity in the variation of spectral reflectivitywith orbital phase among the satellites suggests a similar cause for each. A simple model for J1's spectral variation suggests that some fraction of the bright side ofJ1 must be covered by a material with similar spectral reflectivity but higher albedo than the dark side (such as might be caused by particle size differences ora difference in the amount of the absorbing ingredient). The fraction of surface that must be covered and the exact form of the spectral reflectivity of the added material depends on the albedo chosen for this component. 5) The eclipse brightening observations at two wavelengths indicate that, if this effect is caused by the condensation of some volatile during the eclipse, the condensed materialmust have a very high geometric albedo, probably greater than unity. The simple model applied to the spectral variation, when applied to the eclipse brighteningdata, suggests that the condensed material is not gray in spectral character but has a lower reflectivity at 0.43µ than at 0.56µ.
