Identification of water frost on Callisto

A description is presented of broadband (J, K, and L) and narrowband (3.0-3.8 micrometers) observations of Ganymede and Callisto which were made on the nights of November 7 and November 29, 1976 with a 28-inch infrared telescope. Based on the previous identification of water frost on the surface of Ganymede from shorter wavelength data, it is concluded that there is also water frost on the surface of Callisto, taking into account the presence of similar bands for both satellites in the spectral range from 3 to 4 micrometers.