Identification of water frost on Callisto

Abstract

THE 3.1 -µm absorption band of water frost has been tentatively identified in spectra of the Galilean satellites Ganymede and Callisto. This is the first positive evidence for water frost on the surface of Callisto. The depth of this absorption band on both satellites is about twice as deep as is inferred from the depths of the water frost bands at 1.5 and 2.0 µm. Previous high resolution spectral studies (Δλ/λ∼0.02) of the Galiliean satellites in the 1.0–2.5 μm region have identified water frost on the surfaces of Europa (J II) and Ganymede (J III) from the presence of characteristic water frost absorption bands at 1.5 and 2.0 µm^1–3. All studies failed to positively identify water frost on either Io (J I) or Callisto (J IV) but were used to set an upper limit of 5–25% (surficial coverage) for the amount of water frost on Callisto¹. The spectrum of water frost also has a characteristic band at about 3.1 µm which is stronger than the ones at shorter wavelengths^3–5. Previous studies have shown a decreased reflectance between 3 and 4 µm for Callisto, but water frost could not be positively identified because of low spectral resolution^6,7 and poor signal-to-noise and the problem of using the Moon as a standard because of the onset of lunar thermal emission beyond 2.5 µm (refs 1,2).