Interior and Surface of Titan

Abstract

Few statements can be made with certainty about Titan's interior. The average density 1.88 g cm^(-3) and radius 2575 km (Lindal et al. 1983) are neatly bracketed by the corresponding values for Jupiter's satellites Ganymede and Callisto (Table VII), bodies that are believed to consist of rock and water ice (25-50% by mass) but probably no other ices (Consolmagno and Lewis 1976; Cassen et al. 1980). The term ' rock' refers to silicates and iron (as sulfide, oxide, and/or elemental metal). The large uncertainty in ice content arises because it is not known whether the silicates are hydrated. The similarities between the three satellites listed in Table VII might lead one to suspect that they have similar interior properties. Table VII shows why this would be a premature conclusion: other candidate ices, most notably NH_3 • H_2O and CH_4 • nH_2O, have very similar densities to water ice. If Titan formed at lower temperatures than Ganymede or Callisto then it may have incorporated one or more of these more volatile ices without greatly affecting its average density. Quantification of this possibility cannot be carried out yet with confidence because the densities of these additional ices at relevant internal pressures (up to ~40 kbar), and the processes determining the ratio of silicate to ice in large satellites, are poorly known. All three satellites in Table VII are silicate-enriched relative to cosmic abundances, which predict 60% water ice and 40% anhydrous chondritic rock by mass. The enrichment is even more striking if other ices are included (see Anders and Ebihara 1982, for the most recent compilation of cosmic abundances).

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