Modeling early Titan's ocean composition

Abstract

We model the stability fields of minerals in contact with an interior water-ammonia ocean on Titan. Using constraints on the ammonia abundance from Titan's orbital state, and updated kinetic data, we show that earlier pre-Cassini work by Engel et al. (1994) was qualitatively but not quantitatively correct. We calculate a salinity of the ocean of 1%, consistent with the ocean density derived from Cassini data on Titan's tidal response, and within the range found for Enceladus' ocean. We also consider the sources and sinks of the observed 40Ar in the atmosphere, and conclude that what is observed is only a small fraction of what might be dissolved in the ocean or trapped in clathrate in the crust. Thus, the efficiency of degassing from the core must be much higher than in previous estimates (e.g., McKinnon, 2010) to account for what is observed in the atmosphere. One self-consistent model is that of a partially hydrated core from which much of the potassium has been leached into the ocean, as suggested by the thermal model of Castillo-Rogez and Lunine (2010). Our picture of present-day Titan is that of a slightly salty ocean—about 1% salinity— atop an actively heated and hydrated rock core.