How speed of sound measurements could bring constraints on the composition of Titan's seas

TL;DR

This paper explores how measurements of the speed of sound in Titan's hydrocarbon seas can constrain their composition, using advanced equations of state and an inversion algorithm to improve accuracy in future probe missions.

Contribution

It introduces a detailed method for deriving Titan's sea composition from sound speed data using the PC-SAFT model and an original inversion algorithm, addressing previous simplistic models.

Findings

50 measurements between 90 K and 100 K yield better than 10% composition accuracy

The approach can effectively estimate N2, CH4, and C2H6 proportions in Titan's seas

Presence of propane influences the composition estimates

Abstract

The hydrocarbons seas of Titan, discovered by Cassini/Huygens mission are among the most mysterious and interesting features of this moon. In the future, a possible dedicated planetary probe, will certainly measure the speed of sound in this cryogenic liquid, as it was planned in the case of a Huygens landing into a sea. Previous theoretical studies of such acoustic measurements were based on rather simple models, leading in some cases to unphysical situations. Employed in a vast body of chemical engineering works, the state of the art PC-SAFT model has been recently introduced in studies aimed at Titan. Here, I revisit the issue of the speed of sound in Titan's liquids, in the light of this theory. I describe, in detail, the derivation of the speed of sound from the chosen equation of state and the potential limitations of the approach. To make estimations of the composition of a ternary liquid mixture N2:CH4:C2H6 from speed of sound measurements an original inversion algorithm is proposed. It is shown that 50 measures between 90 K and 100 K are enough to ensure an accuracy of the derived compositions better than 10%. The influence of the possible presence of propane is also investigated.