Evaporation des lacs de Titan : formation d’évaporites Daniel Cordier Journée OSU THETA, 18 Octobre, 2013 CNRS - Institut UTINAM de Besançon (France) OSU THETA Co-workers: Jason W. Barnes, Abel Ferreira and Thomas Cornet

Titan L’objet : Intérêts :  Principal satellite de Saturne (distance au Soleil : 10 UA, rayon : 2576 km)  Atmosphère dense (pression au sol : 1.5 bar)  Température à la surface : 93.7 K ( °C, azote liquide : -196 °C)  compréhension de la structure et de la dynamique.  Prototype d’un berceau d’une éventuelle forme de « vie exotique ».  Informations sur la formation du Système Solaire. Exploration :  Mission Cassini-Huygens (lancement 1997, arrivée 2004, Huygens sur Titan : 2005).

Photochimie, cycle du méthane, pluies exotiques  Photochimie dans la haute atmosphère  brume, aérosols  Pluies « exotiques » : éthane, … Prédiction de l’existence d’un océan global ( Flasar 1983, Lunine et al. 1983)

Les lacs de Titan  Instrument : RADAR de Cassini (Synthetic Aperture Radar Imager [SAR]).  Stofan et al., Nature, (2007) : découverte des lacs.  Réflexion spéculaire : Stephan et al., GRL, 2010

GREY SCALE: RADAROBSERVATION COLORS: INFRA-REDOBSERVATION What are we talking about? Titan’s dry lakebeds probable evaporites What would be the chemical composition of these evaporites ? Barnes et al. (2011)

Ingredients of our model Which solid species could be dissolved in titan’s lakes liquid phase? Which solid species could be dissolved in titan’s lakes liquid phase? How to manage the evaporation of the liquid? How to manage the evaporation of the liquid? What quantities of the considered species can we dissolve in the liquid? What quantities of the considered species can we dissolve in the liquid?

Composition of the snow SpeciesAbundances HCN53.8 % C 4 H % C2H2C2H % CH 3 CN1.8 % CO % C6H6C6H6 0.4 % Mainly from Lavvas et al. (2008)

How to manage the evaporation of the solvent? What is the solvent? What is the solvent? CH 4, C 2 H 6 and N 2 Evaporation rates? Evaporation rates? Not well known & depends on conditions Trick: consider the rates constant = new definition of the time t

What quantities of the considered species can we dissolve? : activity coefficient (from ideal solution, RST, …) : enthalpy of melting : temperature of melting X i,sat : maximum mole fraction of dissolved species i

Model summary Dissolution (solid-liquid equilibrium) Dissolution (solid-liquid equilibrium) Matter conservation Matter conservation + composition of the snow (dissolved species) + composition of the solvent

Which initial chemical composition could we adopt? Type 1 Type 2 Abundances of dissolved solids scaled to their atmospheric precipitation rates = scaled to snow composition uniform abundances of dissolved solids

Results for an ideal solution Type 1 mixture Type 2 mixture ~65 % of C 4 H 10 ~32 % of C 2 H 2 ~ 2 % of CH 3 CN ~47 % of C 4 H 10 ~47 % of C 2 H 2 ~ 4 % of CH 3 CN As the solution is ideal the results do not depend on the respective amounts of CH 4, C 2 H 6 and N 2 in the solvent

Results for an NON-ideal solution mole fraction of C 4 H 10 remains in the range 50 % to 67 % mole fraction of C 4 H 10 remains in the range 50 % to 67 % mole fraction of C 2 H 2 remains in the range 30 % to 50 % mole fraction of C 2 H 2 remains in the range 30 % to 50 %

Discussion The influence of the N 2 content in the liquid remains marginal The influence of the N 2 content in the liquid remains marginal High T (i.e. 95 K!) favors high C 4 H 10 abundances High T (i.e. 95 K!) favors high C 4 H 10 abundances What is robust/uncertain in this work? What is robust/uncertain in this work? o ROBUST: species with high solubility should be in the surface layer o UNCERTAIN: dissolution theory & turbidity Cordier, Barnes & Ferreira, Icarus, 2013, 226, Astro-ph: arxiv.org/abs/ Take-home point: Titan’s evaporites could be rich in C 4 H 10 and C 2 H 2 Take-home point: Titan’s evaporites could be rich in C 4 H 10 and C 2 H 2

Développements Détermination des taux d’évaporation de CH 4 et C 2 H 6 (coll. Université de l’Arkansas, Univ. Toronto) Détermination des taux d’évaporation de CH 4 et C 2 H 6 (coll. Université de l’Arkansas, Univ. Toronto) Détermination des épaisseurs des dépôts, nature des « plages » (coll. Univ. Idaho, Univ. Cornell) Détermination des épaisseurs des dépôts, nature des « plages » (coll. Univ. Idaho, Univ. Cornell) Détermination des solubilités des solides organiques dans CH 4 liquide (UTINAM) Détermination des solubilités des solides organiques dans CH 4 liquide (UTINAM) Formation des dépressions lacustres (coll. ESA Madrid) Formation des dépressions lacustres (coll. ESA Madrid)