Structure of Titan’s evaporites

Abstract

Numerous geological features that could be evaporitic in origin have been identified on the surface of Titan. Although they seem to be water-ice poor, their main properties -chemical composition, thickness, stratification-are essentially unknown. In this paper, which follows on a previous one focusing on the surface composition (Cordier et al., 2013b), we provide some answers to these questions derived from a new model. This model, based on the up-to-date thermodynamic theory known as "PC-SAFT", has been validated with available laboratory measurements and specifically developed for our purpose. 1-D models confirm the possibility of an acetylene and/or butane enriched central layer of evaporitic deposit. The estimated thickness of this acetylene-butane layer could explain the strong RADAR brightness of the evaporites. The 2-D computations indicate an accumulation of poorly soluble species at the deposit's margin. Among these species, HCN or aerosols similar to tholins could play a dominant role. Our model predicts the existence of chemically trimodal "bathtub rings" which is consistent with what it is observed at the south polar lake Ontario Lacus. This work also provides plausible explanations to the lack of evaporites in the south polar region and to the high radar reflectivity of dry lakebeds.

Acetylene on Titan’s Surface

“…C 2 H 2 is highly soluble in Titan's liquid methane and ethane (Glein & Shock 2013;Cordier et al 2015;Singh et al 2015b), intermediate to the solubility of calcite and gypsum in water on Earth's surface. This property implies the existence dynamic surface processes: (1) erosion, through carving of fluvial channels and the formation of karstic lakes at high latitudes on Titan; (2) transport from the surface of continents after a rainstorm by surface runoff; and (3) sedimentation to the bottom of lakes and oceans.…”

“…It appears bright at all wavelengths and exceptionally bright at 5 μm, which has been interpreted as being due to enrichment in evaporite deposits (MacKenzie et al 2014). From thermodynamic calculations Cordier et al (2013Cordier et al ( , 2015 demonstrated that C 2 H 2 should be a major dissolved component in the lakes and should crystallize mainly in the upper part of evaporite deposits present in dried out depressions, possibly mixed with other organics such as butane due to the high solubility in Titan liquids.…”

“…The evaporation and condensation is suspected to be an important process to shape Titan's landscape and redistribute and/or segregate organics on Titan's surface Cordier et al 2015;Mckenzie et al 2014). However, drying-up of a liquid does not always involve the creation of an evaporite.…”

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

“…Moreover, I have checked that PC-SAFT reproduces the reference ternary mixture (N2, CH4, C2H6) of Gabis (1991) provided as supplementary data by Glein & Shock (2013). In the frame of this EoS, each considered species is characterized by the PC-SAFT three parameters, namely the segment diameter σ, the depth of the potential ǫ/kB, and the number of segments per chain m; the values of those parameters have all been given in Tan et al (2013);Cordier et al (2016). Beside this, the binary interaction parameters kij 's are also taken in this article.…”

Where Does Titan Sand Come From: Insight From Mechanical Properties of Titan Sand Candidates

“…Evaporites are also possible candidate materials for Titan sand (Barnes et al, 2015). Titan's evaporites may be made of acetylene, ethylene, or butane (Cordier et al, 2013(Cordier et al, , 2016Singh et al, 2017); however, these materials are not stable solids under room temperatures on Earth, and their solid mechanical properties also have not been measured under low temperatures.…”