Composition, seasonal change, and bathymetry of Ligeia Mare, Titan, derived from its microwave thermal emission

Abstract

For the last decade, the passive radiometer incorporated in the Cassini RADAR has recorded the 2.2 cm wavelength thermal emission from Titan's seas. In this paper, we analyze the radiometry observations collected from February 2007 to January 2015 over one of these seas, Ligeia Mare, with the goal of providing constraints on its composition, bathymetry, and dynamics. In light of the depth profile obtained by Mastrogiuseppe et al. (2014) and of a two-layer model, we find that the dielectric constant of the sea liquid is <1.8, and its loss tangent is < 3:6 þ4:3 À2:1 Â10 À5 . Both results point to a composition dominated by liquid methane rather than ethane. A high methane concentration suggests that Ligeia Mare is primarily fed by methane-rich precipitation and/or ethane has been removed from it (e.g., by crustal interaction). Our result on the dielectric constant of the seafloor is less constraining < 2:9 þ0:9 À0:9 À Á , but we favor a scenario where the floor of Ligeia Mare is covered by a sludge of compacted and possibly nitrile-rich organic material formed by the deposition of photochemical haze or by rain washing of the nearby shores. We use these results to produce a low-resolution bathymetry map of the sea. We also estimate the temperature variation of the bulk sea between February 2007 and July 2013 to be <2 K, which provides a constraint on its net evaporative cooling currently being explored in ocean circulation models. Lastly, we suggest a lag in the summer warming of the northern polar terrains.

Modeling of Seasonal Lake Level Fluctuations of Titan's Seas/Lakes

“…Thus, equation contains P , R , and E but no S and G . This case is based on Figure 11 of Le Gall et al (), who suggested an organic sludge layer with strong cohesion on the lakebed of deep lakes.…”

Titan's atmosphere and climate

“…Neish and Lorenz [] find that Titan's crater distribution indicates widespread wetlands of liquid hydrocarbons at low elevations over much of geologic time. As the north pole moves into summer, the warming is lagging predictions potentially indicating that the north polar region is saturated with liquid [ Jennings et al , ; Le Gall et al , ]. Additionally, there is some evidence of low‐latitude lakes, which are not stable and would require subsurface replenishment [ Griffith et al , ].…”

“…The distribution and composition of evaporites contain information about Titan's climate history, but will require composition measurements that cannot be obtained with Cassini. Measurements [ Le Gall et al , ] and recent models [ Glein and Shock , ; Tan et al , , ] agree that the northern lakes and seas appear to be methane rich, contrary to prior predictions that they should be dominated by ethane [ Cordier et al , , ]. Ethane has been detected in Ontario Lacus from VIMS measurements [ Brown et al , ], and from RADAR Ontario Lacus appears to contain a greater fraction of ethane than the northern lakes and seas [ Hayes , ].…”

Electrical Properties of Tholins and Derived Constraints on the Huygens Landing Site Composition at the Surface of Titan

“…Likewise, the DC conductivity of polyacetylene at 200 K is at most 10 −7 S/m and shows a linear decrease with temperature (Jones et al, ), preventing it from being a major component of this layer. On the other hand, some polymers, such as polyaniline, or in general, nitrile‐rich organic compounds, may be more conductive but their accurate electrical properties remain unknown (Le Gall et al, ). Also, nanophase iron and nanophase hematite which are present in Saturn's rings and on the surface of Iapetus (Clark et al, ) may have relatively high conductivity at low frequencies/low temperatures: Nikolic et al () measured the DC electrical conductivity of hematite doped with Zn, and by extrapolating their results at Titan's surface temperature, the maximum conductivity is found to be 10 −8 S/m; therefore, the possibility that hematite (doped with 10% Zn) as a major component of this layer cannot be excluded but is very low (the highest conductivity value of this component is only slightly higher than the lowest possible conductivity of this layer; additionally, this only applies to hematite with high Zn content, higher than 10%).…”