Composition, seasonal change and bathymetry of Ligeia Mare, Titan, derived from its 2.2-cm thermal emission

Abstract : For the last 10 years, the Cassini RADAR has been exploring Saturn’s moon Titan, the only planetary body besides Earth whose surface presently exhibits significant accumulations of liquids in the forms of lakes and seas. In particular, the passive Radiometer that is incorporated in this instrument has been recording the 2.2 cm-wavelength thermal emission from Titan’s three seas. Radiometry observations provide new information beyond the active radar reflection data. In this paper, we analyze the radiometry observations collected from Feb. 2007 to July 2013 over one of these seas, Ligeia Mare, with the goal of providing constrains on its liquid composition, seafloor nature, bathymetry, and dynamics. In light of the two-layer model we have developed for this analysis, we find that the dielectric constant of the sea liquid is most likely smaller than 1.8, suggesting that the composition of Ligeia Mare is dominated by liquid methane rather than liquid ethane (although a ternary methane-ethane-nitrogen mixture cannot be ruled out). This result is further supported by the value we infer for the liquid loss tangent of 3-5×10-5. This value is in agreement with the one first published by Mastrogiuseppe et al. (2014) based on active radar observation. A high methane concentration suggests that Ligeia Mare is either a sea from which ethane has been removed by crustal interaction, or a sea primarely fed by methane-rich precipitation, or both. For the seafloor, a dielectric constant of 2.6-2.9±0.9 is determined. Though this result is not very constraining, 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. These results are then used to convert the radiometry mosaic of Ligeia Mare into a qualitative low-resolution bathymetry map. Lastly, we establish limits on the physical temperature variation of the sea between Feb. 2007 to July 2013, namely less than 2 K, providing a constraint on the relative amounts of solar heating and evaporative cooling currently being explored in ocean circulation models.
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Submitted on : Friday, December 18, 2015 - 8:05:49 PM
Last modification on : Sunday, July 28, 2019 - 11:58:02 AM

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  • HAL Id : insu-01246645, version 1

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Alice Le Gall, Michael Malaska, Ralph Lorenz, Michael Janssen, Tetsuya Tokano, et al.. Composition, seasonal change and bathymetry of Ligeia Mare, Titan, derived from its 2.2-cm thermal emission. AGU Fall Meeting 2015, Dec 2015, San Francisco, United States. pp.P12B-03. ⟨insu-01246645⟩

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