DIELECTRIC PROPERTIES OF CRYOGENIC LIQUID ALKANES, AND THE FATE OF PROPANE IN SEAS ON TITAN

We have determined the complex dielectric constants of liquid alkanes methane, ethane, and propane at ~90K, using a microwave resonant cavity system operating at ~14 GHz. These measurements are for materials at conditions analogous to Cassini RADAR observations of lakes and seas on Titan. The loss tangents of propane, ethane and methane are found to have a ratio of ~250:4.2:1.0.

Models predict an ethane:propane atmospheric production ratio of ~10:1 (e.g. Lavvas et al., 2008, Planet. Space Sci. 56; Krasnopolsky, 2014, Icarus 236; and references therein), which is approximately consistent with fits to CIRS atmospheric spectra (e.g. Nixon et al., 2009, Planet. Space Sci. 57; Coustenis et al., 2010, Icarus 207). If propane and ethane are present in Titan’s seas at this ratio, then propane absorption would dominate over that of ethane, and therefore for any observed loss tangent (Mastrogiuseppe et al., 2014, Geophys. Res. Lett. 41) the sea would be more methane rich than previous estimates deduced from loss tangent measurements have suggested (Mitchell et al., 2015, Geophys. Res. Lett. 42). Such a compositional ratio is challenging to rationalize with equilibrium compositions determined by past authors (e.g. Tan et al., 2013, Icarus 250) and the detection of ethane in Titan’s seas (Brown et al., 2008, Nature 454).

The existence of an extensive subsurface alkanofer system in contact with crustal ice, at depth, might provide at least a partial explanation for this apparent paradox. We propose that our findings, in the light of remote sensing determinations of the bulk dielectric constants of Titan seas, are consistent with suggestions that higher order alkanes readily substitute for lower order hydrocarbons into clathrate ices (Mousis et al., 2016, Icarus 270, and references therein).

This research was carried at out the California Institute of Technology Jet Propulsion Laboratory under a contract with NASA.