Wind driven capillary-gravity waves on Titan's lakes: Hard to detect or non-existent?
Saturn's moon Titan has lakes and seas of liquid hydrocarbon and a dense atmosphere, an environment conducive to generating wind waves. Cassini observations thus far, however, show no indication of waves. We apply models for wind wave generation and detection to the Titan environment. Results suggest wind speed thresholds at a reference altitude of 10 m of 0.4–0.7 m/s for liquid compositions varying between pure methane and equilibrium mixtures with the atmosphere (ethane has a threshold of 0.6 m/s), varying primarily with liquid viscosity. This reduced threshold, as compared to Earth, results from Titan's increased atmosphere-to-liquid density ratio, reduced gravity and lower surface tension. General Circulation Models (GCMs) predict wind speeds below derived thresholds near equinox, when available observations of lake surfaces have been acquired. Predicted increases in winds as Titan approaches summer solstice, however, will exceed expected thresholds and may provide constraints on lake composition and/or GCM accuracy through the presence or absence of waves during the Cassini Solstice Mission. A two-scale microwave backscatter model suggests that returns from wave-modified liquid hydrocarbon surfaces may be below the pixel-scale noise floor of Cassini radar images, but can be detectable using real-aperture scatterometry, pixel binning and/or observations obtained in a specular geometry.
© 2013 Elsevier Inc. Received 3 March 2012. Revised 12 March 2013. Accepted 10 April 2013. Available online 19 April 2013. A.G.H. would like to the thank the Miller Institute for Basic Research in Science for partially funding this work. R.L., J.I.L., and P.E. were partially supported by the Cassini Project (for R.L. via NASA Grant NNX13AH 14G). T.S. and S.D.G. acknowledge support from a NASA Earth and Space Science Fellowship and a David and Lucile Packard Fellowship. We would especially like to thank and acknowledge H.E. Schlichting of the Massachusetts Institute of Technology for her advice and contributions to this work and Richard Lovelace of Cornell University, whose inquiries supplied the motivation for this project. All authors would also like to thank the Cassini engineering team, without whom this manuscript would not have been possible.