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Mystery of the missing waves on Titan
Mongo
post Jul 27 2013, 01:01 PM
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Mystery of the missing waves on Titan

[ ... ] something has been bothering Alex Hayes, a planetary scientist on the Cassini radar team at Cornell University.
If Titan is really so wet, he wonders, “Where are all the waves?”
Here on Earth, bodies of water are rarely still. Breezes blowing across the surface cause waves to ripple and break; raindrops striking sea surfaces also provide some roughness.
Yet on Titan, the lakes are eerily smooth, with no discernable wave action down to the millimeter scale, according to radar data from Cassini.
“We know there is wind on Titan,” said Hayes. “The moon's magnificent sand dunes [prove] it.”
Add to that the low gravity of Titan – only 1/7th that of Earth – which offers so little resistance to wave motion, and you have a real puzzle.
Researchers have toyed with several explanations. Perhaps the lakes are frozen. Hayes thinks that is unlikely, however, “because we see evidence of rainfall and surface temperatures well above the melting point of methane.”
Or maybe the lakes are covered with a tar-like substance that damps wave motion. “We can't yet rule that out,” he added.
The answer might be found in the results of a study Hayes and colleagues published in the July 2013 online edition of the journal Icarus.
Taking into account the gravity of Titan, the low viscosity of liquid hydrocarbons, the density of Titan's atmosphere, and other factors, they calculated how fast wind on Titan would have to blow to stir up waves: A walking-pace breeze of only 1 to 2 miles per hour should do the trick.
This suggests a third possibility: the winds just haven’t been blowing hard enough. Since Cassini reached Saturn in 2004, Titan’s northern hemisphere (where most of the lakes are located) has been locked in the grip of winter. Cold heavy air barely stirs, and seldom reaches the threshold for wave-making.
But now the seasons are changing. In August 2009 the sun crossed Titan’s equator heading north. Summer is coming, bringing light, heat and wind to Titan's lake country.
“According to [climate models], winds will pick up as we approach the solstice in 2017 and should be strong enough for waves,” he said.
If waves appear, Cassini should be able to detect them. Radar reflections from wavy lake surfaces can tell researchers a great deal.
Wave dimensions, for instance, may reveal the viscosity of the underlying fluid and, thus, its chemical composition.
Also, wave speeds would track the speed of the overlying winds, providing an independent check of Titan climate models.
Hayes is excited about “bringing oceanography to another world. All we need now,” he said, “are some rough seas.”
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.
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rlorenz
post Jul 27 2013, 11:02 PM
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QUOTE (Mongo @ Jul 27 2013, 09:01 AM) *
Saturn’s moon Titan has lakes and seas of liquid hydrocarbon and a dense atmosphere, an environment conducive to generating wind waves.


I recently made a short review of oceanography on Titan, see
http://www.lpl.arizona.edu/~rlorenz/Oceans2013.pdf
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Den
post Sep 15 2013, 10:14 AM
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QUOTE (rlorenz @ Jul 28 2013, 12:02 AM) *
I recently made a short review of oceanography on Titan, see
http://www.lpl.arizona.edu/~rlorenz/Oceans2013.pdf


In the PDF, fig.4 caption is a copy of fig.3 caption. Must be a mistake.
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titanicrivers
post Jun 23 2014, 06:07 PM
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This paper just published yesterday http://www.nature.com/ngeo/journal/vaop/nc...l/ngeo2190.html describes some evidence of waves on Titan’s Ligeia Mare. Some of the data appeared at LPSC 2014 http://www.hou.usra.edu/meetings/lpsc2014/pdf/1841.pdf . Important data from the T92 flyby http://www.ciclops.org/view/7657/Rev194 where radar was prime was used to make the case for waves or rising bubbles or floating solids on Titan. Sorry I can only reference the abstracts (help Mike!?)
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ngunn
post Jun 23 2014, 06:34 PM
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I expect it will turn up here in due course (there is already a great one on specular reflection recently posted): http://www.google.co.uk/url?sa=t&rct=j....69411363,d.d2k
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