September 14, 2006 | David F. Coppedge

Ethane Cloud at Titan: Too Little, Too Late?

Those following the Titan exploration by Cassini-Huygens have wondered where the ethane went.  Oceans of ethane hundreds of meters deep, if not kilometers deep, were predicted but not found, as reported previously (see 04/25/2003 and 10/16/2003 pre-Huygens reports, 01/15/2005 and 01/21/2005 Huygens early results, and 12/05/2005 review; see also New Scientist analysis of the “total revolution” in thinking about Titan going on because of the lack of oceans).  Now, finally, Cassini found something at least: a cloud of ethane at the north pole (see Cassini press release)  Is it enough to blanket the embarrassment of finding Titan to be a dry, young surface with only trace amounts of ethane?
    The findings made by Cassini’s Virtual and Infrared Mapping Spectrometer (VIMS) were published in Science this week,1 and described in a Perspectives article by Cassini atmospheric scientist Mike Flasar in the same issue of Science.2  Indeed, a “vast tropospheric cloud” of ethane was observed over the north pole in a series of observations between Dec. 2004 and Sept. 2006 (this cloud contrasts with the south-polar cloud believed composed of methane).3  The abstract leads one to believe this solves the problem of the missing ethane:

The derived characteristics indicate that this cloud is composed of ethane and forms as a result of stratospheric subsidence and the particularly cool conditions near the moon’s north pole.  Preferential condensation of ethane, perhaps as ice, at Titan’s poles during the winters may partially explain the lack of liquid ethane oceans on Titan’s surface at middle and lower latitudes.

That word “partially” is key, though, as shown in subsequent discussions in the paper.  First, they restate the problem in stark terms, to show the seriousness of the prediction that failed:

Methane (the second most abundant atmospheric constituent after nitrogen) is dissociated irreversibly by solar ultraviolet light, producing primarily ethane and, at one-sixth and one-10th of the ethane production rate, respectively, acetylene and haze, as well as other less abundant organic molecules.  These photochemical by-products precipitate to Titan’s surface.  Titan’s atmospheric composition and photochemical models indicate that ethane accumulates as a liquid (at the equatorial surface temperature of 93.5 K) at a rate of ~300 m (if global) over Titan’s lifetime of 4.5 billion years, whereas solid sediments, including acetylene and haze particles, accumulate at roughly one-third of this rate.  Thus, unless methane is a recent addition to Titan’s atmosphere or ethane incorporates itself into surface solids, it has been reasoned that a considerable fraction of the surface should be covered with liquid ethane.  Titan’s surface reveals dunes of solid sediments, probably including haze particles and acetylene ice.  In addition, the surface is riddled with alluvial features, suggesting the occurrence of methane rain in the past.  Craters are rare, indicating geological relaxation as well as their burial by photochemical sediments.  Yet Titan appears depleted of its most abundant photochemical by-product.  Except for the ethane-damp surface measured by Huygens, no condensed form of ethane has been detected, despite its rapid production in Titan’s stratosphere and the expectation of finding ethane-rich oceans before the Cassini encounter.

The same is admitted by University of Arizona’s Lunar and Planetary Lab in a press release (see EurekAlert): “Ethane is by far the most plentiful byproduct when methane breaks down,” it states.  “If methane has been a constituent of the atmosphere throughout Titan’s 4.5-billion-year lifetime — and there was no reason to suspect it had not — the large moon would be awash with seas of ethane, scientists theorized.”  That’s why most artist conceptions were amply illustrated with liquid before Cassini found a dry desert (05/04/2006) with a few possible lakes at high latitudes (07/24/2006).
    The science team tried to get a fix on the composition of the cloud and the mass of ethane within it from the four oblique views obtained by VIMS.  Best guess by inferring spectral properties, particle sizes and altitude is that the cloud is indeed ethane, that condenses at 30-50 km above the pole and precipitates down, falling at 3 km per month.  “If conditions remain cool enough throughout the year,” they infer, “Titan may accumulate ethane ice each winter at the poles and develop year-round polar caps.”  Direct evidence of an ethane ice cap will have to await future flybys by Cassini (perhaps the high-latitude pass on October 9).  What about the south pole, though, which has been imaged?  No simple answers, unfortunately, and we’ll have to wait to find out, but what we know is not that convincing yet:

Presently, there is no direct evidence of polar caps composed of ethane.  The northern pole has not been imaged.  Cassini images of the southern pole do not indicate the morphology of 2 km of ethane ice, assuming current rates of ethane production over the past 4.5 billion years, accumulated within 35° of the poles.  Yet south polar images suggest flow features, possibly associated with a smaller quantity of ethane ice accumulated on the young surface.  The detection of surficial ethane ice is hindered by the correlation of ethane features and methane signatures, which obscure the visibility of Titan’s surface.  In addition, the polar surface is probably distinct and varied.  Similarly, other hydrocarbons would precipitate preferentially at the poles and pollute the ethane ice, and any lowland methane lakes would dissolve and melt ethane, because the mixture’s eutectic temperature is 72.5 K.  Such lakes might condense out of Titan’s humid lower troposphere during winter.  The surface distribution of liquid or solid ethane, whether corralled into the polar regions by circulation or transported by surface flows to lower latitudes, will be determined with radar and near-infrared images of the geomorphology, radio determinations of the polar temperatures, and infrared measurements of the polar composition, which are scheduled for future Cassini encounters with Titan.

What does Dr. Flasar think of all this?  His commentary focuses primarily on the atmospheric circulation on Titan.  At the end, his reference to this problem is delicately understated:

Until now, clouds of the most abundant product of methane dissociation, ethane, have eluded detection.  The Griffith et al.  identification of polar ethane clouds is reassuring, in that it validates the basic ideas we have about Titan’s meteorology and chemistry: first, that condensation does occur, as expected, in the lower stratosphere, and second, that the inferred altitudes of the ethane cloud (30 to 50 km) are consistent with subsidence in the winter polar region.  This and other clues that we will obtain will help us to sort out the things we still puzzle over.

The biggest puzzle on his mind is, undoubtedly, where is all the ethane?  It’s not enough to answer that it just stacks up at the poles.  The U of A press release spoke with lead author Caitlin Griffith, and reported, “If ethane was produced at today’s rate over Titan’s entire lifetime, a total of two kilometers of ethane would have precipitated over the poles.  But that seems unlikely, Griffith said.”  Why?  The south pole, which should roughly match the north pole, shows no such ethane ice cap.


1Griffith et al., “Evidence for a Polar Ethane Cloud on Titan,” Science, 15 September 2006: Vol.  313. no. 5793, pp. 1620-1622, DOI: 10.1126/science.1128245.
2F. M. Flasar, “Planetary Science: Titan’s Polar Weather,” Science, 15 September 2006: Vol. 313. no. 5793, pp. 1582-1583, DOI: 10.1126/science.1130698.
3Methane, CH4 is exposed to the solar wind in the upper atmosphere of Titan, a continuous process that strips off hydrogen atoms.  The ionized methyl groups CH3 rapidly recombine into ethane C2H6, which has nowhere to go but down.  At surface temperatures on Titan, it should condense as a liquid, or as ethane snow at higher latitudes where it is colder.  Extrapolating the current ethane production rate for the assumed age of the solar system (4.5 billion years) should have yielded deep oceans of ethane at least 300 meters deep, but probably much deeper if methane were more abundant in the past, as is commonly believed.

The problem vanishes when you liberate your mind from the requirement of billions of years.  This is really uncanny.  Everything about Titan screams young, but nowhere do you find anyone questioning the linchpin assumption that Titan is 4.5 billion years old.  It’s almost like they want to downplay this “puzzle” and sweep it under the rug.  Don’t let them.  Most creationists could live with a young or old Titan, though some would prefer the former.  The only people who would be completely scandalized by a young Titan are the evolutionists who absolutely depend on billions of years in which to hide their skeletons.
    The ethane cloud is too little, too late.  After all, vast quantities of ethane must be there.  Dr. Flasar admitted it: he was reassured that the ethane “condensation does occur” and that the basic ideas about Titan’s chemistry have been validated.  It follows logically that this process, going on interrupted for billions of years, would leave the evidence in abundance.  OK, so where is it?  They can’t hide the answer in the future much longer.  Imagine the entire globe covered with an ethane ocean half a mile deep or more.  What happened to it?  Did the interior suck it up like a sponge?  Did aliens take it?  Did creationists steal in a vast conspiracy to support their young-earth views?  Come on; let the facts speak to the unbiased mind.

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Categories: Physics, Solar System

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