October 18, 2006 | David F. Coppedge

New Titan Ethane Theory Proposed

They wonder where the ethane went (see 09/14/2006 and its links).  The case of the missing ethane on Titan has only gotten more puzzling since the Huygens Probe landed last year and found almost none, when oceans a mile deep were anticipated.  In Nature last week,1 D. M. Hunten (U of Arizona) posited a new idea.  It went into smust.  Smust, like smores but not as tasty, is a contraction of the words smog and dust.  Instead of falling as rain, the ethane molecules globbed onto smog particles and slowly descended to the surface, where they piled up instead of liquefying.  That’s where the sand for the dunes (05/04/2006) came from, he thinks.

Titan, has a dense atmosphere of nitrogen with a few per cent of methane.  At visible wavelengths its surface is hidden by dense orange-brown smog, which is produced in the stratosphere by photochemical reactions following the dissociation of methane by solar ultraviolet light.  The most abundant of the products of these reactions is ethane, and enough of it should have been generated over the life of the Solar System to form a satellite-wide ocean one kilometre deep…. Here I explain the mysterious absence or rarity of liquid ethane: it condenses onto the smog particles, instead of into liquid drops, at the cold temperatures in Titan’s atmosphere.  This dusty combination of smog and ethane, forming deposits several kilometres thick on the surface, including the observed dunes and dark areas, could be named ‘smust’.  This satellite-wide deposit replaces the ocean long thought to be an important feature of Titan.

In a note added in proof, Hunten speculated about the ethane cloud at Titan detected by Cassini last month (see 09/14/2006).  The ethane molecules in the cloud amount to only a tiny fraction of the total, he said.  His thoughts did not rely on any tangible evidence; only that “their presence may be compatible with the smust particles discussed here.”

It is entirely reasonable that these few molecules would not reside on the smust particles.  A possible difficulty is that this small amount of ethane vapour would be unable to condense.  On the other hand, if more ethane were available one would expect the cloud particles to grow larger; probably the attachment of most of the ethane to the smust particles is necessary to prevent this.

Another view on the methane that could be the parent of ethane comes from New Scientist.  David Shiga interviewed Cassini scientists in Pasadena who believe there is evidence for volcanic calderas on the large moon.  Some of the radar images have features that resemble liquid-filled calderas with rounded edges, but others feel meteor impacts could be responsible.  Volcanism could belch out methane from the interior, perhaps, but this does not explain where the ethane went.
    The reporter seemed more interested in the idea that volcanism means heat, and heat means liquid.  To an evolutionist, that means life can’t be far away: “What with the methane lakes, perhaps some type of exotic exobiology might not be completely out of the question,” said one.  Shiga took this one suggestion by one scientist as inspiration for the title of his article: “Slushy volcanoes might support life on Titan…. The heat and chemicals associated with these possible volcanoes could provide a niche for life on the frigid moon.”  The bulk of the article was about volcanoes, not life.

1D. M. Hunten, “The sequestration of ethane on Titan in smog particles,” Nature 443, 669-670(12 October 2006) | doi:10.1038/nature05157.

It’s fair to propose an explanation for something, but a little unfair to propose one when it cannot be tested for a long time.  It may be well into the 2030s before another probe returns to Titan, and even then, it may be a blimp-like device incapable of digging several kilometers into the surface to measure the deposits.  What if Hunten’s mechanism works somewhat, and a lander detects these smust particles on the surface, but cannot determine if the layer is only inches deep?  One could not know that there is enough of it to explain the missing billions of years without extremely difficult and costly efforts.  Additionally, it is very difficult to simulate Titan conditions on Earth.  The only lab evidence Hunten mentioned did not support his theory:

It would be desirable to verify in the laboratory the attachment of ethane molecules to smog particles, here deduced from their behaviour on Jupiter.  Such particles, with their fluffy structure, have, however, not been produced in experiments, which instead generate a dense deposit on the walls of the vessel.  It will be a challenge to reproduce this structure along with a realistic composition, and then to expose the particles to ethane molecules.

In other words, this is just an airy, fluffy suggestion on his part.  Even if the particles formed and fell as he suggested, what would keep them from liquefying on the surface after they compacted?  The fact is, Titan was not the oceanic planet that the best scientists predicted.  Speculating after the fact what happened to the missing ethane is not science till verified.  While no one would deny Dr. Hunten his right to speculate based on his own assumptions about the age of Titan, his little story should not be put forth as “the answer” to this huge problem.  Speculation is no substitute for openmindedness that one’s assumptions could be vastly in error.
    As for the relentless suggestions that liquid water or ethane mean the possibility of life, this is so silly it is tiring.  Here’s a better form of entertainment that’s realistic.  For a wild ride down to the surface of Titan aboard Huygens, based on actual photos and measurements, download the animations from the Huygens CD-Rom available from the European Space Agency.  For the download page, click here.

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