State of the Moon Titan Addressed

Each May, a series of articles on major topics of geological interest, written by leading experts in the field, is published in the Annual Review of Earth and Planetary Sciences.  This year’s issue includes a treatise on Titan, the large moon of Saturn, written by the two titans of Titan science, Jonathan Lunine and Ralph Lorentz.  Lunine, a Cassini interdisciplinary scientist, has been studying Titan for over two decades, well before the Cassini-Huygens spacecraft revolutionized our understanding of the cold, haze-shrouded, Mercury-sized moon.  Lorentz is the author of Titan Unveiled, an account of the Cassini-Huygens mission through 2006, interspersed with snippets of his own personal experiences in the investigation of Titan.  This article, then, should provide a reliable overview of current thinking of how Titan came to be the strange world it is.
    The article is entitled, “Rivers, Lakes, Dunes, and Rain: Crustal Processes in Titan’s Methane Cycle,”¹  Since methane is the key to understanding both the atmosphere and crust of Titan, that is the focus of the article.  Two problems have been known for over a decade.  One is that the atmospheric methane, being under erosional bombardment from the solar wind, should be long gone if Titan is 4.5 billion years old.  The second is that the breakdown of atmospheric methane should lead to the accumulation of liquid ethane on the surface.  In the 1980s, some scientists estimated a global ocean of liquid methane and ethane might cover the surface hundreds of meters thick – perhaps even a mile or more deep.  When the Huygens Probe landed in 2005, it found a moist lakebed.  Radar imaging discovered sand dunes of ice particles covering the equatorial latitudes.  Some lakes in the northern latitudes were found, but no global ocean.  Now that Cassini has made over 50 flybys of Titan in 5 years, how do scientists explain this?  A flavor of the answer can be found in the abstract: “The long-term evolution of the methane cycle may have involved episodic resupply of methane to the surface or gradual depletion of a larger surface reservoir of methane, but in either case, removal of large amounts of ethane from the surface remains an unresolved problem.”
    After providing a brief history of Titan exploration, they said, “the lifetime of Titan’s atmospheric methane was straightforwardly calculated to be a few percent of the age of the solar system.”  To them, this “implied that methane must be supplied externally to the atmosphere, from space, from Titan’s surface, or from its interior.”  External supply is out; there aren’t that many impactors with methane likely to hit Titan.  Once it was confirmed there was no surface reservoir, the only place left was under the crust.  Now, though, we know that much of the surface is covered in sand dunes.  There are few craters.  Vertical relief is low in most places (on the order of a hundred meters).  Some places show long, sinuous channels with tributaries, suggestive of rivers, which could be drainage channels from infrequent cloudbursts of liquid methane.  The Huygens probe detected some of these channels on its way down.  Some channels could be outflows from cryovolcanic domes.  Methane, therefore, plays a role in the atmosphere and crust, but not to the extent of early predictions.  A few methane clouds have been seen above the haze layer to move about within days or hours.  Models predict infrequent but powerful outbursts of methane precipitation: one study “showed that tens of centimeters of rain could fall within a few hours, a result confirmed by subsequent modeling”; studies of updrafts in convective plumes showed that while such plumes were little less energetic than those on Earth, the overall flux limitation due to the weak insolation meant that such convection (and by implication, rainfall) had to be rare.”  The average rainfall is estimated at 1 cm per year, though local areas could experience rare, torrential downpours.
    The dunes were another surprise.  “Pre-Cassini expectations were that dunes on Titan were unlikely (Lorenz et al. 1995), an expectation that has been proven wrong (Lorenz et al. 2006a),” the paper said.  The winds were expected to be too gentle to create dunes.  Also, “it was not obvious what processes on such a stagnant world could generate sand-sized particles.”  But there they were, running longitudinally around the equator, dodging obstacles and standing 100 to 150 meters high.  This was all a complete surprise to find that 20% of Titan’s surface is covered in sand dunes.  They expected an ocean, but found “absence of a global ocean on Titan, in place of which are the dunes.”  The surface winds proved stronger than expected (1 m/s instead of 1 cm/s).  Prevailing winds blow east to west.  Dunes are rare outside of the tropics (+/- 30° latitude).  The interdune areas are completely free of sand.  The dune particles apparently contain hydrocarbons, not just water ice.  Scientists wonder how the sand was produced.  It might have precipitated out of the haze, or it might have formed in cycles of wetting and drying in the lakes.  If the latter, the particles had to be lofted into the atmosphere and deposited in the tropics.  Maybe the particles came from impacts or from river channels.  Those sources, however, appear too low to account for 200,000 to 800,000 cubic kilometers of material – larger than all the lake liquid inventory.  Did Titan have more methane at earlier times?

Finally, whereas some observed features can be explained with models of present-day rainstorm precipitation, it is not clear that all can.  There is evidence at the Huygens site of larger-scale features, and radar imagery is revealing progressively larger areas of heavily dissected terrain (badlands).  Cloud models by Hueso & Sanchez-Lavega (2006) indicate that a relative humidity of 80% is required for spontaneous development of convectively driven methane rainstorms on Titan, about twice the present-day relative humidity.  If these models are correct, they imply that the features seen at the Huygens site were formed in a substantially wetter climate than that observed today (Griffith et al. 2008).  The source of the additional methane—if it still exists—remains unidentified.

Some changes may be seasonal.  That’s one thing the Cassini team wants to observe if the spacecraft, now at equinox in the Saturn system, can remain operational till the next solstice (2017).  By then, sunshine will be reaching the north pole.  Maybe the lakes will migrate with the seasons.  Even so, with about 35% of Titan’s surface mapped in radar to date, it appears that only 0.6% of the surface is covered in liquid.  Most of it is in patchy lakes in northern latitudes.  The depth of the lakes is unknown.  If moderately deep (20 m), they could hold two orders of magnitude more hydrocarbons than all the known gas and oil seeps on earth.  Still, that impressive quantity of methane is too low – 1/30 to 1/3 – the amount needed to humidify the atmosphere and produce the rain that has apparently carved the dendritic channels.  Lorentz and Lunine offer two possibilities: the lakes are much deeper than expected, or there are underground reservoirs of methane.  Or, perhaps, the global ocean existed in the past.  If so, we are seeing the waning stages of a drying world.
    But where is the missing ethane?  Although ethane mixes well with methane in its liquid form, it does not vaporize as easily: “the vapor pressure of ethane is more than three orders of magnitude lower than that of methane at the surface temperature of 94 K” (the surface temperature), they said.  “This means that, while ethane mechanically behaves as a fluid identically with methane, and is fully mixed with it, it does not participate in the gaseous phase of the hydrological cycle through evaporation and condensation on the same timescales and with the same mass flux as methane.”  If it has been condensing and dropping to the surface for 4.5 billion years, where did it go?  Lunine’s own prediction was at stake when he noted the “absence of the hundred of meters equivalent depth of ethane expected from methane photolysis over the age of the solar system (Lunine et al. 1983).”  No ocean was found.  The Huygens probe landed on damp sand.  Methane vapor was detected, and rounded cobbles found scattered around on the surface of the landing site in a dry lakebed indicated they had tumbled down the dendritic channels photographed on descent.  Still, it was a much dryer surface than expected.  The probe had been built ready to float on an ocean of liquid ethane and methane.  It even carried an instrument to measure its depth.  It seems the authors’ only escape for the missing ethane was to bury it underground.  The surface ice might be porous, they said; the lakes might be connected by underground aquifers where the ethane escaped out of sight. 

Another unresolved question is the ultimate source of the methane and the disposition of the ethane.  If methane has been photolyzed without major interruptions over the age of the solar system, hundreds of meters equivalent depth of ethane should have been produced during this time.  Disposal of the ethane during volcanic (Mousis & Schmitt 2008) and impact events might explain its relative absence on the surface.  Alternatively, it is possible that much less ethane actually survives to the surface from the stratosphere than is predicted by photochemical models (Atreya et al.  2006), because its vapor pressure is high enough that condensation into aerosols in the lower atmosphere is avoided, or it is incorporated somehow into other organic aerosols (Hunten 2006), or both.  In either case, the surface ought still to be buried under hundreds of meters of solid or solid + liquid debris globally averaged, and the dune fields covering 20% of the surface do not appear to be either extensive enough or deep enough to account for all this material.  Alternatively, material may have been pushed into the subsurface crust, or deeper, as noted above for the ethane.

The authors even referred to other papers that suggested ad hoc scenarios.  “Methane itself might have been manufactured in the deep interior by reaction of carbon dioxide, water, and rock (Atreya et al. 2006)” – but this lacks a transport mechanism to the surface – “or brought into Titan and stored in the deep interior from the beginning, expelled during discrete events in Titan’s history (Tobie et al. 2006) or continuously (Fortes et al. 2007).”  Maybe the ocean was there but is gone.  “Alternatively, the amount of surface methane may never have been sufficient for global coverage, confined instead at most to low points such as the floors of craters and other closed basins.”  If that were true, it should be possible to study the topography and crater floors in finer detail and see if that is true.  At present, however, these suggestions all lack observational basis.  It appears they were engineered to keep Titan old.

1.  Lunine and Lorentz, “Rivers, Lakes, Dunes, and Rain: Crustal Processes in Titan’s Methane Cycle,” Annual Review of Earth and Planetary Sciences, Vol. 37: 299-320 (Volume publication date May 2009) (doi:10.1146/annurev.earth.031208.100142).

We’ve reported these mysteries about Titan for years (insert Titan in the search bar above and see).  This paper is important because it shows that the leading Titan experts have still not been able to come up with credible answers to their falsified predictions despite 5 years of unprecedented data.  Recall the observational facts: no global ocean.  Very little surface ethane.  No plausible supply of methane, which is currently insufficient to account for a perpetual methane cycle.  Indications that methane is being depleted.  Dunes covering 20% of a relatively dry world.  Few impact craters.  Surface flows, indicating remnant heat sufficient to cause surface activity today.  An atmospheric methane abundance only sufficient to last a hundred million years at most (a few percent the assumed age of the solar system).
    All these puzzles can be solved by abandoning the evolutionists’ obsessive-compulsive fetish with that 4.5 billion year age figure.  That would instantly remove the need for all those ad hoc rescuing devices.  Titan would make sense as a young world after all.  If it didn’t cause Charlie D. gastric pains to consider this option, it would certainly be in the direction the evidence is leading.  Aren’t scientists supposed to do that?