Sequestration of ethane in the cryovolcanic subsurface of Titan

Saturn's largest satellite, Titan, has a thick atmosphere dominated by nitrogen and methane. The dense orange-brown smog hiding the satellite's surface is produced by photochemical reactions of methane, nitrogen and their dissociation products with solar ultraviolet, which lead primarily to the formation of ethane and heavier hydrocarbons. In the years prior to the exploration of Titan's surface by the Cassini-Huygens spacecraft, the production and condensation of ethane was expected to have formed a satellite-wide ocean one kilometer in depth, assuming that it was generated over the Solar system's lifetime. However, Cassini-Huygens observations failed to find any evidence of such an ocean. Here we describe the main cause of the ethane deficiency on Titan: cryovolcanic lavas regularly cover its surface, leading to the percolation of the liquid hydrocarbons through this porous material and its accumulation in subsurface layers built up during successive methane outgassing events. The liquid stored in the pores may, combined with the ice layers, form a stable ethane-rich clathrate reservoir, potentially isolated from the surface. Even with a low open porosity of 10% for the subsurface layers, a cryovolcanic icy crust less than 2300 m thick is required to bury all the liquid hydrocarbons generated over the Solar system's lifetime.

I'm a huge fan of hydrocarbon materials sitting in the Sand Sea Basins, then slowly getting absorbed up into porous materials or subsurface reservoirs over time. Or getting incorporated into clathrates at depth (dunno about the kinetics though, but IIRC clathrate formation is exothermic, so that could only help.)

After reading the article, the only question I have is in the sentence: "...the formation process of clathrates strongly reduces the porosity of the crust". IIRC, the formation of clathrates under certian conditions actually causes porous structures to form.

[wild speculation mode: ON]

Would the ices expand on conversion to clathrate, even at pressure under the surface?
Would a porous ice structure get converted and expand into an even more porous clathrate?
Would this set up crustal instabilities due to less density changes at depth?
Could expansion of clathrate causes stresses that induce microcracks and open up even moe new piping under the surface?

These possibilities would only strengthen the author's arguements in the text.

(And begs for some really cool (!) experiments to try to set up in an appropriate laboratory facility.)


These possibilities might also provide a way to sequester ethane/methane without having to invoke recently erupted cryolava as the porous reservoir. But there aren't that many "fresh" cryovolcanic regions observed on Titan; there are a bunch, for sure, but they are not ubiquitous on the landscape. There seem to be more impact craters than cryovolcanoes and cryovolcanic flows. IIRC the isotope ratio for methane suggests that most of the methane is "fresh". In contrast, most of the nitrogen is old, as most of the lighter stuff has escaped leaving behind the heavier forms of N2 gas. So any older cryovolcanic flows may have lost their porosity and no longer be a good absorber of liquids. In the model the authors proposed, there would need to be significant amounts of young cryolavas to suck up the hydrocarbons.

One possibility would be to have the methane/ethane percolate into the depths of the basins along cracks and pipes, filling pore spaces, then in some areas perhaps converting over to clathrate, effectively locking some of the hydrocarbons away (for a subsurface rainy day when the ice melts). This scenario avoids the need for large scale fresh cryolava flows. And it also allows a subsurface microcrack reservoir to function in both the poles and equatorial zones. At the poles it's saturated and peeping out in lakes in some terrains; in the equatorial zones it is drier near the surface (but maybe wetter further down).

If this is true, that would preferentially place larger amounts of hydrocarbons sequestered in the upper 5 km of crust of the Equatorial Sand Seas (and also presumably also under the porous regions of the lake area). Any impacts in these regions might release an impressively large amount of methane gas. (Menrva impact as a major atmosphere generator?)

[/wild speculation mode]

-Mike