[X] My Assistant

[Juramike]

Recent articles have invoked porous ice sands or other crustal grains as an additional potential reservoir for methane on Titan. (Selected examples: Sotin et al., 2009; Mitchell et al., 2009; Turtle et al, 2009,; Hayes et al 2008)

Last week, I set up a very simple laboratory experiment using an analogous system to try to answer the following questions:
1) How much methane could porous sands possibly hold per unit volume?
2) How will it affect the evaporation rate?
3) How could it affect the surface reflectivity?

As a laboratory analog for Titan’s hydrocarbon liquid mix (methane/ethane/nitrogen), I used solvent-grade heptane.
As an analog for Titan’s polar ice grains, I used either Flash-grade silica gel or analytical grade quartz beach sand. (The polar hydroxyl groups of the ice grains being analogous by the siloxy groups of silica)

The set-up
Three standard size 600 mL beakers were used:
Beaker A was charged with 400 mL silica gel
Beaker B was changed with 400 mL sand
Beaker C (control) was left empty.

Here are the initial images:


To initiate the experiment, a volume of Heptane was added to each beaker. Then, images and and weights were taken at key timepoints over a several day period to determine evaporation rate and monitor changes in visual appearance.

[Note: Although the temperature was held constant 298 K, the beakers were placed side-by-side in a fume hood with varying hoodflow (face velocity minimum was 100 cfm).]




Initial References:
Mitchell K.L., et al., LPSC 40 (2009) Abstract 1966. “A global sub-surface alkanifer system on Titan?”.

Hayes, A., et al. Geophysical Research Letters 35 (2008) L09204. “Hydrocarbon lakes on Titan: Distribution and interaction with a porous regolith”. doi: 10.1029/2008GL033409.

Sotin , C., et al. LPSC 40 (2009) Abstract 2088. “Ice-hydrocarbon interactions under Titan-like conditions: implications for the carbon cycle on Titan.”

Turtle, E. P., et al., Geophysical Research Letters 36 (2009) L02204. “Cassini Imaging of Titan’s High-Latitude Lakes, Clouds, and South-Polar Surface Changes.” doi: 10.1029/2008GL036186.

[Juramike]

Assuming a constant surface evaporation rate of the heptanes in the control beaker at 0.4 mL (=cm3) and a beaker diameter of 10 cm (surface area of 78 cm2), we derive an evaporation rate of 0.005 cm/min (=2,693 m/yr) at Earth STP (298 K).

According to Hayes et al., 2008: “The evaporation rate [on Titan] is taken to be 0.3 m/yr, consistent with an average windspeed of 0.1 m/s, methane mixing ratio of 0.35 in the lakes and methane relative humidity of 50% [T = 95 K)

[Putting this in perspective, an evaporation rate of 0.3 m/yr it is roughly equivalent to the aqueous evaporation rate in Needles, California or 3x the evaporation rate in the Piedmont region of North Carolina. Here is a pretty cool link for evaporation rates: http://www.grow.arizona.edu/Grow--GrowReso...?ResourceId=208]

From the simulation, it can be seen that even after 10x the expected lifetime of free heptane at the surface, 10% of the original heptane remains in the silica matrix. Similarly, after 7x the expected lifetime of a quantity of heptanes at the surface, 10% of the original heptanes was measured in the sand matrix.


If these results scale, (and I’d speculate that the lower absolute temperatures on Titan would accentuate the effect), it would be reasonable to expect that a few m of porous substrate would be sufficient to eventually slow the evaporation rate at least 10 fold and allow quantities to remain in the porous matrix longer than would be expected based on the free liquid evaporation rate.

As one implication, Turtle et al, 2009 provides evidence for South Polar lakes that were detected by ISS, but were not observed by SAR RADAR 2.5 years later. They postulate that “enough liquid could have evaporated or percolated into the subsurface during the intervening years to explain the lack of lakes observed by RADAR”.

Considering the change in evaporation rate evidenced by the laboratory simulation above, it is possible that significant amounts of hydrocarbon solvents could even still be trapped in the lake sediments and remain in buffered communication with the atmosphere.

[rlorenz]

Great experiment, Mike. Nice and transparent. I'd have replied earlier
but was out in the field studying transient lakes in high-evaporation regions
(specifically Racetrack Playa in Death Valley) where there wasnt internet!

Evaporation rates from free liquid surfaces are controlled by the vapor pressure excess
(saturation v.p. minus ambient partial pressure), and windspeed. But in a regolith, as
the stuff starts drying out from the surface downwards, the labyrinth of pathways through
the pores becomes the limiting factor (pore size, tortuosity become factors) and the
evaporation rate falls off as inverse square root or log of time or something. Used to
be (maybe still is) a common problem examined for dessication of ice-saturated regolith
on Mars. Long ago Konrad Kossacki and I looked at the large-scale retention of porespace
on Titan regolith as a reservoir for liquid hydrocarbons (though the original idea was due
to Stevenson and Eluskiewicz)

http://www.lpl.arizona.edu/~rlorenz/kossacki.pdf

There's ample geomorphological evidence that Titan rivers can be vigorous, although that
doesnt mean some can be 'muddy'. Whether (or perhaps rather which and when) lakes
are 'tar' vs 'LNG' remains to be figured out, but I'd be shocked if at least some aren't LNG-like

Your basic point that the regolith can be a significant reservoir of even volatile hydrocarbons
is right on. It was basically your heptane-soaked silica (but methane/ethane-soaked ice perhaps)
that got rammed into the warm GCMS inlet at Huygens impact, and got sweated out.