[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]

Volume capacity of porous sediments in the sand sea basins?
Ballparking this conservatively:

Assume 10% of the sand sea basins (most likely the central depths) has a porous sediment depth of 100 m.
Assume the solvent capacity of the porous sediment is 50% (see above post).

Here are estimated areas/volumes of porous sediments in the sand sea basins (conservative, the areal extent is likely underestimated):


1.09E7 km2 total sand seas area (including Mezzoramia) x 10% (basin area with deep sediments) x 0.1 km (sediment depth) x 50% (solvent volume)
= 5.4E4 km3 hydrocarbon solvent could be held in porous sediments in the dune seas.

Compare with estimates of surface lake volume:
6.3E5 km (North polar lake area + 55 dark surface features southward of 69 S area from Turtle et al., 2009) x 0.2 km (200 m depth is 10x higher than Lorenz et al. 2007 estimate of 20 m average volume)
= 1.3E5 km3 hydrocarbon solvent held in surface features.

So even with a fairly conservative estimate of porous sediment capacity in the sand sea basins, it is within an order of magnitude of the amount of surface liquid estimated in the lakes.

Damp porous equatorial dune sediments could be a significant surface reserve for methane and other hydrocarbons on Titan.