[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]

3) How could it affect surface reflectivity?

Here is an set of side-view images of the silica/heptane beaker over time:


(and the same image, contrast enhanced):


Something very interesting happens as the heptane evaporates: the silica very quickly changes appearance (to appear dry), even though the silica still contains a large amount of heptane. This occurs between 30 and 60 minutes, although only a relatively small amount of heptane evaporated during this period (8 mL or 2% of the total.) At the 126 min image, very complex banding can be seen (possibly from evaporation/recondensation?).

In this example, the appearance of the porous material can changes quickly, even though the amount of solvent inside the grains is almost the same.

Only a tiny amount of solvent change is necessary to change the visual aspect!

At 1384 minutes, the silica appears uniformly dry, although it still contains 32% heptanes.
So even though it looks "not damp" it is not dry!

[marsbug]

Mike, would it be possible to place an optical microscope on its side, close enough to observe the grains through the glass, and allow it to move up and down on a platform (maybe a lab jack?) so that you could get some close up images of the grains at various levels? We have a couple of old and unused microscopes around our lab, I'm pretty sure no one would notice if I cannabalised one. I'm wondering if I can get chemistry to loan me some heptane so I can have a go myself!