[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]

2) How will it affect the evaporation rate?

Beakers A (400 mL silica+400 mL heptane), B (400 mL sand+200 mL heptanes) and C (heptane only) were weighed and compared to the weight prior to heptane addition.

Here is a set of images showing how the headspace evaporates in the sand/heptane mixture, then the solvent level in the substrate(indicated by red arrow) can be seen to drop, although the rate slows down:


The chart of measured weights (and calculated volumes) below shows the amount of heptane remaining over time:

As can be seen from the graph, the heptane-only control evaporates at a roughly constant rate, while the sand/heptane and silica/heptane mixtures display a change in evaporation rate over time

The evaporation rate was calculated by calculating the amount of heptane volume change over the previous time period. A log-log graph shows the trend of the evaporation rate over time for the different substrate mixtures:

As the solvent goes deeper into the mixture, the evaporation rate decays exponentially. At all timepoints measured, the sand substrate had the lowest evaporation rate, although it contained less absolute volume.

In the present experiment, a 10-fold drop in evaporation rate was observed in both sand and silica by 2800 min (48 h) when the solvent level in the sand matrix had dropped to 10 cm below the surface.