Yeah, yeah...I know, I haven't done homework #s 1 and 2 yet...Bad Mike!
Well, not really. I did introduce myself to my group leader Geoff Hammond, and got some tips for getting the most out of my expedition trip. I'll consider posting them here, if I get desparate for content. ;)
Anyway, our latest homework assignment is to re-write the soil sampling protocols from the GLOBE Gravimetric Soil Moisture Protocols. We are to "Re-write the protocols so that they enable you to do sterile sampling on Mars.".
It may seem like a scary assignment, but I suspect that it's more of a thought experiment. The GLOBE protocols, despite their complex-sounding name, are really meant for school-age kids as an experiment for a science class period or two. The "Gravimetric" descriptor really just means a weight-based measurement of the soil's moisture content. I suppose it's a fun way to get the kids to add a new word to their scientific vocabulary, and scare their parents. :D
There are three basic experiments. All involve weighing a "wet" soil sample, then cooking it to remove the moisture, and re-weighing it to determine the amount of water lost. Here's the actual protocol for the "Depth Profile Soil Moisture Protocol":
In the Field:
1. Complete the top portion of the Depth Profile Soil Moisture
Data Sheet.
2. Locate your sampling point on the star and cut and pull away
any grass or groundcover.
3. With the trowel, dig a hole 10-15 cm in diameter down to 5
cm. Leave this soil loose in the hole.
4. Remove from the loose soil any rocks larger than a pea (about
5 mm), large roots, worms, grubs, and other animals.
5. Use your trowel to fill your soil container with at least 100 g
of the loose soil.
6. Immediately seal the container to hold in the moisture.
7. Record the container number and mass on the Data Sheet next
to Sample Depth 0-5 cm.
8. Use the auger or trowel to remove all of the soil from the hole down to a depth of 8 cm.
9. In a clean container, collect a soil sample that contains the soil between 8 and 12 cm deep.
Remove rocks, large roots and animals. Seal the container.
10. Record the container number and mass on the Data Sheet next to Sample Depth 10 cm.
11. Continue to auger down to obtain samples centered at 30, 60, and 90 cm. Record the
container numbers and mass values on the Data Sheet.
12. You should have 5 containers of soil taken from 1 hole. Return the remaining soil to the hole
– last soil out, first in.
See? It's not all that difficult!
So...What would I suggest changing for performing the same sample on Mars?
Well, first there is the obvious:
1) If you're seeing grass or other groundcover, stop the experiment immediately! Carefully mark and note your location, and notify as many people as you can. You have just found very strong evidence of life on Mars, and your measurement of water in Martian regolith can wait! This notation would also apply in the case of finding the "large roots, worms, grubs and other animals" mentioned in steps 4 and 9.
2) Remember to take into effect that the force due to Martian gravity is different from that on Earth. This may be of particular importance when using a gravimetric protocol. ;)
Ok, now to be serious...
I'll limit my comments on the GLOBE sampling protocols when applied to the Martian environment to two issues. You wouldn't want me to eat all your bandwidth!
First, we are going to be interested in more than just the water content of a soil sample. Various other liquids or, more likely, volitile solids may exist in Martian soil. In addition, the surface temperature (and pressure...more on that later) of Mars, about 235K, would indicate that water would only appear in solid form. There is also the potential for solid CO2 (dry ice), as well as the possibility of frozen NH3, (ammonia - freezing point at about 200K), or possibly liquid or frozen simple hydrocarbons, or even more complex compounds, like amino acids. The process of heating the sample to "dry" it would also cause these other compounds to vaporize. Now, on Earth we have an extremely large amount of water in the soil, relative to other compounds. So much so, that we can ignore their contributions to the soil's moisture content, particularly with the error margins obtained in the GLOBE experiments.
On Mars, we don't know as much about which compounds will be present, nor do we have an exact expectation of their relative quantities. We expect CO2 and H2O to be the most prevalent, but we need to add an additional step to the measurement process to make our "Gravimetric" process work.
I would like to analyze the compounds that vaporize as a result of heating our sample, by using a mass spectrometer or similar insturment. Granted, we could also use the mass spectrometer to analyze the actual amount of water in our sample (it would certainly be easier!), but that would be "cheating" since it's not using the gravimetric measuring protocol!
Once we know what compounds are present, we can heat our other samples to a point where the compounds with lower vaporization points will evaporate out, but not hot enough to vaporize the water (again, there's the pressure problem...I'll talk about that in a minute). Once the other compounds have been removed, we can take our initial sample weight, and then heat our sample to a point where the water will vaporize out, but not so high as to vaporize compounds with higher vaporization points. We can then weigh our "dry" sample, and determine the water content using our gravimetric protocols.
Now for that little issue of pressure I mentioned earlier...
The atmospheric pressure near the surface of Mars is extremely low. Only about one percent that of Earth's. That means we will have to deal with sublimation of ice and other volitiles. For surface samples, its not too big of a deal anything that could vaporize/sublimate will have already done so. But for our deepest samples, at 90cm, we could lose a significant amount of our volatiles, simply by exposing the sample to the mars atmosphere.
There's a paper (Chevrier, V., D. W. G. Sears, J. D. Chittenden, L. A. Roe, R. Ulrich, K. Bryson, L. Billingsley, and J. Hanley (2007), Sublimation rate of ice under simulated Mars conditions and the effect of layers of mock regolith JSC Mars-1, Geophys. Res. Lett., 34, L02203, doi:10.1029/2006GL028401.) which discusses how ice buried under as little as one meter of regolith can last for as long as 800 years, when it would completely sublimate at the surface.
As long as our sample is exposed to the low atmospheric pressure, we will have skewed results.
My solution would be to either use a mechanical method to "seal" the sample as it is retrieved from the depth - something like an airtight core sampler, as opposed to a trowel, or to have some sort of pressure seal around our sample site, and pressurize the area, using an inert gas (He, Ne, Ar or maybe N2), so that sublimation effects wil be reduced.
Ok - homework assignment done. Just a final word of commentary. Even a "simple" experimental process on Earth is going to become vastly more complicated on Mars (or the Moon, or Titan, or Europa, or...). The best way to get these experiments to work is to get as many people thinking about them, ahead of time, as possible. Our expedition to the Mojave, the previous expedition to the Atacama Desert, and future expeditions expose a wide range of people to just some of the conditions our future explorers will face. Hopefully, we can all add our own small contributions to make things easier for them.
The photo above is of compacted Martian Soil. Credit: NASA's Mars Spirit Rover Team
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