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Planetary News: Mars (2005)The Planetary Society's Mars Analog Blog:
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Credit: Troy Hudson |
Starry Night, part deux
Rock Garden Hill 5:00am
It’s my second night out on the hills waiting for sunrise and it’s a bit more difficult. I don’t think it has to do with how tired I am, more that I’m not using the back cab of the truck to sleep in. I slept at the station until about 4am, then got in the packed truck and headed out here to the hill site that I’d selected earlier in the day (well, yesterday, actually). Setup was easier than last time, since I didn’t have to mess with the TECP.
I expect this evening’s attempt to be different from the last one. I’m focusing on about the same size ground area, but should remain in shadow long after the sun has risen. Not sure how that’ll affect the relative humidity and soil thermal properties, but I’m guessing that the greatest influence comes from solar insolation, and not other ambient conditions. Relative humidity and air temperature change over time, but much slower than soil surface temperature when you’ve got hundreds of watts per square meter beating down on you.
The site we’re looking at this time has both a large solid rock, a bit of rock-free soil, and some rock covered (moderately pavemented) soil. We’re particularly interested in the differences between these types of ‘terrain’.
Current Mood: tired
Current Music: the hum of the electric generator
June 6, 2005 Yungay Station, 4:00pm
Nap time is over.
Rock garden hill field setup
Credit: Troy Hudson |
I collected a good number of samples this morning and am about to start the task of analyzing them on the WP4. I was very tired when I got back to the station this morning and needed a couple of hours rest. Thankfully the bedrooms at the station are pretty much light-tight and I was blessed with a few hours of cool darkness on a rather comfortable cot.
The IR camera worked fine. I think my artificial wetting event will show some interesting things, most notably that you could notice the presence of water lingering the longest on the rock. Not much improvement was to be had in the contrast one sees via the camera, but its ability to do time-lapse is most welcome.
The whole area of the ground I wet, though, seemed to stay visibly darker than the surrounding soil for over an hour after the wetting event, even though we were well into morning by the time I had to pack up to be back at the station. It’s a pity we are short on cars; I would have like to have stayed to see the sun hit this area. The contrast between sunlit and shaded areas would have been too great for the camera (and possibly even my eye) to continue to track the presence of water through the sunrise event. Again, I wish I had a way of monitoring soil temperature over a large area that didn’t rely on reflected light.
Rock garden hill test sight
Credit: Troy Hudson |
The thermal IR camera would be good for full sun or full shade, but for the rather important (it seems) transition between light and shadow, something like a thermocouple array would be most efficacious.
Time to get cracking on those soil samples. The WP4 has to be packed up tomorrow.
Current Mood: Rested
Current Music: Akira, Winds over Neo-Tokyo
June 6, 2005 Yungay Station 9:00 pm
I’ve just finished the analysis of my Rock Garden Hill Sunrise samples on the WP4. I wasn’t sure I’d be able to, and since the device needs to be packed up for return to Decagon tomorrow. I’m very, very pleased that I could.
Preliminary analysis of the data shows a correlation in the Frank Hill samples in water potential and time since sunrise. The potential becomes more negative past sunrise, as expected. What this essentially means is that as the soil loses moisture, that moisture that remains is held even more tightly. You would expect that if the soil were saturated with water, that you’d have a plateau where the water potential would be roughly constant as water was lost, but then at some point when you were down to only a thin film of water, you’d get surface bonding effects coming into play and see a decrease.
Sunset over rock garden hill
Credit: Troy Hudson |
The Rock Garden samples show much less variation, which is not as surprising since they didn’t ever actually see the sun come over their local horizon. The HOBO datalogger also showed that the local relative humidity didn’t vary that much, despite the fact that most everything around the shaded spot of the hill was in full sunlight. It’s interesting to speculate on what happens to nighttime moisture as the sun comes up and casts shadows. To say the moisture dissipates is misleading and meaningless. It has to go somewhere…so where is it going? Surely much of it ultimately gets mixed higher in the atmosphere through convection as the desert surface warms up. But then what happens to the areas in shadow? Do they collect vapor from neighboring areas, essentially cold-trapping surface moisture until the sun finally hits them and forces them to give it up? If so, what size shadowed area do you need? Would you get significant moisture trapping in the morning shadows of small rocks? Would the water content of soils in these sheltered locations increase above the ambient significantly? How high could it get before the ambient conditions of low atmospheric relative humidity pulls away the last dregs of moisture from the rocks?
I wish I’d seen a fog event while I was here. It’d have been interesting to see where the fog lingers the longest. To really determine how the water contents in these various areas change, I’d need a rather elaborate network of sensors and sampling sites. You’d want to have identical conditions, so they’d all have to be collected in the same day at the same times. Setting up relative humidity and temperature sensors in various locations is easy. What’s harder is sampling soils in a time series. You can’t be sure that the soil you sample at site X is holding the same amount of moisture, at the same time, as site Y, which you’ll be sampling in an hour. At these low levels, soil moisture can depend greatly on things such as salt content (some salts like lithium chloride can actually draw moisture out of very dry air), and specific surface area. This latter parameter, the amount of square meters of soil surface per gram of soil, can vary from less than 1 in highly cemented or coarse soils, to well over 100 in soils with high porosity and/or high clay content. Just how heterogeneous are the crusty soils of the Atacama?
The soils I’ve sampled for preservation and later gravimetric water content analysis were visually similar. Hopefully I can justify the assumption that they are, to first order, identical, therefore allowing me to say something about water content over time.
Only once I get these samples back to JPL and do the gravimetric water content analysis will I be able to see if the correlation actually means anything. It’s possible that all I’m seeing is atmospheric moisture physically adsorbed on the soils during the time of sampling. Also, since the analysis of the soils in the WP4 has taken all day, there’s some chance that changing relative humidity conditions in the station effect the measurements. In the future, if we use this device in the field, we’ll have to prepare a glove-box or glove-bag with constant humidity for sample transfer and analysis. It’ would have been very difficult to manage on this trip since all the horizontal surfaces other than the floor are in use. Glove bags are hard enough to work with without having to be on your belly.
It’s barbeque tonight – cheeseburgers, sausages, and hot dogs. Since the WP4 analysis is very much a push-button-and-wait sort of procedure, I was able to contribute to dinner this evening by preparing two types of sautéed onions (one with sugar and curry, the other with red wine) and some sautéed mushroom for the burgers. Hopefully the efforts at petit gourmet will be appreciated.
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