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Planetary News: Mars (2005)The Planetary Society's Mars Analog Blog:
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Credit: Troy Hudson |
All systems are go, all lights are green.
Yungay 8:50 pm
Today was setup day. After a very cold night in the tent (most of me was fine, but my feet froze) and a breakfast of two hardboiled eggs, Chris McKay took several of us newbies out for a tour of the number one sample site, the Rock Garden. It’s about a 10-minute drive from the station, up on a hill overlooking an abandoned and nearly destroyed mining settlement. Most of the land surrounding the ruins has been strip-mined for the near-surface nitrate deposits. When you look up the slope of a hill, there’s a two-meter cliff which indicates the highest point of mining.
With the advent of synthetic nitrates in the 1920s, the bottom dropped out of the market in Chile and elsewhere and many settlements such as this shut down overnight. But prior to this, there were thriving towns like this one all over northern Chile, which still has one of the world’s richest nitrate deposits. It’s still somewhat unclear how all of the nitrates got concentrated here, but the age of the desert (>10 million years, making it one of the oldest on Earth) and the lack of flooding may have allowed what little soil moisture there is to concentrate nitrates in a two-meter layer that’s nearly 90% pure. The ultimate source of the nitrates is still unclear. But the ultimate fate of much of that which was mined was to be shipped overseas to Britain and Germany so they could have World War I.
Rock garden hill
Credit: Troy Hudson |
Rock garden ruins
Credit: Troy Hudson |
Example of rivving in the rocks
Credit: Troy Hudson |
The Rock Garden is up on a hill overlooking one of these abandoned mining towns. Most of the rocks there are a dusty red-yellow color, but greenish on the inside if you break them open. Most likely basalts that have been slightly altered to chlorite, serpentenite and contain some epidote, all of which are usually green. There’s a bit of reddish crust on some of them, most likely due to iron oxide. Often one finds a very low-density rock with a fluffy or fairy-castle appearance. These salts are everywhere, and look a great deal like the remaining nitrate deposits. Since the crystals are so small and I don’t happen to have a portable x-ray diffractometer in my pocket, it’s nearly impossible to tell if they’re nitrate, sulfate (gypsum or anhydrite) or some other salt. They have a somewhat chalky flavor, which is indicative of sulfates (and possibly nitrates), but rules out regular sodium chloride.
After a quick examination of the Rock Garden site, we returned to the station. Most of the rest of the day was taken up with various people running errands to Antofagasta, the preliminary setup of the laboratory, the wiring of the station for lights (it gets dark around 7pm here, being southern hemisphere winter), and me testing my equipment.
I walked across the main road about 200 meters from the station and found a patch of relatively undisturbed soil to practice with my soil physical properties probes. These include a penetrometer and a shear tester. The former is very much like a calibrated piston on a spring. By pushing one end into the soil up to a set groove on the side of the piston, you can read the maximum unconfined (i.e. without walls) compressive stress the soil can handle. The latter device has a flat wheel with a set of perpendicular fins which protrude into the soil. By twisting the spring at the top until the soil fails and the fins snap back into place, you get the unconfined shear stress. These two numbers define another number called the Coulomb criterion, which is essentially a measure of how cohesive a soil is. A dense clay would be very cohesive, while beach sand has no cohesion whatsoever.
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Rock garden hill
Credit: Troy Hudson |
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Rock garden ruins
Credit: Troy Hudson |
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Example of rivving in the rocks
Credit: Troy Hudson |
I was surprised to find how variable the penetrometer results were over a small patch of soil only about 10 cm on a side. Sometimes, the piston broke into the soil rapidly, as if breaking through a thin surface crust after applying only about 0.2 kilograms per square cm. Whereas other sampling spots gave way slowly, reaching a maximum compressive stress of more than 2.5 kg / cm2. The shear tester results seemed more uniform, but I’m not sure what sort of variability one expects for this sort of measurement. Their range covered 0.1 to 1.7, with some outliers at 0.02 (all units in kg per sq. cm).
It gets really hot here in the desert during the day. Now, at about 9pm at night, the temperature is 14°C (57°F) and the humidity is about 49%. During the middle of the day in the sun, the air temperature was 27°C (81°F) and the humidity was only 12.8%. You could tell, too. It’s really easy to dehydrate in this weather and you stay pretty cool even when it’s blisteringly hot. Fortunately, the winds picked up around 2pm, reaching gusts of over 5 meters per second. This kept things cool until the sun went down, when the temperature plummeted. It usually gets down to about 0°C (32°F) at night here.
Oh, and can I just say that the stars are absolutely incredible?!? You don’t even have to get dark-adapted to see the Milky Way…it’s clear as a bell.
I did some other measurements on the soil. I determined it’s color index to be 10YR 7/4 (very pale brown) near the surface and more of a 7.5 YR 7/3 (pink) about three centimeters down. (I’ll explain the color values in a later message). Essentially, it’s a pale yellow/red color, with hints of pink, but gets brighter as you go deeper.
The soil in this desert is unlike most I’ve come across in that there is a desert pavement, but it’s very poorly developed and there’s no desert varnish. Desert pavement is an interlocking mosaic of large stones that forms the uppermost layer in many mature deserts. These surface stones (about 0.5 to 5 cm in size in the Mojave desert) overly much finer, lighter clayey material. Interestingly, it’s thought that the stones trap airborne dust which, through thermal expansion or the capillary action of water, works its way down beneath the stones over long time. Thus, rather than the pavement being a feature formed by deflation, where the underlying dust blows away leaving a lag of densely packed surface stones, it’s thought that the stones get pushed up to the surface by a growing layer of subsurface clay/dust.
The pavement here in the Atacama is not a fine mosaic and there are what appear to be large gaps between the smaller stones. My preliminary guess as to why this is the case is that the soil immediately beneath the ‘pavement’ is cemented together with salt. It’s obvious that the surface here is quite stable: even when stiff winds blow, there’s hardly a speck of dust raised. In some locations, such as where the nitrate deposits have been exposed on the surface, the crust is like concrete. Jumping on a shovel will get you about 4 centimeters down, then it’s jackhammer time. Where I did my soil sampling this afternoon, you could brush away a thin, darker surface layer very easily, while the soil beneath it was somewhat tough. It’s this ‘somewhat’ that I’ll define better when I get to reducing the numbers from the penetrometer and shear tester.
TECP
Credit: Troy Hudson |
Everything else seems to be working well. The IR camera is working fine. I’ve rigged up a front-of-lens mount for the 2.5mm diameter IR filter we got before we left. We’ll see tomorrow if it works at all. The TECP works fine, although Marc never got me the repaired and updated Excel worksheet to reduce the data before we left. Hopefully this’ll be waiting in the mail when I check it.
Chris gave me a very nifty device that you can point at a spot and it’ll tell you its temperature. Great if you don’t want to bend down and stick a thermometer to the surface of the soil. Also fun to point at things and see how hot they are. Right now, the top of my laptop screen is 16°C (61°F), while the base of the screen is 26°C (79°F). This is probably proximity to the computer itself and the waste heat from the processor. My hand is 31°C (88°F) and the floor is 16°C (61°F).
It’s time to sack out. Chris and I will be heading out to the Rock Garden at sunrise tomorrow to set up the camera and see what sort of behavior we get from water we artificially introduce into the soil. (Read: we’re gonna point the camera at the ground, spray it with water, and see what we can see)
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