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Headshot of Emily Lakdawalla

AGU: Some first impressions from today's Phoenix sessions

Posted by Emily Lakdawalla

15-12-2008 20:49 CST


I have neither the time nor the brainpower to write up all my notes from the Phoenix sessions held this afternoon at the American Geophysical Union meeting, but I thought it was worth hitting some of the high points before I ate dinner.

So far, pretty much everything I have written about Phoenix concerns operational details -- where it dug, what trenches and samples were named, on what sols. So here, for pretty much the first time, are some actual results from some of the instruments. Of most interest to people are results from the Thermal and Evolved Gas Analyzer, or TEGA, and the Wet Chemistry Laboratory, or WCL.

An artist's conception of Phoenix

Phoenix Mission, University of Arizona

An artist's conception of Phoenix
TEGA is the instrument that takes in minute quantities of soil, then slowly heats it and "sniffs" the gases that are driven off; it reaches a maximum temperature of 1000 degrees Celsius. The specific temperatures at which gases arrive in TEGA's mass spectrometer can tell you about what minerals or chemicals they were originally part of in the soil, though the results can be ambiguous, as you'll see below. TEGA is also a calorimeter, which means that it "notices" when the temperature quits rising (or rises faster than you'd expect) as they keep putting in heat, which happens whenever something in the oven is going through a phase transition, like when water goes from ice to gas.

For TEGA results, instrument lead Bill Boynton reported the following:

  • There are "three things for which we have good, solid evidence," and by "things" he means "stuff we found in the soil." These are: water ice; calcium carbonate (CaCO3), which is most likely calcite but could be one of the other polymorphs, such as aragonite; and perchlorate salts. (Read all about the perchlorates here.)
  • There are "other cases where we have some information that is just suggestive but we're going to need more laboratory studies to follow up." For example, they saw a low-temperature release of carbon dioxide (CO2). This could be a number of things: iron or magnesium carbonates; adsorbed carbon dioxide; or potentially most exciting, combusted organics.
  • What does "combusted organics" mean? Here's the problem: the presence of lots of perchlorate in the soil. Perchlorate is a potent oxidizer, especially when you start heating up the soil. So, as they heat the sample, the perchlorate could effectively be "burning" with any organic material present in the soil, which could produce the carbon dioxide that TEGA detects at low temperatures, thus hiding the organics from detectability.
  • In other words, they didn't detect organics. But even if there were organics present, they could have been destroyed by the perchlorate during heating before TEGA had a chance to detect them. Unfortunate -- but since the existence of so much perchlorate was a real surprise to the team, it's not something they could have planned to deal with.
  • Water (H2O) was released at both low and high temperatures. There is a huge menagerie of substances that could be responsible for the release of water. For just some examples, the low-temperature release could be from such minerals as goethite, nontronite, kaolinite, iron sulfates like jarosite, or magnesium sulfates like kieserite, just to name a few. The high-temperature release of water could come from other minerals, such as smectites, chlorites, talc, amphiboles, prehnite, serpentine, and more. Take your pick. Only detailed laboratory work to attempt to simulate a martian soil, then drop it into an engineering model of TEGA to try to reproduce the exact results, will eventually result in a more specific match.
  • One very surprising non-detection was that "we saw no evidence for any sulfates in the TEGA data." He said that the presence of sulfates in the soil can't be excluded, though, because it's possible that sulfate might be found in some thermally stable mineral that doesn't decompose even when heated to a thousand degrees. He wasn't specific as to what mineral this could be.
All in all, there aren't very many definitive results beyond what we knew already. But it's informative to know what their instrument showed, and the range of possibilities that they're considering, and that absence of sulfate is new and very interesting. They expected TONS of sulfate, based upon results from other landed missions. No one offered an explanation for why it should be absent in the Phoenix samples. One other fact that surfaced during questioning of a different talk is that Boynton believes that the data is there for them to measure the isotopic ratios of 12C to 13C and 16O to 18O but that it will be "a month or more before we have definitive numbers." Peter Smith acknowledged that because they never got an ice sample into TEGA they are unlikely to get the ratio of deuterium to regular hydrogen from either the water in the soil or, apparently, from the atmosphere.

OK, so the other instrument of interest is the Wet Chemistry Lab, or WCL. I talked about how WCL works at length in this post; in brief, there were four cells, in which they put some water mixed with some small quantities of other chemical species for calibration purposes, then they put in the Mars sample and stir, then they use sensors to determine the quantity of ions present. After that they drop in slugs of other chemicals to test the response of the solution: first an acid, then three different slugs of barium chloride, which was basically a titration intended to determine the quantity of sulfate present in the soil (see, they expected lots of sulfate).

So here are the main results from WCL, as reported by Sam Kounaves.

  • As reported previously, the soil is alkaline, with a pH of 8.3 plus or minus 0.5. The pH they determine is, in theory, partially affected by the partial pressure of carbon dioxide in the air gap above the water in the beaker, but he showed that it wasn't very strongly affected, because of the next thing...
  • The most definite result from WCL is that the soil contains about three percent (by weight) of calcium carbonate. They were able to determine this number using the barium titration. It's very very nice that this basically agrees with the number that the TEGA team determined. Unlike the TEGA team, the WCL team believes that their data shows the presence of a very small amount, about a percent (plus or minus half a percent), of sulfate, as well. No discussion about whether this was consistent or not with the TEGA non-detection.
  • Another thing that the calcium carbonate does is buffer the soil, so when they dropped in the acid pellet, the pH didn't actually change. Calcium carbonate does the same thing in Earth soils.
  • One surprising thing was how fast the salts in the soil went into solution when dropped into the beaker. In "normal [i.e. Earth] soil samples the salts don't come out fast because they're entrapped in the matrix of the material" -- it takes some time for the dissolution to happen. The immediate dissolution of the salts in the Martian soil suggested to Kounaves that -- if I understood him correctly -- these salts were basically formed in liquid water, that is they basically are evaporitic. But he didn't use that word.
  • They attempted to get soil samples from both the surface and from some depth, but they detected no difference between the surface and deeper samples -- probably because they were only 5 centimeters apart in soil depth. Soil from a much deeper trench failed to fall in to the WCL hopper, but Kounaves said that it turned out to be useful to have that cell as a "blank" anyway, so it wasn't a complete loss.
  • He posted a table of weight percents of various ions in the soil, and then said that the numbers were extremely extremely preliminary and should not be taken too seriously and so forth; with those caveats in mind, here are the numbers he posted, which I hope I jotted down correctly. Any errors are, obviously, mine. Don't base a grant proposal on this.
IonWt %
So that's the quick-and-dirty summary of the facts, as they stand, from the TEGA and WCL instruments. Smith said that over the next few months they would be hard at work in the laboratory attempting to develop simulated soil compositions that can reproduce the behavior of the Mars soils in these two instruments, and promised results of that at the Lunar and Planetary Science Conference in March.

Today was an insane day, with no break for me from 8 am until now. Tomorrow should be a little bit less insane -- I hope. I still have much to write about from Titan, Enceladus, Phoenix, and even a little bit of Kaguya -- stay tuned.

See other posts from December 2008


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