Phoenix sol 30 update: Alkaline soil, not very salty, "nothing extreme" about it!
Posted by Emily Lakdawalla
26-06-2008 17:11 CDT
Or: "pHoenix!" (Sorry. MECA team leader Michael Hecht is responsible for that joke.)
There was a teleconference on the status of the Phoenix mission up to sol 30 today. (Like all of these media teleconferences, if you'd like to listen to it after the fact you can do so at JPL's Phoenix Media website.) Here's the news in a nutshell:
- The wet chemistry lab analyzed its first sample, and reported the pH of the surface soil to be alkaline, at between 8 and 9. This was a big surprise to most people.
- The wet chem analysis is about 80% complete. Preliminary results show the presence of cations like magnesium, sodium, and potassium, and anions like chloride. They've not finished the analysis for sulfate yet. Preliminary results indicate it's salty but not extremely so, with salt at the few hundred to a thousand part-per-million level. (The salinity of surface water in Earth's oceans is roughly 35,000 ppm; 1,000 ppm would rate as very slightly brackish.)
- Overall, there is "nothing extreme" in the first wet chemistry lab sample results, which is good news for the question of whether Mars is or ever was habitable.
- TEGA has completed its analysis of its first soil sample. There was no actual water present in it -- no surprise there -- but water vapor was given off at the highest temperatures, indicating water bound within minerals. Examples of minerals on Earth that contain bound water include things like gypsum, clays, and opal, and are generally formed by chemical alteration of other minerals in wet environments of one kind or another. The TEGA team does not yet have an identification for the mineral(s) that released this bound water.
- The TEGA team expressed confidence in their ability to deliver a sample through the narrowly open slit on oven #5.
- There was an earlier press release indicating that the lengthy (hour-long!) shaking of TEGA oven #4 may have caused a short circuit within that oven. This does not harm any of the other ovens, and since the analysis in oven #4 is now complete, it clearly didn't harm that oven. The TEGA team will attempt to avoid a situation in which they have to do an hour's worth of shaking in the future; a few minutes should be sufficient to get a sample into oven #5.
- There is no answer for the question of how TEGA could have been sent to Mars with doors that don't open properly. The Phoenix team is, appropriately, focusing its efforts on making TEGA work now, as is; there will certainly be an investigation of how it arrived at Mars with the door problem undiagnosed, but that effort has not yet started.
Today was all about the wet chemistry laboratory, which is half of the MECA instrument, which itself is one of the two instruments that are really unique to Phoenix (the other being TEGA). Nothing like the MECA wet chemistry analysis has ever been performed on Mars or, for that matter, any other place except Earth. Here's how MECA's wet chemistry laboratory -- which I'll abbreviate "WCL" for this post because I'll be saying it so often, but I'll try not to make a habit of it -- works.
First of all, here's a shot of MECA with the scoop poised over it on sol 30, just before the sample was delivered.Here's a photo of one of those wet chemistry cells, with its parts labeled: And here's a five-step cartoon showing how a sample gets delivered. So, here's what happens. First of all, the WCL gets ready for the analysis run by melting the little block of ultrapure ice it carried from Earth inside a Teflon-coated titanium water tank. A pointy thing punctures a membrane at the bottom of the tank, allowing the water to flow into the beaker. It's 25 milliliters (roughly an ounce) of water. It also drops in the first of five "crucibles," little reagent tablets; the first one is a calibration standard. Then Phoenix collects a sample and brings it over to the cell. When they're ready to go, they slide out the drawer and then dump a bit of sample from the scoop through the grille into the funnel, which funnels it into the drawer. The drawer can hold 1 cubic centimeter of sample. (That would displace approximately a milliliter of water, so you can see there is a lot more water in the beaker than there is sample.) They take a photo of the funnel with the robotic arm camera to make sure the sample got into the funnel and through the grille. The grill wires are 2 millimeters apart, wider than the mesh over TEGA. Then they slide the drawer in, which allows the sample to drop into the water in the beaker; it also scrapes any excess sample away, letting it fall to the deck of the lander below. Here's an animation I put together that I believe shows the funnel before and after the drawer was slid in, delivering the sample to the beaker. Paul Hecht, leader of the MECA team, remarked that they observed the crazily clumpy nature of the soil -- which has been one of TEGA's problems -- when they delivered it; you can see that the excess sample is still sitting inside the funnel even though there's no bottom to the funnel now! Fortunately, it wasn't so clumpy that nothing went into the drawer -- in fact the team seemed perfectly satisfied with the quantity of sample that fell into the instrument.
There's a stirring stick in the beaker, so they stir it up to help stuff in the sample dissolve into the water. Spaced around the beaker are many sensors. From an abstract I found describing the instrument, here's a description of the sensors that are included:
The beaker assembly contains an array of sensors consisting of solid state and PVC (polyvinyl chloride) membrane based ion selective electrodes. These sensors will analyze for inorganic anions and cations, including calcium, sodium, potassium, magnesium, chloride, bromide, ammonium, nitrate, lithium, and sulfate. The array also includes special electrodes for pH, conductivity, oxidation-reduction potential (Eh), anodic stripping voltammetry (ASV) for heavy metals such as Cu2+, Cd2+, Pb2+, Hg2+, chronopotentiometry (CP) for independent determination of chloride, bro-mide & iodide, and cyclic voltammetry (CV) for identifying and analyzing possible reversible and irreversible redox couples.After the first analysis is complete, they drop in another crucible and do more testing. According to the same abstract, "the second contains 2-nitrobenzoic acid, and other three contain barium nitrate for use in determining sulfate."
All clear? MECA team member Sam Kounaves reported that they completed about 80% of the analysis on the first cell. "It's been difficult and slow but it performed flawlessly yesterday," he said, which must have been a huge relief to the team. He went on to say that the team's biggest surprise was the pH being alkaline, with a pH of 8 or 9, rather than acid. (A pH of 7 is neutral.) He said they've also successfully measured magnesium, sodium, potassium, and chloride, though he wasn't yet prepared to give abundances; and he said they haven't finished the sulfate analysis yet, that that'll take another couple of days. He said they can't make a definitive statement about nitrogen yet. He said they "don't see extremely high salt," and that "calcium appears to be extremely low and the chloride appears to be low also. We have to wait for the rest of the analysis to see where the other anions are. There were reasonable levels of salt," hundreds to thousands of parts per million.
Clearly the reporters on the teleconference didn't quite know what to make of this news. The news does have bearing on that key question for the Phoenix mission, the habitability of the Martian environment. The fact that the soil sample wasn't strongly acidic or incredibly salty is good news for habitability. Strictly considering the chemical species that the wet chemistry lab was able to measure so far, it wouldn't be a terrible substrate to try growing some alkaline-happy Earth plants (such as asparagus or turnips or green beans) in. However, MECA can say nothing about whether there are any organics present in the soil -- that's TEGA's job. And MECA's analysis has limited sensitivity; there are a lot of trace elements it's incapable of measuring. But, within its limitations, the news is good for habitability.
In fact, Kounaves said, this looks a lot like soil you find near the surface at one of the locations commonly studied as an analogue to Mars: the Dry Valleys of Antarctica. He said that soil is also alkaline. However, he said, if you dig down 30 centimeters, to the ice table, you can find that the ice-soil interface somehow acts to concentrate a lot of the chemicals that MECA is searching for; he said that at the ice table you can switch to acid soils, and go from near-surface salinities of hundreds of parts per million to sub-surface salinity that's much closer to ocean water, 15, 20, even 30 thousand parts per million. (Again, surface seawater is around 35 thousand.) So it'll be really interesting to see what subsequent samples reveal.
On to the TEGA saga. A press release yesterday mentioned a new problem that was discovered after a sample was finally shaken into oven #4: "Four days of vibration eventually succeeded at getting the soil through the screen. However, engineers believe the use of a motor to create the vibration may also have caused a short circuit in wiring near that oven. Concern about triggering other short circuits has prompted the Phoenix team to be cautious about the use of other TEGA cells." Today, Barry Goldstein clarified this. He said that the short is confined to oven #4, which they're now done with; so the short has no direct effect on the other cells. It's just that the fact that it happened will make them more cautious about doing so much shaking in the future.
It's very, very, very good news from the TEGA team that, despite all the problems, they've successfully completed the analysis of the first sample: Bill Boynton said today that "the scientific data coming out of the instrument is just spectacular; the instrument is working very well in that regard." I think the team deserves congratulations for that. Apparently, reducing the data that they've received from this first analysis will take them weeks. What they can say so far is that the sample was pretty dry (no surprise, since it was from the surface and was hanging out in the sun and air over the TEGA oven for days until it got inside), but that there were small amounts of carbon dioxide and water vapor released during the highest-temperature heating cycle, which means those gases are bound up in the minerals in the rocks. He could say with some certainty that the soil had definitely interacted with water at some time in the past. What he can't say yet is whether that interaction happened at its current location or whether it happened elsewhere on Mars, and what they're looking at is soil that was blown in from elsewhere.I asked how they planned to deliver soil to oven #5. He said that in every trial they've run in their sandbox on Earth, using the new "sprinkle method," they've developed that uses the rasp motor on the back of the digging scoop to vibrate the scoop and sprinkle tiny amounts of soil out, they've gotten soil to flow "very readily" through the tiny opening onto the TEGA screen, and then by vibrating the TEGA screen for a two or three minutes they've gotten sample into their oven. "We're really pretty optimistic that those problems are behind us," he said.
I also asked about the numbering scheme of the ovens -- if the oven on the end is #4 of 8, why is the one next to it #5? The ovens are actually numbered 0 through 7, and not 1 through 8. The ones we're looking at now are 4, 5, 6, and 7. Oven #0 is directly opposite #4. Why would they do this? Probably because it means they only need three rather than four data bits to encode information in their data stream about which oven they're using: numbers 0 through 7 are binary 000, 001, 010, 011, 100, 101, 110, 111. They could call "000" oven number 8 if they wanted to, but it's easier just to translate directly from binary 000 to decimal 0, and binary 111 to decimal 7.
One of the cool things they mentioned today is that they've developed a new skill: they can now take one sample with the robotic arm and, by careful sprinkling, deliver material taken from one spot to all three of the analysis instruments -- TEGA, the MECA WCL, and the MECA microscope. That's not something they flew to Mars knowing how to do; the plans were always just to scoop and dump onto one instrument, then go get another scoop and dump onto the next instrument. But they had to develop the sprinkle technique to make TEGA work, and that's had the positive payoff that now they can analyze the same sample three different ways, so they know that what they measure in the WCL and TEGA is material that looks the same as what they're seeing in the microscope. Every cloud has a silver lining!
Finally, here's a neat view of that pesky, clumpy Mars soil, taken with the robotic arm camera, focused on a tiny fraction of the width of the tip of the scoop. This image has a resolution of 30 microns per pixel -- that is, 33 pixels make a millimeter! I didn't realize the robotic arm camera had this capability. It's like there's two different microscopes on Phoenix, one with low power magnification and one with high power. And there's still the Atomic Force Microscope to come (which I'm afraid I have not heard mentioned once in one of these telecons).P.S. My robotic arm camera image page is now up to date as of sol 30.
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