Phoenix Shake and Bake
Phoenix already has found evidence for water ice beneath the top layer of soil. Chunks of material disappeared four days after Phoenix scooped out a trench in the surface. Scientists believe these dime-size pebbles must have been water ice that sublimed, or turned directly into a gas, after they were exposed to surface conditions. The existence of water makes it more likely that Mars is habitable.
In this interview with Astrobiology Magazine’s Helen Matsos, William Boynton, the lead scientist for Phoenix’s Thermal and Evolved Gas Analyzer (TEGA), talks about how the instrument works, and what it can tell us about the possibility for life on Mars.
So we measure the calories of that – it’s called a calorimeter. When we heat the sample up to 1,000 degrees C (1,832 F), we’ll see if there are other phase transitions. For example, if there are any carbonates, they will decompose at high temperature and give off carbon dioxide. It takes some extra heat to do that, just like it takes extra heat to melt ice. If CO2 is released at around 600 or 700 degrees C, that might indicate the presence of a carbonate, but if CO2 is released at a relatively low temperature, like around 400 degrees C, that would be indicative of organic molecules.
As we heat up the sample, we are “sniffing” the gases that are given off with our Evolved Gas Analyzer. We have a nitrogen carrier gas that we flow over the sample so that as different gases evolve, whether they are organic molecules or just water vapor from the ice, the nitrogen gas carries it to a thin tube that leads to the Evolved Gas Analyzer.
We have a mass spectrometer to measure the different gases given off. We can look for isotope ratios, for example, the carbon in CO2 can exist as C12 or C13, the oxygen as O18 or O16. These give us different masses, and that ratio tells us something about where the CO2 might have come from.
A couple of doors on the instrument gave us some problems in the first few days because they didn’t open completely. Then, when we put a big slug of dirt on the instrument, it clogged up the screen. The instrument has a vibrator, and as we ran the vibrator, that shook the screen. That is supposed to dislodge the dirt so it could pass through the screen into a funnel, which then leads to our little ovens. It was only on the seventh try that we finally got dirt to go through the screen. We don’t know if it finally worked or if the dirt had to dry out a little bit. But we now know that the soil is very cohesive at this location on Mars.
WB: I wouldn’t have known how to answer this until we got those microscopic images back. The microscope shows that the particles are all very small, so I don’t think we are fractionating the sample much, although if there are any small pebbles obviously we will be discriminating against those.
In our first test, after we closed the oven and started flowing the nitrogen gas, we made measurements as we heated the sample up to 35 degrees C (95 F). We just took it to that low temperature to see if any ice was present. At 35 degrees C, water on Mars will boil and that will vaporize any water that might have been in the soil.
There wasn’t any ice in that sample, but we didn’t expect to find any because it came from near the surface and had been exposed to the sun for a long time. Also, the soil was sitting on the roof of TEGA for three or four days while we were vibrating it, trying to get it to fall through the screen. It probably had plenty of time to dry out. Conditions at the martian surface aren’t at the melting point of water, but it is well above the vaporization point. If there were any ice in the sample we would expect it to have turned into a gas.
For our second test we heated the sample up to the mid-temperature of 175 C (350 F). CO2 was given off from the soil. That’s not unexpected because Mars’ atmosphere is CO2, and CO2 is a compound that can stick easily to grain surfaces.
We kept the sample at that temperature for an hour to see if there are any oxidants that would destroy organics. Holding the sample at 175 degrees for that long will decompose some of the oxidants that people have suggested may be present. We want to get them out of the way because we don’t want them to interact with any organics we might release when we heat the sample up to 1,000 degrees. If there’s an oxidant present, organics may get oxidized into CO2, essentially burned before they actually leave the sample. Then we would see a CO2 release rather than a release of some benzene ring or fragments of different organic molecules. We also recognize that there may be some oxidants that are intrinsic to the martian soil, for example ferric iron, the kind of iron that is in rust. That can also potentially oxidize organics.
AM: How will you eliminate the possibility of contamination versus a true reading for organics?
WB: Phoenix has a “blank” that we can analyze. The blank presumably has very low organics -- we heated it to bake all the organics out of it. If we analyze that and find much lower levels of organic contamination in the blank than we do in the Mars samples, then we can say that what we’re finding in the samples comes from Mars, not Earth. On the other hand, if when we analyze the blank we see the same levels of organics that are detected in the soils, then we can say the organics are coming either from contamination on the instrument or the robotic arm scoop.
AM: Isn’t there still confusion over the Viking mission results as to whether or not they did detect organics? It seems like a big mystery.
WB: What people concluded, I think correctly, is that Mars is a very oxidizing environment and that any organic molecules might have been oxidized into CO2. That is perhaps why the Viking experiment didn’t find any.
But it is still a mystery, because even if there weren’t any indigenous organics on Mars, we know there are meteorites impacting the surface all the time and they bring organic molecules with them. If the organic molecules weren’t destroyed by the impact, there should be maybe one percent or so of organics floating around out there.
It’s surprising that Viking didn’t find any organic molecules, but that experiment only heated the samples up to 500 degrees C (932 F). Some of the early break-down products of organics, if you don’t oxidize them all the way to CO2, make something like tars or kerogens. These are very heavy organic molecules, and they won’t vaporize until you get up to about 700 or 800 degrees C (1292 – 1472 F). It’s possible that there may have been a few of these in the Viking area on Mars, but because they don’t vaporize until they are heated to 700 degrees C, they were missed.
AM: Could any of the results from Phoenix be interpreted as biosignatures?
WB: We wouldn’t ever refer to anything Phoenix finds as a biosignature, we would refer to it as an organic signature. Saying that an organic signature is a biosignature is a far bigger step, and we won’t get the data that would allow us to make that second step. We’re mainly looking to see if organic molecules have been able to survive in this colder environment. If there is ice present, it might be a more protective environment than the lower latitudes where Viking landed.
AM: And where would you expect to find organics-- if there are any -- in the ice, or in the soil?
WB: It’s hard to say. It might be in the soil that’s mixed in with the ice, with the idea that the ice might be protecting the organics from being oxidized by the Mars atmosphere. In the polar regions there may be less of the super-oxides present that we believe are made in the atmosphere by the interaction with sunlight. Those oxidants destroy organics, so if there are less oxidants around we may be more likely to find organic molecules.
AM: Will you be taking eight different samples for the eight different TEGA ovens?
WB: The samples will be from the same area, but some will be from a little bit deeper beneath the surface. It might be interesting to see that maybe there are no organics on the surface, but if go down 5 centimeters or so we may find them. We do expect to get ice samples. We’re pretty sure that ice is present because we’ve seen this white stuff that could be ice or could be salt, plus the gamma ray spectrometer on Mars Odyssey found that there are large amounts of ice buried just a few inches below the surface.
WB: No, not at all. We’ll be depositing soil the same way it was given to the optical microscope a few days ago. It will be slowly sprinkled onto the screens and we’ll be running our vibrator at the same time to help bounce the grains through the screen. So I’m optimistic that next time we’re not going to have any problems.
AM: But ice is not a grain. How do you expect that to filter through the screen?
WB: We don’t think we’ll be able to dig the ice because it’s probably going to be too hard at these low temperatures. So we brought along a rasp. It’s about the size of my little finger, and it will grind into the ice and generate a bunch of ice dust, kind of like saw dust if you’re grinding into wood. So the ice sample will be powdery, and it should go through the screen just fine.
AM: And you’re not afraid of it vaporizing before you can get it into the oven?
WB: We are concerned about that, and people have done some calculations that indicate it ought to last for a few hours before vaporizing. We have asked for the ice to be delivered to the oven within ten minutes after we have scooped it.
AM: When will you start digging for ice samples?
WB: We are going to try in the next week or so to dig down to the ice. We don’t really know how we’re going to sample it. My guess is that we may try scraping the ice first, but if it’s too hard, we’ll use the rasp to try to grind out the ice.