For Phoenix, size (and cohesion) matters
Phoenix put its engineering team’s creativity to the test this weekend when the first scoopful of martian soil dumped into one of the Thermal Emission and Gas Analyzer (TEGA) ovens failed to actually make it to the oven. Emily over at the Planetary Society has a great post on this, so I’ll just summarize here.
The scoop of soil was from the top 2-4 centimeters of soil on the north side of the lander, and represents the first scoop analyzed in what was hoped to be a series of scoops taken from increasingly deeper sample locations. Unfortunately, when the soil was dumped into TEGA, it was too clumpy to get through the millimeter-sized grating that covers the entrance funnel. The grating is so fine because particles much larger than a millimeter might clog up the narrow funnel leading to the TEGA oven.
If the soil had made it through the grate, it should have been detected by the laser that is positioned between the grating and the funnel. Unfortunately, the laser detected zilch. This image of the TEGA opening, taken by the robotic arm camera, showed why:
The soil is so clumpy that absolutely nothing passed through the grate!
It’s not well understood how soil becomes clumpy (“cohesive” or “indurated” in geologic terms) on Mars. The usual explanation is cementation by salts or iron oxides. However, both of these processes require either liquid or vapor water, and induration seems to be an ongoing process on the arid surface of Mars today.
So what happens if the engineers can’t figure out how to get the sample into the TEGA oven?
1) These ovens can each only be used one time, and there are 8 total on Phoenix. So now we may be down to only being able to analyze 7 different samples of soil.
2) We may not be able to analyze the top layer of soil for mineralogy, ice content, etc. While this is obviously a problem in terms of getting a complete data set, this also may mean that we won’t be able to as easily compare the results from Phoenix to those of remote instruments. In particular, those that detect composition like OMEGA and CRISM can only sense the top few microns of surface. This will make the results from Phoenix more difficult to extrapolate to other areas on Mars.
3) What if all of the soil in the top 50 cm (the maximum trenching depth for the arm) is clumpy like the surface soil? If this is the case, the engineers will have to figure out a way to break it up before trying to get more of it into the ovens.