Sample Analysis at Mars (SAM) Problems
Did SAM just give us a hint about the real atmospheric pressure on Mars? 9/23/2012
Figure 1 above: Schematic of the Sample Analysis at Mars (SAM).
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Preparations also continue for taking the first sample of the Martian atmosphere. “We are the nose of Curiosity” says SAM (Sample Analysis at Mars) principal investigator Paul Mahaffy. During initial check out tests of SAM, scientists discovered the amount of air from earth’s atmosphere remaining in the instrument after launch was more than expected. As a result, a difference in pressure on either side of tiny pumps led SAM operators to stop pumping out the remaining air as a precaution. The pumps subsequently worked, and a chemical analysis was completed on a sample of earth air.
“As a test of the instrument, the results are beautiful confirmation of the sensitivities for identifying the gases present,” says Mahaffy, who adds the initial indication of methane caused a brief flurry of excitement until the terrestrial origins of the gas were recognized. Mahaffy, who is based at NASA’s Goddard Space Flight Center, says “a few Sols down the road we’re looking forward to getting our first sniff of Mars atmosphere.”
The SAM is a key tool in Curiosity’s search for signs of life, past or present, and is more sensitive and sophisticated than the sensors on the Viking lander which came up negative for organics. The system is designed, for example, to examine a wider range of organic compounds and can therefore check a recent hypothesis that perchlorate - a reactive chemical discovered by the Phoenix Mars Mission – may have masked organics in soil samples taken by Viking."
As is shown on the extract below, there were two surprises in reference to the atmosphere as initially measured by the MSL Sample Analysis at Mars (SAM). The first cause for question was that, “During initial check out tests of SAM, scientists discovered the amount of air from earth’s atmosphere remaining in the instrument after launch was more than expected.” Here the question is, “Why?” It is important to listen to the Principal Investigator for SAM at Goddard: Paul Mahaffy at 42:46 into this video. The full segment in question at the JPL press conference is between 41:53 and 43:50. In discussing why they had to shut down SAM for a while, a transcript of the answer (with a few words undecipherable shown in red font) is roughly as follows:
REPORTER: Be more specific about what caused SAM to quit and take up Mars gas.
ANSWER BY DR. PAUL MAHAFFY: It turns out we had these miniature pumps. We call them wide range pumps but they’re really turbo-molecular pumps on top of the molecular dray stage. The really nice thing about these pumps is they exhaust naturally right at Mars pressure, 10 millibar, 7 millibar. Um, and it turns out there is a very slow leak, uh, into the Tunable Laser Spectrometer and so there was just a little bit of a residual atmosphere in the harriot cell which is the cell where the light bounces back and forth to get a long path length for the methane, the carbon dioxide and the water measurements and so the tens of millibars that we had in there, I think we had 51 millibar and we had assumed that the pump would be fine evacuating that, we routinely evacuate Mars ambient out of the cell but it was just high enough the current sensor on the pump said, nah this is a little bit too high I‘m gonna turn myself off and it did but SAM continued merrily along its measuring path assuming that we had not turned off and so we measured that gas with both the mass spectrometer and the Tunable Laser Spectrometer. It really led to some excitement. The TLS (Tunable Laser Spectrometer) Team, Chris and Greg, their eyes were wide open. They saw all this methane, and it turns out it's terrestrial methane, but it was kind of a good test….
The reason he thought it was a good test were given in the article:
“As a test of the instrument, the results are beautiful confirmation of the sensitivities for identifying the gases present,” says Mahaffy...
The 51 mbar mentioned by Dr. Mahaffy should not be overlooked. That might be the first real clue about how high Martian pressure really is. On Earth that pressure would equate to an altitude of about 63,057 feet or 19,220 meters.