Sample Analysis at Mars (SAM) Problems

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Did SAM just give us a hint about the real atmospheric pressure on Mars? By Barry S. Roffman, 10/24/2015

Update on 3/30/2015: We have the latest information about Methane and other organic chemicals found on Mars at http://davidaroffman.com/photo4_15.html.

Update on December 17, 2014: There is a new report that methane spikes have been found by MSL at Gale Crater. We are just now hearing the report. See the 52-minute press conference at http://mars.jpl.nasa.gov/msl/mission/science/researchpapers/.

 

Previously posted by us on November 28, 2012: The SAM (Sample Analysis at Mars) has apparently found something big that may relate to life on Mars. See the story at http://www.foxnews.com/science/2012/11/28/scientists-speculate-mars-rover-discovery/. SAM is shown on Figure 1 below. Like the REMS (Remote Environmental Monitoring Station), it suffered from some initial confusion with respect to how to interpret data. Below the Figure 1 schematic of SAM is an extract of any article about its initial findings and problems by Guy Norris at  aviationweek.com. The rest of this article was written back on 9/23/2012.

Figure 1 above: Schematic of the Sample Analysis at Mars (SAM).


New Mars Curiosity Landscape Images Surprise Scientists

 

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.

   

       As for confusing planetary atmospheres, terrestrial air is primarily Nitrogen (about 78%) and Oxygen (about 21%). Martian air is over 95% carbon dioxide. Methane is an extremely small proportion of Martian and Terrestrial atmospheres (see Figure 2 below). Given such vast differences in main gas composition, the brief confusion seems curious unless the SAM was only supposed to look for methane (but then again it did distinguish Martian air from Terrestrial air). While all animals and plants on Earth produce methane, the presence of this gas on Mars is so low that it wasn't even known about until relatively recent times. The specific sites on Mars where methane has been detected (Nili Fossae, Terra Sabae and Syrtis Major) are shown on Figure 3 below.

Figure 2 below: Composition of Martian and Terrestrial Atmospheres.

Brief Description

Figure 3 below: Map of Mars showing landing sites through September 2012 and methane plumes.