In the summer of 1976, Viking 1 landed on Mars carrying a Gas Chromatograph (GC). A few months later a second GC arrived via Viking 2. The design of these instruments, and the data and knowledge that were obtained, remain very relevant today. In fact, we can understand quite a bit about the nature of scientific inquiry through the discourse and controversies of the investigation. The discussion tells an interesting story about the importance of instrumental sensitivity and instrument design.
The two GC/MS systems probed the Martian surface using a technique in which the sample is heated in the absence of oxygen (pyrolysis). Organic materials decompose into simpler volatile compounds that are then swept into the GC via a stream of hydrogen gas. The sample ovens could be heated quickly, up to 500 degrees centigrade within 8 seconds. The instrument design, absolutely state of the art for its time, incorporated many new and important features, including a palladium based separator to remove excess hydrogen gas, and was powered by a small nuclear generator.
Scientists have been concerned that conclusions drawn from the Vikings’ data have been overstated, namely that the results of soil analysis there do not show the presence of organic material. Questions about the use of pyrolysis for sampling led, in 2000-2001, to a reevaluation of the limits of detection, while other experimental parameters including the temperature and the iron-rich content of soil further call into question whether the instruments could have conceivably found evidence of organics.
Experts assert that the instruments performed in exactly the way they were designed to perform, and represent a milestone of achievement in scientific instrumentation. The goals of the Viking mission did not, in fact, include a search for biological evidence, and the instruments were not designed for that purpose; thus extrapolation is inappropriate. Yet, the public is interested in the possibility of life on Mars and the media are eager to make the connection. Scientific debate and peer review, while an accepted part of scientific inquiry, are often seen by those outside the scientific community as flaws in the scientific process. As confidence waxes and wanes, important project funding is influenced.
However, critical evaluation of the 1976 data helps us make important decisions about new projects. For instance, the design of NASA’s new Mars rover spacelab, Curiosity, carries a GC instrument as part of its SAM (Sample Analysis at Mars) unit.
The new GC includes extended capacity for detection of a broad range of compounds, and higher temperature (up to 1000 degrees centigrade). Pyrolytic extractors were included in the design, and upgrades include a more robust detection system, in search of organic substances and isotopic measurements of carbonaceous materials.
NASA’s spacelab Curiosity, a Mars rover, carries a GC-MS, a Gas Chromatography-Mass Spectrometer, as part of its instrument panel.
SAM’s ability to extract materials from and into the surface has also been diversified and includes the capacity for wet chemistry derivatization. Dr. Paul Mahaffy, the principal investigator of the SAM project writes, “This extraction method has been utilized in our laboratory to analyze organics from samples of the highly oxidized, organic-poor soil from the driest part of the Atacama Desert. This wet extraction method avoids the potential issues associated with the transformation by oxidation of these organic compounds during pyrolysis.” The Curiosity landed on Mars on August 6, 2012 and is roving about the Gale crater, powered by the same type of nuclear generator as the 1976 Viking landers.
For more information, consult the NASA website: http://ssed.gsfc.nasa.gov/sam/curiosity.html