Mars: The Living Planet Barry E. DiGregorio, with additional contributions by Gilbert V. Levin and Patricia Ann Straat. 365 pages, 1997. Now available on Kindle

Book Review from Spectroscopy Magazine
by Dr. Troy D. Wood

Mars: The Living Planet by Barry E.DiGregorio is a fascinating, in-depth analysis of the life-detection experiments carried on board the Viking landers to the planet Mars and the implications of their results. The author spent five years analyzing data from these experiments, which were at the core of the Viking landers' primary mission to search for life within the Martian soil. These experiments are thoroughly described, and the historical background for their inclusion aboard Viking is presented. Although not a life-detection experiment per se, the gas chromatograph-mass spectrometer (GC- MS) on board the Viking landers was an additional tool used to determine whether any organic molecules existed in the surface soils of Mars.

DiGregorio explains, in impressive detail, the operation of each experiment and describes the differences between the expected results for a blank versus a positive. Most of the discussion focuses on two experiments having results that were most closely examined for the existence of life on Mars. The highly sensitive LR experiments developed by contributors Levin and Straat showed the detection of radioactively labeled carbon dioxide gas after exposure of Martian soils to radio labeled nutrients. In seven of the nine LR experiments conducted on the Viking landers, the radio labeled carbon dioxide was detected, as would be expected if microorganisms were living in the Martian soils. Sterilized soils that were heated did not show generation of radio labeled carbon dioxide. In addition, when Martian soil was held in the dark for several months but not heated, radio labeled carbon dioxide was not released, suggesting that the result could only have occurred because of a biological process.

As DiGregorio elaborates nicely about, another explanation besides the one supported by Dr. Levin — that life is extant on Mars — was put forth by many on the Viking team. To explain Levin's positive LR experiments, an alternative explanation involving exotic "oxidation" chemistry, caused by the presence of peroxides or superoxides in the Martian soil, was proposed. Such oxidation processes, it was reasoned, might be responsible for false positives in the LR experiment (of course, such exotic chemistry does not explain why radio labeled carbon dioxide was not released from the soils held in the dark for several months). It was argued that the absence of detection of organic molecules in the GC-MS experiments supported the oxidants theory because such oxidants would destroy organic molecules through radical reaction mechanisms. DiGregorio points out numerous flaws in the oxidation hypothesis, including the fact that no peroxides or superoxides have ever been detected in Martian soil! In addition, subsequent to the wide dissemination of this (weak) hypothesis, Levin and colleagues showed in a laboratory version of LR that even in the presence of such oxidizing agents, false positives could not be obtained.

One of the real strengths of DiGregorio's book, however, is his examination of the flawed GC-MS experiments. GC-MS instruments consist of two primary components: the gas chromatograph, which separates volatile substances based on their partitioning behavior between the gaseous state and a solid "stationary phase," and a mass spectrometer, which detects charged molecules (or ions) as a function of their mass-to- charge ratio. In the Viking GC-MS, the GC and MS were separated by a palladium separator; the function of this separator was to allow organic molecules transported by the hydrogen "carrier gas" in the gas chromatograph to the mass spectrometer.

Unfortunately, palladium is poisoned by the presence of sulfur, an element found to be in abundance in the Martian soil. Thus, Martian soil containing sulfur was incubated in the oven of the gas chromatograph, rendering the palladium filter inoperative (such palladium poisoning by sulfur is described in the PhD thesis of a student of Dr. Klaus Biemann, the leader of the Viking GC-MS team). Thus, even if the gas chromatograph successfully separated any organic molecules that might be present, they may not be able to penetrate a poisoned palladium filter and would remain undetectable even though they were actually present. Most modern GC-MS use helium rather than hydrogen as a carrier gas. Helium does not require a palladium separator, and thus would be insensitive to the sulfur content in the Martian soils. To date, however, the GC-MS experiments have never been repeated on Martian soils.

From this, DiGregorio rightly points out that the conclusion the scientific community drew about the nonexistence of life on Mars, based on the lack of organic molecules being detected in the GC-MS experiments, is highly flawed. As described by the authors, the exposure of the palladium filter to sulfur rendered any result from the Viking GC- MS in question. As a professional mass spectrometrist myself, I view the Viking GC-MS results as a detection-limit problem. In this case, the mass spectrometer could not detect organic molecules because they could not pass through the palladium filter into the mass spectrometer. Interestingly, subsequent to the Viking missions, in 1996, MacKay and coworkers clearly showed the existence of complex organic molecules in the Martian meteorite ALH84001, using, amongst other tools, a mass spectrometer. This has caused some wide speculation on the possibility of early life in Martian geologic history. Independent of whether early microorganisms might have been present on Mars or not the results of MacKay indicate that organic molecules clearly existed in the earlier geological history of Mars and brings into further question the "oxidant" hypothesis. This by itself weakens the dogma that "no organics equals no life" on Mars.

In the very least, the conclusion reached by DiGregorio in Mars: The Living Planet — that the Viking LR experiment detected microorganisms in the surface soil of Mars — should alert the scientific populace that the earlier autocratic conclusion that "no evidence for life on Mars" was found is simply untrue. Levin's LR experiment never detected a false positive from sterile Antarctic soils, yet detected evidence of radioactively labeled carbon dioxide released in seven of the nine LR experiments conducted by Viking. This, coupled with the lack of any compelling evidence that oxidants in the soil of Mars destroy organic molecules that might be present, should in the very least make every responsible scientist question the dogma taught in undergraduate astronomy courses that Mars is a sterile world. Indeed, DiGregorio and Levin (who, along with Dr. Straat, contributed one chapter to this work) clearly believe that the existing Viking LR results are sufficient to conclude in all likelihood that life, exists on Mars. Although I would make a more conservative conclusion based on the sum of the Viking data and subsequently collected evidence, it is short sighted to state unequivocally that there is no life on Mars. Why so many scientists and NASA would continue to push the "oxidant" hypothesis, in view of the evidence, is puzzling. DiGregorio has his own speculations on this point, and it is left up to readers to decide why the weak "oxidant" hypothesis has so much support.

DiGregorio and his contributors have clearly shown that the conclusions based on the Viking life-detection experiments follow highly questionable suppositions. That being so, DiGregorio's urge that NASA use utmost caution before embarking on its goal to return a Martian soil sample to Earth by 2008 is hardly hysterical. In fact it is a voice of reason. Even if the chances for present-day Martian microorganisms are minute (and based on the LR results, they are certainly far more likely than that), until further experiments are conducted on Martian soils to provide more solid evidence for the existence (or nonexistence) of life, it is irresponsible to bring extraterrestrial soil back to Earth. It may well be that there are Martian microorganisms, and that they may prove harmless; however, in the extreme scenario, Martian microorganisms could be deadly to any number of Terran organisms — bees, algae, and even humanity. They could also cause an ecological disaster not seen on Earth since the mass extinction of the dinosaurs 65 million years ago by an asteroid or cometary impact (DiGregorio puts forth an interesting hypothesis that this impact may have brought along extraterrestrial microbes that contributed to the dinosaurs' downfall).

DiGregorio proposes a sensible alternative to bringing Martian soil to Earth directly, namely to take it to the international space station for analysis until it is proven not to be a biohazard. In view of the possible disastrous consequences if Earth's biosphere is accidentally contaminated by Martian microorganisms, DiGregorio's proposal is entirely logical and should be supported by the scientific community.

Troy D. Wood
Department of Chemistry, State University of New York-Buffalo

November 1999 edition of Spectroscopy Magazine
Copyright © 1999 Spectroscopy Magazine. Used by permission.