Significance of the ALH84001 Research and 1996 Manuscript
Everett K. Gibson, Kathie Thomas-Keprta and Simon Clemett
The August 1996 publication was a benchmark in NASA’s Planetary Exploration Program for Mars. The peer-reviewed paper was based upon more than two years of detailed scientific research by a NASA Johnson Space Center team (co-lead by David S. McKay, Everett K. Gibson and Kathie Thomas-Keprta) and a multi-university group of researchers. The ALH84001 meteorite was from Mars and its carbonate globules offered the opportunity to study the hydrosphere and atmosphere of the red planet and possible biogenic components.
The 1990’s were a period when NASA’s Mars Exploration Program had suffered two major spacecraft failures and was directed to be reduced in scope by the Clinton White House. NASA Administrator Daniel Goldin noted the agency’s planetary exploration program was to be reduced by $6 billion dollars over the next five years. Positions at JPL were being eliminated and exploration programs made “leaner and faster”. The ALH84001 announcement produced interest within the scientific community, legislative branches of government and public. The interest resulted in the Mars exploration program being reinstated. The Mars Exploration Rovers, Mars Reconnaissance Orbiter, Phoenix Lander, Curiosity Rover, Mars Express Mission (ESA) with its Beagle 2 Mars lander along with the orbiters (Maven, Mars Odyssey, etc) origins can all be traced to the ALH84001 research. The excitement of the possibility of potential past or present life on Mars is too important for humans to ignore. The 6 billion dollars were reinstated to the planetary exploration budget and the successful Mars Exploration Program occurred.
The biggest problem arising from the life on Mars debate has been the breadth of knowledge and level of detail required to understand the arguments and interpretation of the analytical data. This can leave many in the general public, and even some in the scientific community, misinformed and/or confused. Unfortunately, in detailed studies of extraterrestrial materials, the old adage remains true ‘the devil is in the details’. The original Science paper noted that within the ALH84001 meteorite’s carbonate globules four lines of evidence existed which offered exciting possibilities for the discovery of past biogenic activity on Mars, if it was ever present. Notwithstanding, we have tried to briefly summarize the original four lines of evidence presented in McKay et al. (1996) and assess how they now stand two decades later.
(1) Presence of complex Ca-Mg-Fe-Mn carbonate assemblages present along fissures and fractures within the orthopyroxene groundmass. These carbonates, while frequently and erroneously described as ‘fracture-filling,’ are in actuality physically inset within only one or other of the adjacent fracture surfaces and show a multifarious chemical and isotopic radial zoning. McKay et al. suggested these carbonates were secondary minerals formed at low-temperatures in the presence of aqueous fluids. The carbonate globules were formed by low-temperature (~25o to 30oC) aqueous processes operating in the near-surface of Mars.
(2) Observation of mineralized micron and sub-micron spherical and elongated ellipsoid structures within altered fracture surfaces and co-localized with carbonate assemblages. McKay et al. suggested these features, spatially associated with the carbonate globules, were ‘biomorphs.’
(3) Detection of organic matter, in the form of polycyclic aromatic hydrocarbons (PAHs), intimately associated with the carbonate assemblages and demonstrably not an artifact of terrestrial contamination. McKay et al. suggested these organics were molecular fossils of more complex (bio)organic species.
(4) Identification of a large population of chemically pure, single-domain magnetite crystals present thorough out the carbonate assemblages and preferentially exhibiting an unusual asymmetric crystal habit, classified as truncated hexaoctahedral. Although the carbonate assemblages contain Fe, and Fe-carbonate decomposes to magnetite when heated, McKay el al., and later work by Thomas-Keprta et al. (2009), reasoned this was not source of the observed magnetite since: (i) while pure Fe-carbonate decomposes to pure magnetite a mixed Fe-carbonate cannot be decomposed to pure magnetite; (ii) carbonate decomposing to magnetite results in a decrease in volume yet the majority of magnetite was not associated with porosity and void space in the surrounding carbonate; (iii) the magnetite size-distribution was strongly centered in the single-domain regime; and, (iv) the unusual crystal habit and favored elongation along a single  crystal axis are not characteristics exhibited by any known population of abiogenic magnetite. However, McKay et al. did note that these characteristics were often collectively exhibited by magnetite formed under biological control within terrestrial magnetotactic bacteria.
McKay et al. (1996) argued that these lines of evidence, when taken collectively, can be interpreted to be a product of biogenic activity. Since 1996, all have been challenged and have been the subject of extensive investigation. The formation temperature of the carbonates initially vacillated wildly but has now converged with the general consensus they formed in a low-temperature, aqueous environment. The observation of ‘biomorph’ like features, although valid, but is not considered indicative of biology. The presence of organic matter embedded within the carbonates has been confirmed by other research groups using different analytical techniques. Finally the observation of a unique population of magnetite crystals, while the most strongly challenged, remains perhaps the most convincing argument for biological activity associated with carbonates in ALH84001. We have continued to research these magnetites and have published numerous papers, with the idea that, if we collected new data and needed to revise our original hypothesis, we would be the first to modify or retract the life on Mars hypothesis. To date, we continue to support our original hypothesis.
The research on the ALH84001 and additional martian meteorites have led to an explosion of interest in the search for past and present life on Mars. Both NASA and the National Science Foundation have established programs to understand the potential of life in extreme environments. The NSF established their program for Life in Extreme Environments. NASA created the Astrobiology Institutes in 1998 to develop the field of astrobiology and to provide a scientific framework for flight missions. As of 2016, the NASA Astrobiology Institute (NAI) has included 26 past and present teams with more than 600 researchers distributed across ~100 institutions. Currently there are 13 international partners with the NAI. The ALH84001 announcement, despite whether you are a believer of the hypothesis or not, has clearly been the guiding idea for the development of the new interdisciplinary field of astrobiology. This must be viewed as positive for the field of scientific discoveries.
Today, the research team believes that the original hypothesis is valid. No scientific data has been presented to date that disproves any of the four original lines of evidence presented in 1996. Interpretation of the data is where the disagreement arises. Research on additional martian meteorites (i.e. Nakhla and Yamoto 000593) has shown that there is verification of indigenous organic materials within Martian materials. The research on the ALH84001 has shown the difficulties of working on trace signatures of past and present life within any geological sample. It is very difficult to believe any remote scientific package delivered to the surface of Mars can answer the question of past/present life. Samples from Mars can be studied today in terrestrial laboratories. The martian meteorites offer excellent opportunities for the scientific community to study samples from Mars. Procedures for distinguishing contaminants from indigenous components can be developed with these valuable samples.
Carl Sagan is often referred to as the “gate-keeper of scientific credibility”. He has been quoted with “Extraordinary claims require extraordinary evidence.” One of the five original reviewers of the ALH84001 manuscript for SCIENCE was Carl Sagan. It has been noted that Carl recommended to SCIENCE that the manuscript be published. His letter to members of The Planetary Society noted the significance of the ALH84001 research and its possible implications for future exploration of Mars.
April 20, 2017