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Biomimetic Properties of Minerals and the Search for Life in the Martian Meteorite ALH84001

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Biomimetic Properties of Minerals and the Search for Life in the Martian Meteorite ALH84001 EA40CH08-Young ARI 23 March 2012 10:48 Biomimetic Properties of Minerals and the Search for Life in the Martian Meteorite ALH84001 Jan Martel,1,2,3 David Young,1,4 Hsin-Hsin Peng,1,3 Cheng-Yeu Wu,1,3,5 and John D. Young1,3,6,7 1Laboratory of Nanomaterials, Chang Gung University, Gueishan, Taoyuan 333, Taiwan, Republic of China; email: [email protected], [email protected], [email protected], [email protected], [email protected] 2Graduate Institute of Biomedical Sciences, Chang Gung University, Gueishan, Taoyuan 333, Taiwan, Republic of China 3Center for Molecular and Clinical Immunology, Chang Gung University, Gueishan, Taoyuan 333, Taiwan, Republic of China 4Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 5Research Center of Bacterial Pathogenesis, Chang Gung University, Gueishan, Taoyuan 333, Taiwan, Republic of China 6Laboratory of Cellular Physiology and Immunology, Rockefeller University, New York, New York 10021 7Biochemical Engineering Research Center, Ming Chi University of Technology, Taishan, Taipei 24301, Taiwan, Republic of China Annu. Rev. Earth Planet. Sci. 2012. 40:167–93 Keywords First published online as a Review in Advance on amorphous minerals, biogenesis, biomineralization, nanoparticles, January 10, 2012 nannobacteria, nanobacteria The Annual Review of Earth and Planetary Sciences is online at earth.annualreviews.org by 219.87.61.9 on 05/03/12. For personal use only. Abstract This article’s doi: The existence of extraterrestrial life was heralded by controversial claims 10.1146/annurev-earth-042711-105401 made in 1996 that the Martian meteorite ALH84001 harbored relics of Copyright c 2012 by Annual Reviews. ancient microorganisms. We review here the accumulated evidence for and All rights reserved against past extraterrestrial life in this Martian meteorite. The main pro-life 0084-6597/12/0530-0167$20.00 Annu. Rev. Earth Planet. Sci. 2012.40:167-193. Downloaded from www.annualreviews.org arguments—the presence of polycyclic aromatic hydrocarbons, magnetite crystals, carbonate globules, and structures resembling terrestrial life-forms known as nanobacteria—can be deemed ambiguous at best. Although these criteria are compatible with living processes, each one of them can be explained by nonliving chemical processes. By undergoing amorphous-to- crystalline transformations and binding to multiple substrates, including other ions and simple organic compounds, minerals—especially those containing carbonate—have been shown to display biomimetic properties, producing forms that resemble bacteria. This simple and down-to-earth explanation can account fully for the existence of mineral entities resembling putative nano- and microorganisms that have been described not only in the ALH84001 meteorite but also in the human body. 167 EA40CH08-Young ARI 23 March 2012 10:48 INTRODUCTION The search for concrete evidence of extraterrestrial life took a great leap forward in 1958 with Polycyclic aromatic the advent of the National Aeronautics and Space Association (NASA) and its space program hydrocarbon (PAH): (Dick 2006). Early on, the NASA space program focused on searching for life on Mars primarily ubiquitous organic because of its proximity to and apparent similarities with Earth. The first exploration of Mars molecule produced consisted of a flyby mission in 1965 made by the Mariner 4 probe. This exploration revealed, from the combustion of organic matter, the among other findings, that the surface of Mars is arid and that its air pressure is rather thin— decomposition of dead less than 1 kPa compared with 101.3 kPa on Earth—suggesting that there is apparently no large organisms, or the amount of liquid or heat-trapping atmosphere on the planet’s surface (Dick 2006). The Mariner heating of simple 4 probe also suggested that Mars does not possess a magnetic field, which would have otherwise inorganic molecules protected the planet against deadly solar and cosmic rays. The Mariner 4 mission was followed by the landing of the two Viking spacecrafts on Mars in 1976. Although the experiments conducted during these missions provided evidence for the presence of oxidizing materials in the Martian soil, the absence of organic molecules in the samples examined led to the general consensus that there is probably no life on the surface of Mars (Dick 2006). However, in 1996, a widely publicized study by David S. McKay and his colleagues at the NASA Lyndon B. Johnson Space Center in Houston (McKay et al. 1996) presented a series of observations that were interpreted as representing signs of past life in the Martian meteorite Allan Hills 84001 (ALH84001). This meteorite, an igneous orthopyroxenite discovered in Antarctica in 1984, was believed to have originated from Mars on the basis of both its mineralogy and the isotope composition of the air that remained trapped inside the meteorite (Gibson et al. 1997, Mittlefehldt 1994). In fragments of the ALH84001 meteorite, McKay and his group found traces of polycyclic aromatic hydrocarbons (PAHs), magnetite crystals, carbonate globules, and putative microfossils of miniature bacteria (McKay et al. 1996). In addition to renewing interest in the search for extraterrestrial life and spearheading the creation and expansion of NASA’s astrobiology program, this report was notable for instigating a major scientific controversy about the prospect of life on other planets as well as a highly publicized debate about what exactly constitutes life (Arrhenius & Mojzsis 1996, Cavicchioli & Ostrowski 2003, Gibson et al. 1997, Grady et al. 1996, Hamilton 2000, Hogan 2003, Young & Martel 2010). The potential impact of these findings prompted President Bill Clinton to declare in an unprecedented White House press conference: by 219.87.61.9 on 05/03/12. For personal use only. “Today, rock 84001 speaks to us across all those billions of years and millions of miles. It speaks of the possibility of life. If this discovery is confirmed, it will surely be one of the most stunning insights into our universe that science has ever uncovered” (Young & Martel 2010). Needless to say, the interest in this meteorite insofar as its implication for biogenesis or the origin of biological life is concerned—on Earth and elsewhere—has continued undeterred in spite of numerous subsequent publications seeking to clarify this controversy. For instance—and this Annu. Rev. Earth Planet. Sci. 2012.40:167-193. Downloaded from www.annualreviews.org sounds like a wistful reminder of all the excitement unleashed by the meteorite findings in the 1990s—the interest in the search for life on Mars and other planets was galvanized again by recent observations that Mars once possessed a magnetic field (Acuna et al. 1999, Solomon et al. 2005) and possibly large amounts of liquid water (Malin & Carr 1999, Malin & Edgett 2000). In addition to the observation that this planet harbors water ice on both polar caps (Solomon et al. 2005), a recent, highly publicized study also suggests that subsurface water may still be flowing on Mars today (McEwen et al. 2011). Here, we review the arguments for and against the hypothesis of past life associated with the Martian meteorite ALH84001. We conclude that the results of the chemical and morphologi- cal analyses performed on the ALH84001 meteorite are insufficient to confirm the existence of past life on Mars. All the proposed biomarkers for past life on Mars can be attributed to lifeless 168 Martel et al. EA40CH08-Young ARI 23 March 2012 10:48 processes, such as mineral growth and simple ionic or particle associations as well as amorphous- to-crystalline mineral transformations or interphases. Finally, we review data demonstrating that mineral formations of known chemical compositions can be shown to display biomimetic proper- ties that enable them to resemble miniature bacteria and other primitive terrestrial life-forms. THE CASE FOR PAST LIFE ON MARS: ALH84001 REVISITED Presence of Polycyclic Aromatic Hydrocarbons PAHs are molecules of fused aromatic rings comprising carbon and hydrogen atoms. Their abun- dance on Earth is attributed largely to their constant production from the slow geochemical degradation of organisms and the incomplete combustion of organic matter (Mackenzie et al. 1982, Simoneit 2002, Simoneit 2004). Although PAHs are ubiquitous on Earth, they are not produced by living organisms and do not possess any specific role in living processes. PAHs can nonetheless affect human life since they represent important environmental pollutants that may induce mutations in DNA and cause cancers (Liu et al. 2008, Møller et al. 1985). In addition to being produced from the degradation of organic matter and from combustion, PAHs can also be produced from nonliving inorganic processes, including from heated mixtures of simple inor- ganic molecules such as CO, CO2,andH2 (Anders 1996, Simoneit 2002, Zolotov & Shock 1999, Zolotov & Shock 2000). For this reason, PAHs are commonly found throughout the universe as well as in interstellar dust and on the surface of other planets (Hahn et al. 1988, Tielens 2008). McKay et al. (1996) described low amounts of PAHs at the level of 1 part per million (ppm) in the ALH84001 meteorite (Grady et al. 1996). These compounds were interpreted as representing the products of the degradation of past microorganisms, a process known as diagenesis (McKay et al. 1996). Because PAHs were present on Earth and could have contaminated the meteorite during its presumed 13,000-year history within Antarctic ice (Gibson et al. 1997), it is first necessary to determine if the PAHs found in the ALH84001 meteorite are indigenous to Mars. Several initial lines of evidence suggest that this is the case. For one, the PAH compounds found in ALH84001 are present at concentrations much higher than those of the compounds measured in the Antarctic ice where the meteorite had been found (Clemett et al. 1998, McKay et al. 1996). In addition, the PAHs identified in ALH84001 are deemed to be structurally different from the ones found in by 219.87.61.9 on 05/03/12.
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