The Alh Case For Life On Mars

ALFONSO F. DAVILA1, ALBERTO G. FAIREN1,

DIRK SCHULZE-MAKUCH2 AND CHRISTOPHER P. MCKAY1

'NASA Ames Research Center, Moffett Field, CA, USA. 2School of Earth and Environmental Sciences, Washington State University, Pullman, WA, USA.

Abstract The Martian Meteorite ALH84001 provided the most recent and controversial chapter in the search for life on Mars. The bold claims of possible traces of Martian biological activity within the meteorite, stirred countless debates regarding some of the basic principles of life, and gave rise to the golden age of Space Exploration that we are witnessing. Regardless of the final verdict, the ALH84001 case for life on Mars exemplifies the importance, the complexity and the excitement associated with the field of Astrobiology, and the quest for the search of life beyond our own planet.

1. A Place in History

The popular belief that Mars may be the nearest inhabited world has persisted for centuries. Early observers of Mars saw through their telescopes oceans and continents (Secchi, 1863; Flammarion, 1892), vegetation (Lowell, 1894), and even complex structures that could only be the making of intelligent civilizations (the famous canali, Schiaparelli, 1878). These interpretations were widely accepted by the public, and readily assimilated by the fiction literature, but met strong opposition within the scientific community. At the turn of the twentieth century, theoretical calculations already indicated that temperatures on the Martian surface were likely well below freezing (Wallace, 1907). At these temperatures the white Martian poles were probably composed of frozen carbon dioxide, rather than water and the dark patches on the Martian surface were likely deserts, not forests. After decades of efforts directed towards the mapping of the Martian surface, the interest of astronomers then turned to determining the composition of Mars' atmosphere and surface. Soon it was established that Mars had a very thin atmosphere mainly composed of CO2, and that the surface of the planet was extremely arid (Kaplan et al., 1964). As new discoveries unraveled, the surface of Mars was slowly becoming more and more inhospitable.

As the era of ground-based observations of Mars declined, the era of robotic exploration slowly rose. Mariner 4 (1965) was the first successful robotic mission to Mars. After the first close up images of the Martian surface were transmitted by the small spacecraft, an unexpected picture of the planet unfolded: No oceans or continents, no vegetation, and worst of all, a dry and heavily cra-tered surface suggesting a long-lasting, violent past. On December 2, 1971 a descent module successfully landed for the first time on the surface of Mars. Mars 2 became the first man-made object to land on another planet. However, contact with the lander was lost within seconds. At the same time, Mariner 9 was quietly orbiting Mars, and conducting a systematic mapping of the surface. Olympus Mons was the first feature to emerge, Hellas and Argyre followed. But the most revolutionizing discovery brought up by Mariner 9 were the images of networks of channels and tributaries that strongly resembled runoff channels and dry riverbeds. Mars was suddenly brought back to life. If there was, or had been, water on the surface, then organisms could have also been present.

The great success of Mariner 9 represented a strong push to what arguably became the most ambitious mission ever sent to Mars: The Viking landers. This mission remains to date as the only life detection experiment conducted outside our planet. Each of the Viking modules consisted of an orbiter and a lander. The landers were equipped with television cameras, meteorological instruments and a miniature laboratory to carry out sophisticated biological experiments. Viking 1 landed on Chryse Planitia on July 20, 1976. Viking 2 landed at Utopia Planitia on September 3, 1976. Both landers started to provide outstanding images and data immediately after touchdown. The long awaited biological experiments also seemed to work according to plan, however they provided confusing results. They had been designed to unambiguously detect organisms thriving in the surface soil, but the results of the experiments were difficult to interpret. After years of debate over the Viking results (Klein, 1999), the overall scientific consensus is that the landers did not provide any hint of Martian life, albeit some voices still argue heartily that there is indeed evidence for life hidden in the data. Independently of anyone's position, after the Viking mission it became clear that the surface of Mars is extremely severe and harsh, and likely not habitable for life-as-we-know-it. Planetary scientists began to recognize the need to better understand the limits of life on Earth, before we attempt to search for life somewhere else. Mars terrestrial analogues such as the Polar Regions or the hyper-arid deserts became the new ground for Astrobiology during the 1980s and early 1990s. These analogue studies provided a wealth of data that increased vastly our understanding of the limits of life on Earth, and allowed to draw realistic limits on the habitability of Mars and the Solar System.

With the 1990s came a renewed interest on Martian exploration. A new armada of landers and orbiters has been sent to the planet over the past 10 years, and three ambitious missions are scheduled between now and 2013; one of them, the Phoenix lander, is right on its way to the planet as these lines are written. This renewed interest on Mars owes to a body of literature and scientific research built up during the two decades after the Viking mission, but also owes to a single event that shocked the public and the scientific community on August 6, 1996:

"A team of NASA and Stanford scientists will discuss its findings showing strong circumstantial evidence of possible early Martian life, including microfossil remains found in a Martian meteorite, at a news conference scheduled for 1:00 p.m., August 7, at NASA Headquarters, 300 E. St. SW, Washington, DC."

Public note from NASA HQ, August 6, 1996

Ten days latter a manuscript entitled "Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001" appeared in the journal Science detailing the different lines of evidence for past Martian life found in the small meteorite. Immediately an unprecedented scientific and public debate burst over these 1.93 g of Mars. The debate became increasingly heated over the months and years that followed the announcement, as it is expected over a question that transcends science, and percolates into the very foundations of our society. While for some the debate is still open, many already speak of the rise and fall of ALH84001. In any case ALH84001 is forever linked to the history of Martian exploration and there are important lessons to be learned from it.

2. The ALH84001 Case for Life on Mars

The Martian meteorite ALH84001 (Fig. 1) was launched into interplanetary space after an impactor hit the surface of Mars around 16 million years ago. It landed on the Antarctic continent around 13,000 years ago, where it was discovered by an American expedition in the Allan Hills region in 1984. ALH84001 is primarily a volcanic, silicate-rich rock composed mainly of orthopyroxene, and smaller amounts of chromite, olivine, pyrite, apatite and Si-rich glass. In their seminal paper, McKay et al. (1996) proposed several lines of evidence for biological activity present in the meteorite: (1) carbonate globules within the meteorite with textures similar to bacteri-ally induced carbonate crystal bundle precipitates, (2) the presence of complex organic compounds, specifically polycyclic aromatic hydrocarbons (PAHs), (3) the coexistence of iron-oxides, iron-sulfides and carbonates, (4) ovoid and bean shaped structures that resemble fossilized ancient microbes and (5) magnetite particles that could have formed through controlled bio-mineralization processes. The authors pointed out that none of their single observations was itself conclusive for the existence of past life on Mars. Each of the observation had reasonable alternative non-biological explanations, but the totality of their observations considered collectively, particularly in view of their spatial associations, was claimed to constitute evidence for relic biological activity on Mars. Since the work was published many scientific and popular articles have focused on the possible evidence of life on Mars. We will resume here the most relevant literature on that respect.

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