Evidence of Past Life in Mars in alh84001 The Role of Contamination

The Role of Contamination

Patricia J. Hoppe, 1611461
For almost forty years, the premier site in the world for finding meteorites has been Antarctica. Beginning in 1969, when nine meteorites were discovered by a Japanese team in the Yamato Mountains, the southernmost continent has been the destination for interplanetary geologists (AntarcWeb). Before that time, there were only approximately 2100 known meteorites, with fewer than 10 new ones being discovered each year. In the next 30 years, visitors to the southern deep freeze would find over 30,000 meteorites, greatly expanding our knowledge of the early solar system (NIPRweb). On Dec 27, 1984, geologist Robbie Score found one such meteorite, a potato-sized dark green rock in the Antarctic snow. As was the custom, the rock was designated by the place it was found, the year of discovery, and whether it was found first, second, third, etc. for that season. Found in the Allen Hills, it became ALH84001. Twelve years later, in August of 1996, a group of NASA scientists astounded the world by announcing they had discovered what appeared to be primitive Martian life in that innocuous small stone. The search for life outside of Earth would never be the same.
The Announcement

The first inkling most of the world had for this potentially earth-shaking discovery was a news conference held on August 7, 1996. Many of the scientists responsible for the investigation were present, including David McKay, Everett Gibson, Kathie Thomas-Keptra, and Richard Zare. Showcased along with them was the rock itself. However, for the scientific world, the real announcement was the paper published in the August 16, 1996 issue of Science (McKay, et.al., 1996). In it, they outlined the series of features that had led them to the conclusion that the rock harbored evidence of primitive life from Mars. While each alone would not serve as pointers to Martian life, the authors argued that the congruence of these various lines of evidence pointed strongly to a biogenic presence in the rock.

First, analysis of the meteorite had clearly shown that it came from Mars. The Matian Viking landers had been able to analyze the ratios of oxygen isotopes in the rocks and soil. ALH80001 clearly fit that profile (PSRDWeb1096). In addition, the rock could confidently be dated as over four billion years old.

Second, when fresh fractures were created, microscopic carbonate globules were revealed. Shaped liked tiny flattened spheres approximately 20 to 250 microns in diameter, most were found on what appeared to be old cracks in the rock. They seemed to have formed from carbon dioxide dissolved in water (Gibson, 1997). That would place the meteorite on Mars during the period when running water was present on the red planet.

Third, investigation into the morphology of the globules revealed a complicated structure containing chemicals that on Earth, at least, are strongly associated with life. Most prominent were polycyclic aromatic hydrocarbons (PAH) and small magnetite grains. The first have been found as decomposition products of ancient microbes. The second, if in the proper configuration, are designated magnetofossils, and found also with ancient bacteria.

Fourth, and visually the most impressive, electron microscope studies of the surfaces appeared to show the remains of ancient bacteria, in shapes similar to those of Archaen cyanobacteria. (See Illustration 1)

Illustration 1. (PSRDWeb 1096)

As would be expected, many in the scientific community were not convinced of the claims made by the McKay group. They offered numerous objections to the different points. Perhaps the only area of agreement was that the rock truly was from Mars.

The source of the globules was a particular area of argument. For them to have been a product of living organisms, they would have to have been formed at temperatures below 115 degrees C. In direct repudiation, Harvey and McSween in 1996 proposed that the globules were the result of an impact, with silicates in the rock interacting with a carbon dioxide rich fluid. The temperature would have exceeded 400 degrees C. in that creation scenario.

As for the magnetites, Scott, in 1999, contended that the various granules did not truly resemble those found in earth magnetobacteria. He asserted that the grains were too small, and lacked the purity of structure in biogenic magnetites. Like Harvey and McSween, he considered the probable origin to be some sort of shock heating.

As for the electron microscope images that appeared to resemble fossil bacteria, Treiman pointed out in 1999 that their size, at 30 to 100 nanometers, is simply to small to contain all the necessary molecules for functionality. In addition, the manner in which the samples are prepared, alone, can produce images that resemble these very small bacteria. Preparation involves coating the sample with metallic vapors of gold, copper, or other metals. These can produce "sample preparation artifacts", a potential source of inappropriately labeled images (PSRDWeb0397).

However, foremost in the minds of many of the scientists was the role of contamination in producing the biomolecules that were so central to the arguments by the NASA scientists. This is the issue that will concern the balance of this paper.

The valleys of Antarctica provide a unique environment for finding meteorites. While no more likely to be hit by a flying piece of rock than anywhere else in the world, Antarctica has attributes not found in warmer climes. When a meteorite lands on a glacier, the heat generated during the trip through our atmosphere melts some of the ice, depositing it deep inside the ice floe. As subsequent snowfalls bury it deeper, it becomes part of the river of ice slowly moving down one of the Antarctic mountains. While many of these glaciers end up dumping their load into the Southern Ocean, some of them meet other mountain ranges. The glacier is forced up, with the edge slowly evaporating in the dry cold. Left behind are the trapped meteorites, some of which may have spent as long as 2 million years buried in the ice (Sawyer, 2006).

Meteorites collected in this manner seemingly should be as pristine as possible on our dirty planet. The heat of entry melts the surface of the rock, creating a distinctive crystalline cover. Having spent thousands to hundreds of thousands of year in deep freeze, the chance of bacterial growth would seem to be remote.

Annually, scientists visit these valleys, looking for the precious tell-tale black on white. The effort is grueling, involving sub-freezing temperatures and howling winds. (Illustration 2). Unfortunately, it would appear that for all their attempts to maintain sterile conditions, it is almost impossible to prevent bacteria from growing, even in an area as cold as Antarctica.

Illustration 2, Collectors Base Camp.

Burckle and Delaney, in 1999, investigated the possibility of bacterial growth in seemingly sterile chondritic meteorites. These had been exposed to temperatures above 300 degrees C. while in space, and were quite obviously metamorphosed. Within microscopic cracks, they were able to find microfossils of both diatoms and opal phytoliths. They speculated that the winds that are a constant problem for Antarctic scientists could also bring dust from Africa or South America. The cracks in the rocks would be an ideal trap for the dust, bringing terrestrial bacteria into direct contact with the interior of the meteorites. Additionally, the ice can act as a direct depository for cosmic dust and other aerosols.
Contamination in ALH84001

While the vision of the potential nanofossils wasintriguing, the existence of organic carbon molecules within the interior of the meteorite was a major point in the attempt to prove that it harbored the remains (or at least the final remnants) of Martian bacteria. The original 1996 paper had emphasized the increasing concentration of PAHs as one moved further from the surface. The authors argued that if contamination was the source of the PAHs, they should have had the opposite configuration, with the surface coated and smaller concentrations further in. The PAHs were strongly associated with the carbonate globules. The authors also carefully delineated the various steps they had taken to ascertain that their own laboratories could not be a source of contamination. In addition, they had attempted to grow bacteria on the surface and cut interiors of the meteorite, without success (Sawyer, 2006).

The Science article as well as the news conference stimulated requests for samples of the meteorite, allowing investigators from all over the world to do their own analyses. Their discoveries quickly put the original investigators on the defensive. An exploration into the amount of carbon-14 in the PAHs molecules resulted in a 1998 paper from A. J. T. Jull in Science. His sample revealed C-14 concentrations consistent with terrestrial origin in approximately 90% of the organic molecules tested. In addition, the carbon 12 and 13 proportions were typical of earth patterns. Only the carbonate minerals appeared to be of Martian origin. In the same year, Bada and his associates were able to show that the amino acids found in the meteorite were also of terrestrial origin. Their concentrations were almost exactly that of Antarctic ice run-off. Further analysis of the carbon compounds in the matrix of the rock by Becker and others in 1999 revealed a kerogen-like compound. These are extremely common in carbonaceous meteorites. A likely source for those molecules was speculated to be late bombardment comets or meteorites.

Surprisingly, the original scientists were the first to report that their initial impressions of the sterility of the meteorite were incorrect. Bringing in an environmental microbiologist (Sawyer, 2006), they revealed in 1999 that a species of actinomycetes could be found on the surface of meteorite. While they were compelled to admit that this could have been the source of the PAHs, they still argued that the lack of interior growth negated that idea. However, Becker, Bada, Glavin, et.al in1997 had shown that the carbonate globules concentrated the PAHs from available melt water. Thus, the fact that the bacteria were only on the surface didn't prove that they couldn't have been the source.

The sum of all these studies would seem to refute the claim that ALH84001 harbors the record of early life on Mars. The sample was not as sterile as they had initially supposed. The concentration of PAHs in the interior could be explained by the affinity of the carbonaceous globules for those compounds. The vast majority of organic molecules bore an Earth carbon tag, not a Mars tag. As much as we would like to have found life elsewhere, this does not appear to be it.
The Legacy of ALH84001

There can be little doubt that the possibility that humankind might have found evidence of life on another planet has profoundly affected the course of the astronomical sciences. NASA's National Astrobiology Institute had been in existence only a year when the furor erupted. They saw an almost immediate increase in their funding. The exploration of Mars intensified greatly after 1996. One of the long term goals of NASA is to place men, not just machines, on the red planet. However, the rock from Mars has gradually receded from the public imagination. As recently as 2002, the 33^rd Lunar and Planetary Science Conference was filled with discussions about ALH84001 (SpaceWeb). Only five years later, the schedule of presentations for the 2007 conference did not mention it by name in any of the program notes. The judgment of science appears to have weighed in on the side of the skeptics. Too many studies were able to find alternative explanations for the features on ALH84001 that figured so prominently in the first paper. Occam's Razor once again has denied a fabulous claim with more mundane answers. Yet the whole tone of the search for life in our solar system has changed. We have a better idea of what will serve as a pointer to primitive life. Thanks to ALH84001, when we do finally reach Mars, we will have a better idea of what to look for, and what we must ultimately reject.

Bibliography
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