THE SPACE EDUCATORS' HANDBOOK

LIFE ON MARS?

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Statement of Dr. David S. McKay

Assistant for Exploration

Earth Science and Solar System Exploration Division
NASA Johnson Space Center

The following statement was given before the Subcommittee on Space and Aeronautics Committee on Science U.S. House of Representatives on September 12, 1996:

Mr. Chairman and Members of the Subcommittee:

It is my pleasure to be here today to discuss the results of the two-year investigation co-led by myself and Dr. Everett Gibson of NASA's Johnson Space Center and Kathie Thomas-Keprta of Lockheed-Martin, with major collaboration of a Stanford team headed by Professor of Chemistry Dr. Richard Zare, as well as six other NASA and university research partners. The results of this research were published in SCIENCE on August 16, 1996, Vol. 273, pages 924-930. A copy of the article is appended to my statement.

We believe that we have found a number of lines of evidence in a meteorite (see above photo) from Mars which could be interpreted as remains of early life on that planet. This meteorite, ALH84001, was found in Antarctica by a joint NASA/NSF field party in 1984. It was brought back to the Johnson Space Center where it was originally classified as another kind of meteorite, a diogenite. This misclassification occurred because only a small chip was used and the chip did not contain some of the unusual features of the whole meteorite. In 1994, David Mittlefehldt, a planetary researcher at the Johnson Space Center studied this rock along with other diogenites and discovered, based on chemical analyses of minerals and detailed study of thin sections, that this meteorite was not a diogenite but was a member of the SNC family of meteorites which has recently come to be accepted as coming from Mars. This family now includes 12 meteorites, half of which were found in Antarctica. Several lines of evidence link these SNC meteorites closely together including their oxygen isotopes which are as distinctive as a fingerprint and are clearly different from any earth rock, moon rock, or other kinds of meteorites. One member of this closely linked family, ETA79001, was discovered to contain gas trapped in small glass pockets by the impact which ejected it from Mars. Detailed analysis of this trapped gas showed that it was identical to the gas of the Mars atmosphere as measured by the two U. S. Viking spacecraft which landed on Mars in 1976. This match was nearly perfect. This meteorite became a kind a Rosetta Stone which linked the atmosphere of Mars to the SNC family of meteorites.

We initiated a detailed study of this meteorite once we realized it was a new Mars meteorite. Using radioactive isotopic age methods, other researchers found that the rock was formed 4.5 billion years ago and was part of the early crust of Mars. The rock was battered by nearby meteorite impacts so that it now contains shattered zones caused by those impacts. At some point in time, estimated to be 3.6 billion years ago by one group, the rock was invaded by water containing mineral salts which were precipitated out in cracks to form small carbonate globules which had intricate chemical zoning. Another group has now estimated that these carbonates may be younger, perhaps between 1 and 2 billion years old. In any case, about 16 million years ago, an object from space, possibly a small asteroid or comet, impacted Mars and blasted off some rocks from the upper few kilometers. The time spent in space is estimated by analyzing a number of isotopes which were formed by the hard radiation, cosmic rays, which are everywhere in space. One of these rocks traveled in space until it got close enough to the earth that it was captured by the earth's gravity and fell to the ice in Antarctica. Age dating using carbon-14 methods showed that this rock has been on the earth about 13000 years, presumably on or inside the ice sheets in Antarctica.

We analyzed the carbonate globules in this rock using two electron microscopes and an electron microprobe at Johnson Space Center. We also sent a small chip of the rock to our colleagues at Stanford University led by Dr. Richard Zare, who analyzed the chips for a specific kind of hydrocarbon, called polycyclic aromatic hydrocarbons or PAHs for short. This analysis was performed using a laser extraction system and laser excitation system which is unique to Stanford and is capable of exceedingly sensitive analysis of these hydrocarbons.

We found that the carbonate globules contained very small crystals of an iron oxide (magnetite) as well as at least two kinds of iron sulfide (pyrrhotite and another mineral, possible greigite). Small crystals of these minerals are commonly formed on earth by various kinds of bacteria, although they can also be formed by completely inorganic processes. Another kind of iron sulfide, pyrite, is also found in the rock but the crystals appear to be a thousand times larger than the pyrrhotite and greigite crystals. In addition, we found a complex zoning in which the manganese was most concentrated in the center of each carbonate globule, and most of the larger globules had rims consisting of iron-rich carbonate, magnesium rich carbonate, and iron-rich carbonate again. The compositional variation of these carbonates is not what would be expected from high temperature equilibrium crystallization, but is more like low temperature crystallization (0 to 80 degrees centigrade). It is also consistent with formation by non-equilibrium precipitation induced by microorganisms.

The Stanford group found an unusually high concentration of hydrocarbons (PAHs) on the chip surfaces where the carbonates were common. These PAHs have a pattern or fingerprint which is unusually simple compared to most PAHs that we are familiar with, including PAHs from the burning of coal, oil, or gasoline or the decay of vegetation. Some other meteorites contain PAHs, but the pattern and abundances are usually rather different from those found in the martian meteorite. The presence of PAHs is by no means an indication of past or present life forms. PAHs can be formed by strictly inorganic chemical reactions and abundant PAHs were formed in the early solar system and are preserved on some asteroids and comets. Meteorites from these objects fall to earth and enable us to analyze the PAHs contained within the parent bodies. While some of these are similar to the PAHs that we found in the martian meteorite, all show some major differences. One reasonable interpretation of the PAHs is that they are decay products from bacteria, and therefore an indication of some kind of past life within the meteorite.

Another feature which we found within the meteorite is the presence of unusual, very small objects or forms, which could be interpreted as the remains of microorganisms or microfossils. These spherical, ovoid, and elongated objects (see photo above) closely resemble the morphology of known bacteria, but many of them are smaller by a factor of 2 to 3 than any known bacteria on earth. Furthermore, microfossil forms from very old earth rocks are typically much larger than the forms that we see in the Mars meteorite. Unfortunately, we do not yet have good chemical data or thin section electron microscope data so that we cannot verify that they are indeed microfossils. We recognize that there may be several other possible explanations. The microfossil-like forms may really be minerals and artifacts which superficially resemble small bacteria. We do not believe this to be the case for most of the forms that we have seen, but additional detailed studies with the electron microscopes may resolve that question. Next, why are they smaller than known bacteria or known fossil microorganisms? Perhaps conditions on Mars such as lower gravity and more restricted pore space in rocks promoted the development of smaller forms of microorganisms. Or perhaps such forms exist on earth and in the fossil record but have not yet been found. If the small objects which we see are truly microfossils, are they really from Mars, the contamination from our lab, or from Antarctica? We have looked at many other kinds of material in our lab and have never seen forms resembling the ones we found in the martian meteorite. We believe that we can eliminate laboratory contamination as a source of these microorganism-like forms. Little is known about microorganisms associated with the big ice sheets of Antarctica, although rocks, soils, and lakes near the coast have abundant microorganisms, and we have studied some of these other Antarctic samples and their microorganism populations. Our studies so far show that the other Antarctic samples do not contain PAHs or microorganisms which closely resemble those found in the martian meteorite.

New data shows that the martian meteorite may contain other types of microorganism-like forms which can be revealed by etching the rocks. These forms include sheath-like hollow spheres, delicate membrane-like material which may be related to cell structure, and other unusual features within the shock-fractured zones of this meteorite. Clearly additional data are needed.

Where are we now? Our paper published in Science contains a number of lines of evidence, each of which can be interpreted as relating to some other cause, but which taken together can be interpreted as evidence for possible early life forms on Mars. We have seen nothing since the publication of the paper to cause us to abandon this interpretation, although other interpretations have been forcefully advanced by the scientific community. We are currently trying to find cell walls in the microorganism-like forms. We are also studying terrestrial microorganisms in a variety of environments, both present day and fossil. We are also looking at bacteria and their products formed in the laboratory. We have found examples of carbonates formed with the help of bacteria and we are comparing these carbonates to the martian carbonate globules. We are pursuing the question of what is the lower size limit for bacteria. We are forming several consortia and will work with scientists from other institutions and other countries to continue this research.

Have we found past life on Mars? The answer is neither yes or no, leaving a strong maybe. We still argue that past life on Mars is a reasonable interpretation of the data on hand. We believe there is considerable evidence in the martian meteorite that must be explained by other means if we are to definitely rule out evidence for past martian life in this meteorites. So far, we have not seen a reasonable explanation by others which can explain all of the data.

In my view, the question of past life on Mars may never be completely resolved by additional detailed studies of this meteorite, although such studies are clearly necessary. Each of the other 12 meteorites from Mars should also be carefully studied. Perhaps they might contain additional or alternative evidence of life on Mars. The question of life on Mars, whether fossil or existing, will never be completely solved until we can bring back the right samples from that planet.

The study of these martian meteorites will set new standards for the search for life in our solar system. These studies will push the state-of-the-art in analytical technology and promote efficient use of very small amounts of material. These studies will develop techniques and force instrument advances that can be used very profitably in the future to investigate samples brought back not only from Mars but from comets, asteroids, and satellites of other planets. Just as analytical instrument technology took a major advance when the lunar rocks were brought back, so will the quest for evidence of life in Mars meteorites and returned samples also cause improvements and advances in chemistry and microbiology.

Mr. Chairman, this concludes my statement. I would be happy to respond to any questions which you or the Subcommittee members may have at this time.

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