By Everett K. Gibson, Jr., David S. McKay, Kathie Thomas-Keprta, Frances Westall, and Christopher A. Romanek

In August 1996, our research team made a rather profound claim: evidence of fossilized life found within a meteorite (ALH84001) blasted off the surface of Mars. Where there's fossils, one can surmise there was once life. To be confident that a sample (from this or any other planet) contains evidence of past life or biogenic activity (i.e. the byproducts of organisms) one must determine beyond a shadow-of-doubt that certain well-established features or "biomarkers" (life's signatures) are present in the sample.

In the case of Martian samples, the criteria for past life have not yet been established yet. The reason for this is simple: if life existed on Mars, we currently have no way of knowing much - if anything - about its nature, ecology, or evolution. Such a detailed history (if it exists) will not be unraveled until we've spent a fair amount of time on Mars with robots and eventually humans.

Since no one has independent evidence about the nature of possible past life on Mars, the scientific community must adopt some criteria in order to guide this avenue of research. Lacking a clear picture of what kind of life may have existed on Mars, the criteria established for evaluating ancient fossil life on Earth will have to suffice - at least until we get to know Mars better.

What might some general criteria be? We feel that the 8 criteria which follow must be met by a sample such as ALH84001 before it can be accepted that one has truly found evidence of past life - on Earth, or another world.

(1) Do we know the geologic context of the sample i.e. where did it come from, how did it form; and is this location one which is compatible with past life?

We are certain that ALH84001 came from Mars. A solid case for Martian origin has been made by comparing both the oxygen isotopic composition and atmospheric gases trapped within the ALH84001 with data from the Viking landers. However, the exact location on Mars where this rock originated is unknown. None the less, ALH84001 has cracks and exhibits a porosity which clearly indicates that the rock formed on a rocky planet. The structure of ALH84001 (cracks and pores) raises the possibility that the rock could have harbored water-born microbial cells and colonies introduced long after the rock cooled, a process which has been shown to happen on Earth.

The presence of secondary carbonate globules within cracks in ALH84001 has been interpreted by most scientists as being indicative of secondary mineralization by a fluid, possibly water, that occurred at relatively low temperature. Thus, the most widely accepted, broad geologic context of this rock is not incompatible with the presence of past life. If these carbonates formed at low temperature, as now seems to be the case, from aqueous precipitation, then their formation is completely compatible with past life. However, such processes do not require life in order to occur.

(2) Do we know the age of the sample and its stratigraphic location within its parent body? That is, what is the age, composition, and history of the surrounding rock, and are these factors understood enough to relate possible life to geologic history of the surrounding material?

An isotopic analysis of ALH84001 shows it to be 4.5 billion years old. Since we're certain that this rock is from Mars, its extreme age suggests that it is a sample of the original Martian crust. Between 3.9 and 4 billion years ago, the sample underwent extensive shocking. Carbonate formation occurred around 3.9 billion years ago. This would have been shortly after Mars experienced a period of extensive bombardment - a time when Mars had abundant water, a thicker atmosphere, and higher temperatures. These events correspond to the time when life appeared and developed on Earth. The rock was ejected from the surface of Mars about 17 million years ago and then orbited the sun until it hit Antarctica 13,000 years ago.

Given the understanding we have of the structure, composition, and history of this rock, we feel that it is possible to relate any putative life forms found within in the rock to the geologic history of Mars. Further, given the similar early histories between Earth and Mars, we feel that a comparison can also be made between ALH84001 and the history of life on Earth.

(3) Does the sample contain evidence of cellular morphology i.e. can the remnants of cells and their internal components be seen?

Structures occur in the rims of the carbonate globules found within ALH84001 that are similar in size and shape to the mineralized casts of modern terrestrial bacteria, their appendages, and by-products. While these structures are very small, they are within the size limit of known nanobacteria (i.e. 50-200 nm). Indeed, when we first presented this data, critics suggested that this was too small for life In the the past few years these size estimates have come down to embrace the size range of these structures. However, firm evidence that these bacteria-like structures are truly the fossilized remains of Martian bacteria has not been found.

Some of the features in ALH84001 we originally identified as being suggestive of fossil organisms may have actually been caused by the process of preparing a sample for electron microscopy. Other features may well be the result of weathering of ALH84001's surface. However other structures are definitely not the result of human or geological intervention. In particular, features we have identified within ALH84001 (e.g., filaments) are common biogenic markers on Earth. The evidence for fossilized microbes and their products is not conclusive. However, much of what we have identified cannot be readily explained by non-biologic processes, and should not be ignored.

(4) What structural remains of colonies or communities or cells or organisms exist within the samples?

We have proposed that some of the features in ALH84001 may be the remains of biofilms and their associated microbial communities. Biofilms are produced by terrestrial bacteria as a protective device against during times of environmental stress. When found in the fossil record, biofilms are considered strong evidence of bacterial colonies in ancient Earth rocks. It is possible that some of the clusters of microfossil-like features in ALH84001 might be colonies, although that interpretation depends on whether the individual features are truly fossilized microbes.

(5) Is there any evidence of biominerals showing chemical or mineral disequilibria? Life's processes frequently result in the selection and concentration of materials that would not occur naturally. Has evidence been found which points to such processes?

The carbonates in ALH84001 contain a number of small particles composed of magnetite. On Earth, magnetite particles in some species of bacteria provide them with the ability to sense magnetic fields. The size distribution and unusual rectangular prism shape of a fraction of these particles are indistinguishable from microbially-produced, terrestrial magnetite, but match no known non-biologic magnetite.

We feel that the formation of these magnetite particles can best be explained by biogenic activity and disequilibria of the iron oxidation potential in the fluid that was the source of the iron. There are other types of irregular magnetite grains that could be either biogenic or nonbiogenic in origin. One example are "whisker"-like magnetite that is quite different in size distribution and shape from the irregular prism magnetite particles.

Overall, the way in which the carbonate globules in ALH84001 are structured, and the extreme chemical variations they contain, are compatible with known biominerals and known disequilibria related to microbial activity on Earth. Clearly, more work needs to be done in general to distinguish true biominerals and biogenically-related chemical disequilibria from totally non-biologic minerals and disequilibria.

(6) Is there any evidence of stable isotope patterns unique to biology? Living systems tend to show a preference for one isotope over another (e.g. Carbon 12 Vs Carbon 13) whereas non-living systems either show no preference or show preferences different to those of life.

The stable isotope patterns within ALH84001 have shown the presence of indigenous carbon components that have isotopic signatures in the direction of known biogenic carbon signatures. Overall, the carbon isotopic signatures of the non-terrestrial, possibly organic carbonates within ALH84001 are similar to the way in which life tends to prefer molecules containing one isotope of Carbon over molecules containing another. However, despite this similarity, this does not prove that living system are responsible for this carbon signature.

(7) Are there any organic biomarkers present? Are there compounds present either alone or in mixtures that do not easily form under non-living conditions?

Possible organic biomarkers are present within ALH84001 in the form of PAHs (Polycyclic Aromatic Hydrocarbons). The PAHs we have found are associated with carbonate globules also found within the rock. We believe that it is possible that some of these PAHs which may be a unique product of bacterial decay. As for the question of contamination: Recent data show that the PAHs are most likely indigenous to ALH84001, whereas the detected amino acids are most likely Antarctic contamination. Much more work needs to be done: exhaustive data must be collected before either component can be used as a biomarker for a specific sample.

(8) Are the features indigenous to the sample? What is the possibility that they have another origin than has been demonstrated, or are contaminants deposited after the sample left Mars?

In our opinion, the recent studies of have shown conclusively that the PAHs are indigenous to ALH84001 and are not contaminants. Based on isotopic compositions and textures, there is absolutely no question or disagreement that the carbonate globules and their included minerals formed on Mars and are indigenous to the meteorite. The possible microfossil structures and some regions of organic carbon that are embedded in the carbonates are therefore almost certainly indigenous. However, other possible evidence for life (e.g. amino acids) may be Antarctic contamination.

Summary:

Clearly, we have not completely satisfied all of the criteria needed for general acceptance of evidence for life in a sample. We believe that we are close to meeting some of the criteria (likely biominerals, possible organic biomarkers, indigenous features) and not so close on others (well-documented geologic context, evidence for cells and colonies). However, not one of the eight criteria have been shown to be violated by any published data on ALH84001 so as to preclude life, and some evidence exists supporting each criterion. Therefore, the jury is still out on early Mars life as revealed by this meteorite. Evaluation against these criteria is still in progress and more data are needed.

In evaluating this sample from another planet for evidence of past life, we have had to re-examine our own views of how one might find evidence of past life on this planet. While finding evidence of life - past or extant, on another world (Mars) would have profound implications, being able to more fully understand life's tenure on this planet is of equal importance. We are confident that our research has added to the understanding of life's presence on both worlds. Indeed, as a result of our findings in 1996, the general consensus regarding how small Earth organisms can be has undergone significant revision - downward, and now embraces the size range of the structures we have observed within a piece of Mars.

Mars has taught us something about Earth.

A comparison of features found within Martian meteorite ALH84001 [(A), (D), and (F) on left] and bacteria fragments found in a sample of Columbia River Basalt [(B), (C), and (G) on right]. All images shown to the same scale ("nm" = nanometers 1 billionth of a meter).

The authors interpret most of the elongated features in the Columbia River basalts to be mineralized filaments and appendages from cells.

The authors also believe that the features of ALH84001 (from Mars) are "remarkably similar in size and morphology [shape] to mineralized filaments and appendages from the Columbia River samples."

The small size of the terrestrial bacterial features illustrates that structures of similar size found within ALH84001 are indeed found within Earth's geological record.