Lesson 7: Life in the Solar System 7.4 From Mars with Love: Mars Meteorite ALH 84001 Figure 7.4.1 Martian meteorites. Mars has sent a letter to Earth, by meteorite mail, reporting on conditions on that planet. It is perhaps to be expected that the message is not easily read: after all, it is from a strange place. The mailbox was the Antarctic ice, a good choice. Whatever arrives in this place is easily recognized as not from the Earth. Furthermore, collection has only started recently. The area has not been picked over by stone age people eager for metallic rocks or, more recently, by souvenir hunters. The particular rock in question, labeled ALH84001, was recognized as a missive from Mars by its composition. It is similar to that of a class of meteorites known as SNC which are assigned to a Martian origin on the basis of gas inclusions resembling the atmosphere of Mars. Also, these types of meteorites are much younger than the ones derived from the ancient solar system debris floating about in space. Figure 7.4.2 Meteorite ALH 84001 ALH 84001 was not recognized as a Martian sample until 1994, when it was re-typed as a probable "Martian" meteorite. Its ancient crystallization age sets it apart from the class of Martian meteorites as whole, and its short space exposure set it apart from most asteroidal meteorites. The most reliable criterion is a certain distribution of oxygen isotopes in its minerals, which completely different from that of any other class of meteorites or of any other object yet known in our solar system, with the exception of meteorites from Mars. The reason ALH84001 became internationally famous (and was written up in the newspapers) was not the fact that it came from Mars (although this is remarkable in itself). The reason is that it contains something that looks like a fossil (sort of). Figure 7.4.3 Electron micrograph of Martian meteorite ALH 84001. Elongated structure at center is similar to bacterial fossils found in Earth rocks. This meteorite was also the first found to contain organic compunds. It is not known if these were produced by inorganic means or by living microorganisms. The finding was announced to a startled world in 1996 by a group of NASA scientists. The "simplest explanation" for the features seen was said to be that these were indeed fossils of tiny Martian organisms. Other scientists in exobiology and related fields have hotly contested this hypothesis. We summarize in the exhibit at: http://exobio.ucsd.edu/Space_Sciences/lifefrom.htm the results of a NASA/ESO 1999 scientific conference on the subject; the consensus was that the evidence for life in this or any other meteorite is far from convincing. Most scientists agree that sample returns from Mars, especially from subsurface probes in likely locations are a high priority. In the course of the conference scientists realized, as a result of the scientific debate, that arguments were fuzzy. No one had a very precise idea about how we can tell, from analyzing a rock, whether the presence of Life is indicated or not. One must look for a property that is characteristic of living matter and not simply consistent with the presence of life. Such a characteristic can then be referred to as a "biosignature". Is the evidence in ALH84001 on the level of a biosignature? Apparently not. The organic compounds seen can in principle result from abiogenic processes. In addition, their presence indicates terrestrial contamination rather than a Martian origin. The association of sub-microscopic grains of iron compounds enclosed in carbonate globules is not diagnostic. The fossil-like features are extremely small; a hundred times smaller than terrestrial bacteria. Although the existence of such extremely small organisms cannot be excluded, the small size weakens the case and also opens the door to a likelihood of mechanical artifacts. The carbonates present do not look biogenic and there is no conclusive evidence that they crystallized at a low enough temperature for life forms to exist. Thus, alas, there is no convincing evidence for past life in ALH84001. The refutation of the claim has been the result of the normal scientific process. Extraordinary claims require extraordinary evidence. The proof here offered has not passed ordinary inspection. Of course, the lack of evidence in this particular rock does not imply the absence of life on Mars, past or present, or weaken any arguments for its presence. It is clear that there is intense interest in the matter. A program of robotic spacecraft missions designed to search for ancient life will be required for definite proof of such life on Mars and elsewhere.