ALH84001 and Occam’s Razor
In the case of Mars, the hypothesis has been much strengthened by David McKay’s team’s analysis of the Martian meteorite known as ALH84001. According to this analysis, the meteorite contains globules of carbonate, polycycle aromatic hydrocarbons (PAHs), magnetite and iron sulfides, and some intriguing structures that some believe might be the fossils of ancient Martian bacteria. We know that the meteorite, which was found in Antarctica, came from Mars because the air trapped inside exhibit the same mix of rare gasses that the Martian atmosphere has. Great care was taken to rule out the possibility that the organic materials found could be the result of contamination (a claim buttressed by the fact that the proportion of those materials increases towards the center of the meteorite). Nevertheless the reaction to McKay’s results, particularly by those scientists considered experts on meteorites, was extremely hostile.
The motivation for the hostility was in no small part the fear of subjecting to ridicule the subdiscipline of planetary science devoted to the study of meteorites, to let it fall into the pit of another “cold fusion,” the big scientific embarrassment of a few years previously. The main argument against McKay’s analysis was methodological. It was based on Occam’s razor, a principle named after William of Occam, the medieval philosopher who insisted that we should accept the simplest explanation available:
All the compounds and structures found in ALH84001 could have been produced by inorganic processes. Therefore, by Occam’s razor, we should eliminate the unnecessary conclusion that we have found evidence of alien life.
As it was often repeated during the debate about ALH84001, extraordinary claims require extraordinary evidence, but hydrocarbons, the magnetite and the minute worm-like features could be explained by ordinary inorganic processes. There is presumably, then, no need to conclude that Martian life caused the phenomena found in the meteorite.
Occam’s razor, however, does not rule against McKay’s analysis. For what we have is a collection of three things in an extremely confined space (a few nanometers across): (1) typical bacterial food (hydrocarbons), (2) structures that look like typical bacteria, and (3) typical excreta of bacteria (magnetite and iron sulfides). One simple hypothesis, life, accounts for all these phenomena and the fact that they are closely packed together: Martian bugs ate the hydrocarbons and left the droppings behind. The inorganic-origins hypothesis requires at least three separate mechanisms and has little to say about why they are together in such a small space.
Some of the strongest criticisms have been made against the claim that the worm-like structures are fossils. Critics have argued, for example, that those structures formed under too much heat (but other studies support McKay on this point). But the most telling criticism was that the structures were much too small to be able to carry out many important organic functions— they came in at less than one-hundredth the size of terrestrial bacteria. This leads to a very important point I will discuss below. For now let me say that, even if those structures are not fossils, the life hypothesis is still more economical: Martian bugs ate the hydrocarbons, left their droppings, but then failed to fossilize — which is perfectly understandable for soft cells. Moreover, inorganic magnetite forms at a temperature about three times higher than that apparently experienced by the Martian meteorite. And to make matters worse, the magnetite in the sample, unlike that produced by inorganic processes, is of an extremely pure form, which on Earth is normally produced only by bacteria. It is far from obvious then that the inorganic-origins hypothesis is simpler. Occam seems to smile on the life hypothesis instead.
This is not to say that I favor the life hypothesis. For I do not believe that a matter of such significance can be decided by one single methodological point -- Occam’s razor. In any event, there seems to be little doubt now that Mars has had organic carbon, and that is a great find after the discouraging results from the Viking experiments. The sensible way to proceed -- and both sides of the controversy seem to agree on this point -- is to go back to Mars and look for evidence of fossils in places naturally protected from ultraviolet radiation.
The prospects for Europa also look good, now that the favored view is that all the origin of life requires is an environment with plenty of liquid water, organic carbon and a source of energy. Europa is ten percent water, much of it apparently in an ocean under the ice; it should have plenty of organic carbon because of its location in an area of the solar system rich in organics; and it obviously has a reliable source of energy, otherwise it could not continue to smooth out its icy surface. Let us be cautions, though, about the prospects for life on the watery moons of the outer planets, Europa for example. It is not enough to have around liquid water, simple organic compounds, and a source of energy. Metals such as iron, zinc, copper, nickel, cobalt, magnesium, and manganese performed catalytic and other crucial roles in terrestrial organic evolution, as they do in the normal functioning of cells today. Without most of those metals the prospects for life in any of the outer moons would be very dim. And so we must ask whether Europa is likely to have them. The answer is yes. Europa is one of the few outer moons with a density (3.0) close to that of our Moon (3.3). Indeed the rocky core of Europa under the watersphere would have a density even closer to the Moon’s. It is a good bet, then, that it would offer a variety of metals similar to that of the Moon — and this is enough to keep Europa as one of the leading candidates in the solar system to harbor alien life.
If we never find life, or fossils of life, in Mars, Europa, of anywhere else in the solar system, astrobiology will have to look at other solar systems for specimens. Unfortunately a mission to the stars would last tens of thousands of years with the technology we can muster today. And even with the best space technology that we can plausibly envision for the next century, it would still take hundreds of years to travel to another solar system. Perhaps fancier technologies (to be described in the next chapter) can get us there faster, but those technologies will not be around in the near future. There is a shortcut available to astrobiology, and that is to make contact with intelligent extraterrestrial life. But that is a long shot, as we will see in Chapter 8.
Nevertheless, even if we find no alien life in the solar system, the pursuit of astrobiology will continue to be worth our while, as we will see in a soon-to-come posting.
 D.S.McKay et al, “Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite AL84001,” Science, Vol. 273, 16 August, 1996, pp. 924-929.
 See R.A. Kerr, “Ancient Life on Mars,” ibid., pp. 864-866. The controversy spread to public arguments in the newspapers; see for example the front page article “Life on Mars: Scientists ‘thrilled’ by prospect,” Seattle Times, August 7, 1996.
 Other rocky moons with possible oceans include the gigantic Jovian moon Ganymede: R. Cowen, “Ganymede May Have Vast Hidden Ocean,” Science News, Vol. 158, December 23 & 30, 2000, p. 404.
. For a fuller treatment of this issue see Chapter 8.