Evidences of Life on Mars - Part 2

So, what was the basis of McKay’s claim? Did they find convincing evidences of life on Mars? In their Science paper, McKay and his eight coworkers pointed to four types of data. Given their extraordinary claims, they were on scientific trial by the rest of the scientific community. Let’s go to the points they made:

1. The meteorite was found to contain organic molecules, including carbon-based compounds called polycyclic aromatic hydrocarbons (PAH). PAH’s commonly form when once living cells are subjected to high temperature. They are not proof of life by any means, but carbon is the key element of life as we know it. The presence of PAH’s distinguished this meteorite from the other Martian meteorites and had an extraordinary significance.

2. The meteorite had microscopic globules of carbonate minerals, similar to those found on walls of caves on Earth. Such carbonates are often deposited through the action of liquid water. Liquid water is the medium for all cells and a necessary condition for life as we know it.

3. The NASA team used an electron microscope to discover and characterize two iron-bearing minerals. Of particular interest were the chains of crystals. The perfect shape of these alien crystals and their unusual chemical purity seemed unlike anything ever seen, except in a few remarkable types of bacteria. They claimed that no known inorganic process could have produced such an ordered crystal array.

4. The meteorite holds tiny sausage shaped objects reminiscent of some species of bacteria. They’re much smaller than any Earthly microbes. These forms were found to be the most convincing evidence by the public. They actually look like fossils. Hundreds of newspapers and magazines reproduced the NASA images with the captions “Martian microbes”.

The main text of the six-page article on Science by McKay and his colleagues conveyed a sober discussion of their findings. They acknowledged that no single line of evidence was enough to trumpet the discovery of alien life. The concluding sentence shifted tone, however: “Although there are alternative explanations for each of these phenomena taken individually, when they are considered collectively in view of their special association, we conclude that they are evidence for primitive life on early Mars.”

To paraphrase the late Carl Sagan: “Extraordinary claims require extraordinary evidence.”

Controversy exploded. Experts aggressively challenged every one of the key points. Opinions ranged from cautious skepticism to outright contempt. Let’s looks at each of the NASA team’s supposed evidences of life on Mars:

Point number 1: the PAH’s. It turns out that PAH’s and other carbon molecules litter the cosmos, notably in the interstellar dust that forms comets and asteroids. These are actually the raw materials that formed Mars. Furthermore, such molecules would have been synthesized in abundance by natural chemical processes at or near the primitive surface of Mars. What’s more, PAH’s are among the most common constituents of pollution on Earth. Analyze the so-called pristine ice from Antarctica and you’ll find PAH’s. That’s why many scientists have concluded that the meteorite could have become contaminated while sitting on the ice.

Even if the PAH’s are from the Martian rock, there is no reason to conclude that these molecules represent remains of living cells.

Point 2: the carbonate globules. Mineralogists quickly pointed out that the carbonate minerals could have formed in many ways other than by circulating water. Carbonates can form in the reactions of rocks with carbon dioxide, which is the most common atmosphere gas on Mars. They can even crystallize directly by mineral processes. Indeed, a number of researches reanalyzed the minerals and found evidence that they formed at temperatures well above the boiling point of water, but that result is still a matter of debate.

Point 3: the chains of crystals. The chain-like arrays of exceptionally pure crystals are unusual indeed, but most observers feel that they are insufficient by themselves to prove the existence of Martian life.

Point 4: those fossil shapes that look like microbes. Many biologists attacked this claim because the fossils are too small, an order of magnitude smaller than any known Earthly bacteria. In fact, they are so small that they can contain no more than a few hundred biomolecules. That’s not nearly enough to support the chemical complexity of any known living cell. There is no reason to characterize these elongated shapes are fossils, since inorganic processes are known to produce similar structures.

The evidences of life on Mars became even less credible when scientists began examining other meteorites, Martian and otherwise. Surprisingly, all meteorites revealed sings of life. It was Earth’s life. Meteorites fall to the Earth’s surface, where microbes are everywhere. Bacteria inevitably contaminate any rock on the surface. Almost every meteorite ever found has been on a contaminated ground for periods ranging from several days to many thousands of years. Once found, most meteorites are usually handled and breathed on, and so exposed to more contamination.

Even meteorites collected in the pristine ice have been exposed to air for centuries. In a matter of months, microbes are able to migrate deep into a meteorite’s interior. Given such a contaminated Earth environment, how can anyone ever be sure that the Allan Hills supposed microbes are evidences of life on Mars?

Right from the start, one of the most vocal critics of the Martian claim was UCLA paleontologist Jay William Schopf. He is a leading expert of microfossils and an authority on Earth’s most ancient fossil life for at least 40 years. He was outraged. It was surprising that Schopf was invited by NASA to participate as an objective dissenting voice at the well publicized press conference in 1996 in which the discovery was announced. Schopf described the event in his popular book “Cradle of Life” (not the famous videogame).

He said he didn’t want to publicly humiliate the NASA crowd, so he was somewhat restrained in that public forum. He tried to be reasonable, even gentle. He underscored his criticism of the NASA work in an addendum to his book.

The majority of scientists at this point are unconvinced by the evidences of life on Mars found in the Allan Hills meteorite. Everyone, however, is wildly enthusiastic by getting more data.

The hunt for life on Mars features conflicts between our vivid imagination and the cold hard scientific facts.

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