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In the summer of 1996, NASA scientists (Science, August 16, 1996) announced that they had found evidence of primitive life on Mars, in meteorite ALH84001 discovered in Antarctica. Not little green men of course, but an incredible announcement nevertheless. Is this possible, and is it good or bad science? You be the judge.
In order for something to be considered science it must meet the following criteria:
What do we do if there is more that one possible explanation for the observed data? Then we apply the principle known as Occum's razor.
| Occum's razor: If there are several possible explanations for some observation, and no significant evidence to judge the validity of those hypotheses, you should always use the simplest explanation possible. Extraordinary claims require extraordinary evidence. |
In other words, be sure that there isn't a more ordinary explanation before claiming that life once existed on Mars. Don't be surprised that there is a great deal of controversy amongst scientists. Discussion of contradictory ideas is absolutely essential if we are going to reach valid conclusions. Both skepticism, yet open-mindedness and being willing to look at the evidence are essential qualities of science.
Did these meteorites come from Mars? The composition of the gases found in the meteorite matches those found on Mars by the Viking Martian lander. This is quite different from the composition of minerals found on Earth. The age of the rock in the meteorite is very old, about 4.5 billion years. At this time Mars may have been a very wet planet.
How did they get to Earth? Approximately 15 million years ago, an asteroid impacted with Mars. The collision threw enormous amounts of material into space. Eventually, some of this material intersected Earth's orbit. About 13,000 years ago, meteorite ALH84001 landed in Antarctica.
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Are there really fossils in these meteorites? Tiny microscopic deposits of carbonate minerals appear under electron microscopy to be very similar in appearance to fossilized Earth bacteria. However, just looking like a fossil is not conclusive proof. Similar carbonate globules might have formed in other ways that did not involve life at all. For example, carbonate globules are known to form on Earth in rocks subjected to high temperatures. They could have originated on Mars in water heated to a high temperature, for example by a volcano. Other evidence cited is that there are tiny magnetic grains of iron oxide very similar to those found in Earth bacteria. However, magnetic grains are commonly found on earth in processes that do not involve life.
What other evidence is there for life? The features that appear to look like fossils occur very close to organic molecules called polycyclic aromatic hydrocarbons (PAHs). PAHs are large molecules containing carbon that are known to be formed on earth by living organisms, especially when they decompose. However, there is also significant evidence that PAHs can be formed by non-living processes as well. For example, PAHs are found in many non-Martian origin meteorites, and in interstellar dust. No scientists claim that these PAHs have a living origin.
Does the extraordinary claim that these meteorites show fossilized life have supporting extraordinary evidence? Making the claim that life once existed on Mars on the basis of just a few meteorites is very speculative. We really need more evidence that we can only collect by getting a large number of samples from Mars itself, and returning them to Earth for analysis. If we really want to know the answer we need a lot more evidence, evidence that can only be obtained by going to Mars and examining its geologic history.
Suggested reading:
Donald Goldsmith, The Hunt for Life on Mars, Dutton Books, 1997, ISBN 0525943366
