In 1996, a team of scientists announced that they had discovered signs of single-celled bacteria in a piece of Mars rock, indicating that the Red Planet had its own extraterrestrial life a few billion years ago. At the time, the announcement caused a sensation. But today, further research has led most scientists to dismiss the claim.
Meteorites are chunks of rock that have fallen to Earth from space. Nearly all of them are bits of asteroid, which formed about 4.5 billion years ago when the gases and dust of the early solar system condensed to produce solid chunks of rock in orbit around the newly-formed Sun. Because they are made of the same early material that formed the planets, meteorites are of great interest to geologists and astronomers, and a lot of scientific effort goes into finding and studying new meteorites.
One of the best places to find meteorites is Antarctica. The vast ice sheet makes it easy to spot the dark meteorites lying on the surface, and the cold dry conditions help to preserve the space rocks and reduce their contamination by terrestrial dust and atmospheric gases. Since the 1970s an international group of scientists known as ANSMET (Antarctic Search for Meteorites), partially funded by NASA and the US National Science Foundation, has been collecting meteorite rocks every year during the Antarctic summer. So far, they have found over 7,000 samples.
In December 1984 (which is the middle of summer in the southern hemisphere), ANSMET found a greenish-gray rocky meteorite in the area of Antarctica known as the Allan Hills. It was a little smaller than a brick and weighed just over four pounds. Because it was the first find to be catalogued from the Allan Hills area in 1984, it was assigned the identification number ALH84001. It was packed back to the US and identified as a diogenite, a relatively rare but rather uninteresting type of stony meteorite. The rock was filed away and forgotten. Ten years later, though, space sample ALH84001 suddenly became a lot more interesting.
Because the asteroids and planets all condensed from the early solar system cloud at the same time, they all give about the same age when they are radio-dated–about 4.5 billion years. But then a puzzle appeared–in a very small number of meteorites, radio-dating gives a much younger age, by billions of years. Something was unusual about these rocks. It wasn’t until the 1970s that the answer was found: around half a dozen of these unusual meteorites chemically matched the Apollo samples that had been brought back from the Moon. They were pieces of Moon rock. And even more surprising, about a dozen meteorites contained tiny bubbles of atmosphere that matched the data sent back by the Viking landers on Mars. They were pieces of Mars. And when the ALH84001 meteorite was finally examined by NASA in 1994, it was discovered that the original classification had been mistaken. The rock was not a diogenite and was not just an asteroid–it contained tiny bubbles of Mars atmosphere.
Once the Allan Hills meteorite was recognized as a piece of Mars, pieces of it were sent out to a number of scientists for further study. Over the next couple years, it was determined that the meteorite had been born about 4.5 billion years ago when, in the early solar system, it crystallized from molten lava as part of the planet Mars. About 4 billion years ago, the now-solid rock was compressed by a heavy shock wave, probably from a nearby meteor impact. At this time, Mars was not the cold dry nearly-airless world that it is now–it was much warmer, had a thicker atmosphere, and had liquid water on its surface, which flowed into cracks inside ALH84001 (produced by the impact shock wave) and left some distinct chemical deposits inside. Over the next several billion years, Mars gradually lost its atmosphere, its surface water dried up, and it became geologically dead. Then, about 16 million years ago, the ALH84001 was again compressed and shocked by another meteor impact–enough to partially melt it. This time, the force was enough to throw the rock into the air with enough speed to escape Mars gravity and enter space. Now, the potato-sized chunk of debris orbited the Sun until, about 13,000 years ago, it came too close to Earth, was pulled in by our gravity, and fell to the surface as a meteorite, landing in Antarctica.
This history was enough to make ALH84001 interesting, but in 1996 a team of scientists led by Dr David McKay made an announcement that was absolutely stunning: they declared that they had found biochemical signs of life, including fossilized bacteria, inside the rock, showing that in its ancient past, when conditions were more suitable, Mars had been inhabited by living organisms. McKay cited four bits of data in support of his conclusion: a class of chemicals called polycyclic aromatic hydrocarbons (PAHs) that are produced by the decay of living organisms, some other chemicals found inside carbon-rich globules that are also characteristic of life, a number of tiny magnetite crystals similar to those produced by certain bacteria, and most spectacular of all, tiny structures inside the rock that look like fossilized bacteria. Taken together, McKay announced, these four findings conclusively demonstrated that primitive one-celled extraterrestrial life once existed on Mars.
But one of the greatest strengths of science is that NO finding is ever accepted on the basis of a single study–every scientific discovery is subjected to the process of verification as others repeat the experiments in an attempt to confirm the findings. And when other teams of scientists began to examine ALH84001, they quickly found that McKay’s conclusion was very far from certain. Each of the four categories of data cited by McKay’s team could also be explained by chemical processes that did not involve any biological life.
The PAHs are indeed a normal product of biological decay, but not exclusively–they can also form under certain chemical conditions from plain carbon. PAHs have been found inside other carbon-bearing meteorites which came from ordinary asteroids where no one has ever postulated that life existed.
The carbon globules inside ALH84001 contain chemicals that are produced by living cells, but it is known that at least some of the carbon in these globules came from bacteria that entered the rock after it landed on Earth, indicating that at least some of these chemicals are the result of contamination by terrestrial sources. And like the PAHs, these carbon compounds can also be produced by non-biological chemistry.
ALH84001 does contain a number of very tiny crystals of magnetite, a magnetic form of iron. On Earth, similar crystals are manufactured by certain groups of bacteria, who use them to help orient themelves to the planet’s magnetic field. But these crystals can also be made by heating siderite (iron carbonate), a mineral that is present in Mars rocks. So the magnetite in ALH84001 could also have been formed when the rock was heated by the impact that threw it into space.
Most of the controversy has been around the scanning electron micro-photographs which depict microscopic round or oblong objects, some of them in chains. McKay argued that they are the mineralized remains of Martian bacteria. But there are problems with that hypothesis. Only a dozen or so of these objects have been found and photographed. There is little consistency to the objects in size or shape: this might indicate a wide variety of Martian bacterial life, or it might indicate that the objects are just mineral precipitates, perhaps forming around magnetite crystals, and are not bacteria at all. Because the objects are mineralized, they cannot show any internal structure that would mark them as biological. They are also extraordinarily small–less than one-thousandth the size of a typical terrestrial bacterium and only one-tenth the size of the smallest known Earth microbe. Some have speculated that they are “nano-bacteria”, but there are no observed cellular organisms that are this small in size.
So there the matter rests. The data does not conclusively rule out the possibility of Martian life in ALH84001, but each bit of evidence can also be equally well-explained by non-biological mechanisms. Under the scientific principle of “Ockham’s Razor”, in which no new explanatory mechanism can be invoked unless it is necessary to explain the observed evidence, there is no need to postulate the existence of Martian life to explain any of the data. So the scientific consensus is that those who claim the existence of Martian life in ALH84001 do so because they WANT it to be true, not because there is any data or evidence which makes it necessarily true.
The apparent absence of clear signs of life in the Allan Hills meteorite does not, of course, conclusively mean that there was never any life on Mars–it just means there is no conclusive evidence for it in the meteorite. The clear indications that Mars once had an inhabitable climate, with running water and a thicker atmosphere, leaves open the possibility that the planet may indeed have harbored life at some time in its past, and one of the primary goals of planned future missions to Mars is to search for evidence of such possible life.