The little piece of rock called “Allan Hills 84001” began its long journey many millennia ago on the planet Mars, where it crystallized from molten rock 4.091 billion years ago, a time when liquid water is thought to have existed here. There it lay for millions of years, just another rock on the surface of the red planet, until around 17 million years ago, when a meteor impacted with the surface to propel it out into the sea of stars. Once in space, it was inexorably drawn into orbit around our sun, where it floated through the void for eons, before having its orbit slowly change, gravity and perhaps other asteroids smashing into it all conspiring to move it inexorably closer to Earth and farther away from its place of birth. Then, 13,000 years ago it was grabbed by our gravity and fell from the sky to land in the frozen badlands of Antarctica, upon the Allan Hills Far Western Icefield, from which it gets its name. This was the end of its epic journey to our world, but only the beginning of the mysteries it posed.
During the 1984-1985 Antarctic summer, Roberta Score, Lab Manager of the Antarctic Meteorite Laboratory at the Johnson Space Center and part of an Antarctic expedition of meteorite hunters, found the rock while out on his snowmobile in the frigid wasteland and brought it in. It wasn’t much to look at, about 6 inches long by 4 inches by 3 inches in dimension, 4.3 pounds, and looking like a big potato dipped in tar, but through its chemical composition it was ascertained to have originally come from Mars. This was very exciting, as there were only 11 other such meteorites, called the shergottite–nakhlite–chassignite (SNC) class of meteorites, ever found, and this one was by far the oldest of them all. This was exceptional, because its estimated age would make it the only known Martian meteorite to have formed on a “wet Mars,” meaning at a period in its natural history when liquid water was thought to have been present on the planet surface. Of course, this meant that if there had ever been some sort of microbial life on Mars, then this would be our best chance of proving it, and this is where the story gets interesting.
In 1996, a team of scientists led by David McKay, Everett Gibson and Kathie Thomas-Keprta from NASA's Johnson Space Center in Houston, came forward with a rather sensational and earth shattering announcement. According to their findings, they had uncovered evidence of what appeared to be fossilized microbial life deep within Allan Hills 84001. The team claimed that they had observed within the meteorite myriad microscopic structures, extremely tiny at only 20–100 nanometres in diameter, but which were more similar to some sort of fossilized nanobacteria that any known natural process. Indeed, the structures superficially resembled terrestrial bacteria, although on an even smaller scale. Some of these structures almost seemed to be making up a colony of organisms the way they were interwoven with each other. In addition, there were found traces of organized magnetite crystals, which are significant as on Earth these are only known to be formed through biological processes and could be seen as a very strong biosignature. At the time the announcement was sensational, covered by news agencies all over the world, and then U.S. president Bill Clinton even made a formal televised announcement on these amazing findings. For many this was it, the final proof we needed that we are not alone in the universe, but controversy and skepticism were brewing even from the beginning.
One of the main arguments against this being evidence of alien life was that it was seen as more likely that this was terrestrial contamination of some sort, as this had been found in other Martian meteorites as well. Another strike against them was that over the years it would be found that non organic processes could produce results extremely similar to what is seen in Allan Hills 84001, some of which have been recreated in lab settings. In the opinion of skeptics, there was nothing present in the meteorite that could not be explained by explanations other than alien life. On top of this was the idea that the structures were simply way too small to feasibly contain what is needed to support life, and if real they would be by far the smallest lifeforms ever seen. However, the team defended their research, saying that the structures were too melded to the rock itself to have been contaminants, and that the magnetite crystals, when taken with the observable morphology of the structures, were more likely to have been made by biological processes than through the theorized natural methods through which they could be theoretically produced. As for the laboratory reproductions of some of the processes, McKay argued that they had used unrealistically pure raw materials as a starting point and could not adequately explain all of the features. Indeed, there has still been no demonstrable way to explain simultaneously all of the properties that have been suggested as possible biogenic properties in the Allan Hills meteorite or how it could all be produced by nonorganic processes in the combination they show, and so the debate has raged on.
Over the years the discussion has continued, and the mystery of the Allan Hills meteorite has still not been totally solved. Indeed, there have been occasional new discoveries made with the rock that further point to the presence of fossilized organisms. For instance, in 2010 it was announced that more data had been found to support the fossil theory, and in 2020 the meteorite was found to possess what appear to be nitrogen-bearing organics. Adding to it all is that there has been similar evidence for life found in other Martian meteorites since. In 2019, a team of scientists in Hungary found what appear to be the biosignature of filamentous iron-oxidizing bacteria, the mineralized remains of ancient microbes and their alteration of minerals in the rock, in another Martian meteorite from Allan Hills, called ALH-77005. So, is any of this true proof that life exists, or at least existed, on Mars? At the moment, no, but it has still generated much debate, and the legacy of Allan Hills 84001 is that it really pushed to the forefront the study of astrobiology, which is the theoretical science of life on other worlds. Everett Gibson, one of the scientists from the original team that studied the meteorite has said of this:
The interest resulted in the Mars Exploration Program being reinstated. The Mars Exploration Rovers, Mars Reconnaissance Orbiter, Phoenix lander, Curiosity rover, Mars Express mission with its Beagle 2 Mars lander — all can be traced to the ALH 84001 research. The excitement of the possibility of potential past or present life on Mars is too important for humans to ignore. The ALH 84001 announcement, despite whether you are a believer of the hypothesis or not, has clearly been the guiding idea for the development of the new interdisciplinary field of astrobiology. This must be viewed as positive for the field of scientific discoveries.
What is there locked in this piece of rock? Is the answer to the ultimate question of if we are alone to be found here? Or is it just wishful thinking and misidentification? As we push further into the reaches of space, we will undoubtedly continue to make new, groundbreaking discoveries, perhaps even finding life. In the meantime we have that meteorite, which has held its secrets for millions of years and might still do so for some time to come.