In the search for life on Mars, certain rare meteorites found on Earth—rocks blown off the Martian surface by past impacts—play a unique role. Because they represent our only samples of Mars (at least so far), meteorites allow scientists to assess whether habitable conditions might once have existed on the planet and whether microbial life might still be found there today.
The most famous of all Martian meteorites, ALH84001 (ALH stands for Allan Hills in Antarctica, where the rock was found), was the subject of a 1996 announcement that researchers had found evidence of past Martian life. The claim was taken seriously enough that former president Bill Clinton held a press conference, NASA stepped up its Mars exploration program, and the agency established the NASA Astrobiology Institute. Ultimately, though, most scientists found the evidence inconclusive.
Now a new paper by Ildikó Gyollai from the Research Centre for Astronomy and Earth Sciences in Budapest, Hungary, and colleagues, claims that there might be clues to Martian life in another Allan Hills meteorite, this time ALH77005. They base their conclusions on morphological and geochemical indicators—including the presence of organic material—which lead them to speculate on the past presence of iron bacteria in this Martian rock. While iron bacteria would be consistent with an early Martian environment, I don’t find the presented evidence convincing.
Minerals can have shapes very similar to those of bacteria, and the presence of organic material alone does not guarantee a biological origin. This is true even for rocks that originated on Earth. And the bar for establishing life on Mars is, justifiably, set very high. In the case of the famous 1996 paper by David McKay and colleagues about Martian meteorite ALH84001, most scientists concluded it did not provide evidence for biology, but simply interesting chemistry. Personally, I’m not so sure about this apparent majority conclusion. I find the close spatial association of reducing and oxidizing regions within the meteorite, which is typical of microbial interactions, and the magnetite chains—indicative of a certain type of bacteria called magnetotactic bacteria—intriguing. I think there is still something to McKay’s argument that the most parsimonious explanation of their findings is biology.
Many other Martian meteorites also contain organic material. One of the most famous is the Nakhla meteorite, which, according to some accounts, killed a dog when it came down in Egypt in 1911. That raises another problem we often face with meteorites: the possibility of terrestrial contamination, particularly when most of them were exposed to Earth for quite a while before they were found.
In my view, the question of whether there is life on Mars can not be answered conclusively based on the analysis of Martian meteorites. What can be said, though, is that some regions on early Mars were habitable based on the many studies conducted of ALH84001, as well as on other thorough and elaborate studies such as the one by Elias Chatzitheodoridis of the National Technical University of Athens, Greece on the Nakhla meteorite. These kind of detailed investigations will be essential until we’re able to bring well-chosen Martian samples back to Earth sometime in the 2020s, or send a new life-detection mission to Mars.