When the Mars meteorite ALH-84001 discovered in 1984 in the Antarctic, was announced in 1996 it got a lot of media attention. Even President Clinton commented. At the time it was very exciting for those who were looking for life outside the Earth, and for some it even suggested a possible source for life on Earth itself.
While many still believe this detection is genuine, others have cast doubt, suggesting that before this discovery can be accepted as evidence of past life on Mars, it must more convincingly pass a set of criteria that includes factors like contamination from life on Earth. Since it is believed that the ALH-84001 meteorite probably landed on Earth about 13,000 years ago, can scientists be certain the evidence is not coming from life on Earth rather than Mars?
Of great concern for NASA is the possibility that the spacecraft being sent to Mars for the detection of life is itself contaminated with Earth-life. Enormous effort is expended in assembling these spacecraft and their instruments in clean rooms that have been thoroughly decontaminated. Many of the techniques used to accomplish this task have been adapted from hospital operating rooms and biotechnology labs.
In a paper recently published in the European journal FEMS Microbiology Ecology, several authors have applied a new technique for detecting the presence of contaminating bacteria. They used samples collected from clean room facilities at the Jet Propulsion Laboratory, Kennedy Space Flight Center, and Johnson Space Center. Prior to this study, techniques used to test the clean rooms included culturing samples for various likely contaminants. According to the paper, culture studies are limited because only a small fraction of possible contaminating bacteria can be grown in a culture. In contrast, these authors have used a different technique they call molecular rRNA gene sequence analysis. This method provided a far broader and more sensitive inventory of possible bacterial life by using a genetic marker within the bacterial cell machinery itself to indicate its presence.
Their results showed significant levels of a diverse community of bacteria still present in the clean rooms tested. The researchers concluded that
The presence and persistence of microbial contaminants on spacecraft and in their assembly facilities underscores the need for comprehensive cleaning and maintenance protocols and frequent surveys of bacterial communities.
This all goes to show that we may have already contaminated Mars with Earth-life from the few spacecraft NASA has landed on Mars.
At the time of the original meteorite discovery, Reasons To Believe called attention to the fact that, in addition to material having been blasted off the surface of Mars and eventually finding its way to Earth, there is also a large amount of material that has been driven from the surface of Earth, eventually landing on Mars. Consequently, RTB’s position has been that it seems reasonable to conclude any evidence of life found on Mars or in Mars meteorites is more likely to have come from Earth—the planet teeming with life—rather than the other way around. Based on this view we have predicted that it is highly probable that evidence for life will, in fact, be found on Mars. However, with the likelihood of additional contamination from spacecraft, if life is detected on Mars, we’ll have even more reason to believe it originated on Earth.