There are many objectives of the Mars mission, although one objective is to collect some samples and bring these back to Earth. But what happens if there is indeed life on Mars? What measures can be taken to protect our planet from extra-terrestrial sources of microbial contamination?
In addition, it is important not to mistake Earth life for indigenous life brought back from Mars.
These considerations have led to a consideration as to how optimally to examine the effects of desiccation and freezing on microbial ionizing radiation survivability on Mars. Many types of bacteria, and fungi possess several strategies that enable them to survive desiccation in the form of spores or when simply dried as vegetative cells.
With the research, the aim was to better understand the impact of the Martian surface on microbial dormancy and survivability. To do this, the scientists replicated the conditions on Mars. This involved testing the influence of desiccation and freezing on the ionizing radiation survival using six model microorganisms.
The organisms were: Vegetative cells of two bacteria (Deinococcus radiodurans, Escherichia coli), a strain of yeast (Saccharomyces cerevisiae), and vegetative cells and endospores of three Bacillus species bacteria (B. subtilis, B. megaterium, B. thuringiensis). The scientists found that desiccation and freezing increased the radiation survival of vegetative microorganisms. Here there is a strong relationship between the processes of desiccation, freezing, and radiation survivability.
For example, the research discovered that desiccated and frozen cells of the bacterium Deinococcus radiodurans can survive high levels of ionizing radiation (this organism demonstrated by far the highest levels of radioresistance). This is to a level equivalent to hundreds of millions of years of background radiation on Mars. This include levels up to 140 kGy for organisms in the freeze-dried state. One ‘gray’ is defined as the absorption of one joule of radiation energy per kilogram of matter. Sterilization is typically achieved at 15-25 kGy.
From this it can be concluded that if Martian life ever existed then organisms like D. radiodurans cells could survive the equivalent of 280 million years in the frozen Martian subsurface. In addition, their macromolecules would survive much, much longer.
The implications of these findings are that if life ever evolved on Mars, there is a strong chance that it will be revealed in future missions.
On the other side, given that some microorganisms can survive on spacecraft and within the spaceship environment, there exists a very real possibility of Earth microbes contaminating specific landing sites on Mars.
It is also possible that some microorganisms introduced from Earth to Mars (Martian survivors) could also be transported back to Earth. This will present a complexity in deciphering whether these organisms were originally from Earth or are originally Martian.
The study results have been published in the journal Astrobiology. The research is titled “Effects of Desiccation and Freezing on Microbial Ionizing Radiation Survivability: Considerations for Mars Sample Return.”
