Human Mission to Mars: Brief Review on Infection Risks
October 29, 2010 3 Comments
Liquid water has almost certainly been a feature on Mars in its earlier history, and the presence of extinct or present life on Mars cannot be excluded. However, based on our current understanding of host-pathogen relationships and evolutionary processes, we may conclude that the chance of a human mission to Mars to encounter pathogenic microorganisms is small, albeit not zero. A set of safety measures to prevent, diagnose and eventually treat infections with Martian microorganisms should be considered. If the history of space exploration can be used as a lesson, the highest risk for such a mission will be catastrophic vehicle failure. However, visiting an alien planet will also come with a certain degree of biological risks, in terms of infection or contamination of either the astronauts, the technical crew on the ground, or even Earth ecosystems upon return of the mission.
The important question is here, before delving much into the topic, is whether life on mars is possible. Mars mapping by Mariner 9and by Viking 1 and 2 revealed channels resembling riverbeds,and information collected by the Mars Global Surveyor (MGS) strengthened the case for early surface water on Mars. Even more striking, data from Mars Exploration Rovers discovered round pebbles scattered on the surface of Meridiani Planum, suggesting that this region has once been submerged.
Studies on the Martian meteorite ALH84001 that reported the discovery of carbonate granules resembling microfossils, have been hailed as the first probable direct evidence of life outside Earth. Within the same meteorite, magnetite crystals with properties compatible with biogenic terrestrial magnetite have also been found. However, these reports remain controversial, as non-biological processes have also been proposed to explain the features found in ALH84001 meteorite, yet all such evidences boost the possibility of probable micro-life forms. The detection of methane and formaldehyde in Mars’s atmosphere could be another indication that microbes exist on Mars. Although the presence of methane could be a mere sign of geographical processes, it is well known that most of the methane in Earth’s atmosphere is produced by microbes. Thus, a straightforward aspect for martian life is that it would more likely be microscopic. Viruses are obligate intracellular organisms, needing a host to replicate and transmit genetic information, which make them additionally vulnerable to extreme environments.
How likely is that a Martian bacteria would be pathogenic for humans, or disruptive for an Earth ecosystem? The chance of a Martian microbe, adapted to extremely slow, cold and anaerobic conditions having the ability to attach to cells of a terrestrial host and invade its cells or tissues, and hence produce infection, in full competition with terrestrial microbes, is very small. Less likely is even transmission to a second ‘vulnerable’ host.
However, a pathogenic potential of Martian microbes cannot be excluded either. Even if they were not capable of directly invading the host and causing infection, Martian microbes could still have pathogenic potential by secreting toxins that could indirectly harm the astronauts (e.g. through wounds, contaminated food). Examples of powerful microbial toxins secreted by terrestrial microbes abound. Still, one has to recognize that the majority of such toxins of terrestrial bacteria are proteins, which in turn are recognized by specific cellular receptors, again requiring a history of previous interaction between the pathogenic agent and the host. Would such putative toxins of Martian microbes also be proteins, would they have similar biochemistry, would they even be made of the same aminoacids? Although it is possible that horizontal gene transfer may play an important role in injecting genes and genetic exchanges.
A different aspect of the biohazard potential of Martian microbes is the capacity of such microorganisms to disrupt Earth ecosystems, should contaminated material from a Mars mission reach the environment upon return. This risk is most likely also small, as environmental conditions such as temperature, humidity, chemistry, atmospheric pressure,and nutrients fundamentally differ between Earth and and Mars. From an evolutionary point of view, it is highly unlikely that a Martian microbe that in Earth terms would be characterized as an extremophile would be able to compete successfully with terrestrial microbes which are optimaly adapted to environment through millions of years evolution. Despite the low probability of pathogenic microorganisms as indicated above, it cannot be excluded that Mars harbors microscopic life, and the possibility that astronauts would come in contact with it necessitate precautionary measures to insure safety of the crew and Earth habitats upon return of the mission.
[Ref: Infection Risk of a Human Mission to Mars by Mihai G. Netea]