Earth Microbes Not Likely to Contaminate Mars, Scientists Say
This image shows the Mars rover Curiosity being assembled in one of the Jet Propulsion Laboratory's 'clean rooms.' The team members are all dressed in special head-to-toe white suits.
Credit: NASA/JPL-Caltech

Scientists have long been concerned with the possibility that their search for any signs of life on Mars could be thrown off-course by wayward microorganisms from Earth that hitch a ride to the red planet on man-made spacecraft. But a new study suggests that fear may be less likely than researchers once thought.

While some bacteria could potentially survive the harsh trip through space, Mars atmospheric entry and landing, they would most likely die soon after landing on Mars because of the planet's harsh atmosphere, the study found.

Ultimately it is unlikely such microorganisms will be able to replicate once on the Martian surface, the research suggests.

"Without replication, terrestrial microorganisms are very unlikely to contaminate a landing site," Andrew Schuerger, one of the study's researchers, told "Thus, it is unlikely that spacecraft microbes will compromise the search for organics or the search for life on Mars."

Mars has been one of the primary places that scientists have expanded their quest for extraterrestrial life, and while Curiosity is not intended to be a life-seeking mission, it is still important for a rover to have minimal bacterial impact on the red planet.

NASA's next Mars mission ? the huge rover Curiosity ? is slated to launch in 2011. Protecting against microbial contamination will be a vital part of that mission's preparation.

Simulating Mars on Earth

The new study was conducted by researchers at the University of Central Florida to examine whether two bacterial species common to spacecraft could survive and replicate under simulated Mars-like conditions.

The week-long study observed the behavior of Escherichia coli (E. coli) and Serratia liquefaciens (S. liquefaciens), two bacterial species that appeared to be tolerant of low pressures, under Martian conditions.

"In a previous study with 37 bacterial strains, the E. coli and S. liquefaciens strains appeared able to grow and replicate at 25 millibars," Schuerger said. "The pressure on Mars is 6.9 millibars. Thus, the bacterial strains were hypothesized to be "potentially" capable of growth at the Martian pressure of 6.9 millibars."

The Mars Simulation Chamber, built by Schuerger, recreated the planet's soil, atmosphere, pressure, temperature and UV irradiation.

S. liquefaciens was found to be highly sensitive to the desiccating, or drying, conditions prior to being introduced to the Mars Simulation Chamber, said Schuerger, and was therefore not included in the final simulation. On the other hand, E. coli was able to survive desiccating conditions and Martian conditions for seven days within the confines of the experiment, but was unable to grow or replicate.

Has Earth already spoiled Mars?

But, could Mars already be contaminated from previous missions to the red planet? The study's researchers say it is highly unlikely.

"We really can't extrapolate a seven-day study to speculate on the survival rates of what could already have made it to Mars; a process that is highly unlikely given the extremes of spacecraft assembly and interplanetary travel," Bonnie J. Berry, one of the co-authors of the study, told

In order for scientists to conclude that Mars is contaminated, they would need data that demonstrates that microbes could first be dislodged from spacecraft and then could survive, grow and replicate on the planet, said Schuerger.

"While a few microorganisms might still be attached to spacecraft components, if the microbes cannot grow and replicate, then their spores will stay attached to the spacecraft and essentially not impact the pristine nature of the rest of Mars," Schuerger said. "So it might be accurate to state that the immediate landing site on Mars does have a few microbes associated with the spacecraft, but it would then be wrong to extrapolate that point to the conclusion that Mars is contaminated."

Protecting Mars from Earth

In fact, there are already stringent policies and procedural requirements in place to protect against the biological or organic contamination of Mars and other bodies in the solar system.

NASA's Planetary Protection Program aims to prevent cross-contamination that could result from the space agency's exploration missions by regulating how "clean" spacecraft have to be before launch.

The program's standards vary, depending on the function of the spacecraft and the goals of the specific mission. In other words, landers and rovers are required to meet a different standard than orbiters.

The 1976 Viking missions, the first life-detection missions to Mars, operated under more probabilistic measures, said Karen Buxbaum, Planetary Protection Manager for the Mars Program at NASA's Jet Propulsion Laboratory. Since then, the rules have maintained a conservative approach, but have become far more mission-specific,

Currently, NASA policy states that fewer than 300,000 viable microbial cells should be present on the entire spacecraft at the time of launch. For hardware that will also be impacting the surface of a celestial body, the rule is more stringent ? there should be less than 500,000 cells in those cases.

For comparison, a typical gram of soil contains between 1 million and 100 million viable microbial cells, said Schuerger. So, the number of spores allowed on the entire surface area of a spacecraft is roughly equivalent to the number of microbes "found in less than 1/10th a gram of soil."

Standards for the Curiosity rover's mission go even further. The scientific instruments on the new rover are so sensitive that NASA also has to control for molecular contamination.

"The scientific payload is so sensitive. There's a very strict contamination control plan," Buxbaum told "Engineers are building the spacecraft in an extremely clean assembly environment. They use very strict gowning and cleaning protocols, so regardless of what the requirements are going in, the reality is that [Curiosity] is very clean."

Buxbaum also agreed that the possibility that Mars is already contaminated is doubtful, even though some microorganisms may have been previously exposed to the Martian surface.

"There's been a certain amount of microbes that may have gotten to the Martian surface, but always in regions of the planet considered to be too dry, too cold, too exposed to ultraviolet radiation to allow for those microbial contaminants on Earth to multiply and spread," Buxbaum said. "So, the short answer is: yes, terrestrial microbes have been on spacecraft that have gone to Mars. But, the rest of the answer is that that was done in compliance with a very conservative planetary protection policy intended to protect Mars from potential harmful contamination."

100 percent Earth germ free

In the future, scientists are hoping to develop methods to be able to build spacecraft that have no threat of contamination, especially for any upcoming life-detection missions

"It's important for exploration to any destination in the solar system where we think we might encounter life ? so, if we found a warm, wet region of Mars, or missions to Europa or Enceladus, for example ? that we come as close to absolute sterility as possible," Buxbaum said. "But right now, we currently do not build spacecraft that are compatible with an absolute sterility requirement."

In the meantime, scientists and the Planetary Protection Program are doing what they can to control the threat of contamination prior to launch, while simultaneously developing ways to recognize and account for microorganisms that are already known to originate on spacecraft.

The result is a microbial inventory that catalogues genetic material taken from samples swabbed from the spacecraft and the room in which it was built. So, if organic material is discovered in space, scientists can use the inventory to compare the specimens with the DNA of the organisms that were present at the time of launch.

"This is our version of DNA forensics," Buxbaum said.