Biological Stowaways on Mars
Current and upcoming missions to Mars hope to find some sign of past or present life in martian soil. But a constant worry is that biological contamination on the spacecraft will lead to a false detection.
New research adds to these concerns with evidence that ATP — an energy-storage molecule vital to life on Earth — could survive for months or even years onboard a martian probe.
Andrew Schuerger of the University of Florida and colleagues used a martian simulator to measure the degradation rate of ATP (adenosine triphosphate). This complex organic molecule transports chemical energy through the cells of all terrestrial organisms. It undoubtedly has found its way onto every spacecraft that has ever flown.
"It turned out that under normal equatorial Mars conditions the ATP was a lot more stable than we anticipated," said Schuerger.
If ATP stowaways can survive as long as Schuerger’s team observed, they could wind up in life-detection instruments, thereby confounding efforts to detect organic molecules inherent to Mars.
Most scientists believe that if martian life exists, it depends on some form of complex organic molecules. The 1976 Viking missions were the first to search for these compounds in martian soil but were unable to detect any.
NASA’s Phoenix mission, which landed on Mars on May 25, will try again. But Phoenix is examining ice-rich soil from deeper down than Viking looked. The forthcoming Mars Science Laboratory and ExoMars missions also plan to dig for organic molecules.
Schuerger and his colleagues, writing in the March issue of the journal Icarus, believe that prelaunch cleaning protocols on these missions may need to be strengthened to minimize ATP contamination.
In Schuerger’s lab sits the Martian Simulation Chamber (MSC), a half-meter-wide cylinder in which temperature, pressure and radiation levels are controlled to mimic conditions on the Red Planet. Special attention is given to reproducing the ultraviolet light from the sun, which easily penetrates Mars’ ozone-less atmosphere and is particularly damaging to biomolecules like DNA.
In previous work, Schuerger and his colleagues placed different bacteria samples in the MSC and found the organisms could not survive more than a few hours in simulated martian sunlight.
"We expect a spacecraft surface will be sterilized on the first day after landing on Mars," Schuerger said.
The short life expectancy of terrestrial microorganisms on Mars is reassuring, but Schuerger and colleagues wondered what would happen to the "dead bodies" and other biological residues that may contaminate the surface of a space probe.
Past studies looked at simple organic molecules, but Schuerger’s group is the first to measure how long a biologically-specific molecule – in this case ATP – might survive the harsh conditions on Mars. The researchers expected it to remain for only a few martian days (or sols) but were surprised when the MSC results implied that ATP would take 158 sols to disappear from sun-exposed surfaces, and as long as 32,000 sols (nearly 50 martian years) from shaded regions.
The authors are careful to point out, however, that actual residence times could be shorter, since their simulation did not account for oxidizing chemicals that are likely to play a role in degrading organic molecules on Mars.
Schuerger’s work was supported by NASA’s Planetary Protection Office, which is responsible for avoiding contamination of extraterrestrial environments. Much of the attention is on preventing an "invasive species" from hitching a ride to Mars.
Typically, a Mars lander must have fewer than 300,000 bacterial spores on its outer surface before launching, "which is fewer organisms than you would find in a pinch of dirt in your garden," said Cassie Conley, NASA’s planetary protection officer.
However, planetary protection policies do not regulate non-living organic material.
William Boynton of the University of Arizona is the lead scientist on the Phoenix mission’s Thermal and Evolved Gas Analyzer (TEGA), which will look for organic molecules on Mars. His team built their instrument under very stringent clean room conditions, but "making it any more rigorous than this would have slowed down the construction to the point we could not have delivered the instrument in time for launch," he said.
Conley thinks the amount of ATP on Phoenix and other missions is probably small to begin with, but if it does survive long enough to show up in experiments, "scientists will need to do more homework to interpret their results."