To date, a total of a dozen U.S., Soviet, and European spacecraft have landed or crashed on Mars. They were all sterilized before leaving Earth, but were the procedures good enough to wipe out every last stowaway Earth bug?
If we were to determine Mars has been biocontaminated by microscopic colonists from Earth, it would open a Pandora's box of astrobiological issues. Did we "play God" by inadvertently seeding another world with life that mutated into a new type of organism that adapted to alien conditions?
There is reason to worry about biocontamation. In 2006 it was reported that a common soil bacterium, called Bacillus, remained healthy and viable on a spacecraft that had been sterilized with ultraviolet light.
Perversely, sending microbes to Mars could become one of the greatest accidental science experiments of all time: the introduction of an organism on another planet that tests the power of Darwinian evolution, and offers a window into Earth's early history.
"We have known from the rock record that complex life is amazingly resilient. Despite repeated near annihilation, complex life has never failed to adapt to new environments. We believe that once life got started on this planet, it survived one way or another," writes Janet Siefert of Rice University in Astrobiology.
The author argues that based on the fossil record it seems inevitable that microbes will eke out a living by beating all odds to accumulate the right genetic material to adapt quickly. But can they pull it off on a dry, irradiated, and hostile place like Mars?
A lab experiment in 2012 subjected microorganisms to Mars-like conditions. The bugs had a rough time coping with a daily thawing and freezing cycle, lack of oxygen, and sparse water. It slowed down their growth and the microbes ultimately perished.