Mars is a primary target in the search for evidence of extraterrestrial life, past or present. This planet appears to possess all of the materials and some of the environments associated with life on Earth. While the surface of Mars is now cold and dry, some scientists believe that conditions on the planet may have been much like those on Earth early in solar system history (though others speculate that Mars may have been cold and dry in this early period). And Mars subsurface conditions are now, and may always have been, quite different than those on the Martian surface.

In recent years, NASA’s Mars Global Surveyor (MGS) and Mars Odyssey spacecraft have made observations that show evidence of plentiful water ice. More recently,NASA’s Mars Exploration Rovers,Spirit and Opportunity, and the European Space Agency’s Mars Express orbiter have collected evidence of past liquid water on the planet’s surface. Some scientists claim observations also reveal signs of possibly recent liquid water flows on the surface of Mars. Others speculate that Mars may have been cold and dry, as it appears to be today, for billions of years. Scientists have long considered, as well, that liquid water may have existed, and may still exist today, beneath the surface of Mars. They have also explored the possibility of interplanetary transfer of microbial life from Mars to Earth or Earth to Mars. Such transfers could have resulted from the high-velocity ejection of soil and rock caused by planetary impacts of comets and other small bodies early in the history of the solar system, when such events were frequent. Planetary Protection considerations for missions to Mars are thus substantial, and sterilization requirements for spacecraft landing on Mars are strict. Planetary Protection requirements for MGS and Mars Odyssey are intended to ensure that the orbiting spacecraft do not inadvertently drop out of orbit and crash onto Mars after their missions are completed. Planetary Protection requirements for the Mars Exploration Rovers are intended to ensure that these landers did not transport Earth microbes to the surface of Mars.

Planetary Protection techniques that may be applied to spacecraft bound for Mars include clean manufacturing processes for spacecraft components and the use of cleanroom techniques during spacecraft assembly, test and launch operations. Biological contamination loads may be reduced by methods such as alcohol wiping, dry heat microbial reduction and hydrogen peroxide sterilization. Radiation treatment is an option for spacecraft components that can tolerate it, and newer molecular-level contamination detection methods may also be employed to characterize biological loads. Cultivation-based microbial assays are still the standard forward contamination control method, in part because of the lack of comparability with molecular methods and also because of additional personnel, training and requirements associated with molecular methods. These newer methods are attractive, however, because a biological load measurement that could take three days to complete using a microbial cultivation assay could be made in less than one hour using a molecular detection method.

Missions to Mars may be designated Category III, IVa, IVb or IVc for Planetary Protection purposes, depending on their aims. Mars orbiters such as MGS and Mars Odyssey are classified Category III and subject to biological burden limits or orbital lifetime requirements to ensure against inadvertent contamination of the planet in the event of a crash. Mars landers not carrying instruments designed to search for evidence of extant Martian life are designated Category IVa and subject to biological contamination limits: their biological burden is restricted to a level no greater than the Viking-lander pre-sterilization level.

Mars landers carrying instruments designed to search for evidence of extant Martian life are designated Category IVb and subject to more stringent sterilization requirements: The entire landed system must be sterilized at least to Viking Lander post-sterilization biological burden levels or to levels of biological burden reduction driven by the nature and sensitivity of the particular life-detection experiments, whichever are more stringent — or, alternatively, subsystems to be used in acquiring, delivering and analyzing Martian samples for in-situ life detection experiments must be sterilized to these levels, and a method of preventing recontamination of sterilized subsystems and contamination of material to be analyzed must be in place.

Mars lander missions intended to explore so-called “special regions,” areas where liquid water is present or likely to occur — even if they are not designed to carry life-detection instruments — are designated Category IVc and subject to stringent sterilization requirements, in whole or in part, determined by the parameters of the mission, such as landing method, landing site or rover mobility.

Visit NASA's websites for Mars and Solar System Exploration for more information.