London, June 7 : Two new studies have suggested that NASA's Phoenix Mars Lander may have been coated with dozens of species of bacteria when it left Earth, which might have hopped on to Martian surface when the probe touched down on the Red Planet.
According to a report in New Scientist, the only chance for any stowaway microbes on Phoenix to flourish is during experiments that warm and moisten Martian soil samples.
But, researchers have said that the parts of the Lander that will contact water ice on Mars, which might provide a toehold for life, have been carefully sterilized and protected with a protective film called a 'biobarrier', minimizing the chances that terrestrial life could colonize the planet.
NASA has long realised that spacecraft could potentially seed other planets with terrestrial life.
To cut the chances of transporting microbes to space, probes such as Phoenix, which landed on the northern plains of Mars on May 26, are now assembled in clean rooms ventilated with filtered air.
NASA also swabs the craft to measure the levels of particularly hardy spore-forming bacteria, which can lay dormant for decades and withstand extreme temperatures.
But the agency doesn't routinely check for less resilient bacteria or microbes that can't be cultured, since harsh ultraviolet radiation on Mars is thought to quickly kill most such organisms.
To fill this gap, a team from NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, US, took a census of all microbial life living in the Phoenix assembly room as the mission progressed.
About four months before launch, in April 2007, at least 100,000 microbial cells - including 132 different kinds of bacteria - covered each square metre of the room.
In June, the JPL team found evidence for at least 35,000 cells per square metre, belonging to 45 different kinds of bacteria - a decrease most likely due to stepped-up cleaning efforts.
Kasthuri Venkateswaran, a microbiologist at JPL, also tested what would happen to microbes that happened to make it to Mars on Phoenix, a likely prospect.
To simulate the planet's conditions, the researchers put bacteria into special growth chambers that simulated the air pressure, temperature and UV radiation of the Red Planet.
Within five minutes, three species of bacteria had died, including one resistant to radiation.
But when the JPL team added Mars-like soils from the Atacama desert and a Hawaiian volcano that could shield the microbes from dangerous radiation, some bacteria managed to survive.
"All these data suggest that yes, you'll get some killing, but you're not going to get 100% sterility," said team member Rocco Mancinelli, a microbiologist at the SETI Institute in Mountain View, California, US.