For several years now there has been increasing speculation that the first life on Earth may have arrived on meteors from Mars. Now, three studies have suggested that the first modern life on Mars may come in the form of microbes from Earth.

All three papers focused on the risk of an interplanetary exchange of organisms using research gathered on the International Space Station (ISS). Currently, spacecraft bound for Mars or other bodies in our solar system must meet the requirements for a maximal allowable amount of microbial life.

These bioburden levels were established based on studies of how well various organisms respond to the rigors of space travel. These conditions are incredibly harsh and might include extreme cold or heat, high levels of radiation and the airless vacuum of space.

“If you are able to reduce the numbers to acceptable levels, a proxy for cleanliness, the assumption is that the life forms will not survive under harsh space conditions,” explains Kasthuri J. Venkateswaran in a statement. Venkateswaran is a researcher with the Biotechnology and Planetary Protection Group at NASA’s Jet Propulsion Laboratory and a co-author on all three papers.

However, recent research has shown some organisms to be much hardier than expected. Spores of Bacillus pumilus SAFR-032, for example, have shown high resistance to the techniques used to clean a spacecraft, such as UV radiation and peroxide treatment. They have also survived in a simulated Mars environment for 30 minutes, when most spores in that environment died after 30 seconds. In one experiment, the spores were exposed for 18 months on the European Technology Exposure Facility (EuTEF), a test facility mounted outside the ISS.

“After testing exposure to the simulated Mars environment, we wanted to see what would happen in real space, and EuTEF gave us the chance. To our surprise, some of the spores survived for 18 months,” says Venkateswaran.

In another experiment, spores of the bacteria along with another spore-forming organism, Bacillus subtilis 168, were dried onto spacecraft-quality aluminum and subjected to the vacuum of space as well as radiation and a simulated Martian environment on EuTEF. Most of the organisms were killed but those that were kept in the dark were 50 percent more likely to survive.

That means that if the spores were situated in the right location, one that shielded them from solar radiation, they could survive the trip to Mars.

The third experiment tested lithopanspermia, which is the theory that life can travel from one planet to another on meteorites. Rock colonizing cellular organisms specially adapted to extreme environments on Earth were placed in the EuTEF facility and exposed to an even more extreme environment for 1.5 years. Many of these organisms did survive, though actual lithopanspermia might take thousands or even millions of years.

These experiments can help space explorers to avoid contaminating alien worlds accidentally and will prevent researchers from mislabeling Earth-based micro-organisms as alien. However, it is entirely possible that bacteria from Earth will colonize Mars along with Mars One, whether they are invited on the expedition or not.