Today, the Moon is as dead as you can get, but scientists contend that this might not have been the case billions of years ago. According to Dirk Schulze-Makuch of Washington State University and Ian Crawford from the University of London, there were two periods shortly after the Moon formed when it could have been possible for life to have existed on the lunar surface.

The modern Moon is a terrifyingly inhospitable place. Without an atmosphere or a magnetic field, the satellite is subjected to meteorite bombardments, daily swings in temperature from the far subzero to boiling, and blasts of solar and cosmic radiation. It's also so dry that the only water is in the form of ancient ice at the bottom of polar craters and perhaps deep in the lunar mantle. It's a world where, if any life does exist, it's in the form of a few spores brought by American and Soviet lunar missions during the Cold War.

But Schulze-Makuch and Crawford claim that things could have been very different four billion years ago when the Moon first formed from a debris disk believed to result from the proto-Earth colliding with a Mars-sized object. As the newborn Moon coalesced, there would have been a lot of water sealed inside it, much as in the early Earth. As it cooled, the Moon would have eventually formed a primitive atmosphere protected by a magnetic field generated by the still-molten core.

Under these conditions, water could have existed on the lunar surface under conditions that could support some forms of microbial life. This wouldn't have lasted long, and after a few million years the atmosphere and the water would have been gone. However, five hundred million years later, the Moon's volcanic activities were at their peak, belching billions of tons of gas out to form a second temporary atmosphere and a second watery habitat that would have lasted a few more million years.

"It looks very much like the Moon was habitable at this time," says Schulze-Makuch. "There could have actually been microbes thriving in water pools on the Moon until the surface became dry and dead."

The question remains, where would such microbes come from in such a brief time? According to Schulze-Makuch, the most likely source is Earth – 3.8 to 3.5 billion years ago, our planet's most advanced known life form was cyanobacteria, which can live without oxygen. This was also a time when the Earth was being constantly bombarded by comets and asteroids. It's entirely possible that some of these bacteria could have been blasted off the Earth and then rode protected inside meteors to eventually rain down on the Moon.

At the moment, this is largely theoretical, but Schulze-Makuch says that future lunar missions could seek out samples that might show the presence of not only ancient water, but organic molecules and other biosignatures. He also says that experiments on using simulated lunar environments could test how well life might have survived on the early Moon.

The research was published in Astrobiology.