Credit: NASA.

With a stark appearance dotted by thousands of craters and almost no atmosphere, the moon is perhaps one of the last places in the solar system you’d call ‘habitable’ — but that’s not to say it’s been like this forever. According to simulations run by astrobiologists at Washington State University and the University of London, the moon may have fostered the necessary conditions for life to survive on its surface during its distant past.

Lunar life?

Despite ambitions to one day colonize the moon, not much serious consideration has ever been given to the thought of a once habitable moon. Recent developments, however, have shown that there is much to learn about the bright white bulb that lights the night’s sky.

Until not too long ago, scientists used to think that the moon had virtually no atmosphere. However, recent studies have confirmed that our moon does indeed have an atmosphere, consisting of gases like sodium and potassium, which aren’t found in Earth’s atmosphere, or those of Mars or Venus, for that matter. It is true that it’s extremely thin — fewer than 1,000,000 moles can be found in each cubic centimeter of the lunar atmosphere, which is comparable to the density of the outermost fringes of Earth’s atmosphere, where the International Space Station orbits — but it’s still something.

Perhaps the most important revelation about the moon is the presence of water. Almost a decade ago, researchers reported that our moon holds at least hundreds of millions of metric tons of water ice on its surface. Far more water is thought to be present in the lunar mantle, trapped by minerals that formed very early in the moon’s history.

There is also reason to believe that an early moon may have been protected by a magnetic field, which could have shielded surface-dwelling life forms from the harmful touch of solar winds.

These sorts of developments — some backed by solid evidence, others more speculative — form the basis of a new study that examined the habitability potential during the moon’s geological history.

Dirk Schulze-Makuch, an astrobiologist at Washington State University, and Ian Crawford, a professor of planetary science and astrobiology at the University of London, performed simulations that suggest very simple lifeforms may have lived on the moon — and it could have happened twice in its distant past.

One habitable episode may have occurred shortly after the moon formed around four billion years ago from a debris disk. The second may have occurred around 3.5 billion years ago during a period of heightened lunar volcanic activity.

Outgassing in both periods may have formed pools of liquid water on the moon’s surface but also a thick atmosphere that could have been stable enough to last for millions of years.

“If liquid water and a significant atmosphere were present on the early Moon for long periods of time, we think the lunar surface would have been at least transiently habitable,” Schulze-Makuch said.

If the moon was indeed habitable at some point, what are the odds of life surfacing there? There’s really no way to tell, but if the moon ever had any life one of two things might have happened. Life could have spontaneously appeared from chemical building blocks under just the right conditions, as it could have on Earth. Alternatively, microorganisms like cyanobacteria may have been transported by meteorites. The early solar system used to be extremely chaotic and Earth was frequently bombarded by small and giant cosmic bodies alike. It’s possible that meteorites containing bacteria could have been blasted off the surface of Earth onto the moon.

A future program of lunar exploration might be able to determine just how likely it was for life to arrive on the moon. One line of inquiry would be obtaining samples from deposits that correspond to the period of heightened volcanic activity. In parallel, experiments that simulate early lunar environments on Earth and the International Space Station could study the survivability of microorganisms.

The findings appeared in the journal Astrobiology.