A new paper by Ian Crawford from the University of London and myself, just published in the journal Astrobiology, claims that Earth’s Moon might have been habitable about one billion years after its formation, when pools of liquid water may have existed on the lunar surface. Today, of course, the Moon has no atmosphere and no liquid water. It’s uninhabitable and certainly lifeless. But 3.5 billion years ago, a billion years after it formed, the lunar environment was quite different.

During this period of extreme outgassing from lunar magma, the Moon is estimated to have had an atmospheric pressure of 10 millibar, or one percent of Earth’s current atmosphere. This is thicker than the current atmosphere on Mars, and would have been substantial enough for liquid water to pool on the lunar surface, perhaps for many millions of years.

Combine this with recent findings that lunar rocks are more water-rich than previously thought, and we can hypothesize that lakes, even an ocean, could have stably existed on the Moon for a substantial amount of time. There is also evidence that the early Moon had a magnetic field, which might have partially protected its surface from solar and cosmic radiation. This would have resulted in a temporarily habitable world, at a time when life on Earth had already gained a foothold.

If early terrestrial life spread to the Moon via meteorite impacts, it could have thrived there if the lunar surface was indeed habitable. Simulations show that a transfer of microbes from Earth to Mars via meteorites, referred to as lithopanspermia, was possible—although just barely, given the distance between the two planets. The Moon is much closer than Mars, however, so an Earth-Moon transfer would have been much easier. And the Moon was even closer to Earth 3.5 billion years ago than it is now.

The image of liquid water teeming with microbes on the lunar surface completely shatters the current paradigm of the Moon as a dead rock in space. Of course, we have to be careful not to get carried away with the idea. After all, we don’t see any of the water-modified topography on the Moon that we see on Mars. Then again, would we really expect to, considering that the Moon has been pounded by solar wind, cosmic radiation, and micrometeorites for several billions of years?

Crawford and I therefore propose ways to test our hypothesis rigorously. We suggest searching for water-rich minerals in geological layers trapped between lava flows dating from this period. We also propose a more aggressive future program of lunar exploration, where state-of-the-art instruments are placed on the Moon’s surface and lunar samples are returned to Earth for analysis. Further, we recommend conducting experiments in laboratories on Earth that simulate the early lunar environment to observe whether microorganisms can remain viable under environmental conditions predicted to have existed on the Moon 3.5 billion years ago.

With this intriguing new idea in mind, we’ve already expanded our lunar research group and started to plan some of the required experiments.