As locations for life go, the Moon would seem to be a non-starter — especially given what we suspect about the circumstances of its creation. “In the beginning,” our hypothetical sacred text might read, “The Earth was smashed by another planetesimal, Thea. This impact turned the surface of our hapless rock back into magma, may have created a shared molten rock atmosphere between Earth and Luna, and oh yeah, briefly created life on one of the most barren lumps of rock in the solar system.”
It doesn’t exactly read like a bestseller — but it could’ve happened. That’s the finding of a study on lunar geology, which suggests that in the immediate aftermath of its formation, the Moon may have contained large quantities of water. Studies of lunar flood basalts (magma deposits) suggest that they retain water concentrations at several hundred parts per million, while studies of water vapor retention during the impact that created the Moon suggest, again, that substantial deposits may have been preserved.
It’s easy to understand how the enormous heat of either planetary accretion or massive volcanic activity could temporarily lead to liquid water being present on the surface of the Moon — but in the absence of an atmosphere, liquid water wouldn’t persist for any length of time. At the same time, however, the enormous amount of gas being given off by the moon in either of these two states would effectively form a temporary atmosphere. The authors note:
Gases derived from lava outpourings may have built up an atmosphere of about 10 mbar, which is above the triple-point pressure of water and about 1.5 times the present atmospheric pressure on Mars (and about 3 times as massive as the current Martian atmosphere, given the difference in surface gravities).
The far side of the Moon, and Earth, hanging in space
And a “temporary” atmosphere in geological terms can still translate to an atmosphere that’s awful damn permanent from our point of view. The Moon’s temporary shield might have lasted 70 million years. 70 million years ago on Earth, the dinosaurs were still four million years from extinction, just to give that time frame a sense of scale. The question is, would this have been enough time for life to evolve? And the answer, honestly, is that we don’t know. Some have estimated that cyanobacteria could have appeared within 10 million years from the appearance of the building blocks of life — which would mean life could have emerged on the Moon independently of Earth and then gone extinct once the conditions changed. This isn’t a crazy concept given that we’ve seen similar theories proposed for Venus and Mars.
Life was unambiguously present on Earth by 3.8 – 3.5B years ago. It may have been present on Earth as far back as 4.26B years, which would be virtually as soon as our planet formed. But one of the problems of tracing the origin of life on Earth is that Earth’s active geology precludes this. The oldest part of the seafloor is only 280 million years old. Rocks on land have been subjected to billions of years of erosion and other shaping forces. It’s not that we don’t have any rock dating back to the beginning of life on Earth, but we don’t have very much.
Another possibility for life on the Moon is that it might have been carried there by sufficiently large impact events on Earth. This isn’t exactly crazy, either. The reason we know as much as we do about Vesta’s internal structure, for example, is because there are pieces of Vesta on Earth that arrived here after being knocked off the asteroid in a collision. Vesta is over 150 million kilometers from Earth. The Moon, in contrast, is about 240,000 miles — and that’s the current distance. When it formed, the Moon would’ve been an estimated 14,000 miles from Earth, which would’ve made the asteroid transit difference trivial.
But either way, it’s an interesting argument. Finding evidence that life arose repeatedly across the solar system, even temporarily, would change our understanding of how life appears and evolves. It would suggest that life should occur abundantly across the universe and that the reason we see no advanced alien life other than ourselves is not simply because life almost never comes into existence in the first place. This might not seem like much of a step forward given how many huge questions would be left unanswered, but given that we don’t currently know any of the answers, any step forward is an improvement.