One thing the Apollo missions drove home about our nearest celestial neighbour is that it is a very harsh place with almost no atmosphere. Dust that’s more like asbestos than what we have on Earth, and zero liquid water on the surface. A harsh mistress indeed to quote Heinlein, an utterly dead landscape. And it’s been that way for billions of years. But this may not always have been so, to the point that the moon once might have been an abode of life.
In a paper by Dirk Schulze-Makuch and Ian Crawford, “Was There an Early Habitability Window for Earth’s Moon?” they detail that as the study of the moon has progressed in recent years. It has become evident that the moon does possess more water than previously thought. Not just locked up in ice in shadowed craters at the poles, but deep down within its interior.
About 4 billion years ago, shortly after the moon formed after proto-earth’s collision with Theia, there was a time where it was completely molten. This is thought to have led to a period of outgassing. And it might have allowed the new moon to build up an atmosphere of water vapour. And in fact, at one point it may have been more substantial than Mars’ current atmosphere. Also possible at this time was that the moon may have been generating a protective magnetosphere.
As the moon cooled, this might have allowed for liquid water to be present on the surface of the moon for a time. Or in protected environments just beneath the surface. Also happening during this period were lots of impacts, which might have delivered the building blocks of life to the moon. This allows for two possibilities:
- Panspermia from Earth: The first is Panspermia from Earth. If life arose here, which may have happened as long ago as 4.1 billion years, and coincided with the moon’s period of habitability, then it’s possible that a meteorite blasted off the surface of the Earth might have contaminated the moon. And for a time, Earth’s microbial life might have inhabited two worlds instead of just one.
- Native moon life: The other possibility is native moon life. We know from Earth that as soon as life could get going on this planet, it did. This may also have been the case for the moon. This leads to several exciting possibilities. The earliest life on Earth is still here. When the plants began photosynthesizing, it made the Earth’s atmosphere poisonous to these first primitive microbes. But they didn’t go away, they simply retreated to environments where they could continue to thrive, including underground. If any life present on the moon likewise abandoned underground as the surface became uninhabitable and the moon still has significant amounts of water and heat below its surface, that life in principle could always be there. Or fossilized evidence of it. Europa or Enceladus the moon is not, but it’s still a maybe if a long shot. And, it’s also possible that life present on Earth now actually originated on the moon. Meaning that the Apollo missions were technically earth life returning to the homeworld after a long period of being marooned on Earth.
Unfortunately detecting the presence of past life on the moon would be difficult. The moon shows no evidence of the action of liquid water. Any such evidence would have been destroyed. Both by volcanism early in the moon’s history, and also the process of gardening that occurs on the moon as meteoroids invariably impact it and slowly resurface it. To find out for sure, geologic evidence from very early in the moon’s history would be needed. But it is possible to find it. Such as ancient rocks that might be preserved below a solidified lava flow. Still, it will be difficult. But this reasonable habitable period for the moon is exciting to think about. As is the idea of the moon with a substantial atmosphere. I wonder what that looked like from here.
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