For over a decade, Mars has been considered the most promising, fertile locale for life away from Earth. The moon, on the other hand, has long been considered an uninhabitable chunk of rock.
But a new study published Monday in the journal Astrobiology tells a different story. After analyzing data from lunar probes and reviewing previous studies of the moon’s topography and atmosphere, astrobiologists Dirk Schulze-Makuch and Ian Crawford of Washington State University and the Birkbeck University of London, respectively, concluded that it was possible—even likely—that life could have existed on the moon billions of years ago.
As the pair reviewed data, a pattern emerged. “It became more and more apparent to us that the moon could have harbored an ocean, or liquid water on the surface, during certain time periods,” Schulze-Makuch told The Daily Beast.
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Schulze-Makuch and Crawford believe they found two such time periods where there may have been water, and the moon’s environment could have supported possible life. The first occurred approximately four billion years ago while the moon was still forming; the other occurred 500 million years later during the peak of lunar volcanic activity. In both cases, according to a press release on the study, the moon was likely “spewing out” hot, volatile gases from its interior.
One of these gasses was probably water vapor.
“We know that there [were] a lot of volatiles—a lot of gasses—in the magma,” Schulze-Makuch said. “Carbon dioxide, water, sulfur gasses [...] Since we approximately know what the gas composition of magma is, [we] can come up with how much volatiles were gassed out at the time.” They estimate that during the first period, there could have been a layer of surface water one kilometer (.62 miles) deep at most (although they acknowledge that this is “very optimistic”), and that the second was likely much shallower, citing another team’s estimate of water vapor outgassing that would equate to three millimeters of water, or about a tenth of an inch.
Schulze-Makuch and Crawford aren’t the first to publish evidence of water on the moon. In 2009, the press release noted, an additional team of scientists discovered evidence of hundreds of millions of metric tons of water ice. There is also “strong evidence” that water once existed in the lunar mantle, according to another cited study.
Schulze-Makuch and Crawford’s study, however, is the first to find strong evidence of liquid water on or near the moon’s surface—the kind necessary to sustain life.
This is because, the authors note, the moon likely had the proper conditions to keep this water in a liquid, habitable form. The paper cites a previous study from Needham and Kring showing that the moon had an atmospheric pressure of about 10 millibars during both periods—about one percent of the Earth’s current atmospheric pressure and 1.5 times more than the current pressure on Mars. With that amount of pressure, Schulze-Makuch said, “liquid water would be stable on the surface.”
He also noted that the moon likely had a magnetic field at that time, which would have “protected” any potential water from harsh solar winds. “We basically had what we would call habitable conditions,” he said. “Or at least the most basic parameters for that.”
These findings are “especially interesting” during the more recent period, Schulze-Makuch said, because that period occurred after there was documented life on Earth (which began about 3.7 billion years ago). It’s quite possible, even “likely,” he explained, that an asteroid could have hit the Earth during that time, shooting rock—and with it, microbial life—to the moon.
That life would be incredibly basic, Schulze-Makuch noted: microbes, anaerobic bacteria, and at most, photosynthesizing cyanobacteria. And as of today, there is no evidence that this life existed—just that for a few tens of millions of years, it was possible.
To try and find evidence of such life, Schulze-Makuch has two ideas for further exploration. First, he said, scientists could take samples from designated spots on the moon, with the aim of finding regions with evidence of past oxidation or hydrothermal activity. Or on Earth, he added, researchers could use existing “moon simulation chambers” that can reproduce the moon’s early atmosphere to see which (if any) microbes are able to survive.
Unfortunately, no matter what further studies discover, it’s unlikely that humans will be moving to the moon anytime soon. Besides being what Schulze-Makuch called a “dead rock in space,” the study noted that moon’s surface has no protection from cosmic or UV radiation, and is subject to massive temperature fluctuations that would make life there all but impossible. And after the atmosphere eroded and the water dried up after the second possible habitable period billions of years ago, Schulze-Makuch said, “there’s no liquid water anywhere.”
“The only thing the moon has going for it is that it’s close by. If you want to live on the moon today, you basically need to dig yourself under the bottom meter of rock,” he added. “That is not really great.” Mars still remains the most viable option for human colonization, he said, “or for life in general at this time.”
Regardless, Schulze-Makuch maintains that these findings are vital for our understanding of the moon, and of the development of early life. It’s possible, he notes, that lunar samples could even contain evidence of the last universal common ancestor for all life on earth. But even if that’s not possible, he said, researchers are likely to learn more about the “biological treasure trove” of early development, and make new discoveries about how life on Earth came to be.
“It changes the paradigm,” he added. “Okay, the moon is just a dead rock in space … but 3.5 billion years ago, it may have actually looked quite different.”
