The moon may have once been able to support life — but learning more will require 'an aggressive future programme of lunar exploration'
- The moon could have supported life twice in its history, according to a new study.
- It's possible that 4 billion and 3.5 billion years ago, the moon had liquid water, an atmosphere, and a magnetic field. It could have even had organic material.
- These conditions may have lasted for tens of millions of years — long enough for life to emerge.
We may not need to travel far from our home planet to find a spot in our solar system that could once have supported life.
Long ago, Earth's moon may have had conditions in which life could arise, according to a study published on Monday in the journal Astrobiology.
In fact, such conditions could have arisen on the moon during two different periods, each tens of millions of years long, the study suggests. The authors are not saying that life ever existed on the moon — just that the conditions that make life as we know it possible seem to have been in place billions of years ago.
When we look for signs of life on other planets and moons, clues that can indicate a climate supportive to life include liquid water, an atmosphere that would help keep water stable on the surface, a magnetic field offering protection from solar and cosmic radiation, and organic compounds that could make up life's building blocks.
According to the study's authors, at least some of those key conditions could have existed simultaneously on the moon.
"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," Dirk Schulze-Makuch, a Washington State University astrobiologist and co-author of the study, said in a statement.
But astronauts and rovers have never found any evidence of life on the moon, and even if organic material did once exist on our planet's satellite, we don't know if any traces remain.
How the moon could have supported life
The idea that the moon could once have been habitable is based on a series of discoveries, mostly made within the past decade, that show the moon isn't as dry as we thought. There's probably still water ice in polar craters and water deposits trapped in the moon's interior.
Billions of years ago, there could have been good amounts of liquid water on the surface, the new study says.
To understand why, a bit of lunar history is needed. Sometime around when our solar system settled into its current layout — about 4.5 billion years ago — a proto-Earth and another planetary body likely collided and were vaporised, according to a paper published earlier this year.
As this theory goes, the super-heated doughnut of molten, vaporised rock and liquid — called a synestia — cooled, then the moon emerged, after which the remaining cloud of vapour condensed to form the Earth. For a long time after its formation, the moon was largely molten, with an ocean of magma spewing gases into its sky.
Those gases could have been enough to create an atmosphere. As that molten ocean finished solidifying (around 4 billion years ago), there could also have been deep pools of liquid water on the moon's surface. That time period, the new study suggests, was the first time conditions on the moon could have supported life.
The second time was during a period of intense volcanic activity 500 million years later — 3.5 billion years ago. That activity could have created an even more dense atmosphere with more water on the lunar surface, the study says.
According to calculations cited in the paper, there could have been liquid water on the surface for 70 million years during that period, especially if there was a magnetic field protecting the moon from solar winds.
Where early life could have come from
During both of these time windows, life may have already existed on Earth. We still don't know how organic material first appeared on our home planet. It could have been delivered to Earth by tiny meteorites, or life could have been the result of a chemical transformation at volcanic vents in Earth's oceans. Scientists also still don't know how common it is throughout the universe for conditions that support life to exist.
Some of the oldest evidence of life we have on Earth comes from fossilised microbes known as cyanobacteria. Somehow, certain precursor molecules — the chemical building blocks for life — fused together to form organic materials, which evolved eventually into those cyanobacteria.
We don't know exactly how long that process took, but some researchers have estimated that it was less than 10 million years.
By that logic, there could have been enough time for something similar to happen on the moon. If organic material was there, life could have emerged during these two windows.
Even if there was no organic material on the moon during those years, they were periods of intense meteoric activity. Schulze-Makuch and co-author Ian Crawford wrote in the study that it's "expected that meteorites blasted off the surface of the Earth will have landed on the moon." So those meteorites could have brought micro-organisms with them, which might have survived the crash if slowed down by an atmosphere.
But many doubts still linger
Although the idea of life on the moon is intriguing, we don't know if the factors noted in the study ever came together to enable life on the moon. Any efforts to find out more would involve an "aggressive future programme of lunar exploration," the study authors wrote.
Plus, even if there were relevant evidence on the moon, chances are it's been destroyed by billions of years of cosmic radiation, solar winds, and meteorite strikes. Future missions to the moon could, however, collect samples from layers of the moon that might provide evidence about these periods of volcanic activity. Lunar explorers could also eventually collect samples from the craters that still might hold ice.
Further research could also involve simulation chambers that would mimic the moon's conditions to see if life could have survived.
Regardless of any potential next steps, the researchers behind the study think their work at least shows life could have existed on the moon during these two periods.
"It looks very much like the moon was habitable at this time," Schulze-Makuch said. "There could have actually been microbes thriving in water pools on the moon until the surface became dry and dead."
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