Life may have flourished for millions of years – on the moon, suggests new research.
The barren surface was once covered by lakes where primitive organisms could have emerged, according to the study.
Scientists say that would have been during two time periods in the distant past soon after its formation.
The findings, published in the journal Astrobiology, suggest there could be evidence of extra-terrestrial life much closer to home than previously thought.
NASA says the likeliest places to find life beyond Earth are the ocean worlds of Europa and Enceladus – moons of the far off gas giants Jupiter and Saturn respectively.
But planetary scientists Professor Dirk Schulze-Makuch and Prof Ian Crawford say our moon wasn’t always as uninhabitable as it is today.
Conditions were sufficient to support simple lifeforms shortly after the Moon formed from space debris four billion years ago and again during a peak in lunar volcanic activity around 3.5 billion years ago.
During both periods the moon was spewing out large quantities of superheated volatile gases from its interior – including water vapour.
This could have formed pools of liquid water on the lunar surface – and an atmosphere dense enough to keep it there for millions of years.
Prof Schulze-Makuch, of Washington State Univerity, said: “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.”
Working with Prof Crawford, of the University of London, he drew on results from recent space missions and sensitive analyses of lunar rock and soil samples.
These show the moon is not as dry as previously thought, reports Astrobiology.
In 2009 and 2010 an international team discovered hundreds of millions of metric tons of water ice on the moon.
There is also strong evidence of a large amount of water in the mantle believed to have been deposited very early on in its history.
The early moon is also likely to have been protected by a magnetic field that could have shielded lifeforms on the surface from deadly solar winds.
Life could have originated much as it did on Earth. But the more likely scenario is it would have been brought in by a meteorite, said Prof Schulze-Makuch.
The earliest evidence for life on Earth comes from fossilized cyanobacteria that are between 3.5 and 3.8 billion years old.
During this time the solar system was dominated by frequent and giant meteorite impacts.
It’s possible meteorites containing simple organisms like cyanobacteria could have been blasted off the surface of the Earth and landed on the moon.
Prof Schulze-Makuch said: “It looks very much like the moon was habitable at this time.
“There could have actually been microbes thriving in water pools on the moon until the surface became dry and dead.”
He said determining if life arose on the moon or was transported from elsewhere “can only be addressed by an aggressive future program of lunar exploration.”
One promising line of inquiry for any future space missions would be to obtain samples from deposits from the period of heightened volcanic activity to see if they contained water or other possible markers of life.
In addition, experiments could be conducted in simulated lunar environments on Earth and on the International Space Station to see if microorganisms can survive under the environmental conditions predicted to have existed on the early Moon.