Throughout human history – and outside of flights of fancy – the Moon has always been thought of as an airless ball of rock, tidally locked to Earth so that it shows the same, almost never-changing face to us in the night sky. But it may not always have been so.
In recent years, our perceptions of the Moon have been changing as a result of a number of studies and missions. In 2009, for example, India’s first lunar mission, Chandrayaan I, produced a detailed chemical and mineralogical map of the lunar surface, revealing the presence of water molecules in the lunar “soil”. In that same year, NASA launched a pair of missions to the Moon, the Lunar Reconnaissance Orbiter (LRO) mission and the Lunar Crater Observation and Sensing Satellite (LCROSS).
LCROSS was a small satellite designed to follow the upper stage of the rocket used to launch it and LRO to the Moon and analyse the plume of debris created by the impact of the upper stage with Cabeus crater in the Moon’s south polar region. The impact came with a kinetic energy equivalent of an explosion created using 2 tons of TNT, and LCROSS recorded strong evidence of water within the resultant impact plume.
For its part, LRO entered lunar orbit to commence a comprehensive campaign of mapping, imaging and probing the Moon’s surface and environment. In doing so, it further confirmed the presence of abundant concentrations of water in the lunar south polar regions. At the same time and LRO has been studying the Moon, an ongoing analysis of the rock samples brought back by the Apollo astronauts has revealed strong evidence for a large amount of water being present in the lunar mantle – possibly as much as is present in Earth’s upper mantle.
These results and findings have given rise to the idea that very early on in the Moon’s history conditions could have been very different to how it is now. In the immediate period following the Moon’s creation (roughly four billion years ago), there are a period when it was very volcanically active (about 3.8-3.5 billion years ago), releasing considerable amounts of superheated volatile gasses, including water vapour, from its interior. This outgassing could have given rise to an atmosphere around the Moon dense enough to support that water vapour condensing out into liquid on the surface which could have conceivably lasted for several million years whilst the atmosphere remained dense enough to support it, before it either (largely) evaporated or retreated underground to eventually freeze.
In their new study, published in July 2018, Dirk Schulze-Makuch, a professor of astrophysics at Washington State University, USA, and Ian A. Crawford, a professor of planetary science and astrobiology at Birkbeck College, University of London, UK, review the evidence for liquid water to have been present on the Moon and examine the potential for it to have been life-bearing. In particular, they note that when all is said and done, if the early conditions on the Moon did give rise to a dense atmosphere and a water-bearing surface, then the conditions there wouldn’t have been that different to those being experienced on Earth when life here was starting up, and would have occurred in the same time frame.
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.
Dirk Schulze-Makuch, co-author of Was There an Early Habitability Window for Earth’s Moon?,
quoted in Astrobiology Magazine
So does that mean life, however transient, got a start on the Moon? Possibly; however, some have suggested rather than giving rise to life directly, the conditions on that early Moon might have been ideal for life from Earth to gain a toe-hold.
As noted, the period when the Moon may have had its dense atmosphere coincided with life starting on Earth in a period referred to as the Late Heavy Bombardment, (4.1 and 3.8 to 3.5 billion years ago). During that time, bacteria such as cyanobacteria were believed to be already present on Earth, even as it was being bombarded by frequent giant meteorite impacts (hence the period’s name). So the suggestion is that this bombardment could have thrown chunks of bacteria-laden rock into space, where they were “swept up” by the Moon, transferring the bacteria to its surface, where it might have taken hold.
It’s unlikely that if it go started, life on the Moon got very far; within a few million years after the end of the Moon’s volcanic period the atmosphere would have been lost, and conditions would have become far too harsh for life to endure. However, in noting this, Crawford and Schulze-Makuch use their study as a call for a more robust study on the potential ancient habitability of the Moon, including a hunt for possible biomarkers.
Such an endeavour would likely be focused on the lunar south polar regions, simply because of the potential abundance of subsurface frozen water there. And as it is, NASA, India and China are already committed to studying the region in great detail. NASA will initially do so from orbit, while the Indian Chandrayaan-2 mission will attempt to place a lander and rover close to the Moon’s south pole in 2019. Also in 2019, China will send its Chang’e 5 mission to the Moon’s north polar regions to gather and return around 2 kg of rock samples for detailed analysis on Earth.