An Outrageous Hypothesis Re-Evaluated.
In 2018, Ian Crawford from the University of London (UK) and I put forward the hypothesis that the early Moon might have been habitable and hosted life. We based our hypothesis on new results about the Moon, which indicated an enormous amount of volcanic activity on the Moon about 3.5 billion years ago. This volcanic outpouring led to many of the mare basalts on the Moon (the dark areas visible with the naked eye, which were thought to be oceans by early astronomers) and a tenuous atmosphere of about 1/100th of the pressure of Earth’s atmosphere today. However, that amount of pressure is still more than on Mars today, and it would be sufficient for water on the lunar surface to remain liquid. A startling vision of puddles, lakes, and streams on the early Moon became feasible. Four years later, we can ask: how has the hypothesis fared so far?
First, we have to admit that the envisioned environmental conditions billions of years ago are in stark contrast to today when the Moon has no atmosphere and no liquid water, and its surface is uninhabitable and lifeless. However, there has been a recent paper by Manasvi Lingam and Abraham Loeb, from the Florida Institute of Technology and Harvard University, respectively, pointing out the possibility of liquid water and even life deep in the Moon's subsurface.
Nevertheless, when looking at the Moon, it is difficult for us to overcome the perception of a big dead rock in space, which by all likelihood it is – today. However, there is evidence that Earth´s Moon could have been very different long ago. Not only may the Moon have had a significant atmosphere but also a magnetic field to shield it. A team led by James Green from NASA Headquarters modeled the magnetic field of the early Earth and the early Moon and found that their magnetic fields were connected, protecting the early atmospheres of both planetary bodies.
The presence of a magnetosphere is critical for life on the surface of a planetary body to be sheltered from harsh cosmic radiation. Green elaborated that the Moon generated its own global magnetosphere from about 4.25 to 2.5 billion years ago and that at peak lunar magnetic intensity (about 4 billion years ago), the Moon was only at a distance of 9 Earth diameters away from Earth.
What does it mean for our hypothesis? There are two possible habitable time periods for the Moon. The first one was very early on, shortly after the Earth and the Moon formed, at a time when early Earth and Moon would be better described as a double-planet system. The Earth-Moon system was likely enshrouded by a common atmosphere. The problem is that the origin of life appeared later (as far as we understand), by about 4.1 billion years ago. However, since the study by Green showed the peak lunar magnetic activity associated with volcanic activity was about 4 billion years ago, this may still have worked for early lunar habitability and life.
The other peak of volcanic activity, which is thought to have provided an atmosphere of about 10 mbar, was timed at about 3.5 billion years ago, based on the NASA scientists Debra Needham and David Kring. By then, life had originated on Earth and was widespread on our planet, and according to Green, the Moon still had a magnetic field. If habitable conditions were present on Earth’s Moon, then life on Earth can be expected to have spread to the Moon via asteroid impacts by ejecting rocks from Earth that later landed on the Moon. And if the Moon was not habitable, remnants of early life on Earth could have still been preserved on the Moon in icy craters. The Moon is very special in this regard because it is the only repository where we might still find evidence from the early evolution of life on Earth.
Crawford and I proposed to test our hypothesis by having future lunar missions look for water-rich minerals in geological layers sandwiched between ancient lava flows, which would be evidence of near-surface water during a potentially habitable time period. We still have to wait for suitable missions, but we can now conduct experiments in lunar simulation chambers in laboratories on Earth. These experiments aim to observe whether microorganisms can maintain viability under the environmental conditions predicted to have existed on the early Moon. We are currently doing these experiments and are excited to see what results we obtain.