The moon is one thing we all have in common. I’ve always loved looking up at it. Whether it’s from a religious, mythological or scientific perspective, Luna holds a powerful mystique regardless. The story of the moon is written into the story of life itself.
What does the colossal impact taking place in the above picture have to do with the moon? Because it’s likely the moon formed via a process of accretion.
Around four and a half billion years ago, earth itself had only just coalesced from a cloud of gases and dust that eventually gave rise to the entire system.
Earth is believed to have formed without a moon. In fact earth as we know it today formed as a result of the moon.
Picture this. Earth is newly formed. It’s a toxic planet with vast tracts of it’s surface covered by a magma ocean.
From the outer solar system it comes. An object roughly the size of mars slams into Earth 1.0. The object has been named Theia. This impact is catastrophic, essentially tearing away the outer surface of our world.
Where does all of this crust go? Into space, forming a ring around the newly resurfaced earth. It is this ring, consisting of the fragmentary remains of both our world and Theia, that will accrete to form the moon.
That’s the moon in a nutshell. It’s influence on the course of life has been fundamental, with a critical role in climate and seasonality via the key role it plays in tides. For over four billion years the moon has stared down upon the world, seeing the march of life with all of it’s ups and downs.
Has the moon itself been lifeless all this time? It’s been our closest neighbour for practically forever. We have always thought of the moon as a dead, hostile place. Today it certainly is. With no atmosphere to speak of, negligible water and lethal solar radiation bombarding it’s surface, the consensus of opinion is that the moon is completely devoid of life.
Image Credit: NASA/GSFC/Arizona State University
But it may not always have been like this.
It may be a stretch, but several studies have suggested that at least for a time the moon may have been at least habitable. Perhaps not an oasis of life, but a place that could harbour it.
How is this viable? As noted, we all know the moon is hostile to all life. However, the moon is now an inert world, devoid of any geological activity.
Once, though, the moon was anything but inactive. In the period after the moons formation, around four billion years ago it was highly volcanically active.
Intense volcanism can be a source of atmospheric gases. This is definitely a factor on earth. Many atmospheric gases, including several trace greenhouse gases are pumped into our skies by volcanoes. Greenhouse gases are pivotal in regulating climate on earth. On the moon all those billions of years ago, volcanoes may have done something similar, bulking up the lunar atmosphere and enabling this tiny world to retain some heat. In addition, a thick atmosphere provided protection against solar radiation and an environment amenable to liquid water. Water is, as we know, crucial to all life on earth. “Follow the water” is one of the central catch cries of astrobiology. Find water, the reasoning goes, and life may be there.
This isn’t always the case though. Water exists almost everywhere in the solar system. There is even water vapour on the sun! There is plenty of water on the moon, locked up as ice in several craters in permanent darkness.
How would all this water have arrived on the moon? Prevailing theory regarding the origins of earth’s water held that much of it was delivered by cometary impacts. This is certainly reasonable. Recent discoveries though hint at vast reservoirs of water locked up deep within the planet itself. Water may be replenished over the eons by outgassing from volcanoes for example. This could have happened on the moon. Several studies of lunar composition have demonstrated that there may be similarly vast amounts of water locked up within the moons core. The ancient moon may have gained a thick watery atmosphere from centuries of volcanic activity partially terraforming it.
So, to put a long story short, water by itself is no guarantee of habitability. The moon, however, may once have been a very different place. With a thick atmosphere providing protection from cosmic rays and allowing pools of liquid water to form, life could have quite easily gained a foothold there. Most likely this life was in the form of unicellular organisms which may have arrived via lithopanspermia. This is a process whereby worlds at close proximity can exchange life or it’s building blocks via impact or volcanic ejecta.
This very concept is being applied to crowded systems of exoplanets such as the TRAPPIST-1 system, and is an exciting avenue to explore. In such a system, the possibility of interplanetary ecosystems could exist! This is, of course, very theoretical, but damn what an interesting idea!
What do you think? Was the moon ever habitable?
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