The origin of the Earth-Moon system and its implication on evolution
*Disclaimer: I’m a biologist turned self-taught computational biologist and now digging into computer science, artificial life, astrophysics, and the evolution of the earth for my Ph.D. about the evolution of circadian clocks. All of the information is researched to the best of my knowledge.*
We are so used to a 24 hour day in our life’s that it doesn’t come to our minds that it might not have been always like this. But how do we measure time after all? The base unit of time as we measure it is a second. Sixty seconds is a minute and sixty minutes make an hour. A day or more precisely a solar day is currently about 86,400.002 seconds long, which is almost equal to 24 hours. A day is not a good measure of time but rather defined as the time it takes for the earth to rotate fully around its own lateral axis. You might begin to wonder why I write so cautious about the length of a day. The reason for this is that the length of day can vary by a few milliseconds between different days of a week. Variability on the length of a day stems from a few different reasons, e.g. earthquakes, strong winds, or mostly the tidal friction of the moon.
Origin of The Earth-Moon System
There once was a time where the moon did not exist, but this is more than 4 billion years ago. Even though the giant impact theory for the origin of the moon is currently the most accepted one, multiple other theories persist (Fig. 1). All of these theories, including the giant impact theory, have problems explaining certain aspects of the Earth-Moon System and its origin.
Fission
The oldest theory for the origin of the moon was proposed by George Darwin (1879), the son of Charles Darwin. He envisioned a proto-Earth spinning so fast that a body of the size of the moon would detach from the Earth by centrifugal forces and form the moon. This theory was later disproved, however, was revived in the late 1960s by Ringwood and Wise and is still a viable hypothesis for the formation of the Moon. The fission scenario, however, has two problems with the angular momentum (rotation) of the Earth that is necessary to form the Moon. First, it is based on an Earth spinning multiple times faster than today with a rotation of around three hours with is close to the minimal rotation speed before the Earth itself would collapse. Second, the resulting Earth-Moon system has a much higher angular momentum than observed today. Thus, variations of this theory arose claiming that a fast-spinning Earth would not fission off one body but rather multiple smaller ones that later accreted to form the Moon. Furthermore, scenarios with a fast-spinning Earth that ejects a moon-sized body rely on additional forces that pushes out the proto-Moon like nuclear explosions (de Meijer et al. 2013). These scenarios relax the constraints of rotational speed a little bit making such scenarios more likely.
Giant Impact
The Giant Impact (GI) scenario for the origin of the moon was first established in the mid-70s by Hartmann and Davis (1975), and Cameron and Ward (1976). It is based on the idea that two planetary bodies, i.e. proto-Earth and Theia (proto-Moon), collided. The size of the proto-Moon varies between versions of the Giant Impact scenario. According to the “canonical impact” scenario, which was established by Canup, the proto-Moon had a size similar to Mars. The latest insights into the GI are based on numerical hydrocode simulations, which provide the opportunity the test different scenarios by changing parameter like the size of the proto-Moon, the speed and angle with which the proto-Moon hits the proto-Earth as well as the potential rotations of the proto-Earth prior to the impact. In contrast to the fission scenario, the GI theory relies on a powerful impact, which produces a large enough disk of material that later forms the moon by accretion processes. The disk size is one limiting factor of the GI scenario, however, not every suitable disk also forms a moon with the right isotopic compositions. Based on analyses of moon sample from the Apollo mission, the Earth and Moon are almost identical regarding their isotopic composition. This fact presents the biggest hurdle for GI theory because first simulations predicted that the moon formed mostly from the proto-Moon impactor. This is in principle not a problem, however, given the fact that the impactor and proto-Earth share almost identical isotopic composition is highly unlikely, it makes the formation of today’s Moon problematic. Another option could be a thorough mixing within the material disk after the impact, which would result in an Earth and Moon with similar isotopes. Such scenarios were first convincingly proposed by Thompson and Stevenson in 1988 and later refined by Ward and others.
Impact-Aided Formation And Multiple Impact Scenarios
Cuk and Stewart proposed in 2012 a scenario, which combines aspects of both the fission and GI theories. In their scenario, a fast-spinning Earth is hit by a small impactor. This results in a disk large enough to form the Moon with an isotopic composition similar to the Earth. However, the resulting Earth-Moon system has again a higher angular momentum compared to today’s situation. They proposed that a mechanism like the evection resonance between Moon and the sun can reduce the angular momentum of the Earth-Moon system.
Another recent adaptation to the GI scenario was proposed by Rufu and colleagues in 2015 and 2017. They argued that the likelihood of a single GI is rather small giving the speed and angle constraints and that multiple impacts with smaller impactors could be more plausible. They showed in their simulations that it is possible to form a material disk around the Earth that forms a Moon with similar isotopic composition. Multiple impacts would also relax some of the mixing constrains within the disk if these impacts are in close time distance. However, according to their simulations around 20 impacts are needed to form such a disk able to form the Moon, which is arguably as unlikely as the single GI scenario.
Tidal Acceleration And The Changes In The Length Of Day
All of the above-mentioned theories for the origin of the Moon result in a Moon being much closer to the Earth than today and an Earth rotating faster than today. The mechanism that explains the changes towards our current system is called “Tidal Acceleration”. Tidal acceleration describes the process in a two body system by which the smaller body is moving away from the larger one while the larger one’s rotational speed is slowing down. Tidal acceleration is due tidal friction between the two bodies. The rotational speed of Earth around its own axis determines the length of the day, i.e. the faster the Earth rotates the shorter are the days on Earth. The Moon’s gravity raises tides on Earth and causes the water of the oceans to bulge out. This effect causes the rotational speed of the Earth to decrease. However, since energy is conserved in a closed system this energy is transferred to the Moon resulting in an increasing distance of Earth and Moon. Today a day is roughly 24 hours, however, analyses of sediments and historical eclipses showed that the length of day has been around 18–20 hours before and is increasing. This will continue until Earth is tidally locked with Moons orbit, meaning that Earth’s rotation is as fast as Moons orbit with the result that the Moon will be “overhead of a single fixed point on Earth” [Wikipedia — Tidal Acceleration].
Why that matters to me
You might have asked yourself why all this is of interest to a biologist. During my Ph.D. I have been studying the evolution of circadian clocks. The model organisms I’m working with are cyanobacteria — as they are the oldest and simplest know organisms to have a circadian clock. Cyanobacteria are one of the oldest living organisms and are mostly responsible for the oxygenic atmosphere we are living in. In addition, parts of the circadian clock in cyanobacteria are estimated to be close to three billion years old and are thus among the first proteins in living organisms. Since evolution is also associated with environmental conditions, it is of special importance to learn more about the environmental conditions on Earth over the last four billion years. Sunlight is supposed to be the driving force in the evolution of circadian clocks, especially in organisms that solely need sunlight to produce energy and metabolites. Thus, the length of day during the evolution of the Earth is important to understand the evolutionary trajectories that favored a circadian clock to evolve. Furthermore, because cyanobacteria and their circadian clock are almost as old as life itself on Earth, it is not sufficient to go back a couple of million years in time. This brings me to the evolution of Earth-Moon system and its subsequent effect on length of day, which is around four billion years ago and thus roughly only a couple hundred million years before life on Earth began. So far there are no sediments available that give us insights into the length of day four billion years ago, however, all of the above-mentioned simulations should result in rotational speeds of Earth and Moon as well as the distance between them. From there it should be possible to calculate the change in the length of day from then until now. I’m currently not sure how much effort this endeavor costs so I need some help or tips to get a better understanding of the magnitude of this question and whom to contact. So please if you are interested or if you know anyone who can help me, please get in contact with me.
Thank you for reading.
*A big Thank You goes to Eric Young for always reading and commenting on my drafts.*
