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The Origins of the Earth and the Moon – Why geochemistry doesn’t get the Credit it Deserves

This piece belongs to my blog series at https://edingeoslife.com where I discuss the seminars I attended hosted by the University of Edinburgh. The first blog entry can be found at: Why All students, from All years, from All subjects should attend Seminars.

Title of the Talk: The Origins of the Earth and the Moon

Speaker: Miki Nakajima

The Origins of our Moon

Miki Nakajima is a postdoctoral fellow at the Carnegie Institution for Science. In her talk she laid out her work on trying to understand the origins of the Earth-Moon system.

Understanding the chemical makeup of the Moon is important for terrestrial science as the two systems are so interlinked. It would help us to gain a better understanding of tidal interaction between the Earth and the Moon. The data would clean up our chronological understanding of the formation of the Earth.

The original theory was that a small impactor, a Mars size object grazed our planet, kicking out considerable amount of surface mantle material, creating the Moon. There is chemical evidence for this theory from the Moon rocks returned by the Apollo missions.  Both planets have nearly identical isotopic ratios. However several aspects of the Moon’s chemistry are very different to Earth’s. The Moon’s mantle is very Iron poor as well as being low on volatiles and water. The Moon’s overall density is much lower compared to Earth’s. The Moon’s chemistry is partially heterogeneous therefore either a lot of internal changes happened inside the Moon, post collision or the colliding planetoid left more material behind than thought. Clearly the canonical theory on the formation of the Moon does not explain everything.

In order to understand what truly happened in the past Miki and her team ran several simulations in order to answer the questions: Why is the Moon so Iron poor? Where did the volatiles go? Why is the chemistry of the Moon so heterogeneous despite having similar isotopic ratios with Earth?

In her series of simulations she ran the canonical model simulating the heat of individual elements after the collision, tracking how the material mixed and how the volatiles behaved. Different types of collisions have a different effect on planetary material mixing and post formation chemistry. She looked at the other two models proposed for the formation of the Moon: The fast spinning model where the colliding planet completely merges with Earth, the momentum throwing out the debris which formed the Moon. And the the Collision of two half Earths where the two entities collide several times. The two proposed models both show mixing of material. The fast spinning Earth produced partial mixing while the two half-Earths colliding produced complete mixing. The results from the simulations can be seen bellow.

The current canonical Moon-forming impact. The simulation bellow demonstrates the formation of the moon, the possible distribution of material and the cooling of each individual elements:

The possible scenarios were broken down into 3 separate models illustrated bellow:

The 3 main models

Canonical Moon formation:

Moon formation with a fast spinning Earth:

Moon formation with two half-Earths colliding:

 

While simulations allow us to draw up a working theory, physical evidence is important to confirm it. As the formation of the Moon was a very long time ago and simulations are based on mathematical assumptions, we don’t have much evidence to work with. However there is one key element of Earth that can help us: the magnetic field of our planet. There is evidence that the magnetic field formed very early on. In order to have a magnetic field, a lot of heavy elements need to be molten and in convection inside the core. In order to keep the heavy elements at the core, the right amount pressure and temperature is needed constantly. If the canonical theory is correct, the magnetic field would have formed much later. If the planet would have only grazed Earth, the temperature and pressure balance would have changed, disrupting the iron equilibrium in the core. The only way the collision and the early magnetic field could have happened if the Mars sized planet collided with Earth head-on. This would have caused full core mixing but not much mixing of the mantle. This would explain where the iron went, as the larger core of the Earth would have absorbed much of the original core of the other planet. The missing volatiles could have been burned away by the ionising radiation of the sun when the debris was forming into the Moon.  The large magnetic field of Earth could have come from the extra iron absorbed by the Earth mid-collision (however there is still plenty of debate around the idea).

However after the simulations were run and the volatile loss was checked, the burn-off of the the volatiles was insufficient to account for the missing amount, therefore another past process was at play. The current theories are looking at water absorption by Earth’s mantle to account for the missing material.

The new proposed theory speculates that while the impactor was Mars sized, it didn’t graze the planet but collided head-on with it, allowing the cores of the two planets to mix. Earth absorbed most of the core of the other planet taking the iron with it. The final outstanding question is: Where did all the water and volatiles go from the colliding planet? If there was an insufficient loss from the formation disk in the simulation, was the water absorbed by the mantle of the young Earth?

Planetary Demolition Derby

New information Changes everything

There is one thing that I deeply appreciate about science: everything is not set in stone. If new data comes in challenging the old theory, the old theory goes and the new one gets accepted as standard. This is an amazing system, very unique to the world of Empirical science. In any other fields, the orthodoxy usually stays very dominant, chocking off progress. In science if you have valid data you can change entire fields including the theories on the origins of our planet and our moon. This talk very much connects to that idea. The small impactor theory as the origin of the Moon was very much standard in the past, yet due to the simple inconsistency of geochemical data from the lunar rocks, has been reset to the large impactor theory with further room to understand why the Moon is so depleted in water and Iron. Before going to university I didn’t know about geochemistry (or was very conscious on how important it was). It wasn’t much talked about in school. I haven’t seen it mentioned in public media. Yet after attending several seminars through the years and studying some aspects of it, I can’t stress enough the field’s importance. Modern geological science would be nothing without Geochemistry. The methods within the subject help us pinpoint geological events, understand geological processes and build chronologies.

The department should concentrate more on promoting geochemistry in the media and to the public. It is a science that does a lot of the heavy lifting within geology.

Future Research

Miki in her future research seeks to better understand mantle evolution of the Moon post cooling, planetary accretion of volatiles into mantle, the origins of the Martian moons and other giant past impacts around the Solar System.

Further Reading

Inefficient volatile loss from the Moon-forming disk: Reconciling the giant impact hypothesis and a wet Moon