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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

What temperature is the surface of the Moon and why should You care?

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: What temperature is the surface of the Moon and why should I care?

Speaker: Neil Bowles

Our weird Neighbour

If you would be an alien browsing across our Solar System you would find certain oddities. No it wouldn’t be our gas giants. Or the giant rings of Saturn. Your focus would shift on the planet third in line. A blue dot in the sky that is not like the other planets in our system. This bright ball in the sky is mostly blue with green and brown blotches running across her surface. After taking a closer look, you the alien would realise that warm water runs across this planet. You would see, it is teaming with Life, great and small. You would quickly realise that an oxygen rich atmosphere reaches across this planet, like a great space suit, keeping everything alive. Your instruments would detect a giant magnetic field, like a shield, defending the planet from the fiery dragon breath of the sun. This sphere would certainly be a great interest to you. But you would notice, this odd fella is not a lone traveller. It has a partner, a moon. However this companion couldn’t be a greater contrast to the blue ball of life. It is barren. Craters spawn its surface. No life whatsoever. No flowing water. The temperatures are hellish. On the sunny side you fry. In the dark, you freeze stone solid. No air to breathe. No magnetic field to shield you from the ionising radiation ripping up your DNA. The place is Death. Sitting opposite to the space ship of Life. An odd couple indeed. However, if you are an alien of a scientific mind, you would notice that the two are deeply interconnected. This moon is relatively close to the big blue ball. Their orbital paths are so synchronised that it only shows one face towards the planet of life. This moon is very large compared to other similar systems. If you would take rock samples from both planets you would learn that this moon formed hot and has a lot of chemical similarities to the planet of life. Taking your knowledge of the formation of other systems you would guess that this system formed when a past large impactor collided with the blue planet forming the moon. A violent beginning to such a peaceful dancing formation today. You as alien would certainly be hungry to find out more.

Neil’s research focuses on trying to understand this strange big rock, our heavenly neighbour. More precisely the origins of the Moon, the reasons for the wide temperature contrasts on the surface and how the craters play a key role in preserving potential water ice on the surface. He argues that we should send more probes to the Moon especially to the polar regions to understand the weird temperature fluctuations there. He is curious to see how good the shady craters are to preserve water ice. He thinks if we find plenty of water ice in the polar regions, future generations could set up Moon bases there and use it as a jumping platform for our journey into the cosmos.

Lunar Reconnaissance Orbiter – Measuring the surface temperatures of the Moon

Historical Context

The Moon was a focal point of the Space Race of the 1960s. It represented the prize for both the Americans and the Soviets. Both sides visited it extensively with manned missions and robotic probes bringing back plenty of rock samples. Plenty of studies were done on the Moon using the samples in the 60s and 70s. However the aims of most Moon missions were not scientific but political. The United States and the Soviet Union were locked in a deadly Cold War. They saw getting to the Moon as a political victory over the other. Therefore the geologically interesting areas of the Moon were not visited, only the flat relatively mountain free areas to make the landings as risk free as possible. Scientific data is limited about the Moon despite us visiting for such a long time. Newer probes such as the Lunar Reconnaissance Orbiter or the SELENE probe while did make some broad observations and yielded good surface temperature data, did not provide the complete picture. The current aim of Prof. Neil is to convince ESA and NASA to send a lander, similar to the Martian InSight probe to directly study the surface of the lunar poles. Therefore despite popular belief our work on the Moon is far from over.

Surface Temperatures and Hidden Ice

The temperature of the Moon greatly fluctuates much more than a normal interplanetary body. It has no atmosphere so there is no gas absorbing or releasing heat. The top surface has poor heat conduction, as it underwent heavy asteroid bombardment. On the sunny side the average temperature can be as high as 60 C degrees while on the shaded side it reaches -180 C degrees. These heavy temperature fluctuations suggest poor hydration of the rocks therefore a water depleted lunar mantle. The surface temperatures are measured by satellite looking at the amount of heat radiated back into space. The rate of cooling and heating was calculated based on thermal inertia.

One of the most interesting locations on the Moon are the Polar regions. They are covered in wide and deep craters and due to the sun’s low angle they are in darkness all the time. This maintains the low, bellow freezing temperatures through the lunar year, allowing the preservation of water ice. The stability of ice on the moon is influenced by several factors. Erosion of the lunar surface driven by impact of micro-meteorites and irradiation of heat by crater walls.

The areas with the highest potential for water have been mapped by the Lunar Crater Observation and Sensing Satellite (LCROSS) based on hydrogen emissions of the area. While evidence for water was poor, the South Pole yielded the best evidence for a good stability field for subsurface water. With a new satellite solely focusing on the area, we could learn more about the location of the water ice.

Maps of measured and model-calculated surface and subsurface temperatures in the lunar south polar region – Ideal place for a Human colony?

Why should we go back?

We still have a limited understanding of our own neighbour. Going back would enable us to expand our scientific knowledge. If we find water ice in the poles, it would provide us raw materials for oxygen generation, rocket fuel and drinking water. The rocky body of the Moon is rich in minerals and Helium3 (a source of fuel for safe nuclear fission reactors) which would enable us to build a civilisation and increase the reach of mankind. The Moon is safer to navigate than asteroids therefore it would make a better platform for Mars missions and planetary exploration. The gravity of the Moon is low, while predictable (unlike an asteroid’s) allowing the launches of rockets at a much cheaper cost than from Earth. The Moon would be a great stepping stone for mankind to further reach into the heavens. Understanding the surface temperature fluctuations and explaining them would go a long way in preparing us for a journey to Mars and beyond.

The best way to summarise why should mankind return to the Moon and beyond is summed up perfectly by astronaut and commander of the Apollo 15 mission, David Scott:

“As I stand out here in the wonders of the unknown at Hadley, I sort of realize there’s a fundamental truth to our nature, Man must explore . . . and this is exploration at its greatest.

For when I look at the Moon I do not see a hostile, empty world. I see the radiant body where man has taken his first steps into a frontier that will never end.”

The plaque left behind by Apollo 17 – “May the spirit of peace in which we came be reflected in the lives of all mankind”

Further Reading

Diviner Lunar Radiometer Observations of Cold Traps in the Moon’s South Polar Region