Unsolved Mysteries in Space Exploration; Why can’t we agree on the depth of the Lunar Magma Ocean of the Moon?
I think we can all agree that understanding ‘the unknown’ brings so much excitement and pride because so many possibilities stem from it. Let’s take the exploration of the Moon as an example.
Attempts of exploration started back in 1959 with the ‘First Flyby’ of the Moon. A couple years later came an event I am sure we all remember or have come to learn about. On July 20, 1969 was the first successful human exploration of the Moon, the U.S. mission Apollo 11. This historic event cast so much hope across the globe for the advancement of human life. Whether it be understanding extra-terrestrial planets scientifically or the potential economic benefits of mining materials from other planets, but, the historical events did not stop there. The first robot lunar rover to land on the Moon was the Soviet vessel Lunokhod 1 on November 17, 1970 and many more successful missions have been recorded since then. These missions have equipped us with extensive understanding of the Moon.
Origin of the moon
Like everything in science, there are multiple hypotheses when trying to prove something and the origin of the Moon is no different. For the purpose of this article I would only mention the most popular one, in that, the creation of the Moon came to be when a Mars-sized body collided with Earth approximately 4.5 billion years ago, and the resulting debris from both the Earth and the impactor accumulated to form what we know now as the Moon. This newly formed Moon came in a molten state which was named the ‘Lunar Magma Ocean’(LMO). In the first few millions of years, the molten moon underwent crystallization resulting in the formation of the lunar crust. The lighter minerals floating to the top to be the lunar crust we see today, and the heavier minerals sinking to the bottom forming the residual magma ocean.
Here’s where the confusion comes in. The studies of the initial bulk composition of the Moon and the depth of the LMO has not come to a general consensus. Some studies suggest that the LMO was shallow and that it consists of the Lunar Primitive Upper Mantle (LPUM) that lies between the crystallized LMO and the metallic core. On the other hand, some studies suggest that there is no Lunar Primitive Upper Mantle, and that the LMO is in fact deep. All that consists in the deep lunar magma ocean, is the lunar crust and the metallic core.
The debate; Shallow or Deep Lunar Magma Ocean
There are some variants to take into consideration before you pick a side.
The thickness of the crust and the chemistry of lunar samples are useful to investigate the bulk composition and depth of the magma ocean. To do this, you would need data from lunar samples to generate models for the estimation of the depth.
Within the LMO, an adiabat lies between the liquidus (lowest temperature at which everything is liquid) and the solidus (highest temperature at which everything is solid). The adiabat here just means a line or curve that represents the pressure and temperature of matter during an adiabatic process. If the adiabat of the LMO lies between the liquidus and solidus at all depths it means that you can find crystals at all depths of the magma ocean. This is great, but different minerals crystalize at different depths because of their chemical properties. This created a window for many compositional models to come in and explain the molten Moon. We have models like the Lunar Primitive Upper Mantle model, the Taylor Whole Moon model and the O’Neil model who all intended to stimulate the composition of the totally molten Moon minus the metallic core. They have done this by establishing the crystallization sequence for the LMO, taking into account the mineral densities, melt viscosity and how the minerals behave under certain pressure and temperatures. With each model comes different parameters, like whether the mantle went through an equilibrium crystallization or a fractional crystallization, or both! This impacts the minerals that are reported and in turn, creates a different bulk composition. A simple Google search or even one read of an academic paper would show at least 9 different estimates of the Bulk Moon composition.
Studies show that the depth of lunar magma ocean ranges from shallow to deep; 200–1000 km (Charlier et al., 2018; Elkins-Tanton et al., 2011; Taylor and Jakes, 1974; Solomon and Chaiken,1976)
Why is there no definitive answer?
We agree that the lighter minerals float to the top to form the crust and that the heavier minerals sink, but what we do not agree on is the bulk composition of the magma ocean and the depth of it. The uncertainty of the ocean’s depth stems from the lack of understanding of the bulk Aluminium oxide (Al2O3) content of the magma ocean and the true thickness of the lunar crust. No agreement has been reached on whether there was a development of a primitive upper mantle below the magma ocean or if the moon’s interior solely consists of the metallic core and the magma ocean.
What do we do next?
More research and experiments need to be done to either disprove or strengthen arguments so that a general consensus can be reached.
Useful links and references:
· Nasa: https://moon.nasa.gov/exploration/history/
· Nasa: https://moon.nasa.gov/
· Charlier, B., Grove, T., Namur, O. and Holtz, F.,2018. Crystallization of the lunar magma ocean and the primordial mantle-crust differentiation of the Moon. Geochimica et Cosmochimica Acta, 234,pp.50–69.
· Elkins-Tanton, L., Burgess, S. and Yin, Q., 2011.The lunar magma ocean: Reconciling the solidification process with lunar petrology and geochronology. Earth and Planetary Science Letters, 304(3–4), pp.326–336.
· Taylor, S. R. and Jakes P., 1974. The geochemical evolution of the Moon. Proc. 5th Lunar Science Conf. 1287-l 305.
