Today we will be metaphorically travelling back in time to visit early Earth. Most rocky planets (like the Earth) went through a magma ocean phase, where all or most of their surface was made of molten mantle. The mantle, remember, is the section of rock between the crust and the core of a planet.
The early Earth saw a lot of collisions with other bodies, like hunks of rocks and wanna-be planets. Pieces of these collected and eventually formed what we know today as the planets, in a process called accretion. A giant impact could have resulted in the entire mantle being melted, resulting in a global magma ocean at some point in this accretion stage. That’s what today’s study investigates. Published in Journal of Geophysical Research: Planets in 2013. Authored by T. Lebrun et al., “Thermal evolution of an early magma ocean in interaction with the atmosphere” looks at a hypothetical global magma ocean. One important question is ‘how long did it take to cool?’, which has impacts on the start of tectonics and planet habitability. While this article also discusses Venus and its possible past, I choose to focus on Earth and its past.
The presence and composition of an atmosphere on a planet with a magma ocean would factor into how long that magma ocean lasted, due to considerations like the greenhouse effect and how much radiation escaped through the atmosphere to space. The authors of this study created a model that included both the magma ocean and a water vapor-carbon dioxide atmosphere. This model was simplified, including some effects and excluding others. While simple, it gives a ballpark range and a starting point for future investigations.
It is possible that Earth underwent cycles of magma oceans: a magma ocean that cooled, continents and possibly even water oceans forming, before another large impact destroyed the progress and recreated the magma ocean. The magma ocean would have solidified from the bottom (near the Earth’s core) up, towards the surface. This gave way to two distinctive behaviors, where the mantle behaved like a liquid and where it was more solid-like, depending on temperature-related properties of the magma within the mantle. The line between these was termed by the authors as the ‘rheology front’ (rheology is the study of how matter flows). This rheology front slowly moves towards the surface; when it reaches the surface, the magma ocean phase is considered to be over. Without taking into account the atmosphere, this takes about 4,000 years: with the atmosphere included, it takes 1.5 million years. This is further complicated by the factors of the amount of volatiles (materials easily turned into gas, such as water), the initial amount of carbon dioxide in the system, and possibly quickly decaying radioactive elements.
As mentioned before, the model used was a simplified one. A more complex one could include the effects of smaller impacts and the process of atoms escaping from the atmosphere. Still, this gives us a framework for future research.
Astronomy Picture of the Day. Marchi, Simone, SSERVI, NASA. Nemiroff, Robert and Bonnell, Jerry, ed. Four Billion BCE: Battered Earth. Astronomy Picture of the Day. 5 Aug 2014. Web.
The illustration used in today’s article.
Lebrun, T., et al. “Thermal evolution of an early magma ocean in interaction with the atmosphere.” Journal of Geophysical Research: Planets 118.6 (2013): 1155-1176.
Today’s main article.
Marshak, Stephen. Earth: Portrait of a Planet. 3rd ed. W.W. Norton & Company. Print.
My geology textbook, used to refresh my memory on Early Earth and the magma ocean.