Back in October 2021, my first paper was published in the Journal of Geophysical Research: Planets. I’d been working on the paper for about a year and a half so it was very exciting and a big relief to finally see it published! The aim of this piece of research was to understand how different gases, or volatiles, affect the ascent of magma in the Moon’s crust. Volatiles (such as water, hydrogen, carbon monoxide, carbon dioxide, sulphur dioxide etc.) play a key role in driving volcanic eruptions. At depth and high pressure, these volatiles are dissolved in magma in the crust. However, if this magma rises, the volatiles separate from the magma solution to form bubbles of gas. This makes the magma more buoyant and causes it ascend through the crust more quickly and, if the magma reaches the surface, it can lead to a volcanic eruption. If there is a high concentration of volatiles dissolved in the magma, the magma can ascend more rapidly and could erupt more violently; this link is extremely useful for understanding what’s going on inside the Moon’s crust and mantle.
Planetary and terrestrial scientists are very interested in understanding the Moon’s interior because the Earth and Moon likely formed at the same time following some sort of giant impact. Based on the “giant impact” hypothesis, the composition of the Earth and Moon are very likely to be linked, therefore, understanding the composition of the Moon helps us understand the composition of the Earth, especially in terms of the abundance of different elements and volatiles. However, it is difficult to study the interiors of planets, therefore, in this study, we modelled the ascent of magma in the Moon’s crust to try and decipher the volatile content of the lunar mantle.
To understand this link further, the authors used a numerical magma ascent model to see how different initial volatile contents would affect magma ascent. The magma ascent model has previously been used to model eruptions at Stromboli (Italy) and Sunset Crater (Arizona, USA). In order to apply the model to eruptions on the Moon, the model had to be adapted to account for the different conditions on the Moon compared to Earth, for example the gravity on the Moon is about 1/6th of that on Earth so the relevant parameters in the model had to be modified to take account of this difference.
By completing 20,000 model simulations with either water and carbon monoxide, or hydrogen and carbon monoxide initially dissolved in the magma, we found that the initial water concentration does not have a strong control on magma ascent and eruption compared with carbon monoxide and hydrogen, at least for the ranges of volatile content we considered. The figure below is called a Sobol index plot; the Sobol index is a quantitative measure of how much a model input affects a model output. Here the plot shows how different model inputs of water/hydrogen/carbon monoxide content, pressure, temperature, and conduit radius, affect the model outputs of gas volume fraction, mass flow rate, exit velocity, and exit pressure. Each model input is represented by a different colour and the more there is of that colour, the more the input affects the output. For example, yellow represents the radius of the conduit that the magma is rising through and because the yellow bar is very long for mass flow rate (panels b and f), we can say that the conduit radius has a very strong control on the mass flow rate. Conversely, the initial water, hydrogen (both red), and carbon monoxide (brown) concentration has a strong control on the exit velocity of the magma (panels c and g).
These results shed some light on how different volatiles present in lunar magmas may have affected magma ascent and eruption on the Moon, and could be verified with data and images of real volcanic deposits on the Moon’s surface. I’m still working on using these model results for my PhD research, so stay tuned for further developments!
The full paper is available open access (free to read and download) in the Journal of Geophysical Research:
Earth & Solar System 