The missing magnetism of the old moon

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This illustration shows how a lunar dynamo could generate a magnetic field. Credit: Hernán Cañellas, courtesy of Ben Weiss

The simulations rule out plasmas caused by meteorite impacts as the source of lunar magnetism, supporting the proposition that the old moon generated a dynamo core.

Today, the moon does not have a global magnetic field, but it has not always been so. Satellite measurements of the lunar crust and lunar rocks recovered by the Apollo missions contain residual magnetization that formed 4 to 3.5 billion years ago in a magnetic field comparable in strength to that of the Earth. . Scientists argued that the source of this was a dynamo – a magnetic field generated by the moon’s molten, rotating metal core. However, research indicates that the alleged small moon core may not have been able to generate enough energy to support the old magnetic field that planetary scientists have deduced from its rocks.

In a recent Scientists progress paper, researcher Rona Oran and professor of planetary sciences Ben Weiss of MIT The Department of Earth, Atmospheric, and Planetary Sciences examined the plausibility of an alternative hypothesis that has existed since the 1980s that could produce the residual magnetization in the lunar crust: the transient plasmas generated by the impacts of meteorites. Here they describe some of their findings.

Evolution of plasma flow and magnetic field

Plasma flow models (left) and magnetic field evolution following an impact of pool formation. Credit: Image courtesy of the authors.

Question: What is the “impact plasmas” hypothesis and why is it still considered a potential mechanism to explain the ancient magnetism of the moon?

Oran: Two main hypotheses have been put forward to explain the ancient magnetic field of the moon. One is that the moon has already generated a dynamo. The main challenge of this theory was that the moon is much smaller than the Earth and that it does not have enough energy to generate a surface magnetic field with the high intensity deduced from the analyzes of the samples and the crust of the Earth. ‘Apollo.

Weiss: A long-held alternative hypothesis is that the source of the field was not the interior of the moon itself but rather meteor impacts on the surface. In particular, impact plasmas – highly conductive fluids produced by vaporization of the lunar surface – have been proposed to have spread around the moon and engulfed it. In doing so, the plasmas compressed and amplified the interplanetary magnetic field, known as the solar wind. The fields would then be induced in the lunar crust, and the enhanced field signal would then be seen in the ground on the other side of the moon. This hypothesis is supported, in part, by observations of four young large craters that have strong and important magnetic signals on the opposite site of the moon.

Question: In examining the impact plasma model, how did you examine its plausibility and why were you able to exclude it as a prime suspect?

Weiss: We tested this idea by performing the first impact plasma simulations that consistently take into account the physics governing the generation and decay of the magnetic field.

Oran: One of the reasons why this hypothesis has not yet been tested in this way is that the tools we have used belong to the discipline of space science; no one has actually applied them to this problem before. Then Ben, who is researching paleomagnetism, and I joined forces to work together and showed that the impact plasma hypothesis cannot work.

The evolution of magnetized plasmas is a complex process where the flow of plasma and electromagnetic fields change in response to each other. It is only by simultaneously simulating the plasmas and the magnetic field that one can have a realistic view of the process.

We have found that whatever you do, however you play with it in terms of impact location, direction and direction of the initial field, you cannot create enough magnetic energy from these. impact plasmas. This is because we can think of the lunar body as this gigantic spherical resistance that essentially kills all the currents that these magnetic fields are trying to induce there. Then instead of having strong magnetic fields in the crust caused by the impact, we generate these fields, but they dissipate in a matter of minutes, so you end up heating the rock. So, we saw this effect completely opposite to what we had initially sought to find.

Question: What does your discovery tell us about the evolution of the moon, its magnetism and similar planetary bodies? And what questions remain?

Weiss: If the impact fields hypothesis were correct, it would mean that the residual magnetization we find on the moon’s surface would tell us essentially nothing about the geophysical and thermal evolution of its interior. This in turn would have had profound implications for tracing the magnetic history of the moon, and even for understanding the record of residual magnetization found on other airless bodies like Mercury, which has craters, and asteroids, which meteorites suggest may have crustal magnetization. Now that we have shown that the impact fields hypothesis is unlikely to explain most of the lunar magnetism, this supports the dynamo nucleus hypothesis for magnetism on the moon and other bodies.

Oran: Since we now favor a lunar dynamo, the strong fields we see on the moon still call for an explanation, as a dynamo like the one we have on Earth, in which the core rotates due to its own cooling, may not suffice. In recent years, some alternative dynamo theories have been developed which could generate stronger fields, for example, the agitation of the nucleus by the oscillation of the overlying solid mantle.

Our most immediate follow-up study is to repeat the same type of simulations but, instead of a non-magnetically active body, we would allow the moon to generate its own central dynamo, and then examine how impact plasmas would interact with such a field. Another issue to consider is whether you can create a footprint on the impact site itself. Either of these scenarios could give us a better match for the magnetizations we see on the moon’s surface.

Former MIT Visiting Professor Yuri Shprits of the German Research Center for Geosciences GFZ, former EAPS postdoctoral fellow Katarina Miljković of Curtin University and Gábor Tóth of the University of Michigan also participated in the study.

This research was funded in part by the ">Nasa Solar System Workings Program, the NASA Solar System Exploration Virtual Institute and the Skoltech Faculty Development Program for their support.


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