Lunar Magnetic Fields

This morning I’m sitting in a session titled, “Lunar geophysics.” A more accurate might have been, “Lunar Magnetic Fields.” So far the dominant theme has been trying to determine if the moon once had a nature magnetic field driven by a lunar dynamo, or if all magnetic fields fields found on the moon were induced by impact events. This is a complex question that I will try to address, but first let me do a bit of book keeping. This is probably the last coverage of the Moon we’re going to bring you from this meeting due to conflicts with future moon sessions. Currently Rebecca is sitting in a session on fluvial (liquid) systems on Mars, and this afternoon one of us will be catching up on Enceladus and Venus while the other sits in on more Mars geography. There is another poster session tonight. Tomorrow we will only catch the first session do to a conflict with Rebecca’s flight and me confusing which airport is in South Houston (that would be Hobby) and which is in North Houston (that would be Bush), and booking her flight out of the most distant airport in a moment of stupidity. While we can, tomorrow morning we’ll be covering comparative planetology (my favorite topic), and environmental analogues. I will try to find someone to cover the Titan session for us in the afternoon. (If you are here and are interested, please email me).

Now, on to the lunar magnetism.

Below is a detailed map of the magnetic field strength across the surface of the moon (credit: Lunar Prospector). The origins of the field, and the details of this field are extremely confusing, To try and make sense of this, I’m going to integrate content from across many of the morning presentations.
Lunar Prospector
The overall structure of this magnetic field is fairly noisy, although the structures in the magnetic field do correlate roughly with topographic features. Here are the same maps showing topography (credit: Clementine)
There is rough correlation between the magnetic strong points matching the far-side basin and the lowest magnetic field corresponding with the lunar mare. Looked at in detail, there is also correlation between central up-wellings in craters and magnetic field strength.

What is missing from our understanding is the source of this magnetism. At one point in its past the moon was geologically active with lava flows. It is possible the moon once had a magnetic dynamo inside like the one the Earth has. While this dynamo could have died as the Moon cooled, the magnetic field that dynamo could have created may have become frozen in the magnetic rocks formed during that period of high magnetism. As the rocks cooled, the atoms in the liquid material would have aligned along the magnetic field and they would have stayed aligned as the rock cooled, and then hung out on the moon for a few billion years. Heating material, exposing it to a magnetic field, and then cooling it in the magnetic field is one way magnetic can be made commercially.

At the same time, it’s possible such a dynamo never existed, and the magnetism came from hitting the moon with large impactors. This is the equivalent of magnetizing a nail by hitting it with a hammer. The shock can knock the atoms into alignment if it is hard enough.

It is possible that both methods of creating magnetic fields takes place (and if there was a dynamo, it is most probable that the modern magnetic observations reflect both the dynamo and the impacts.

The best way to try and understand if there was a lunar dynamo is to look for patterns in the magnetic properties of individuals rocks as a function of when they formed. A rudimentary study of rocks that have published data shows that between 3.0 and 3.6 billion years ago sampled rocks have much higher magnetic field. When a detailed study is done to figure out if these are primary magnetic fields, there are problems with all the samples. Problems include measurements not being reproducible, the magnetic fields changing between measurements, and single samples having materials with different field directions/strengths. In careful analysis using many methods, not one rock could be found to have clear cut, consistent across methods, measurements. This could have to do with reheating (heat erases magnetic fields), or the magnetic fields just not coming from an ancient lunar magnetic field (called a paleomagentic field).

While some scientists are struggling to prove paleofields in old rocks, others are finding there are some rocks with planer magnetite (magnetic material) that have huge magnetic values. Magnetic anomalies in these rocks are over quartze grains next to ferromagnetic materials. This raises the question, can this type of rock play a role in the magnetic central uplifts in craters (magnetized central peaks). The answer seems to be yes, and the central magnetized peaks in to craters seem to best be explained either as material shocked into a magnetized state during impact, or that became magnetized after melting during impact, and aligned into a magnetized state.

Many talks, many questions, and no clear answers. Magnetic fields are in physics, astronomy, and geophysics one of the hardest things to understand. For now all we can really say is the Moon has fossil magnetic fields, and some of them (but maybe not all of them) were induced during impacts.

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