Solving the Puzzle of Apollo 12’s Mysterious Magnetic Moon Rocks

Article written: 16 Mar , 2012
Updated: 23 Dec , 2015


Ever since their discovery by the Apollo 12 crew, scientists have been puzzled by strongly magnetized rocks found on the Moon. Most Moon rocks that were brought back by the Apollo missions have very little iron, and therefore lack the ability to be strongly magnetized. At first, the magnetic oddities didn’t appear to be related to any lunar geology such as craters or lava flows. Over time, additional lunar missions have provided more data showing that only some portions of the Moon’s crust have magnetic fields. A team of scientists now theorize that the magnetized “patches” on the lunar surface may be the remains of an asteroid that crashed into the Moon shortly after its formation nearly 4.5 billion years ago. The impact crater, known as the South Pole-Aitken basin is one of the largest known in our Solar System.

Mark Wieczorek (Paris Institute for Global Physics), describes the South Pole-Aitken basin as, “this huge, whopping crater that’s roughly half the size of the U.S,” and says it may hold the answers to the mystery of the Apollo 12 rocks.

The Apollo 12 landing site as seen by LRO. Credit: NASA/GSFC/Arizona State University

Studies of the basin show that it is elliptical which suggests the impact was by a large object that hit at an oblique angle. Wieczorek speculates that the impactor was 10% to 30% iron by weight and about 100 times more magnetic than the lunar regolith. Interestingly enough, the theorized impact angle would have flung debris from the object in a pattern very similar to the observed magnetic anomalies. The material could have been magnetized as it cooled by a magnetic field that may have existed early in our Moon’s history.

Wieczorek and his team set out to test their theories with computer simulations of different types of impacts. The research led to a scenario where an object struck the Moon at about a 45 degree angle with a velocity of 15 kilometers per second. The team’s best impact model was described as normal by Wieczorek who stated, “We don’t require improbable conditions.”

Now the team needs to address one other question: How and when did a magnetic field develop on the Moon?

Wieczorek offers a simple solution: Go back to the moon and collect samples.

Source: NASA Lunar Science Institute

16 Responses

  1. Mihir Panchal says

    As magnetism is an alignment of the atomic magnetic dipoles, and any strong shakes disturb that, would not a major impact like that have demagnetized the asteroid?

    • Torbjörn Larsson says

      That is a good question.

      However, the part about shaking a permanent magnet disturbing its magnetism is wrong. It is however true that shocks, like hitting it with a hammer, does disturb it.

      Here I would think the main effect would be the heat created taking the iron above its Curie temperature, the temperature where its ferromagnetism (ordered magnetism) is lost to paramagnetism (random magnetism), at ~1 100 K. That and the impact shock could explain the absence of magnetic anomaly in the center of the crater under the proposed model.

      Presumably the ejecta, which will mainly consist of impact material IIRC, was still magnetized in some fragments and those are what we observe, according to the proposed model.

    • Torbjörn Larsson says

      I was clearly wrong on the last two points below, see ITSRUF’s comment above which points out where the magnetic field of the impactor remains comes from.

      I don’t know why the center of the impact wouldn’t be magnetized. It would be covered by regolith perhaps, and if thick enough the resulting field would look like the overall field over regolith.

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  3. OlynthusDam says

    The impactor hit the moon at a velocity of only 9 meters per second? My dog runs faster than that. How big would my dog need to be to create such an enormous crater?

  4. Torbjörn Larsson says

    It could be a viable theory, but as it explains groupings individually (here “he moon’s largest grouping of magnetic anomalies”) it has many observations to further predict.

  5. ITSRUF says

    I understood the article to say that the asteroid material magnetized as it cooled, so the impact may have de-magnetized the asteroid. The asteroid delivered a lot of iron just waiting to be magnetized as it cooled.

    • Torbjörn Larsson says

      I saw that, but there is three problems with that:
      – The article also states that the _impactor_ was ~100 times more magnetic than the regolith. Presumably that goes into the model at some point.
      – Where is the magnetization at the major impact mass (center of impact)?
      – The remagnetization scenario relies on the Moon primordial field being larger. Not a bad assumption, but was it large enough? The article doesn’t say. (And I can’t remember the involved field strengths as I write.)

  6. Sanjay Kumar Chauhan says

    Magnetic or non-magnetic, I am not teleporting there myself to crawl on someone’s body down here. I am too old for that, yes too old.

    • Sanjay Kumar Chauhan says

      Me too, I am also too old for a mission so & so….. however, do consider us as reserve cavalry for the War Galactica though….. there isn’t much to loose for us oldies, might as well wriggle somewhere in the lonely canoe…..

  7. Sanjay Kumar Chauhan says

    And I forgot to mention Little Miss Moon got bitten by a Bug on her thighs about 4.5 years back, and need to see a doctor. you now must correct your report & the typo, & stop talking in codes (using billion years for years just few) to talk just between yourself, raising much curiosity in the public, keeping them in the dark about your real dark motives, and other things (Blaah Blaah Blaah) to justify your expenses on space exploration. When there is nothing there that needs to be explored, the lights up there are just the signs of the life-forms down here. And why should the people other, personal life & diaries their with you, should they share ?…………………..

    And your own LCROSS mission dived into her right tit, and she has got her lower frontal teeth missing, and got six fingers on one of her toes. Even I did wonder, why it is so. We just happened to intercept her cries for help, SOS & MayDay. Aye you must now stop messing around, watching others in the heaven & playing hyper-dimensional voodoo with modern high-tech names. You all are sicko. Aren’t you just happy to see the friendly stars shining on you ?.

    And be aware, what goes up does eventually fall back on the sources of those woes to others.

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