Moon’s Mini-Magnetosphere

Many objects in the solar system have strong magnetic fields which deflect the charged particles of the solar wind, creating a bubble known as the magnetosphere. On Earth, this protects us from some of the more harmful solar rays and diverts them to create beautiful aurorae. Similar displays have been found to occur on the gas giants. However, many other objects in our solar system lack the ability to produce these effects, either because they don’t have a strong magnetic field (such as Venus), or an atmosphere with which the charged particles can interact (such as Mercury).

Although the moon lacks both of these, a new study has found that the moon may still produce localized “mini-magnetospheres”. The team responsible for this discovery is an international team composed of astronomers from Sweden, India, Switzerland, and Japan. It is based on observations from the Chandrayaan-1 spacecraft produced and launched by the Indian Space Research Organisation (ISRO).

Using this satellite, the team was mapping the density of backscattered hydrogen atoms that come from solar wind striking the surface and being reflected. Under normal conditions, 16-20% of incoming protons from the solar wind is reflected in this way.

For those excited above 150 electron volts, the team found a region near the Crisium antipode (the region directly opposite the Mare Crisium on the moon). This region was previously discovered to have magnetic anomalies in which the local magnetic field strength reached several hundred nanotesla. The new team found that the result of this was that incoming solar wind was deflected, creating a shielded region some 360 km in diameter surrounded by a “300-km-thick region of enhanced plasma flux that results from the solar wind flowing 23 around the mini-magnetosphere.” Although the flow bunches up, the team finds that the lack of a distinct boundary means that there is not likely to be a bow shock, which would be created as the buildup becomes sufficiently strong to directly interact with additional incoming particles.

Below energies of 100 eV, the phenomenon seems to disappear. The researchers suggest this points to a different formation mechanism. One possibility is that some solar flux breaks through the magnetic barrier and is reflected creating these energies. Another is that, instead of hydrogen nuclei (which composes the majority of the solar wind) this is the product of alpha particles (helium nuclei) or other heavier solar wind ions striking the surface.

Not discussed in the paper is just how valuable such features could be to future astronauts looking to create a base on the moon. While the field is relatively strong for local magnetic fields, it it still around two orders of magnitude weaker than that of Earth’s. Thus, it is unlikely that this effect would be sufficiently strong to protect a base, nor would it provide protection from the x-rays and other dangerous electromagnetic radiation that is provided by an atmosphere.

Instead, this finding poses more in the way of scientific curiosity and can help astronomers map local magnetic fields as well as investigate the solar wind if such mini-magnetospheres are located on other bodies. The authors suggest that similar features be searched for on Mercury and asteroids.

12 Replies to “Moon’s Mini-Magnetosphere”

  1. It is my understanding that Mars has similar magnetic dipoles in various places.


  2. If up to 20% of the solar wind protons that hit the moon are reflected, this means that 80% of the solar wind protons are absorbed. Where do they go on the moon? The solar wind sends at least some 50000 trillion protons per second to the moon´s surface. Since the sun started producing the solar wind, many seconds have passed….

  3. Where’s Ivanman when you need him? The first paragraph in this article needs so much help in the grammar department my head literally hurt trying to read it. :^P

  4. I am thinking if this magnetic anomaly can be attributed to another cause:
    I suppose that moon had a magnetic field in the distant past, just like Earth has one now (because also the Moon had molten lava on the surface, and I presume also in it’s interior). If some magnetic material (lava rich in iron for example) was solidified during the time that the Moon still had a magnetic filed, it may be that the molecules in the solidified lava oriented themselves in the N-S direction and remained in the same way since. Now, the strong magnetic field of the moon has disappeared (the core has solidified meanwhile) but those magnetized molecules remained oriented in the same direction and could generate locally a very weak magnetic field…

    Just my 0.02$….

  5. @Gbendt: It’s my understanding that they stay there. I seem to remember that it becomes embedded in some of the oxygen bound to the rock to produce a form of oxygen that’s rare on Earth, but makes great rocket fuel if we ever want to go and scoop it up.

    @Quasy: The article didn’t explore a cause for the anomaly, but I was thinking along the same lines

  6. Quasy, there’s at least two works that finds a Moon local dipole that fits a harmonic function.

    The one paper I like the model in, since it fits with predictions from modeling Earth’s tectonics, is that it is the frozen remain of the first mantle convection riser that is presumed to have started the Earth convection/tectonic plate conveyor belt. It fits size, geological time, field strength et cetera. [I have the ref, if you want it.]

    Now this seems totally odd (to me) compared to the confused set of local dipoles that Mars shows, as LC mentions. Again the Moon places well in the concept of “the Earth-Moon system”; Mars, not so much.

  7. “the first mantle convection riser that is presumed to have started the Earth convection/tectonic plate conveyor belt.”

    Um, I meant “the first mantle convection riser that is presumed to have started the convection/tectonic plate conveyor belt on Earth.” Moon # Earth, last I looked.

  8. Torbjorn Larsson OM: It is not surprising if these local dipoles are high order terms in a Legendre multipole field expansion.

    The lunar magnetic field dynamo has probably closed down. What magnetic field there is is due to a permanent magnetization of lunar material. The liquid core-mantle boundary probably has electrical currents which generate the Earth’s magnetic field. The moon probably does not have an active core.

    jimhenson: Nothing you write makes any sense.


  9. Ah wonderful Muppet.

    Another absolutely brilliant joke! Please stop it, oh please stop it, my sides are really hurting from so much laughing. This must be the funniest thing I read in ages, and so original and good, David Letterman may have found his replacement! Who knew Universe Today could be so hilarious!

    Jon. Please talk to Fraser for me. We should have a weekly comedy section in Universe Today, with Muppet here a the star! It’d be a classic!

    Muppet, you a genius, mate!! Even I couldn’t dream of anything so stunning sublime. Even the long-gone Anaconda now even appears mainstream! Keep it up, please!

  10. Oh god, the Muppet is still prattling on!
    I’m starting to think the guy might be a bit of a madman.


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