Neptune: Which Way Is North?

Something’s wrong with Neptune’s magnetic field.

Nv07 Neptune's Compass

Not all planets have magnetic fields. For the planets that do, we can generally expect two things:

  • Magnetic north and south should roughly correspond to geographic north and south.
  • The center of the magnetic field should run through the planet’s core.

Neither of these statements are true for Neptune (or Uranus, by the way). In 1989, when Voyager 2 visited Neptune, it found a magnetic field skewed 47º from the planet’s axis of rotation, with the source of the magnetic field offset by 0.55 Neptune radii (placing it not in the planet’s core but in a seemingly random point in the mantle).

Nv07 Neptune's Magnetic Field

More recent computer simulations suggest that the situation on Neptune could be even more complicated than Voyager 2 observed, with the magnetic field in constant flux, rotating and changing in ways that planetary magnetic fields ought not to do.

But new research, published earlier this year in Nature Communications, might help explain what’s going on. Remember those exotic forms of water ice, like ice VII, ice VIII, and ice X? Those weird “hot” ices believed to exist in the high-pressure interiors of Uranus and Neptune? Now we can add superionic ice to that icy mix.

In superionic ice, water molecules are under so much pressure that the oxygen and hydrogen atoms dissociate from each other. Oxygen forms a tight lattice structure which ionized hydrogen atoms can then move through freely. Much like free flowing electrons, these free flowing hydrogen ions (also known as protons) would generate an electric current.

Superionic ice located in the mantles of Neptune and Uranus would conveniently explain why these two planets have such wobbly, lopsided magnetic fields. Yes, that would be very convenient. Too convenient, perhaps, to be the whole story.


Scientists Predict Cool New Phase of Superionic Ice from Science Daily.

Magnetic Fields: On Other Planets from Joly Astronomy.

Neptune’s Badly Behaved Magnetic Field from Astronomy Magazine.

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Today’s post is part of Neptune month for the 2015 Mission to the Solar System. Click here to learn more about this series.

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