Planetary magnetic fields beyond Earth

6 problems

• How does Ganymede sustain a core dynamo today?

  Open

  Jupiter's moon Ganymede is the only moon known to make its own magnetic field. Models of how such a small body cools suggest its core should have stopped stirring long ago.

• Why are Uranus's and Neptune's fields strongly tilted and multipolar?

  Open

  The magnetic fields of Uranus and Neptune are tipped far from the spin axis and lumpy, unlike those of other planets. This suggests the dynamo (the field generator) runs in a thin layer, but the interior is poorly known.

• How did the Moon's small core make a strong ancient field?

  Contested

  Some old Moon rocks record a magnetic field about as strong as Earth's, though the Moon's iron core is tiny. Either an unusual power source existed or the measurements are misleading.

• When and why did the Martian dynamo stop?

  Contested

  Mars has strongly magnetized ancient crust but no global magnetic field today, so its core dynamo (the flowing-iron field generator) stopped long ago. When it stopped, and whether it ever restarted, decides how long Mars was shielded from the solar wind.

• Why is Mercury's magnetic field so weak and shifted north?

  Open

  Mercury has an active dynamo (a field generator made by flowing liquid iron) but its field is about a hundred times weaker than simple scaling predicts, and its center is shifted toward the north pole. Its large iron core should do better.

• Why is Saturn's magnetic field almost perfectly symmetric about its spin axis?

  Open

  A theorem says a dynamo (the flowing-metal field generator inside a planet) cannot keep up a field that is perfectly symmetric around the rotation axis, yet Saturn's field is tilted by less than a hundredth of a degree. Something must hide the asymmetric part.