The gas planets (Jupiter, Saturn, Uranus, and Neptune) have no surface to speak of, so how was it determined how long a day is?

Question

The gas planets (Jupiter, Saturn, Uranus, and Neptune) have no surface to speak of, so how was it determined how long a day is?

In: 549

Answer 1

IIRC you map the magnetic field of the planet and use that. No idea what you would do if you found a planet where that field was also independently rotating.