QUESTION:
Why do you think that Jupiter spins so fast?  Generally, is there any
systematic account of planetary spin rates?

ANSWER from Glenn Orton on March 8, 1997:
To my knowledge, there is no systematic accounting for spin rates, outside the
theoretical possibilities allowed by various models for the formation of
planets.   In general, all the outer planets have internal spin rates closer
to 10-15 hours.  Mars and the Earth are near 24 hours, and Venus and Mercury's
rates are much, much longer.   For the terrestrial planets, it is clear that
lots of forces such as tidal ones from the sun (or the Moon, in the Earth's case)
could, might have interfered with the initial spin rates to perturb them to
what we see today.   For all the planets in general, the spin is an expresssion
of the local value of the angular momentum of the pre-solar-system nebula or
gas and dust disk which formed around the sun, now preserved in the rotation of
each of the planets.