Re: Planet rotation

[Leigh Palmer <palmer@SFU.CA>]

>      Is it possible for a planet not to rotate?

There is no dynamical reason forbidding the zero angular momentum
state for a planet. Venus rotates only once in 243 days, and in a
retrograde direction at that. There is no reason to believe it was
ever in a state of exactly zero rotation, of course.

> I believe from a dim recollection of past readings that planet rotation is a
> residual effect of the accretion of particles that formed the planet bringing
> momentum through collisions.  I know it can be affected by tidal forces.
> Can it be zero? Why or why not?

Tidal interaction with the Sun would tend to produce a rotation
period commensurable with the revolution period, as is the case
for Mercury, which has a year that is a day and a half long. In
Mercury's case this is a stable condition because the orbit is
quite eccentric. Tidal friction in this case will tend to
circularize the orbit, disrupting the present commensurability,
but that won't happen on a timescale less than the lifetime of
the Sun.

For a planet with a nearly circular orbit tidal friction could
only produce a stable condition of zero rotation with respect
to the primary, typified by the Earth and the Moon (and many
other planetary satellites). This is not a zero rotation state
of course.

Leigh