Are there any theories on why the moon only rotates once its axis per
orbit around the earth? Is this a characteristic of all moons in the
solar system?

Hello.

In short, the earth's gravitational pull upon the moon has slowed down
the moon's rotation to its current once-per-orbit. This is a
characteristic of most (but not all) moons in our solar system. Here
are three good explanations of what's going on:

From Cornell University:

"Almost all moons in the Solar System keep one face pointed toward
their planet. (The only exception we know of is Hyperion, a moon of
Saturn.)
...
You probably know that the Moon's gravity affects the Earth's oceans.
Well, the Earth's gravity also affects the Moon. It distorts the
Moon's shape slightly, squashing it out so that it is elongated along
a line that points toward the Earth. We say that the Earth raises
'tidal bulges' on the Moon.
The Earth's gravity pulls on the closest tidal bulge, trying to keep
it aligned with Earth. As the Moon turns, feeling the Earth's gravity,
this creates friction within the Moon, slowing the Moon's rotation
down until its rotation matches its orbital period exactly, a state we
call tidal synchronization. In this state, the Moon's tidal bulge is
always aligned with Earth, which means that the Moon always keeps one
face toward Earth."
http://curious.astro.cornell.edu/question.php?number=142

From Physlink.com:

"The mass and speed of rotation of the Earth influence the moon in
that some of its rotational energy is actually transferred to the
moon. The result of this being that the moon rotation is slowed while
also being placed continually into a higher orbit and thus slowing its
revolution. The net effect of this gravitational relationship is that
the moon?s rotation has been slowed to match its orbital period.
Ironically, since the Earth is giving up some of its rotational energy
to the moon, the Earth and moon will, in the far distant future, reach
a synchronization of rotational periods, as Pluto and its nearer to
its mass moon Charon have already done."
http://www.physlink.com/Education/AskExperts/ae390.cfm

From University of Arizona:

"The asymmetric nature of this gravitational interaction is also
responsible for the fact that the Moon rotates synchronously, i.e. it
is locked in phase with its orbit so that the same side is always
facing toward the Earth. Just as the Earth's rotation is now being
slowed by the Moon's influence so in the distant past the Moon's
rotation was slowed by the action of the Earth, but in that case the
effect was much stronger. When the Moon's rotation rate was slowed to
match its orbital period (such that the bulge always faced toward the
Earth) there was no longer an off-center torque on the Moon and a
stable situation was achieved. The same thing has happened to most of
the other satellites in the solar system. Eventually, the Earth's
rotation will be slowed to match the Moon's period, too, as is the
case with Pluto and Charon."
http://seds.lpl.arizona.edu/nineplanets/nineplanets/luna.html

I should note that Saturn has another satellite, Phoebe, that does not
always show the same face to the planet. Source: NASA
http://solarsystem.nasa.gov/features/planets/saturn/phoebe.html

search strategy: "moon rotates", "other moons"

I hope this helps.

---

Clarification of Answer by juggler-ga on 26 Sep 2002 00:35 PDT

Just one more note...

Not all of Jupiter's satellites rotate synchronously with Jupiter. For
an explanation, visit the Monterey Institute for Research in Astronomy
http://www.mira.org/fts0/planets/099/text/txt002x.htm

Also visit NASA's Jupiter pages:
http://solarsystem.nasa.gov/features/planets/jupiter/jupiter.html

3 Stars for a very good qualitative answer. 4 stars would be given for
a basic quantitative answer, 5 stars for a dynamic model which
indicated inital starting condition, final state, and effects due to
perturbations such as meteorite impacts on moons. Good Job!