Our present picture of the Moon's interior is that it has a crust about 65 km
thick, a mantle about 1000 km thick, and a core that is about 500 km in radius.
A limited amount of seismic data suggests that the outer core may be molten.
There does appear to be some amount of differentiation, but not on the scale of
that of the Earth. It has no magnetic field to speak of, but magnetization of
Lunar rocks suggests that it may have had a larger one earlier in its history.
The planet Mercury
is very difficult to study from the Earth because it is always so close to the
Sun. Even at elongation, it is never more than 28 degrees from the Sun in our
sky. It is the second smallest planet (it was believed to be the smallest until
the discovery that Pluto is actually much smaller than originally thought), and
also the fastest in its orbit since it is the innermost planet. In fact, the
name Mercury derives from its speed in moving around its orbit.
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