Mercury-1

The Mercury of Roman mythology
was an erratic and volatile character;
and this Swift moving messenger lent his
name to one of the solar system's most
mysterious worlds, the planet Mercury.
Mercury is really a weird planet! The
normal terrestrial planets all the rest
have a relation between how big they are
are how dense they are. Mercury is not
following that rule: it is much more dense
than what you would expect for its size.
So something went wrong in the formation
of Mercury that we don't know. One of the
reasons why Mercury remains mysterious
is that it's so close to the Sun.
Telescopes find it difficult to observe
such a small planet against such ferocious
light as is coming from the sun.
It's a missing piece of the jigsaw which
we need in order to understand the
evolution of our solar system.