Why Mercury has no moons?
Because it doesn't. That's just the way it worked out. Mercury
has no natural satellites that we know of. Alternatively, Jupiter
has at least 62 known moons. It is not for any reason, it's just
the way things turn out.
I am a different person and say there are none.
I think that there is more to it than that. If one was to
hypothetically place
a natural satellite in any reasonable orbit around Mercury then
it
becomes possible to calculate the gravitational force between
that satellite
and firstly Mercury, secondly between the satellite and the
Sun.
When I did this calculation I learned that the gravitational
force between the satellite and the Sun was greater than that
between the satellite and Mercury.
This tells us that if there had ever been a satellite around
Mercury then the Suns influence would have pulled it away from the
planet into some sort of orbit around the Sun.
This may have happened during the formation of the Solar System
or it may not.
Any satellite pulled out of such an orbit would have become just
another
planetismal in the chaos that was the Solar System at the
time.
The satellite would have collided with some other body, been
ejected from the
Solar System or been pulled into the Sun itself.
It is long gone.
Carrying out such calculations for Venus and the Earth is also
quite instructive.
Venus cannot hold onto a natural satellite either.
The Earth and the Moon present a different problem.
The gravitational force between the Moon and the Sun is greater
than
that between the Moon and the Eath.
How then is it possible for the Earth to have retained the Moon
for over
four billion years ?
One has to consider that there is another set of related forces
acting between
bodies as well as the straightforward gravitational force.
This is the Tidal Force.
Calculating the maximum tidal forces for the above cases shows
that it is only in the case of the Earth and the Moon that there is
enough force for one of the Solar Systems inner three planets to
hang on to a natural satellite.
We could easily have lost the Moon long ago had things like the
mass of the Moon or the distance from the Earth to the Sun been
different.
Isaac Asimov wrote a short article in the 1960's on this very
topic.
He considered all of the planets and their known satellites at
the time.
It is still relevant and interesting today and well worth
reading.