It is reasonable to believe Mercury once had a moon as it is a normal thing for planets.
That after aeons in orbit this moon impacted onto Mercury and was absorbed and that evidence
of this event is the Caloris Basin. For want of a name, this theoretical moon is named Caloris.
Mercury's parameters
Mercury has a 3:2 spin:orbit resonance, rotating three times (3 × 58.7 days) for every two revolutions
(2 × 88 days) around the Sun; the eccentricity of Mercury’s orbit makes this resonance stable —
at perihelion, when the solar tide is strongest, the Sun is nearly still in Mercury’s sky. The orbit of
Mercury has the greatest eccentricity of all the Solar System planets, but has the smallest axial tilt.
It so happens that every second orbit around the sun, perihelion is exactly over the Caloris Basin.
At perihelion on the in-between orbits, the Caloris Basin is at the antipodal point.
Mercury ----- Caloris ----- Sun
Caloris ----- Mercury ----- Sun
If Caloris had been a random object flying through space and had hit Mercury, it is IMPOSSIBLE
it could have impacted at the mathematical centre of perihelion.
When the Solar system formed into planets and minor planets, moons were also formed, and Mercury
acquired a moon, Caloris. Mercury is 4878 km dia and as the Caloris basin is 1550 km dia, it is
assumed that Caloris moon was about this same diameter. This means that Mercury:Caloris was 30:1 by
volume. Mercury and Caloris would both have orbited around their barycentre and likely both rotated
on their axes. Over time, Caloris would have stopped rotating because of tidal effect between
Mercury and Caloris, and so end up like Earth's Moon in always showing the same face to its
planet. Mercury and Caloris would continue to revolve around their barycentre once every 58.7 days.
At some time, during perihelion, the gravitational attraction reached its maximum and, possibly
coupled with perturbations from the other planets, caused both Caloris and Mercury to fall to the
barycentre. At this stage, the barycentre would be inside Mercury. The collision would cause Caloris's
mass and an equal mass of Mercury at the point of impact to have a gigantic increase in temperature,
sufficient for the combined quantity to become completely molten. Gravitation forces within Mercury would
cause this mass to be absorbed and re-establish the globular shape of Mercury we see today.
