Mercury’s rotation has slowed down to a rate where its days are 4226.6 hours long
and this means that the sunny side of the planet has time to get very hot (480 degrees
C) while the dark side cools to much lower temperatures. This irregular heating effect
will cause pockets of vapour to form within the planet and reduce its density.
Mercury is in its final stages of Planetary Metamorphosis. Eventually it will get
to a critical distance from the Sun (between 30 and 40 million km) where it will
be affected by the Sun’s gravitational tides. Its rotation will almost stop and the
resultant higher temperatures will cause even greater levels of cavitation and violent
out-gassings on one side of its surface, only. Its orbital rotation is also very
eccentric and so these destructive forces could reach a maximum during which the
planet could suddenly be jet-propelled away from the Sun’s hold. This may have happened
to Pluto, our Moon, and the other 16 ‘previous planets’ listed below.
It can be seen from the above graph that planets reaching the 130 million km distance
from the Sun begin a process of degradation and their diameters progressively decrease
in size. From this point the planet starts to ‘cook’ and violent out-gassings reduces
the amount of mass in the mantle area below the crust. This causes the crustal surface
to contract and form the characteristic lobate scarps found on the surface of Mercury
and the Moon. The loss of mantle material eventually renders the metallic core as
the predominant part of the planet’s structure. Mercury has a metallic core which
is considered to be 60% of the planet’s volume.
But some of this metallic core also yields to the powerful effects of the Sun’s energy.
Cavitation causes powerful bursts of vapours through the crust and it is here where
the craters are formed. Cavities in the core of the planet reduce the overall density
of the old planet.
A planet reaching a proximity of 45 million km to the Sun will have a surface temperature
of about 900 deg.C (See graph of Solar energy) a temperature which is high enough
to melt rock. Lavas in Hawaii which have a glow of dark red to cherry red usually
have a temperature of 630 deg C. while those with an orange to yellow glow have a
temperature of 900 deg C (USGS).
Mercury’s density of 5427 kg/m2 can be compared with the following previous planets.