Continuing the Sequence of Planetary Metamorphosis
As planets gradually move closer to the Sun, they become exposed to an increased
level of solar energy. The inverse square law is used to calculate the level of insolation
the planet receives relative to its distance from the Sun.
The graph below shows the rate of increase in maximum temperature in relation to
distance from the Sun and it can be seen that temperatures begin to rise steeply
once they reach the position now occupied by Venus.
Venus receives a maximum of 2600 watts/square metre of solar light, nearly twice
the amount the Earth receives. Mercury, further in towards the Sun, receives more
than ten times (14440 watts/m2) the energy the Earth receives.
As planets get closer to the Sun their rotation velocity decreases also and this
means that certain areas of their surface are exposed to solar energy for increasingly
At these higher temperatures the planet’s atmosphere begins to lose its lighter gases.
As we would expect nearly all the water vapour, nitrogen and oxygen on Venus has
been lost to its outer atmosphere, displaced by the heavier carbon dioxide (96%)
and sulphur dioxide (150 ppm) creating an atmospheric pressure 90 times that on Earth.
Venus’ maximum equatorial temperature has been measured at a much higher level than
that one would expect from its distance from the Sun, but this could be due to the
greenhouse effect of carbon dioxide.The graph predicts a temperature maximum of 84
degrees C, but space probes have measured it at around 460 degree C. Volcanism may
also attributable to these high temperatures.
The above video describes Messenger’s mission to Mercury and observations.
Mercury is 60% metal. It has a magnetic field.
It has sodium, potassium and calcium in its tenuous atmosphere.
It mentions that there are signs that its surface has contracted over time (leaving
lobate scarps) and there has been much more volcanism than previously considered.
These observations are consistent with a planet which has been eroded and ‘cooked’
by the solar wind as a consequence of its proximity to the Sun.