The Solar System and the Planet Earth 295
contracted to form the Sun. When discussing star formation we
mentioned that as the protostar contracted its rate of rotation increased
making it susceptible to losing matter and angular momentum. The
matter that the Sun lost during its formation condensed to form the
planets and the satellites. A more detailed understanding of the formation
of the Solar System unfortunately does not exist. We will, therefore,
proceed to describe the various features of the Solar System beginning
with the Sun’s closest companion the planet Mercury.
Mercury
Mercury orbits the Sun in a highly elliptical orbit that ranges from 46 to
70 million km (0.31 to 0.39 AU) once every 88 days rotating on its axis
once very 59 days. Mercury, the tiniest of the planets, is barely larger
than the Moon with a mass only 1/20 that of the Earth. Because of its
weak gravitational field the planet has no atmosphere. It resembles the
Moon in many ways with its many meteor craters and volcanic
mountains. The surface temperature is a scorching 600 K in the day and
200 K in the night. Mercury has no moon.
Venus
Venus is the brightest object in the sky with the exception of the Sun and
the Moon. The planet orbits the Sun every 225 days at a distance of 0.72
AU. Venus rotates on its axis once every 243 days in a retrograde
fashion, from east to west instead of the usual west to east manner of all
the other rotations of the planets (Uranus excepted) and their moons. The
size, mass and density of Venus are very similar to Earth. The planet has
no moon, however. Venus has a very thick cloudy atmosphere consisting
of carbon dioxide and hydrocarbons, which obscures the features of its
surface. The atmosphere is so heavy that the pressure at the surface is
92 atmospheres compared to 1 atmosphere at the surface of the Earth.
The surface temperature is surprisingly high, 735 K on the bright side
and 550 K on the dark side. These temperatures are attributed to a
greenhouse effect. The heavy atmosphere allows solar radiation in
but absorbs the radiant energy that is reflected from the surface of
the planet.