| The inner planets
The inner, or terrestrial planets (Mercury, Venus, Earth, and Mars)
are rocky in composition with densities greater than 3 grams per cubic
centimetre (g/cm3). The realm of the inner planets is the region
inside the asteroid belt. They lack ring systems, and have few or no satellites.
Venus, Earth and Mars have atmospheres, but Mercury is too small and too
close to the Sun to hold on to any gases. Among the planets of the inner
Solar System, only the Earth has a strong magnetic field and is therefore
probably the only planet that still has an active inner core of molten
iron and nickel. They all show signs of previous volcanic and geological
activity of one form or another.
Formation of the inner planets
Close to the forming Sun, in the pre Solar System gas nebula, the
temperature was too high for water and ice to form, while at the distances
of Jupiter and beyond, ice could form. The planets forming close to the
Sun could not hold onto their lighter more volatile molecules and atoms,
leaving the heavier molecules and elements like iron and the silicates
to form the inner planets. The formation of the inner planets, including
the Earth, was complete within 100 million years of the collapse of the
interstellar gas cloud. The bombardment of planetary and satellite surfaces
by debris left over from the initial formation of the planets continued
for another 600 million years, adding small quantities to the original
size of the planets. The surfaces of Mercury and the Moon are witnesses
to the ferocity of this age of meteors.
The energy released during meteor impacts was sufficient to cause
vaporisation and extensive melting of the planet surfaces, transforming
the original surface and producing volcanoes, craters, cracking and mountain
formation on the early inner planets. Models of the planet-forming process
suggest that several planets the size of the Moon or Mars must have been
present in the early inner Solar System in addition to today’s planets.
Collisions between the planets and moons would have had dramatic effects
and could have produced many hard-to-explain phenomenon such as the high
density of Mercury, the thinness of the Martian atmosphere, the retrograde
rotation of Venus and formation of our own Moon.
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Formation of the inner planets
