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 todays
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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