Geology Reference
In-Depth Information
Geo-inSight
The Terrestrial and Jovian Planets
The planets of our solar system can be divided into two major
groups that are quite different, indicating that the two underwent
very different evolutionary histories. The four inner planets-
Mercury, Venus, Earth, and Mars—are the terrestrial planets;
they are small and dense (composed of a metallic core and
silicate mantle-crust), ranging from no atmosphere (Mercury)
to an oppressively thick one (Venus).
The outer four planets (Pluto, is now considered a dwarf
Planet)—Jupiter, Saturn, Uranus, and Neptune—are the Jovian
planets; they are large, ringed, low-density planets with liquid
interiors cores surrounded by thick atmospheres.
Sun
Jupiter
Saturn
Neptune
Earth
Mercury
Venus
1. The relative sizes of the planets and the Sun.
(Distances between planets are not to scale.)
Mars
Pluto
Uranus
100,000 km
2. Venus is surrounded by an oppressively
thick atmosphere that completely
obscures its surface. However, radar
images from orbiting spacecraft
reveal a wide variety of terrains,
including volcanic features, folded
mountain ranges, and a complex
network of faults.
3. The Moon is one-fourth the diameter of
Earth, has a low density relative to the
terrestrial planets, and is extremely dry.
Its surface is divided into low-lying
dark-colored plains and light-colored
highlands that are heavily cratered,
attesting to a period of massive
meteorite bombardment in our solar
system more than 4 billion years ago.
The hypothesis that best accounts for
the origin of the Moon has a giant
planetesimal, the size of Mars or larger,
crashing into Earth 4.6 to 4.4 billion
years ago, causing ejection of a large
quantity of hot material that cooled and
formed the Moon.
4. Mercury has a heavily cratered
surface that has changed very
little since its early history. Because
Mercury is so small, its gravitational
attraction is insufficient to retain
atmospheric gases; any atmosphere
that it may have held when it formed
probably escaped into space quickly.
JPL/NASA
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