- The planets are often divided into two groups: the inner planets and the outer planets.
Which planets do you think are in each of these two groups? What do members of
each group have in common?
25.2 Inner Planets
Lesson Objectives
- Describe key features of each of the inner planets.
- Compare each of the inner planets to Earth and to one another.
The Inner Planets
The four planets closest to the sun - Mercury, Venus, Earth and Mars are the inner planets,
also called the terrestrial planets because they are similar to Earth.Figure25.7shows the
relative sizes of these four planets. All of the inner planets are small, relative to the outer
planets. All of the inner planets are solid, dense, rocky planets. The inner planets either do
not have moons or have just one (Earth) or two (Mars). None of the inner planets has rings.
Compared to the outer planets, the inner planets have shorter orbits around the Sun, but all
the inner planets spin more slowly. Venus spins the slowest of all the planets. At one time,
all the inner planets have been geologically active. They are all made of cooled igneous rock
with inner iron cores.
Mercury
Mercury, shown inFigure25.7, is the planet closest to the Sun. Mercury is the smallest
planet, and it has no moon. As Figure25.7 shows, the surface of Mercury is covered
with craters, like Earth’s moon. The presence of impact craters that are so old means that
Mercury hasn’t changed much geologically for billions of years and, with only a trace of an
atmosphere, has no weather to wear down the ancient craters.
Because Mercury is so close to the Sun, it is difficult to observe from Earth, even with a
telescope. However, the Mariner 10 spacecraft, shown inFigure25.8, visited Mercury in
1974–1975. In January 2008, the Messenger mission returned to Mercury and took much
more detailed pictures. One of these images can be seen inFigure25.9.
Short Year, Long Days
Mercury is named for the Roman messenger god, who could run extremely fast. Likewise,
Mercury moves very fast in its orbit around the Sun. Ayearon Mercury—the length of