Inner Planets

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Inner 4

Copyright Calvin J. Hamilton

Inner Planets

The inner four planets are called terrestrial planets. That means that they are like the earth in some ways. The terrestrial planets Mercury, Venus, Earth, Mars, and the earth's moon have similar compositions and densities. These planets are close to the sun, rocky, and dense. They are fairly small. They have few moons and no rings. Their evironments are oxidized; that is, oxygen dominates the chemistry. Meteors, tectonic activity, and erosion have modified the surfaces of the terrestrial planets.

The atmospheres of the terrestrial planets have changed as a result of sunlight (which produces chemical reactions, such as those that break apart water molecules), escape of light gases, volcanic activity, and in the case of our planet, biological activity.
The main stages of evolution of the terrestrial planets are:


courtesy of NASA


Mercury is the planet closest to the Sun, but surprisingly not the hottest. Because it has almost no atmosphere, there is no greenhouse effect to trap the heat, so temperatures vary from extreme hot to extreme cold. In fact it has the widest day/ night temperature change in the solar system. Its surface of craters resembles the Moon because it has been bombarded by comets etc. The planet was named for the Roman god Mercury, a winged messenger, and it travels around the Sun faster than any other planet. Mercury is difficult to see from Earth.

Venus is the brightest planet in the sky because it is very reflective - it is covered in clouds. It has a tremendous greenhouse effect that does not allow the heat to escape leaving the temperature to hover around 880°F which makes it the hottest planet in the solar system. Venus is often called Earth's twin because the two planets are close in size, but that's about the only similarity. Venus' rotation is not only extremely slow but it is also backward or retrograde. Venus is named after the Roman goddess of love and beauty and appears as a bright, white disk from Earth.


courtesy of JPL/Caltech/NASA


courtesy of NASA

Earth is the third planet from the sun and is 4.6 billion years old as determined by radioactive dating. It is 5.5 times more dense than water on average. It's inner core is made up of solid iron with a molten outer core that produces it's magnetic field. Earth's atmosphere is made up of 78% Nitrogen, 21% Oxygen, and traces of other elements. Our greenhouse effect regulates the temperature on Earth and is essential for our survival. Also, Earth is not perfectly round; it bulges at the equator and is flatter at the poles.

Mars is the fourth planet from the Sun, situated between Earth and Jupiter. The surface of Mars contains silcon dioxide and iron oxide. The iron oxide is rust, which gives it a reddish color. Mars has two moons Phobos and Deimos. Like our moon, the two moons keep the same side toward Mars as they revolve around the planet. They are small and appear to be captured asteroids. Mars has the largest volcanic mountain in the solar system, Mt. Olympus, which is 2.5 times the size of Mt. Everst. The poles of Mars are covered with frozen water and frozen carbon dioxide producing white polar ice caps. There can be no liquid water due to the thin, low pressure atmosphere which allows water to vaporize.


courtesy of NASA

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