The Inner Rocky Worlds

The interior four planets of the Solar System are small worlds composed primarily of rock. These four planets are Mercury, Venus, Earth, and Mars. A total of only 3 moons are orbiting, with the Earth having one natural satellite and Mars having two, while Mercury and Venus have none. All four inner planets have relatively warm surface temperatures due to their close proximity to the Sun, and only one of those worlds is presently within the "habitable zone" of our Solar System. One planet, Mercury, looks like our Moon, is airless, boasts the greatest temperature change between day and night, and the only planet to spin on its axis completely upright. Venus is the hottest planet surface in the Solar System and the only one that spins upside down, hence making the Sun appear to rise in the west and set in the east. Earth is the only planet with current active plate tectonics that are responsible for recycling chemicals necessary to support life, and the only planet where water exists simultaneously as solid, liquid, and gas. It is also the most dense of all nine planets, something counterintuitive to how one might expect the planets to have formed. Mars has more legends attached to it than any other planet, and it is certain that copious water once flowed on its surface in the past. There is a possibility that life may have once gained a foothold on Mars, but the evidence to prove so is presently lacking.





1st from Sun

2nd from Sun

3rd from Sun

4th from Sun

You are encouraged to start your study of the planets with Earth. This is a separate unit, and should have been completed by the time you arrive at this page. However, I know that people do not like to follow directions or move in an order suggested to them, so you might be here at this page ahead of yourself. If so, please go to the Earth introduction first and learn about our own home as a planet. The information contained in that unit is necessary to give you a basis for comparison to the rest of the planets and their moons. If you have already completed the Earth Unit, then move forward to Mercury.

Move forward to Mercury or back to the Planet Introduction, or the Syllabus.


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