Mercury, Venus and Mars Composed mostly of rock. Have atmospheres composed of a mixture of gases. Mercury has a very thin atmosphere.
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Jupiter, Saturn, Uranus and Neptune Composed almost entirely from gases.
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Mercury, Venus, Mars and Earth are sometimes called the rocky planets because they are composed mostly of rock. They can also be called the inner planets because they are closest to the sun. Jupiter, Saturn, Uranus and Neptune are sometimes called the gas planets because they are composed mostly of gas. They can also be called the outer planets because they are furthest from the sun.
36.4 Comparing gravity The force of gravity is strongest at the centre of any object, including space objects. The gravitational force gets weaker as you move away from the centre of the object. Scientists always refer to the gravity on the surface of the planets when comparing the gravity of planets. This is called relative surface gravity.
Relative surface
gravity is the surface gravity of one object compared to another.
Relative surface gravity =
Surface gravity of space object Surface gravity of Earth
If you look at a video of astronauts walking on the moon, you’ll notice that they seem to bounce as they walk on the moon’s surface. Even though they wore space suits with a mass of more than 80 kg, the astronauts could still jump much higher than they could on Earth. They also dropped back down much more slowly than they would on Earth. Astronauts appear to bounce on the moon because there is less gravity pulling them down on the moon than there is on Earth. In other words, relative to Earth, the moon has a lower surface gravity.
