Aidan Broome, of Dunedin, asks :-

Why is gravity on Mercury different to gravity on Earth?

Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.

Gravity on Mercury is different from gravity on Earth because Mercury is smaller than Earth. The gravity pull of a planet depends on how heavy a planet is. A planet twice as heavy as Earth has a gravity pull twice as strong as Earth's gravity. (Strictly, when we say 'heavy' we mean how massive the planet is.)

Mercury is small. It would take 18 Mercurys to make one Earth. So a million kilometres from the Earth the gravity pull of Earth will be 18 times stronger than Mercury's gravity is a million km from Mercury.

A planet's gravity also changes with distance. The pull gets weaker as you move away from the planet. If you double your distance from a planet then the gravity pull is one-quarter of what it was before. At three times the distance the pull is one-ninth.

We can think of a planet's gravity as though all the mass or weight was at the centre of the planet. So when you are standing on a planet the gravity depends both on how heavy (massive) the planet is, and how far you are from its centre.

When you are standing on Earth you are 6400 km from Earth's centre. When you are standing on Mercury you are 2400 km from Mercury's centre. Because you are closer to Mercury's centre that increases the pull of its weaker gravity. On Mercury's surface the gravity is about 4/10ths of what it is on earth. 0.38 is the accurate figure. So a 100 kg person on Earth weighs 38 kg on Mercury.

The same rules work on the moon. It takes 81 moons to make an Earth. But on the moon you are only 1700 km from the moon's centre. So the gravity on the moon is 1/6th of what is is on Earth. A 100 kg person weighs 17 kg on the moon (plus space-suit, of course.)