10 Facts About the Inner Planets

The inner planets of our Solar System are made up of Earth, Mercury, Venus and Mars. They occupy a region known as the inner Solar System. Since the Earth and Moon are covered in a separate piece, here we will look at 10 facts on the rest of the inner planets: Mercury, Venus and Mars.

1. Mercury is the most cratered planet of the Solar System


It has no significant atmosphere and so the craters are not weathered.

2. Mercury has a magnetic field

Cross section of the planet Mercury.

Planets with magnetic fields usually spin quickly and have a molten core. Mercury spins only once every 59 days and was thought to be completely solid until this discovery by the spacecraft Mariner 10.

3. Mercury is the fastest moving planet in the Solar System

Mercury orbits quickly.

It has an average orbital speed of 112,000mph (180,000kph) taking it round its highly elliptical orbit in just 88 days.

4. The planet Venus has a retrograde rotation


It rotates anti-clockwise which is unusual and may be the result of a large collision in its early history.

5. A day on Venus is longer than its year

Orbit of Venus.

Venus takes 243 Earth days to spin once on its axis but it orbits the Sun in only 225 days.

6. It has the highest average surface temperature (about 470oC) of all the planets


It is subject to a ‘runaway greenhouse effect’ because of the thick atmoshpere of mainly carbon dioxide. The image above was taken by the Russian spacecraft Venera 9 and was the first image to be taken on the surface of another planet.

7. The surface of Venus was mapped using radar

Radar map of the surface of Venus.

During 1990 and 1991, the Magellan spacecraft used radar to ‘see’ through the dense atmosphere. There are two highland areas, but 80% of the surface consists of volcanic plains.

8. In 1877 Giovanni Schiaparelli, an Italian astronomer, produced the first detailed map of Mars

Mars map.

The map showed what he called ‘canali’ which has been mis-translated as ‘canals’, suggesting that Mars supported an advanced civilisation. ‘Canali’ translates as channels and they were an optical illusion.

9. There are permanent polar ‘ice’ caps on Mars

Mars polar ice cap.

These are in fact caps of frozen carbon dioxide at the north pole. The cap is around a metre thick, but the southern cap is thicker at about 8 metres.

10. The largest mountain in the Solar System is on Mars


Olympus Mons is a huge volcano that looms nearly 25km above the surface, that’s almost 3 times more than Mt. Everest.

Taught science for 16 years at a secondary school in the East Midlands.