Mars is the fourth planet from the Sun and the last of the four Inner Planets. Mercury, Venus and Earth are the other Inner Planets. It is the second closest planet to Earth and is about half as wide. In mythology, Mars is the Roman god of War, a name possibly inspired by the planet's red (blood) colour. Because of its colour, Mars is sometimes known as the Red Planet.
The length of a day on Mars is not much longer than a day on Earth. It takes 24 hours and 37 minutes for Mars to spin once on its axis, compared to 23 hours and 56 minutes for Earth to do the same. A Martian day is often called a Sol. To complete a full journey around the Sun, Mars takes 687 days. Therefore, a Martian year is 687 days long.
Mars has two small moons - Phobos and Deimos. These moons are very different from the Moon orbiting Earth. They are irregularly-shaped, not round like Earth's Moon, and are possibly captured asteroids. They orbit Mars quite closely and travel around the planet quickly. Phobos is 9,377 kilometres (5,827 miles) away from Mars and orbits it every 7 hours, 39 minutes. Deimos is 23,460 kilometres (14,580 miles) away and completes an orbit in just over 30 hours.
Mars' surface is covered in ancient craters, thought to have been formed by meteorite impacts over billions of years. Countless rocks of all shapes and sizes are scattered across the surface of the planet, covered in a dusty layer of rust (iron oxide). This is what gives Mars its red colour. The planet was once volcanically active and is home to the largest volcano in the Solar System - Olympus Mons. Olympus Mons is 27 kilometres (17 miles) high. In comparison, Earth's highest mountain, Mount Everest, is 8.8 kilometres (5.5 miles) high. Another feature of Mars' surface is Valles Marineris, a 4,000 kilometres (2485 miles) long canyon which stretches almost a quarter of the way around the planet. It is the largest known surface canyon in the Solar System.
Scientists believe that water was once abundant on the surface of Mars and quite possibly plentiful enough to form oceans similar to those on Earth. Nowadays, Mars is a dry planet with only small signs of the present existence of water. So where did all of the water go? Nobody knows for certain, but scientists certainly aim to find out through missions to Mars. For almost half a century, a number of space craft have visited Mars. Some have simply flown past it to take pictures, others spend years orbiting the planet to collect data, and some have even landed on its surface to take in its sights and sounds, probe its soil and poke its rocks. Even now, there is a rover on Mars travelling very slowly across its surface and making discoveries.
The main reason for visiting Mars, apart from the fact that it is fairly easy to get to, is that there is a small chance that the planet could have supported life at some point in its long history. There's even a remote possibility that it could support life in some form today. The presence of water is a key factor in this which is why scientists get quite excited whenever they find some there. So far, all missions to Mars have been unmanned, and all discoveries have been made by robots. However, at some point in the not-too-distant future (hopefully), people will travel to Mars. Who knows what they'll find when they get there?