Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System after Mercury. Because of the iron oxide on Mars' surface, which gives it a reddish appearance, it is often referred to the 'Red Planet'. Mars is a terrestrial planet with a thin atmosphere, with surface features reminiscent of the impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth. The days and seasons are comparable to those of Earth. Mars is much colder than Earth due to its greater distance from the Sun. The average temperature is about minus 60 degrees Celsius, although it can vary from minus 125°C near the poles during the winter to as much as 20°C at midday near the equator. The carbon dioxide rich atmosphere of Mars is also about 100 times less dense than Earth's on average, but it is nevertheless thick enough to support weather, clouds, and winds. The Red Planet is home to both the highest mountain and the deepest, longest valley in the Solar System. Olympus Mons is roughly 17 miles high, while the Valles Marineris system of valleys reaches as deep as 6 miles.