Mars

Mars is the fourth planet from the Sun in the Solar System and the second-smallest planet. Mars is a terrestrial planet with polar ice caps of frozen water and carbon dioxide. It has the largest volcano in the Solar System, and some very large impact craters. Mars is named after the mythological Roman god of war because it appears of red color.

Space probes such as the Viking program landers are the main tools for the exploration of Mars.

Mars is a terrestrial planet and made of rock. The ground there is red because of iron oxide (rust) in the rocks and dust. The planet's atmosphere is very thin. It is mostly carbon dioxide with some argon and nitrogen and tiny amounts of other gases including oxygen. The temperatures on Mars are colder than on Earth, because it is farther away from the Sun and has less air to keep heat in. There is water ice and frozen carbon dioxide at the north and south poles. Mars does not have any liquid water on the surface now, but signs of run-off on the surface were probably caused by water.

The average thickness of the planet's crust is about 50 km (31 mi), with a maximum thickness of 125 km (78 mi).

Mars does not have a global magnetic field. Despite this, observations show that parts of the planet's crust have been magnetized. This suggests that polarity reversals have occurred in the past. This paleomagnetism is similar to the magnetic striping found on Earth's ocean floors. One theory is that these bands suggest plate tectonic activity on Mars four billion years ago, before the planetary dynamo stopped working and the planet's magnetic field faded.

Mars has a very thin atmosphere with barely any oxygen (it is mostly carbon dioxide). Because there is an atmosphere, however thin it is, the sky does change colour when the sun rises and sets. The dust in the Martian atmosphere makes Martian sunsets somewhat blue. Mars's atmosphere is too thin to protect Mars from meteors, which is part of the reason why Mars has so many craters.

Mars is home to the highest known mountain in the Solar System, Olympus Mons. Olympus Mons is about 17 miles (or 27 kilometers) high. This is more than three times the height of Earth's tallest mountain, Mount Everest. It is also home to Valles Marineris, the third largest rift system (canyon) in the Solar System, 4,000 km long.

A Martian day is called a sol, and is a little longer than an Earth day. Mars rotates in 24 hours and 37 minutes. It rotates on a tilted axis, just like the Earth does, so it has four different seasons. Of all the planets in the Solar System, the seasons of Mars are the most Earth-like, due to their similar axial tilt. The lengths of the Martian seasons are almost twice those of Earth's, as Mars's greater distance from the Sun leads to the Martian year being almost two Earth years long.

Martian surface temperatures vary from lows of about −143 °C (−225 °F) (at the winter polar caps) to highs of up to 35 °C (95 °F) (in equatorial summer). The wide range in temperatures is due mostly to the thin atmosphere which cannot store much solar heat. The planet is also 1.52 times as far from the Sun as Earth, resulting in just 43% of the amount of sunlight.