Mars is the fourth planet from the Sun, rocky body about half the size of Earth and gravity of 38% that of Earth. Volcanoes, impact craters, crustal movements and atmospheric conditions such as dust storms have altered the surface of Mars as with the other terrestrial planets - Mercury, Venus, and Earth. Mars has two small moons, Phobos and Deimos, that may be captured asteroids.
Like Earth, Mars experiences seasons due to the tilt (25.19 degrees) of its rotational axis. Its orbit is about 1.5 times farther from the Sun than Earth's and is slightly elliptical, that affects the length of Martian seasons. Its orbital period is 687 Earth days. The solar day on Mars is only slightly longer than the Earth day. There are polar ice caps, that appear to be made mainly of water, growing and receding with the seasons. Liquid water cannot exist on the surface of Mars due to low atmospheric pressure, which is about 100 times less than that on Earth. Volcanism was active more than 3 billion years ago. Some of the giant shield volcanoes are younger, having formed between 1 and 2 billion years ago. Mars has the largest volcano in the solar system, Olympus Mons with a height of nearly 22 km (16 miles). There is, as well, a spectacular equatorial canyon system, Valles Marineris. More than 4,000 km (2,500 miles) long, 200 km (120 miles) wide and up to 7 km (4.6 miles) deep the Valles Marineris system is one of the largest canyons of the Solar System.
The diameter of Mars is approximately two times smaller than that of the Earth, and its surface area is only slightly less than the total dry land area on Earth. The red-orange appearance of the Martian surface is caused by ferric oxide (Fe2O3 ), more commonly known simply as rust. Mars lost its magnetic field 4 billion years ago, thus the solar wind interacts directly with the Martian ionosphere, lowering the atmospheric density by stripping away atoms from the outer layer. However, there are multiple umbrella-shaped magnetic fields mainly in the southern hemisphere, which are remnants of the ruined global magnetic field. In northern hemisphere of Mars evidences of widespread auroras are detected below 100 km above the surface.
The atmosphere of Mars consists of about 96% carbon dioxide, 1.93% argon and 1.89% nitrogen. It is quite dusty, which gives the Martian sky a tawny color when seen from the surface. Martian surface temperatures vary from lowest of about −143 degrees C (−225 degrees F) at the winter polar caps to highest of up to 35 degrees C (95 degrees F) in equatorial summer.