Mars

Mars is the fourth planet from the Sun and the second-smallest terrestrial planet in the Solar System, being larger than only Mercury. In English, Mars carries the name of the Roman god of war and is often referred to as the "Red Planet". The latter refers to the effect of the iron oxide prevalent on Mars's surface, which gives it a reddish appearance distinctive among the astronomical bodies visible to the naked eye. 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. However, it is half the size of the Earth, with a size of 6,780 kilometres.

The days and seasons are comparable to those of Earth, because the rotational period as well as the tilt of the rotational axis relative to the ecliptic plane are similar. Mars is the site of Olympus Mons, the largest volcano and highest known mountain on any planet in the Solar System, and of Valles Marineris, one of the largest canyons in the Solar System. The smooth Borealis basin in the Northern Hemisphere covers 40% of the planet and may be a giant impact feature. Mars has two moons, Phobos and Deimos, which are small and irregularly shaped. Mars has a diameter of 6,779 kilometres, which is half of the Earth, and merely a mass of 6.410 x 10^23 kg, ten times lighter than Earth.

Orbit
Mars is the fourth planet from the Sun, with a distance orbiting to the Sun of 227,940,000 kilometres. Its orbit is not parallel to the orbit of Earth, as there are considerable fluctuations between Mars and Earth. They are in closest proximity when they are in opposition. This will likely result in Sun-Earth-Mars forming a line. Mars' orbit is more elliptical than circular as there is variation in distance during opposition. During opposition at the point of greatest distance from Sun, known as aphelion, most recently on 12 February 1995, the distance between Mars and Sun is 249,830,000 kilometres (about 100 million kilometres to Earth). During the opposition at the point of shortest distance from the Sun on 7 November 2005, known as perihelion, the distance between the Sun is 197,470,000 km, about 55.8 million kilometres to Earth. As a result, Mars orbits the Sun 687 days (~2 Earth years) to complete 1 Martian year.

Rotation
A Martian day has 24 hours and 37 minutes, as how long it takes for the planet to rotate its axis. The rotational axis is tilted 24 degrees towards the orbital plane. An axial tilt, comparable to that of Earth, is 1.7 degrees (about 1.8 degrees to ecliptic), causes seasons. During a given season, certain areas are also twice as long. It takes Mars 687 Earth days (1 Martian year) to orbit the Sun.

Surface
The most noticeable characteristics of the Martian surface is reddish tint. This is caused by iron oxidation in the rocky sediment and dust, referring as "Red Planet" or "Dusty Planet". Mars has diverse landscapes, with cratered plains and mountains, with candidate of deserts, icy polar caps, huge faults, and volcanoes. The craters were created by meteorites, in which some of the craters are partially eroded because of the of the strong winds and dust, as these form the reddish tint in the Martian atmosphere. Sand storms are the most common storms in this planet. However, there are no continental plates that can collide or drift apart. The Martian crust consists of a single plate. This facilitated the formation of extremely high mountains or volcanoes. The highest mountain (or volcano), is Olympus Mons, it is the highest mountain in the solar system, with a length of 600 kilometres, and height of 22 kilometres. It is three times larger than Mount Everest, as a highest mountain on Earth. Near the equator, there is a huge plain, the Valles Marineris. It contains canyon-like ravines that are three times deeper and ten times long as the Grand Canyon. However, the maximum temperatures can rise above 10 °C. The ice on the south pole disappears almost entirely, because there are considerable amount of liquid water on Mars. The permafrost layers on Martian landscape is comparable to Earth tundras.

Moons
Mars has two moons named Phobos and Deimos, named from two companions Terror and Fear in fiction. Asaph Hall discovered these moons in 1877. Their peculiar shape earned the satellites the nickname 'potato moon'. Their appearance suggests that they have been originated in the Kuiper belt and were captured by Mars. Deimos has a diameter of 12 kilometres. Its dark rock has only a few craters and its surface was smoother than Phobos, while the mean distance to Mars is 23,500 kilometres, corresponding to a 30-hour orbital period. Phobos is the largest of the two moons in Mars (also known as Martian Moons), with a diameter of 22 kilometres, as it orbits Mars in a mere 7 hours and 40 minutes. It is literally littered with craters. The largest of these, called Stickney, has a diameter of 9 kilometres. However, it is predicted that Phobos is moving closer to the surface of Mars, it will finally hit the planet 50 to 100 million years from now.

Exploration
Mars has been explored by several uncrewed spacecraft. Mariner 4 was the first spacecraft to visit Mars; launched by NASA on 28 November 1964, it made its closest approach to the planet on 15 July 1965. The Soviet Mars 3 mission included a lander, which achieved a soft landing in December 1971; however, contact was lost seconds after touchdown. On 20 July 1976, Viking 1 performed the first successful landing on the Martian surface. On 4 July 1997, the Mars Pathfinder spacecraft landed on Mars and on 5 July released its rover, Sojourner, the first robotic rover to operate on Mars. The Mars Express orbiter, the first European Space Agency (ESA) spacecraft to visit Mars, arrived in orbit on 25 December 2003. In January 2004, NASA's Mars Exploration Rovers, named Spirit and Opportunity, both landed on Mars; Spirit operated until 22 March 2010 and Opportunity lasted until 10 June 2015. NASA landed its Curiosity rover on August 6, 2012, as a part of its Mars Science Laboratory (MSL) mission to investigate Martian climate and geology.