Mars

Mars is the fourth planet from the Sun and the second-smallest planet in the solar system. It is often called the Red Planet because its surface is covered with iron-rich dust that has rusted to a reddish colour. Mars is a rocky, cold and dry world with a thin atmosphere, polar ice caps and the tallest volcano and one of the largest canyons known anywhere in the solar system. It is the most intensively explored planet after Earth.

Because Mars shows strong evidence of having once had flowing water and a thicker atmosphere, it has long been a prime target in the search for possible past life and as a candidate for future human exploration.

Orbit and seasons

Mars orbits the Sun at a greater distance than Earth, taking roughly 687 Earth days, or about 1.9 Earth years, to complete one orbit. Its rotation period is close to Earth's, at about 24.6 hours, so a Martian day is only slightly longer than ours. Mars has seasons like Earth because its axis is tilted, but they last longer and are more extreme because of the planet's elongated orbit. The planet has two small, irregularly shaped moons, Phobos and Deimos, which are thought to be captured asteroids or remnants of debris.

Surface features

Mars has a varied landscape. It contains giant shield volcanoes, including Olympus Mons, the largest known volcano in the solar system, which rises far higher than any mountain on Earth. It also has Valles Marineris, an enormous system of canyons stretching thousands of kilometres. The northern hemisphere is generally lower and smoother, while the southern hemisphere is higher and more heavily cratered. Both poles have ice caps made of water ice and frozen carbon dioxide that grow and shrink with the seasons. The surface is dry and dusty, with sand dunes and vast plains.

Atmosphere and climate

The Martian atmosphere is thin, with a surface pressure far lower than Earth's. It is composed mainly of carbon dioxide, with small amounts of nitrogen and argon. Because the atmosphere is thin, Mars cannot retain much heat, so temperatures swing widely between day and night and can drop well below freezing. Dust storms are common and can sometimes grow to cover the entire planet, lasting for weeks. Despite the cold, liquid water cannot persist on the surface today; under present conditions it would quickly freeze or evaporate.

Water and the search for life

Evidence suggests Mars was warmer and wetter billions of years ago. Orbital and rover observations have found features that look like dried river valleys, lake beds and deltas, along with minerals that typically form in water. Today water exists mainly as ice in the polar caps and below the surface, and there may be briny liquid water in some places. Because liquid water is essential for life as we know it, these findings make Mars a key target in astrobiology. So far, no definitive evidence of life has been found, but missions continue to search for signs of past or present biology.

Exploration

Mars has been visited by many robotic spacecraft, including orbiters, landers and rovers. Rovers have studied rocks and soil, taken photographs and analysed samples to understand the planet's geology and climate history. Some missions have also tested technologies relevant to future human exploration, such as producing oxygen from the atmosphere and flying a small helicopter. Plans for crewed missions are discussed by several space agencies, though the challenges of distance, radiation, life support and safe return remain significant.

Potential for human settlement

Mars is often proposed as the most plausible destination for human settlement beyond Earth, though it presents enormous difficulties. The thin atmosphere offers little protection from radiation, temperatures are extreme, and there is no breathable air. Supporters point to accessible water ice, useful resources in the soil and the possibility of producing fuel and oxygen locally. Critics stress the cost, health risks and ethical questions involved. For now, any human presence would depend on extensive infrastructure and resupply from Earth.

Common misconceptions

One myth is that Mars has visible canals built by an advanced civilisation; the "canals" seen by early observers were optical illusions and misinterpretations. Another is that Mars is close to Earth in astronomical terms; even at its nearest, it is tens of millions of kilometres away. People sometimes imagine Mars has a warm, Earth-like climate, but it is far colder and drier. Finally, the Red Planet's colour comes from iron oxide dust, not from heat or fire.

Key points