
Mars is the next planet out from the Sun from Earth. It is roughly half the Earth’s diameter and twice the diameter of the Moon. Its year is about twice as long as that of Earth, but its day is very similar to ours. The surface gravity is only about a third that of Earth. Its axis is tilted to the plane of its orbit slightly more than Earth’s tilt. This means that Mars, like Earth, has seasons.

Mars doesn’t come as close to Earth as Venus does. Of all the planets in the solar system it is the only one with any similarity to Earth as it does have an atmosphere. This means that the sky is not black as it is on the Moon or Mercury. However, the atmosphere on Mars is unbreathable, being mostly carbon dioxide and with less than a hundredth the atmospheric pressure on Earth.
Photographs taken on the surface of Mars by landers show a desert-like planet, with sand and rocks. Its horizons that resemble those on Earth, with the more distant features being increasingly hazy.

Mars is bitterly cold. Although on a summer day at the equator the surface temperature could reach 20º C or more, and air temperatures as high as 35º C have been recorded, the average temperature is –65º C and night temperatures are typically –100º C, even in mid latitudes.
Despite these low temperatures, Mars has occasional clouds of ice crystals high in the atmosphere. However, although snow has been observed to fall from these clouds, it turned to vapour before reaching the surface. Hoar frost has been seen in parts of the planet.

Mars has polar caps, which are largely composed of water ice like those on Earth, but with carbon dioxide ice as well. The north polar cap can disappear in northern summer but this doesn’t leave pools of water: the low atmospheric pressure means that the ice turns directly to gas. The south polar cap never completely melts in summer.
And although the atmosphere is thin, it can support dust storms. In fact these can cover the entire planet, as happened notably in 1971 when the spacecraft Mariner 9 arrived and sent back puzzlingly blank photos. But then features started to appear as the dust subsided. Rovers have often photographed dust devils whipping across Mars’s surface.

Surface features

The predominant colour on Mars is, not surprisingly, red. This is the result of ferric oxides in the dust or sand that covers large areas of the planet. This dust blows around in the thin atmosphere, with the result that surface features change over a period of time. Before the era of spacecraft, the changing patterns on the planet were interpreted as vegetation of some sort changing with the seasons. In addition, the contrast of the darker features against the reddish lighter areas made them appear greenish. But modern photographs show that Mars really is just different shades of red. And when spacecraft drill into the soil, in many areas grey rocks show through very quickly, showing that the red colour is only a veneer on the surface.
Craters
The most obvious large-scale surface features on Mars are craters of all sizes, resulting from impacts from space throughout its history. These are much more eroded than the craters on airless bodies such as the Moon, as a result of the martian atmosphere, but the fact that they are there in such numbers shows that Mars has not been subjected to plate tectonics, as on Earth where the result has been vast mountain chains, nor to global effects of water erosion.

Water features
In some areas there are clear signs of water erosion in the form of great canyons, the most notable being Valles Marineris.
Rovers have been sent to these areas, and have obtained evidence of sedimentary rocks, clays and other features that result from surface water. Present conditions on Mars rule out any permanent bodies of surface water. However, there is a lot of evidence that at some point in the quite distant past, Mars was warmer and wetter and there were lakes and channels of flowing water, and probably oceans. And there is no shortage of evidence of water ice at the poles and below the surface, particularly surrounding the poles.

Although the large bodies of water were probably in existence billions of years ago, it is possible that in some parts of Mars there was permanent surface water only a few million years ago. The orbit of Mars is currently very eccentric compared with that of Earth, and changes in the eccentricity and axial tilt will have changed the conditions of Mars over its history. But the greater distance of Mars from the Sun means that it will always have been colder than the Earth.
Volcanoes
There are several great volcanoes on Mars, together with lesser evidence of lava flows, some of which have taken place over the past 200 million years. One of the volcanoes, Olympus Mons, is 21 km high, and puts Earth’s volcanoes in the shade. Whether it is actually the largest volcano in the solar system, as is often claimed, depends upon how you define largest, because some others on Mars have a greater spread but are not as high. None of the volcanoes are currently active.

Names of features
The dark and light markings visible from Earth were given Latin names in the 19th century, as was common at the time. Many are based on classical names of earthly features. So we have such places as Syrtis Major, which was the Roman name for what is now known as the Gulf of Sidra on the Libyan coast; Amazonis and Arabia; and the more romantic Utopia Plantitia, or Plain of Paradise. Star Trek fans will know that this was the home of the shipyards where the Starship Enterprise was built in the 24th century.
The Latin tradition has been continued into the modern era as the true nature of features has become known. So for example the white spot previously known as Nix Olympica, or Olympian Snows, is now Olympus Mons, or Mount Olympus. A large region of ridges and trenches is known as Medusae Fossae, Medusa being a Greek and Roman mythological figure and fossae being Latin for trenches.
