Temperature and seasons

The two Viking landers functioned as weather stations for two full Martian years. Their data, together with information from the orbiters, has given us a good picture of the weather on Mars.

Mars has a greater average distance from the Sun than does Earth, and because of this it has a lower average surface temperature (-60°C). At perihelion, Mars receives about 45 per cent more solar radiation than at aphelion. As a result there is a large variation in surface temperatures during the Martian year. The lowest temperatures, of -125°C, occur in winter at the south pole; this temperature is the freezing point of carbon dioxide. The highest temperatures, of around 22°C, occur during summer in southern mid-latitudes. The large difference between equatorial temperatures and polar temperatures produces brisk westerly winds and low pressure systems, similar to cyclonic systems on Earth.

Mars is tilted on its axis at 25.2°, which is similar to Earth's tilt of 23.5°, and so it experiences four seasons. Each season lasts about twice as long as Earth's because Mars's orbit is much larger.

In Mars's northern hemisphere, spring and summer are characterised by a clear atmosphere with little dust. White clouds may be seen at sunrise near the horizon and at higher elevations. During winter, falling temperatures around the northern polar ice-cap cause carbon dioxide from the atmosphere to condense to renew the ice-cap. The carbon dioxide ice comes and goes with the seasons, but a permanent ice-cap of water ice remains.

The southern hemisphere summer occurs when Mars is closest to the Sun, and so southern summers are hotter than northern summers and winters are colder. During summer the southern polar ice-cap shrinks but a core of water ice remains.

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