Seasons on Mars

Mars' rotational axis is inclined 25° 11' with respect to the axis of its orbit compared to 23° 27' for the Earth. So the influences which cause Earth's seasons are comparable to those on Mars. But Mars has the additional influence of an orbit eccentricity that is over five times as large as that of the Earth. This moves the planet significantly further from the sun during summer at the south pole - so the south pole is colder than the north pole. The main visible change in the appearance of Mars is the cycling of the polar caps.

Mars' Polar Caps

Mars North Pole Image acquired by Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) on March 13, 1999. The light-toned surfaces are residual water ice that remains through the summer season. The north polar cap is roughly 1100 kilometers (680 miles) across. 1999年3月13日，火星全球勘测者（MGS）火星轨道相机（MOC）拍摄的火星北极图像。图像中较亮的区域是夏季仍残留的水冰。北极冰冠的直径约为1100公里（680英里）。

Mars South Pole This is the south polar cap of Mars as it appeared to the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) on April 17, 2000. The polar cap from left to right is about 420 km (260 mi) across. 火星南极 这是2000年4月17日火星全球勘测者（MGS）火星轨道相机（MOC）观测到的火星南极帽。从左到右，极帽宽约420公里（260英里）。

Mars has bright polar caps which make dramatic changes with the seasons. In the winter season for a pole, the cap becomes much larger as carbon dioxide freezes on the surface (dry ice). This happens when the temperature drops to about 150K. The cap gradually increases in size, extending down to about latitude 50° by the beginning of spring.

During the Martian summer, the caps recede but never completely disappear. The permanent cap at the Martian north pole is formed not of dry ice, but of ordinary water ice. The residual north polar cap has been measured to be about 1000 km in diameter. Measurements from Global Surveyor indicate a thickness of about 3 km for this ice cap.

The south polar cap is quite different: it is about 350 km in size and thicker than the north cap. It is formed of dry ice with an unknown thickness of water ice. For the south cap, the temperature never gets above 150K, so the dry ice survives the "summer". The caps are different because of the eccentricity of the Martian orbit. Its eccentricity is over five times that of the Earth, and larger than all planets except Mercury and Pluto. This results in the planet being significantly further from the sun during summer at the south pole.

Atmosphere of Mars

The thin atmosphere of Mars produces an average surface pressure of only 0.007 bar or about 5 mmHg, less than 1% of Earth's atmospheric pressure. This pressure would correspond to that about 30 km above the Earth's surface. The Martian atmosphere is primarily carbon dioxide (95%) with smaller amounts of nitrogen(2%) and argon(2%). These proportions are similar to the atmosphere of Venus.

The atmospheric gases were measured by NASA's Curiosity rover in October 2012. NASA/JPL-Caltech, SAM/GSFC 2012年10月，NASA的“好奇号”漫游车测量了大气气体。NASA/JPL-Caltech，SAM/GSFC

Two types of clouds form in Mars' atmosphere: water ice clouds and high altitude dry ice clouds. The water ice clouds often form around mountains. Examples of this type of cloud were imaged by the Viking orbiters around Olympus Mons. The CO₂ clouds are in the form of thin hazes at temperatures around 150K. In addition to the clouds, there are sometimes spectacular dust storms which cover a large fraction of the planetary surface.

Dust Storms on Mars

Even though the atmosphere of Mars is very thin, less than 1% of Earth's atmospheric pressure at the surface, it has been observed to produce spectacular dust storms. These dust storms are sometimes almost global in extent. When the Mariner 9 spacecraft arrived into martian orbit on November 13, 1971, it discovered an enormous dust storm which completely obscured the surface for weeks.

Magnetic Field of Mars

Mars has no detectable global magnetic field, indicating that it has no internal dynamo like the Earth. Mars has a diameter of just over half that of the Earth. It has a density of 3.9 g/cm³ compared to 5.5g/cm³ for the Earth. This suggests that its composition is mostly silicates, but the density is high enough to suggest a small metallic core. With a rotation period similar to the Earth, 24 hours and 37 minutes, it would be expected to have some magnetic field if the metalic core were liquid, providing a rotating conductor.