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Astronomers in the 19th century inferred that Mars has an atmosphere because they could see white clouds developing at different regions and during different seasons, and they could see giant dust storms.
When the Viking landers went to the Martian surface, they analyzed the composition of the atmosphere and measured the temperature and pressure with height. Like on Earth and Venus, there is a troposphere and a stratosphere. Because of the thin atmosphere and the temperature fluctuations with location, season and daytime/nighttime, the troposphere can be 10-20 km high during daytime to non-existent in nighttime. For example, at the Viking landsite, during noon and summer daylight, the ground temperature was about freezing, but the air temperature could reach -30 C, creating convection patterns and a troposphere. At night, both ground and air temperatures dropped to -100 C, therefore convection doesn't take place and the troposphere disappears.Interior
Models of the interior of Mars lead to two possibilities that at present cannot be tested from Earth. One is that it has a relatively small, dense iron core, and the second is that it has a larger core consisting of a mixture of iron, iron sulfide and magnetite. The Martian mantle probably has the same density as Earth's and is made up of iron-magnesium silicate, iron oxide and some water.Magnetic Field
No spacecraft has detected a magnetic field from Mars. This may be due to the fact that, like our Moon, Mars is small enough to have cooled off from its formation and no longer has a molten core. Mercury is also small, but denser than Mars, and may therefore have a large iron core that produces its magnetic field.The Polar Caps
The north and south poles of Mars are covered with solid ice caps, much like Earth. The size of the caps vary seasonally as Mars orbits the sun. The regions of the caps that wane and wax with the seasons are called seasonal polar caps, and are composed of dry ice (frozen CO2). The permanent ice is called the residual solar caps.
During the Martian winter, the seasonal caps extend toward lower latitudes, then they shrink during spring at about 20 km/day and are smallest during early summer. Whenever the temperature falls below the freezing point of CO2 (150 K), CO2 in the atmosphere is available to condense on the surface.
The northern and southern residual polar caps are different. The southern one is about 350 km across with a surface temperatureof 150 K (the freezing point of CO2). Water ice is also present. The northern permanent cap is 1000 km in diameter. Measured surface temperatures are approximately 200 K, indicating that the majority of the ice cap is water ice. Why does the southern residual cap stay colder than the northern one, since the summers are hotter in the southern hemisphere? It may be because the dust storms happen during southern summer, at the time when the north cap is forming. Thus, dust mixes with the north cap, makes it darker and able to absorb more sunlight which heats the CO2 and allows it to escape into the atmosphere.Water on Mars, Past and Present
Most of the water in Mars is the form of ice in the polar caps and in permafrost in the crust for latitudes greater than 40°. The permafrost layer gets thicker with latitude, until it emerges from the crust at higher latitudes and forms the caps.
Small amounts of water vapor are present Mars's atmosphere (only about 3%). However, the relative humidity of the atmosphere is very high, as it holds as much water as it can, given its temperature and pressure conditions. This allows the formation of water clouds, but not of rain.
Mars has no liquid water at present, but the channels on its surface point to the existence of running water in the past. Two types of channels are observed: those formed by rivers that dried, including their tributaries, and those formed by catastrophic water runoffs. The first type are called runoff channels and are only found in the cratered uplands of the southern hemisphere. By counting the craters, astronomers can date the time of formation of these channels to 4 billion years ago. The second type are called outflow channels and are found only near the equator, running from the southern uplands into the northern plains. These were formed 3.5 billion years ago.
With liquid water, a warmer climate and a thicker atmospheret, it is very exciting to think about the possibility of past life existing on Mars. On Earth, the earliest evidence of life dates back to 3.9 billion years. However, if life existed on Mars, liquid water was only present durning a short period of time and therefore life would only be able to evolve to initial stages.The Surface of Mars
NASA Martian rovers show a surface littered with rock, boulders, gravel, sand, and silt. Many of these rocks originated from volcanoes, the largest being Olympus Mons, which measures 600 km at the base and rises 90,000 feet, two and a half times higher than Mt. Everest! Its area at the base compares to the area of the state of Missouri. The volcanoes formed about a billion years ago. Such large volcanoes require a crust thicker than the crust on Earth, which supports much smaller volcanoes. The thickness of the Martian crust is estimated to be 120 km thick, about twice as thick as Earth's.
A large topographic feature on Mars' surface is the Tharsis bulge, rising about 10 km and located on the equatorial region. This is a volcanic region as large as North America. It formed between 3 and 2 billion years ago from the uplifting and bending of the crust by tectonic forces, as indicated by large fractures on the crust pointing away from the center of the Tharsis bulge.Bibliography
Moons and Planets, W. Hartmann, 1993, 3rd edition, Wadsworth Publishing Co.