and Interior

The atmosphere and (probably) the interior of Mars differ substantially from that of the Earth. The atmosphere is much less dense and of different composition, and it is unlikely that the core is molten.

The Martian Atmosphere

The atmosphere has a pressure at the surface that is only 1/200 that of Earth. The primary component of the atmosphere is carbon dioxide (95%), with the remainder mostly nitrogen. Seasonal heating drives strong winds that can reach 100 mph or more, stirring up large dust storms. Clouds form in the atmosphere, but liquid water cannot exist at the ambient pressure and temperature of the Martian surface: water goes directly between solid and vapor phases without becoming liquid.

Variation in temperature at the Viking 1 landing site

The preceding image shows the variation of the surface temperature over a period of 50 Martian days at the Viking 1 landing site (data source). Notice the large variation between night and daytime temperatures (associated with the low density of the atmosphere) and the almost constant high and low temperatures for this period. Compare this, for example, with the daily temperature variations for Nome, Alaska (note however that the Nome plot is in degrees Fahrenheit, not Celsius).

Here is a graph of Martian atmospheric temperature variations as recorded over a period of days at the Pathfinder (1997) landing site compared with data from the Viking 1 site over a similar period in 1976 (in the these graphs a Sol is a Martian day, which corresponds to 24 hours and 37 minutes of Earth time). At the time of these observations, the night temperatures drop to around -90 degrees celsius, but at the Pathfinder site the day temperature approaches a relatively balmy -10 degrees celsius at it peak.

The Martian Interior

The density of Mars is about 25% less than that of the Earth, suggesting a proportionally larger concentration of lighter materials relative to the core. It is probably intermediate in composition between the Earth and the Moon. Though Mars is probably at least partially differentiated, there is little evidence for large-scale tectonic motion (but there is smaller scale motion such as that responsible for the Valles Marineris system). The core is thought to be iron sulphide; the absence of any detectable magnetic field even though the rotation period is comparable to that for Earth suggests that the core is probably not liquid.

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