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.
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.
Mars is farther from sun and so it likely formed with more volatiles than that of the Earth. While it is also smaller than the Earth, and therefore had less heat to drive geological activity, the outgassing occurred faster (remember that smaller volume means smaller ratio of volume to surface area, and smaller gravity, so it is easier for smaller planets to lose heat and gas).
The amount of atmosphere a planet retains is determined by how fast it releases internal heat, and how fast it loses gas. The faster these processes, the less dense the atmosphere.
The rate of losing gas increases with temperature and increases with decreasing planet mass. Why?: Because the smaller mass, the smaller the gravity, and the lower the escape speed. The hotter the gas, the more gas particles that have speeds above the escape speed.
Escape speed on mars is less than 1/2 that of Earth. Gas escapes more easily from Mars. See fig 22-15 for illustration of loss of planetary gases. The dots there show the escape velocity for material on each of the planets where the temperature of the top gas layer is measured. The curves plot the relation between temperature (a measure of thermal energy) and velocity for different mass molecules. Since temperature is a measure of energy, higher mass molecules have the same energy at lower velocities than lower mass molecules. Thus the lines below the dot of Mars show that heavy molecules do not escape, but that lighter molecules do.
Water is heavy enough to be kept by Mars, but it can get dissociated by UV rays into hydrogen and oxygen. The hydrogen escapes. The oxygen gets locked into the soil thus giving it the red color, when combined with metals.
Presence of 1.6 percent argon suggests a much denser atmosphere (100 times denser) in the past.
Mars seems to have only a weak magnetic field emanating from it, and this is thought to be the cause of the absence of CO2: the solar wind blew it all away after any magnetic field decay since the magnetic field is the only thing to deflect the solar wind.
The core is thought to be iron sulphide; the absence of strong magnetic fields away from the surface even though the rotation period is comparable to that for Earth suggests that the core is probably not liquid now. However, surface magnetic fields measured by Mars Global Surveyor found weak residual crust magnetic fields. Thus core was probably molten at one time.
No liquid water on Mars today but life depends on it and it is also important in planet making so we'd like to understand the history of the water.
Channels seem to indicate large amounts of liquid water, floods with flows 10,000 times that of the Mississippi and riverbeds with flows for millions of years. These are thought to be formed from water frozen as permafrost in the soil.
Riverbeds seem to be present in the older southern hemisphere near old craters, perhaps highlighting rivers lasting 10million years.
The number of craters in these regions tell us that water was probably present around 2.5-3.5 billion years ago. (The pressure was higher then and could support liquid water. This was probably the result of a greenhouse effect which trapped carbon dioxide, before it was blown off by the solar wind.)
Meteorites from Mars show that the magma from which the basalts have solidified on Mars' surface contained 1.4 percent water. This would have been enough to form a surface covering ocean 200m deep (recall for Earth oceans would be 3000m deep if spread uniformly over surface).
Can also see evidence for minerals which formed in the presence of water from Hubble telescope observations of subtle differences in surface color.
Most recently we have the Mars Rovers (latest updates are here ). As of March 2004, there are 4 key pieces of new evidence that water existed on Mars in the past. This evidence that water once at least seeped through the rock comes the Opportunity rover currently on Mars:
(1) Small round features in the Meridiani outcrop (surface rocks) near in the Meridiani plains seem to be "concretions" = mineral deposits precipitated from water.
(2) Tabular holes in the outcrop look like what happens when crystals of minerals form from liquid water in a rock. They later dissolve or erode away leaving the holes.
(3) A large amount of sulfur appear in the outcrop suggesting a plentiful sulfate salt in the rock, which is very hard to produce without water.
(4) The mineral "jarosite" is seen in the rock, which is an iron sulfate hydrate. This requires water for its formation.
There is also evidence that the outcrop rock was actually flowing in water (not just water seepage, but full-fledged-flowing water...). This evidence is from "cross bedding" seen in the rocks: layered structures of rocks that are not parallel to each other--as if the layers had been disturbed or deposited into sediments within a flowing fluid.
In short, it is now (Mar 2004) believed that the liquid water provided habitable epochs on Mars. 2.5-3.5 billion.
Differentiation Stage: Core, mantle and crust formed. No evidence for plate tectonics on Mars. Weak magnetic fields suggest core was molten at one time.
Cratering Stage: Southern hemisphere of Mars shows evidence for this cratering about 4 billion years ago.
Flooding stage: Flooding is likely due to both Lava and water. It is thought that Mars may go through climate cycles induced by slow changes in its inclination angle to the Sun. This could melt the carbon dioxide on the poles periodically, and induce a greenhouse effect which would warm the planet and initiate some surface flooding. (Something like ice ages) This might explain layered terrain at the poles of Mars.
Surface Erosion: Today surface erosion is weak, Mars seems not to be volcanically active and there is little water.