Mars has a very tenuous atmosphere. As mentioned before, at the surface, the pressure is only 0.636 kPa, 200 times less than that of Earth. Because of the low pressure, any liquid water on the surface of water would be immediately vaporized. Mars may at one time had a more substantial atmosphere, but over time, the gas was stripped by the solar wind because of the lack of a magnetosphere. Other gases escaped because their average speed is faster than the escape velocity required to leave Mars. We do know that Mars had a thicker atmosphere in the distant past because of evidence of flowing water (see previous post).
The atmosphere of Mars is mostly carbon dioxide, much like Venus. However, that is where the similarities end. Venus' atmospheric pressure is 90 times that of Earth. Because Mars is so much thinner, it did not undergo a runaway greenhouse effect. Besides carbon dioxide, Mars' atmosphere also contains argon, nitrogren, oxygen, carbon monoxide, and trace water vapor.
Mars would have difficult time maintaining a thick atmosphere because of the lack of a magnetosphere.
The magnetosphere prevents solar wind from reaching the outer layers of the atmosphere and stripping the gas in Earth's atmosphere. The magnetosphere is created by the rotation of the molten core in Earth's interior. Our molten core is made of iron and nickel which are ferromagnetic materials, i.e. they can become magnetic. As the core rotates, it turns the Earth into a giant magnet which pushes the solar wind away from Earth. Mars' core does not rotate, so it does not have a magnetosphere. Not only does the magnetosphere keep Earth safe, but it also creates the beautiful aurora we see near the poles of Earth.
If we wanted to terraform Mars, not only would we have to find a way to thicker the atmosphere, we would also have to find someway to get the core to rotate again or some other way of creating a giant magentosphere around Mars to save the atmosphere from the solar wind.
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