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All the scientific information gathered from the Mars landers (Curiosity, Spirit, Opportunity) indicates the presence of an atmosphere 1 Billion years after the planet Mars was created. Presence of water and air co-existed so when Mars lost its water it also lost most of its atmosphere.
Even from the first close pictures of the Mars landscape it was clear that Mars had water at one time because of the numerous aluvial fans and dried up river beds. However there are several reasons why Mars has lost its atmosphere.
1) Mars is only 11% of Earth's mass, and has a thin atmosphere (only 1% density of Eath's atmosphere).This makes Mars more susceptible to meteorite impacts blasting away its atmosphere.
2) Mars does not have a huge dynamo that produces a massive magnetic field to protect its atmosphere from solar winds. Perhaps due to its size, the core of Mars does not produce the necessary magnetic field to protect its atmosphere so it is more likely to loose air to the vacuum of space. Contrary to Mars, Earth has a huge solid iron core and a liquid iron outer core. Movement in the liquid iron outer core around a central iron solid core is thought to create an electric current that in turn produces a massive magnetic field that shields the Earth from loosing its atmosphere. This is the Earth's dynamo that protects our precious atmosphere. Without it, the Earth would have lost its atmosphere long ago.
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