Mars has nearly no atmosphere because the solar wind blew it away. The magnetic field acts as a shield that prevents the atmosphere from leaking out into space,

Mars has nearly no atmosphere because the solar wind blew it away. The magnetic field acts as a shield that prevents the atmosphere from leaking out into space, basically [1]. [1] http://m.esa.int/Our_Activities/Space_Science/Cluster/Earth_s_magnetic_field_provides_vital_protection http://m.esa.int/Our_Activities/Space_Science/Cluster/Earth_...