terrestrial_planet = {
    'Mercury' : {
        'mean_radius' : 2439.7,
        'mass' : 3.3022E+23,
        'orbital_period' : 87.969
    },
    'Venus' : {
        'mean_radius' : 6051.8,
        'mass' : 4.8676E+24,
        'orbital_period' : 224.70069
    },
    'Earth' : {
        'mean_radius' : 6371.0,
        'mass' : 5.97219E+24,
        'orbital_period' : 365.25641
    },
    'Mars' : {
        'mean_radius' : 3389.5,
        'mass' : 6.4185E+23,
        'orbital_period' : 686,9200
    }
}

  print(terrestrial_planet['Mars']['mean_radius'])
