In 1965, the Mariner 4 probe discovered that Mars had no global magnetic field that would protect the planet from potentially life-threatening cosmic radiation and solar radiation; observations made in the late 1990s by the Mars Global Surveyor confirmed this discovery.
Mariner 4 je 1965. godine otkrio da Mars nema magnetosferu koja bi ga štitila od kozmičkih i sunčevih zraka opasnih po život; promatranja koje je Mars Global Surveyor proveo krajem 1990-ih potvrdila su to otkriće.