The long-held theory that Earth's magnetic field protects its atmosphere from solar winds has some new challengers. "The controversy stems from recent observations that show Mars and Venus are losing oxygen ions from their atmospheres into space at about the same rate as Earth. This came as something of a surprise, since only Earth has a strong dipolar magnetic fieldthat can prevent solar wind particles from slamming into the upper atmosphere and directly stripping away ions. 'My opinion is that the magnetic shield hypothesis is unproven,' said Robert Strangeway from UCLA. 'There's nothing in the contemporary data to warrant invoking magnetic fields.'"