Is terraforming Mars impossible? The Christian Science Monitor

It looks like humanities hope of turning Mars into a second Earth may never translate into reality thanks in part to the red planet’s lack of a magnetic field. Scientists have discovered that our Sun’s solar radiation may thwart all attempts at increasing the atmospheric pressure of the crimson world, which means we may never get the chance of witnessing a green Mars, let alone a blue one. Although this means that Mars may never become a second eden (unless we can create a global magnetic field), it does not mean that humanity will never settle the planet en mass.
Future colonists will have to adapt to living within specialized biospheres (with portable magnetic shields to protect them from radiation), although doing so is probably much cheaper than terraforming the entire planet.