Important Mars news

Mars is safe from radiation – but the trip there isn't - space - 21 November 2012 - New Scientist

I've never been 100% sure on this point - just how safe would the Martian surface be for astronauts from a radiation point of view?   Now it seems the answer is a bit clearer:

You needn't fry on Mars. Readings from NASA's Curiosity rover suggest radiation levels on the Red Planet are about the same as those in low Earth orbit, where astronauts hang out for months on the International Space Station. A Mars visit would still be dangerous though, due to the years-long return trip.

Unlike Earth, Mars has no magnetosphere shielding it from solar and galactic radiation. But it does have a thin atmosphere, and readings from two of Curiosity's instruments suggest this provides some protection.

"This is the first ever measurement of the radiation environment on any planet other than Earth," Curiosity team member Don Hassler said at a press briefing on 15 November. "Astronauts can live in this environment."

The overall picture is still not rosy, though:

The biggest threat to Mars voyagers would be the cumulative radiation exposure during the long trip. NASA estimates that a return human mission to Mars would take three years. During that time astronauts might receive more than seven times the radiation dose they get during six months on the ISS.

And:

Solar flares would also be a problem. On Earth these eruptions of charged particles from the sun are largely deflected by the magnetosphere. But Mars enjoys no such protection, and since Curiosity has yet to see a flare, it is unclear how much shielding the thin atmosphere would provide. '

Dartnell suggests that a base or colony on Mars could be built underground to avoid surface radiation. Or, with enough advance warning, astronauts could retreat to protective shelters during a flare. But is all that trouble worth it just to send humans where robots already thrive?

 As I have argued before, if you're going to have to live underground on Mars, in an atmosphere that is barely there, why would you bother travelling so far when you could do the same on the Moon, and always be just two days away from seeing a Broadway show?

I think water is the key difference, and if it is on the Moon in any useful quantities, I'm just not sure that Mars is worth it.