Following the study, which was published in the Advancing Earth and Science Journal, the researchers found that it would not possible for humans to stay on Mars long-term.
They found that human exposure to radiation threats, including particle radiation from the Sun, distant stars, and galaxies, would exceed safe levels after four years on the planet.
Is this on the surface, or below ground? I thought it was a common assumption that until we get a magnetic shield up and do a bit of terraforming, any long-term habitats would have to be subsurface.
The article was disappointingly lacking, but would radiation still not penetrate Mars crust to a certain degree? Having the terraforming capability and magnetic shield in place within a 4 year period per individual seems to be the constraints implied.
It does, but a thick layer of matter is actually a pretty good radiation shield. Material is rated based on its “halving distance” - how many centimeters of stuff it takes for the radiation passing through it to be reduced by half. It never quite blocks all of it, but if you keep piling on additional halvings you can get the radiation levels down as small as you want.
This article has a table of values for how well various types of material blocks gamma rays, for example. Sand has a halving distance of 2.9 inches, and solid stone is 2.2 inches, so a couple of feet thickness will provide thousand-fold reduction in radiation.
Other kinds of radiation penetrate heavy elements better, but those kinds of radiation actually get blocked by light elements instead, such as the hydrogen in water. Mars has a relative abundance of water so you can incorporate that into your shielding too.
