I want a short little aside here to talk about a little pet peeve of mine:
People talk as if Mars’ atmosphere does basically nothing to reduce the radiation dose as compared to free space. This is definitely not true, but the confusion comes from a few areas, but largely because people have not bothered to do some basic math and geometry.
1) People use the datum or even higher altitude sites to calculate the surface pressure. The pressure at the datum (the sort of average height on Mars, analogous to “sea level,” but not really) is 636 Pascals (6.36mbar http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html ). But the scale height of Mars is 11.1km. Scale height is the constant used to determine pressure given a simple exponential model of the planetary atmosphere. The lower altitude, the higher pressure, determined by this equation:
P = P0 *e^(-z/H)
Where P is the pressure at the altitude “z”, and P0 is the pressure at “zero” altitude, and H is the scale height.
So at Mars, P0 = 636Pa, H=11.1km, and the lowest point on Mars is in a corner of Hellas Basin at z=-8.2km (i.e. 8.2km below the datum), whereas pretty much all of Hellas Basin is ...