the state of astrophysics and biology is so so awful they don't even concern themselves with the problem of blocking radiation from the sun. the real main reason earth has life on it is that magnetic field. almost everything else would arise because of the protection of DNA from being scrambled. even the changing of the atmosphere to a composition like we have now, entirely depends on algae and other tiny plants growing in water. doesn't need to be a lot of water to start with, but sunshine, radiation shielding and water. that's it.

there is not very much water on mars. it doesn't have a geomagnetic dynamo, because it's not big enough, it has an induced field but life could not be surviving on the surface, it would have to be at least several feet under the soil, it's too cold because there is not enough atmosphere to form the electrical circuit that drives precipitation, and the simple fact that CO2 is the main component of the polar ice tells you the planet is just plain baked by x and gamma rays and UV and there is probably life especially underground near volcanic vents where there is more heat and sources of energy.

it's not just because of the heat. if heat was the only factor, mars would be covered in plants. it's not so cold it couldn't have plants. the plants would just lose genetic integrity. there is only one organism i know of that has a shield on it that could survive being outside of the protective atmosphere, that's fungal spores, many of them have very dense high carbon shells on them that block radiation enough to allow the DNA inside to survive. macroscopic forms that can survive constant ionizing radiation have very few parameters where they can absorb other forms of energy at the same time as blocking those damaging energies.

as well as all this, the heat is a factor. earth is just at a level where it spends around 20% of the time not frozen over. it's one of the most frustrating things about the gLoBaL wArMiNg bullshit that actually, the biggest danger is that it gets too cold. it was almost too cold 10000 years ago. the geological record and ice cores show that the planet has been mostly a popsicle for much of its existence. carbon dioxide actually helps a lot, as does water vapor and methane, to retain enough heat during winter that everything doesn't ice over. in actual fact, human population and matching increased CO2 and methane output would dramatically increase the fertility of the planet.