As the other reply mentioned, this is a millions of years sort of problem. But there are two straightforward solutions. First, in the new atmosphere make sure

As the other reply mentioned, this is a millions of years sort of problem. But there are two straightforward solutions. First, in the new atmosphere make sure there's enough oxygen to breath so that an ozone layer forms. That's something we want to do anyways. Also, give Mars a magnetic field. That's a stupendously large but straightforward engineering problem, sort of like terraforming is in the first place but smaller.