It's a good question. It seems there are meany sources, including production on earth[1] and in space. It seems a lot of commonly available chemicals in space

It's a good question. It seems there are meany sources, including production on earth[1] and in space. It seems a lot of commonly available chemicals in space if hit with light or electricity will generate them. The open question has been what does it take to generate RNA/DNA. There has been suggestion that RNA would be unlikely to form in open, active water like found on early Earth. However, Mars would have had shallow oceans with large amount of shore area for chemicals to combine in quiet. The Panspermia theory is that RNA/DNA formed there and was ejected by asteroid hits to Earth. Support for that theory is that life appears to have formed within a few hundred million years of it being possible on Earth, which seems far too short of a time for RNA/DNA to form spontaneously in an ocean Earth. If this is all true, life may be a fluke and requiring a rather stable set of environments to work. All of this is hard to know with a sample size of one, though. [1] https://en.wikipedia.org/wiki/Miller%E2%80%93Urey_experiment https://en.wikipedia.org/wiki/Miller%E2%80%93Urey_experiment