The reason for the high level of Oxygen in the atmosphere today is due to the billions of years worth of photosynthetic prokaryotes releasing it as a waste product. Of course, in the higher levels of the atmosphere, oxygen becomes ozone, filtering the worst of the Sun's UV radiation to levels tolerable to humans.
I was thinking of a world that humans have settled upon. In many respects, the planet is similar to Earth, but perhaps younger, and crucially, no indigenous life, much less photosynthetic lifeforms
My question is this -
In a world devoid of oxygen-producing lifeforms, is there any alternative to the creation of an ozone layer, or an alternative substance with the same effect?
Additionally, are there any methods that can be used to create an artificial ozone layer, and what is the timescale involved?
Any answers would be greatly appreciated.