Mars is rusty, but how did it happen?
The origin of banded iron formations of iron oxide (rust) on Earth coincides with the onset of ecological dominance by massively oxygen-producing colonies of cyanobacteria. So despite Earth having no ozone layer back then, the ultraviolet photodessociation of water was not enough to produce large amounts of oxygen to make iron rust. This means that the even lower UV levels on early Mars (being further from the Sun) certainly were not anywhere near enough to produce even a fraction of the rust it has today. Even if we factor in longer periods of time and a brightening Sun, we still end short of how much rust Mars has today.
This means that there is, or at least have been, an oxygen source far more efficient than ultraviolet photodessociation of water on Mars. This supports the idea of life on early Mars.