Cleaning systems
I think I read something a long time ago about how they discarded cleaning ideas for the rovers due to weight. Also, their planned lifetime didn't really call for one (who knew?)
Electrostatic cleaning systems would be ideal for desert locations. A water system, even a recycling one, would lose a lot of water just to evaporation.
Ammonia is a natural substance, and is used as a fertilizer. In fact, about 80 percent of all ammonia is used this way. Back in the Midwest, big pressure tanks of ammonia were everywhere.