Rings of Neptune
The best photos that came back from Voyager 2 showed that there are clumps of particles in the rings, but no-one can tell whether they are made of fine dust, or if there may be bigger particles in the clumps as well. All of Neptune's rings have a lot of microscopic dust but particularly the arcs of the Adam's ring. Scientists know this because they looked very bright in the photos that Voyager 2 took with the sun shining behind them.