Microwave propagation experiments performed with Voyager 2 at Neptune revealed sharp layers of electrons in Neptune's lower ionosphere with densities of ∼10 4 per cubic centimeter. These layers are reminiscent of the sporadic-E layers in the Earth's ionosphere, and when taken together with data from the other giant planets, these data confirm the importance of the magnetic field in layer formation. A photochemical model that incorporates species produced by meteoroid ablation predicts that singly ionized magnesium is the most likely metal to be found in the layers, although laboratory data on the kinetics of metallic atoms and ions in a reducing environment are lacking. The metal chemistry discussed here is directly relevant to the abundant metals observed at the impact site of the G fragment of comet Shoemaker Levy 9 on Jupiter.