Infrared observations of the dark side of Iapetus

Broadband (J, H, K, L, and L′) and narrowband (2.8–3.8 μm) circularly variable filter photometric observations were made of the dark side of Iapetus (Eastern Elongation). The data show a 3-μm absorption feature which is too deep to be a residual water frost feature from the bright material around the poles. Laboratory simulations show that the dark material must have a strong 3-μm absorption and could be a hydrated silicate mixed with a small amount of water frost.