Decoding the domino: The dark side of Iapetus

We present new spectra of the leading and trailing hemispheres of Iapetus f rom 2.4 to 3.8 mum, We have combined the leading hemisphere spectra with pr evious observations by others to construct a composite spectrum of the dark side (leading) hemisphere from 0.3 to 3.8 mum. We review attempts to deduc e the composition of the dark material from previously available spectropho tometry, None of them (numbering more than 20 million!) leads to a syntheti c spectrum that matches the new data, An intimate mixture of wafer ice, amo rphous carbon, and a nitrogen-rich organic compound (modeled here as Triton tholin) can ht the entire composite dark side spect rum. Observations in t his spectral region have not revealed this mix of material on any other obj ect observed thus far, We propose that this dark material may have originat ed on Titan, where atmospheric photochemistry has been producing nitrogen-r ich organic compounds for 4.5 GY. (C) 2001 Academic Press.