Near IR Spectroscopy of Iapetus : Search for Organic Matter

R. Wittemberg (DESPA, Obs. Paris-Meudon, France), A. Coustenis (DESPA, Obs. Paris-Meudon, France), B. Schmitt (L.G.G.E., Grenoble, France), J.-G. Cuby (ESO, Garching, Germany), E. Quirico (Univ. Nantes, France), S. Douté (L.G.G.E, Grenoble, France)

One of the most mysterious among the icy Saturnian satellites, Iapetus, is unique in its range of surface albedos, from 0.5 on the trailing side (typical of icy objects) to 0.02-0.04 in the inner parts of the leading hemisphere.

This albedo asymmetry is not unique in the solar system, as Titan and Hyperion present such a property, which might be indicative of a common origin.

The nature and the origin of the dark component has not been elucidated yet, but according to some models, this is possibly connected with the presence of organic material on the surface.

We have tried to detect organic lines on the leading side of Iapetus with the Infrared Spectrometer (ISIS) at the CFHT in the 1.4-1.8 micron range, where some signatures of most of the expected organic components (Polymers, tholins and other hydrocarbons) are located. The observations were made with R=2000 on 26 & 27 August 1996.

We have compared the observations with laboratory measurements made at the L.G.G.E (Grenoble, France) of C tex2html_wrap_inline12 H tex2html_wrap_inline14 , C tex2html_wrap_inline16 H tex2html_wrap_inline18 , C tex2html_wrap_inline16 H tex2html_wrap_inline22 & C tex2html_wrap_inline16 H tex2html_wrap_inline16 (Quirico and Schmitt, 1997, Icarus, in press).

No obvious identification was achieved, but we set constraints on the abundance of some organics on Iapetus' surface.