As recently discovered, planetary satellites exhibit a diversity
 of physical phenomena that has opened a new and active field of
 research. We propose to make a comparative study of three most
 interesting couples of objects in the outer solar system, i.e. Iapetus
 and Hyperion; Pluto and Triton; Titania and Oberon using low-resolution
 spectra of these objects in the 2.5 - 12 microns waveband. Such data
 have never been obtained because Voyager spacecrafts did not include
 the adequate instruments and because of the too low angular resolution
 of I.R.A.S. The present scientific goal is manifold:
  - a search for spectral signatures from organic and non organic
 compounds such as CO2, CO, carbonates, silicates, sulfates, etc. to
 investigate the nature of the dark material in the outer solar system;
  - a comparative study of these satellites and a comparison with
 other asteroids and planetary rings material;
  - a study of the behaviour of ices at low temperature and
 pressure, far from conditions on Earth;
  - a better understanding of satellite surface asymmetries; and
   - a preparation and complement to the future infrared data from
 the Cassini probe.