Chemical and physical properties of the variegated Pluto and Charon surfaces

We present new photometric and spectroscopic observations of the Pluto-Charon system carried out at the VLT-ESO (Chile) with two 8-m telescopes equipped with the FORS2, ISAAC and SINFONI instruments. The spectra were obtained in the 0.6-2.45 μm range with a spectral resolution from 300 to 1500. The SINFONI data were obtained using adaptive optics, allowing a complete separation of the two bodies. We derive both objects' magnitudes in the near infrared and convert them into albedo values. These first near infrared photometric data allow to adjust the different parts of Pluto's spectrum, provided by the three instruments. We run spectral models in order to give chemical and physical constraints on the surface of Pluto and Charon. We discuss the dilution properties of the methane ice and its implications on Pluto's surface. The heterogeneities of the pure and diluted methane ice on Pluto's surface is also investigated. The high signal-to-noise level of the data and our analyses may support the presence of ethane ice on the surface of Pluto, which is one of the main products of the methane irradiation and photolysis. The analyses of the spectra of Charon suggest that the water ice is almost completely in its crystalline form and that the ammonia compound is hydrated on the surface of this satellite.