1. 1970-2010: The Golden Age of Solar System Exploration TNOs: Four decades of observations. F. Merlin M.A. Barucci S. Fornasier D. Perna

2. The key dates: - First discovery of a Centaur: 1977 (Chiron by C.Kowal) - First discovery of cometary like activity: 1988 (Chiron by D.Tholen) - First discovery of a TNO : 1992 (1992 QB1 by D.Jewitt) - First discovery of a binary object : 2001 (1998 WW31 by C.Veillet) - First discovery of a family object: 2006 (Haumea family by K.Barkume) The discovery of the minor icy bodies in the solar system F. Merlin

3. Need to classify the thousands of objects: - With actual orbital elements + Study of the orbital stability over 10 m.y. (Gladman et al. 2008) If yes... The different dynamical groups of TNOs and the Centaurs 1) Member of a resonance? -> Resonants 2) Tj Comets 3) a Centaurs 4) a>2000 ? --> Inner Oort cloud 5) Scatters ? --> Scattered 6) e > 0.24 ? --> Detached 7) 39.4 < a < 47.8 ? Main Classical Belt F. Merlin

4. The four taxonomical classes, first evidence of variegated surface Barucci et al. 2005 Barucci et al. 2011 Presence of icy compounds on all BB members, possible observational bias? IR members seem to be depleted of ices. The bimodality of the Centaurs cannot be explained from the icy content of their surface. Among the TNOs and Centaurs, visible and near infrared colors show a quasi continuous trend

5. Are there any links between dynamic and taxonomy? BR bi-modality of the Centaurs: Due to evolution processes? - Cometary like activity: Possible - No disruptive impacts. Unprobable during the Centaur life  Seems to be the case for the small TNOs If similar causes, we can exclude cometary activity Cold and hot classical populations Very red with e<0.05 and i<5°  The atmosphereless bodies are mainly governed by the space weathering.  No disruptive collisions (collisions should occur with small velocities) should occurred and have greater effects on small bodies - compatible with dynamical models from Dell’Oro and Davies&Farinella Peixinho et al. 2012

6. The big picture Irradiation acts on every atmosphere less bodies. - destroy or modify the icy content of the surface  Dark or and red crust Refreshment processes act in different ways on the different objects - Icy compounds are revealed  Cometary activity resplenishes the whole surface (Centaurs)  Thin atmosphere redeposit (Big TNOs with volatile)  Collisions excavate internal compounds (all TNOs and Centaurs) Remote laboratory to investigate the effect of space weathering Jewitt

7. Clues for strong evolution of the surface Brunetto et al. 2006 Merlin et al. 2012 Irradiation of thin sample :  Reddens the visible reflectance  Removes the absorption bands  Generates new absorbtion bands  Reduces the visible albedo This explains the observation of dark, red and featureless objects. In details, can constrain the initial composition Population of objects covered by methanol. - Centaur, Resonant and classic - CH3OH is assumed to be primordial CH3OH might be destroyed by radiolysis  Refreshment processes in the last Gy. - Possible cometary like activity for Centaurs - Probable collisions for the TNOs

8. Unveiling the physical and chemical properties of the surface Need high quality spectroscopy to retrieve: Chemical composition, using absorption bands of icy compounds and spectral models. Detection of C 2 H 6 on Pluto in addition to N 2, CO and CH 4. Detection of ammonia hydrates on Chiron Detection of methanol on Pholus and on 2002 VE 95. Spectral modelling using a wide wavelength range is able to give constraints on the particle size. Here, the case of Pluto with sub mm and cm size particles. Merlin et al. 2010

9. Unveiling the physical and chemical properties of the surface Need high quality spectroscopy to retrieve: The surface temperature can be retrieved from the peak position of one or several absrption bands. From the peak position of the 1.65µm band of crystalline water ice (Merlin et al. 2006). From the peak position of several CH4 absortption band (Grundy et al. 2002). Spectral models can be used for icy mixtures. The dilution state of the ices can be estimated from the position of several absorption bands too. CH 4, CO, C 2 H 6 in N 2 for Pluto CH 4 on N 2 for Eris NH 3 hydrates for Charon, Orcus  Need accurate laboratory analogs to disentangle between temperature and dilution Merlin et al. 2010 Merlin et al. 2006

10. New Horizons, the future of TNOs exploration Departure Date: January 19th 2006 Arrival date at Pluto system: July 14th 2015 Instruments: Imager, spectrometer (UV, VIS, nIR) Spectral resolution: 2 angstroms (UV) up to 500 (nIR~2.20μm) NASA NASA-ESA Map the CO, CH4 and N2 species (temperature, dilution, abundancies, depth) Investigate the closest environment of Pluto (5 satellites) Follow his route toward possible new target…