1. EPSC 2012, Madrid, 25 September 2012 Search for Water Vapour Emission from DAWN target (1) Ceres with HERSCHEL M. Küppers 1, L. O’Rourke 1, S. Lee 2, D. Bockelée-Morvan 3, P. van Allmen 2, J. Crovisier 3, B. Carry 1, D. Teyssier 1, R. Vavrek 1, T. G. Müller 4, M. A. Barucci 3, B. G. González García 1 and the MACH-11 team 1 European Space Astronomy Centre, European Space Agency, Villafranca del Castillo, Spain 2 JPL, Pasadena, USA 3 Observatoire de Paris, Meudon, France 4 Max-Planck Institut für Extraterrestrische Physik (MPE), Garching, Germany

2. EPSC 2012, Madrid, 25 September 2012 Introduction: Water on Ceres Some models predict an ice rich crust or mantel on Ceres A large surface fraction of water ice can be excluded The search for water vapour or its dissociation products is the most sensitive method to infer small amounts of water ice on the surface

3. EPSC 2012, Madrid, 25 September 2012 Water measurements so far Based on measurements of OH UV emission at 308 nm Observations with small slit targeted at poles Most sensitive to localized water production Post-perihelion, north polar region Pre-perihelion, south polar region Pre- perihelion, both polar regions

4. EPSC 2012, Madrid, 25 September 2012 Herschel Observations Search for water ground state line at 557 GHz with HIFI –Part of MACH 11 guaranteed time program (PI: L. O’Rourke) 1.1 hours of dual beam-switching obs. Took place on 23 Nov. 2011, at 2.94 AU pre- perihelion (close to aphelion) Second observation will take place in October 2012 at ~2.7 AU

5. EPSC 2012, Madrid, 25 September 2012 Herschel Observations (2) Observations with large beam: Sensitive to wide range of atmospheres/exospheres Ceres and beam HIFI beam size to scale!

6. EPSC 2012, Madrid, 25 September 2012 Result Ceres’ continuum was detected, but no water line Continuum level (6.83 ± 0.5 Jy) in agreement with thermal model

7. EPSC 2012, Madrid, 25 September 2012 Modelling (Cometary analogue) Main difference to cometary case: Ceres is BIG –Line expected to be seen in absorption and emission –Part of the water is expected to fall back to the surface (expansion velocity ~ escape velocity of 516 m/s) We conservatively assume that particles that fall back disappear –But no atmosphere in thermal equilibrium (no collisions) Cometary radiative transfer model (Bockelee-Morvan et al., Biver) and Monte Carlo model (S. Lee et al.) were used –Both models agree for spherical expansion at constant velocity –Monte Carlo model also used for “realistic” density distribution

8. EPSC 2012, Madrid, 25 September 2012 Upper limits Production rate: 10 25 s -1, 1-1.5 σ

9. EPSC 2012, Madrid, 25 September 2012 Upper limits (2) Production rate: 10 26 s -1, 5-6 σ

10. EPSC 2012, Madrid, 25 September 2012 Conclusions Water vapour around Ceres not confirmed – Upper limit ~3 – 5 x 10 25 s -1 If a significant fraction of Ceres crust is water ice, it must be covered by a thick dust layer –Minimum thickness centimeters to 10s of meters depending on thermal conductivity Sporadic and/or localized evaporation still a possibility Upcoming Herschel data will provide deeper search and cover lower heliocentric distance Can DAWN provide the final answer?