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Search for Water Vapour Emission from DAWN target (1) Ceres with HERSCHEL

Search for Water Vapour Emission from DAWN target (1) Ceres with HERSCHEL.

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Search for Water Vapour Emission from DAWN target (1) Ceres with HERSCHEL

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  1. Search for Water Vapour Emission from DAWN target (1) Ceres with HERSCHEL M. Küppers1, L. O’Rourke1, S. Lee2, D. Bockelée-Morvan3, P. van Allmen2, J. Crovisier3, B. Carry1, D. Teyssier1, R. Vavrek1, T. G. Müller4, M. A. Barucci3, B. G. González García1 and the MACH-11 team 1European Space Astronomy Centre, European Space Agency, Villafranca del Castillo, Spain 2JPL, Pasadena, USA 3Observatoire de Paris, Meudon, France 4Max-Planck Institut für Extraterrestrische Physik (MPE), Garching, Germany

  2. 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. 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. 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. Herschel Observations (2) • Observations with large beam: Sensitive to wide range of atmospheres/exospheres Ceres and beam HIFI beam size to scale!

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

  7. 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. Upper limits Production rate: 1025 s-1 , 1-1.5 σ

  9. Upper limits (2) Production rate: 1026 s-1 , 5-6 σ

  10. Conclusions • Water vapour around Ceres not confirmed • Upper limit ~3 – 5 x 1025 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?

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