Adsorption of water on olivine and the origin of Earth's water

Adsorption of water on olivine and the origin of Earth’s water Luca Vattuone1 Marco Smerieri 2, Letizia Savio, 2 Abu Md Asaduzzamann 3, Krishna Muralidharan4 Mario Rocca 1 1DIFI University of Genoa & IMEM-CNR (Italy) 2IMEM-CNR (Italy) 3School of Science, Engineering and Technology , Pennsylvania State University (USA) 4Dept. Of Material Science and Engineering & Lunar and Planetary Observatory University of Arizona (USA) It is generally agreed that it was too hot at 1 astronomical unit (AU) for hydrous minerals to be stable in the accretion disc. Boss A. P. 1998. Temperatures in protoplanetary disks. Annual Reviews of Earth and Planetary Science 26:26–53. Earth’s water is conventionally believed to have been delivered by comets or wet asteroids after the Earth formed BUT •wet asteroids and comets have elemental and isotopic properties that are inconsistent with those of the Earth Drake MJ. 2005 The Leonard medal address: origin of water in the terrestrial planets. Meteoritics Planet. Sci. 40, 519–527. •It was thus proposed that water was introduced during planet growth in the accretion disc in a form stable under high-temperature conditions. Bethell T, Bergin E. 2009 Formation and survival of water vapor in the terrestrial planet– forming region. Science 326, 1675–1677. L. Vattuone The presence of water on Earth is an enigma… Only carbonaceous chondrites contain significant amounts of water BUT Osisotopes in Earth are quite unlike any hydrous carbonaceous chondrite meteorite falling to Earth today. D/H ratio of Earth’s water not compatible with known D/H raio in comets ! & The last meteorites were anydrous (Walker et al.2002 Geochimica et Cosmochimica Acta 66:4187–4201) L. Vattuone The hypothesis of water delivering from comets / meteorites is not satisfactory… Theoretical Montecarlo calculations Indicate that : 1) A significant H2O coverage is possible 2) Such coverage can account for (more than) the amount of water present in Earth’s oceans L. Vattuone M. Drake’s idea: direct adsorption of water onto grains prior to planetary accretion L. Vattuone An experimental test: H2O adsorption on olivine 100 1 mm thick slab of natural Pakistani olivine (Fo90) cut from a single crystal parallel to {100} and polished on one side only to 1 µm smoothness H2O delivered by Supersonic Molecular Beam (H2O vapour seeded in He, E=0.30 eV) Adsorption on: non polished (model for real olivive) polished olivine Sticking probability estimated by Temperature Programmed Spectroscopy Sticking probability estimated by King & Wells method L. Vattuone et al. 2013 Phil Trans R Soc A 371: 20110585. L. Vattuone Experimental results: 300 s H2O (0.30 eV) at 138 K The peak at 165 K is due to H2O multilayer The peak at lower T is evident only on non-polished side and is due to H2O nucleation at defects After each cycle the sample is flashed to desorb the multilayer and then exposed to H2O again The amount of desorbing H2O decreases in subsequent uptakes ! S < 0.05 S0.2 Polished side Nonpolished side The surface is passivated due to H2O adsorption and desorption cycles DISSOCIATIVE ADSORPTION ! L. Vattuone Experimental results: testing H2O adsorption at high T (423 K/ 560 K) Electron spectroscopies not suitable due to the insulating nature of the surface H2O adsorption at 136 K used as probe of surface state If olivine is exposed to H2O at high T (423 K/560 K ) and then cooled down and exposed to H2O at 136 K For the polished side: TPD after the second uptake shows no desorption For the non polished side: KW shows no adsorption in the probe experiment Polished olivine Nonpolished olivine H2O adsorption MUST have occurred during the high T uptake: with S > 0.015 for the non polished side with S> 5 10-4 for the polished side L. Vattuone Hints from theoretical calculations (DFT) Vienna ab initio simulation package (VASP) within the framework of DFT and the generalized gradient approximation functional PBE . We have used the projected augmented wave as provided with the VASP package . A cut-off energy of 400 eV was chosen for the plane-wave basis. 𝐸𝑑𝑖𝑠𝑠 =𝐸ℎ𝑦𝑑 − (𝐸𝑢ℎ𝑦𝑑 + 𝐸𝑊 ) Ehyd hydrated surface energy Euhyd unhydrated surface energy EWcohesive energy of a single H2O molecule Ediss=-270 kJ/mol For the molecular (i..e non dissociative) adsorption on the hydroxylated surface: E= - 33.7 kJ/mol , less stable than condensed water (-40 kJ/mol) Thus explaining the inertness of the hydroxylated surface •Significant water uptake takes place on olivine when dosing at 138 K, leading to multilayer adsorption. •When annealing, part of the water layer desorbs and part of it must dissociate forming OH and H. Dissociation is possible only for molecules in contact with the olivine surface. •Water uptake at 560 K (i.e. inside the range expected for the accretion disk: 500-1500 K) causes the formation of a passivating layer of hydroxyls. The so-produced surface is passive with respect to low T water adsorption and stable up to at least 900 K, since annealing to this temperature does not restore the initial reactivity. A way to produce H2O from OH is required to make the accretion disk mechanism viable. Three ways can be envisaged: 1)Recombinative desorption with H given by H2: not suitable since H is weakly adsorbed on olivine (102 kJ/mol) 2) Reaction with protons provided by the solar wind (strongly exhotermic by 511 J/mol and barrierless) 3) Recombination of OH with OH to form H2O and liberate oxygen in planetary magma (also exhotermic by 66 kJ/mol) L. Vattuone Conclusions and open issues…