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Title: DYNAMIC DEUTERIUM ENRICHMENT IN COMETARY WATER VIA ELEY–RIDEAL REACTIONS

Abstract

The deuterium-to-hydrogen ratio (D/H) in water found in the coma of Jupiter family comet (JFC) 67P/Churyumov–Gerasimenko was reported to be (5.3 ± 0.7) × 10{sup −4}, the highest among comets and three times the value for other JFCs with an ocean-like ratio. This discrepancy suggests the diverse origins of JFCs and clouds the issue of the origin of Earth’s oceanic water. Here we demonstrate that Eley–Rideal reactions between accelerated water ions and deuterated cometary surface analogs can lead to instantaneous deuterium enrichment in water scattered from the surface. The reaction proceeds with H{sub 2}O{sup +} abstracting adsorbed D atoms, forming an excited H{sub 2}DO* state, which dissociates subsequently to produce energetic HDO. Hydronium ions are also produced readily by the abstraction of H atoms, consistent with H{sub 3}O{sup +} detection and abundance in various comets. Experiments with water isotopologs and kinematic analysis on deuterated platinum surfaces confirmed the dynamic abstraction mechanism. The instantaneous fractionation process is independent of the surface temperature and may operate on the surface of cometary nuclei or dust grains, composed of deuterium-rich silicates and carbonaceous chondrites. The requisite energetic water ions have been detected in the coma of 67P in two populations. This dynamic fractionation processmore » may temporarily increase the water D/H ratio, especially as the comet gets closer to the Sun. The magnitude of the effect depends on the water ion energy-flux and the deuterium content of the exposed cometary surfaces.« less

Authors:
;  [1]
  1. Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 (United States)
Publication Date:
OSTI Identifier:
22664001
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 835; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ATOMS; CHONDRITES; COMETS; DETECTION; DEUTERIUM; DUSTS; ENRICHMENT; FRACTIONATION; HEAVY WATER; HYDROGEN; ISOTOPE RATIO; JUPITER PLANET; MOLECULAR IONS; OXONIUM IONS; PLATINUM; SUN; SURFACES

Citation Formats

Yao, Yunxi, and Giapis, Konstantinos P., E-mail: giapis@cheme.caltech.edu. DYNAMIC DEUTERIUM ENRICHMENT IN COMETARY WATER VIA ELEY–RIDEAL REACTIONS. United States: N. p., 2017. Web. doi:10.3847/1538-4357/835/1/67.
Yao, Yunxi, & Giapis, Konstantinos P., E-mail: giapis@cheme.caltech.edu. DYNAMIC DEUTERIUM ENRICHMENT IN COMETARY WATER VIA ELEY–RIDEAL REACTIONS. United States. doi:10.3847/1538-4357/835/1/67.
Yao, Yunxi, and Giapis, Konstantinos P., E-mail: giapis@cheme.caltech.edu. Fri . "DYNAMIC DEUTERIUM ENRICHMENT IN COMETARY WATER VIA ELEY–RIDEAL REACTIONS". United States. doi:10.3847/1538-4357/835/1/67.
@article{osti_22664001,
title = {DYNAMIC DEUTERIUM ENRICHMENT IN COMETARY WATER VIA ELEY–RIDEAL REACTIONS},
author = {Yao, Yunxi and Giapis, Konstantinos P., E-mail: giapis@cheme.caltech.edu},
abstractNote = {The deuterium-to-hydrogen ratio (D/H) in water found in the coma of Jupiter family comet (JFC) 67P/Churyumov–Gerasimenko was reported to be (5.3 ± 0.7) × 10{sup −4}, the highest among comets and three times the value for other JFCs with an ocean-like ratio. This discrepancy suggests the diverse origins of JFCs and clouds the issue of the origin of Earth’s oceanic water. Here we demonstrate that Eley–Rideal reactions between accelerated water ions and deuterated cometary surface analogs can lead to instantaneous deuterium enrichment in water scattered from the surface. The reaction proceeds with H{sub 2}O{sup +} abstracting adsorbed D atoms, forming an excited H{sub 2}DO* state, which dissociates subsequently to produce energetic HDO. Hydronium ions are also produced readily by the abstraction of H atoms, consistent with H{sub 3}O{sup +} detection and abundance in various comets. Experiments with water isotopologs and kinematic analysis on deuterated platinum surfaces confirmed the dynamic abstraction mechanism. The instantaneous fractionation process is independent of the surface temperature and may operate on the surface of cometary nuclei or dust grains, composed of deuterium-rich silicates and carbonaceous chondrites. The requisite energetic water ions have been detected in the coma of 67P in two populations. This dynamic fractionation process may temporarily increase the water D/H ratio, especially as the comet gets closer to the Sun. The magnitude of the effect depends on the water ion energy-flux and the deuterium content of the exposed cometary surfaces.},
doi = {10.3847/1538-4357/835/1/67},
journal = {Astrophysical Journal},
number = 1,
volume = 835,
place = {United States},
year = {Fri Jan 20 00:00:00 EST 2017},
month = {Fri Jan 20 00:00:00 EST 2017}
}
  • The dynamics of a direct reaction between a gas phase H atom and an H atom adsorbed on a Cu surface are explored. This exothermic Eley--Rideal reaction is studied by implementing both 3D quantum and quasiclassical scattering calculations. The metal surface is assumed to be flat. The effects of substituting either or both of the H atoms with D are examined in detail for five different model potential energy surfaces. The reaction cross sections, and the translational, rotational, and vibrational state distributions of the product molecules are computed. A process is considered in which the incident atom transfers enough ofmore » its kinetic energy normal to the surface into its motion parallel to the surface and into the adsorbed particle to become trapped. These trapped ``hot`` atoms can go on to react with other adsorbed atoms, giving ``hot`` products, as in the Eley--Rideal reaction. We examine the dependence of this mechanism on isotopic substitution and incident energy, in an effort to determine how one might distinguish between the two processes. {copyright} {ital 1995} {ital American} {ital Institute} {ital of} {ital Physics}.« less
  • Quasiclassical molecular dynamics studies are made of H or D atoms incident from the gas phase onto D or H-covered Cu(111) surfaces. Two detailed model potential energy surfaces are used, both based on the results of extensive total energy calculations using the density functional method. The incident H (D) atoms can react directly to form HD via the Eley{endash}Rideal mechanism, or trap onto the surface. These trapped hot atoms can react with the adsorbates to form HD or can eventually dissipate enough energy through collisions with the adsorbates to become immobile. We also observe the formation of D{sub 2} (H{submore » 2}). Probabilities for these various processes, as well as the rotational, vibrational, and translational energy distributions of the products are computed and compared with experiment. Hot-atom pathways to product formation are shown to make significant contributions. One of the potentials gives excellent agreement with experiment, while the other is less successful. {copyright} {ital 1999 American Institute of Physics.}« less
  • CO{sub 2} is one of the most abundant components of ices in the interstellar medium; however, its formation mechanism has not been clearly identified. Here we report an experimental observation of an Eley-Rideal-type reaction on a water ice surface, where CO gas molecules react by direct collisions with surface OH radicals, made by photodissociation of H{sub 2}O molecules, to produce CO{sub 2} ice on the surface. The discovery of this source of CO{sub 2} provides a new mechanism to explain the high relative abundance of CO{sub 2} ice in space.
  • We have performed first-principles total-energy calculations of low-dimensional sections of the electronically adiabatic potential energy surface (PES) that are relevant for the Eley{endash}Rideal (ER) reaction of H atoms on a rigid Cu(111) surface. These calculations were performed within density-functional theory using a plane-wave and pseudopotential method and the generalized gradient approximation for the exchange-correlation energy. The calculated energy points for various configurations of one and two atoms on the Cu(111) surface were used to construct a model PES that can be used in ER reaction dynamics calculations. {copyright} {ital 1999 American Institute of Physics.}