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 −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 2 O + abstracting adsorbed D atoms, forming an excited H 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 3 O + 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.

Dynamic molecular oxygen production in cometary comae

“…Laboratory experiments were carried out in an ultra-high vacuum ion scattering system connected to an ion beam line, as described in detail elsewhere 41 42 . Positive ions were extracted from an inductively coupled plasma of H 2 O or D 2 O in Argon carrier gas, operated at 5 mtorr and 500 W of radio-frequency power supplied at 13.56 MHz.…”

Eley–Rideal, the Other Mechanism

“…The high D/H ratio in water found in the coma of Jupiter family comet 67P/Churyumov-Gerasimenko clouds the issue of the origin of Earth's oceanic water. L-R reactions between accelerated water ions and deuterated cometary surface analogs led to instantaneous deuterium enrichment in water scattered from the surface [37]. The instantaneous fractionation process may operate on the surface of cometary nuclei or dust grains, composed of deuterium-rich silicates and carbonaceous chondrites.…”