Context. The synthesis of water is one necessary step in the origin and development of life. It is believed that pristine water is formed and grows on the surface of icy dust grains in dark interstellar clouds. Until now, there has been no experimental evidence whether this scenario is feasible or not on an astrophysically relevant template and by hydrogen and oxygen atom reactions. Aims. We present here the first experimental evidence of water synthesis by such a process on a realistic grain surface analogue in dense clouds, i.e., amorphous water ice. Methods. Atomic beams of oxygen and deuterium are aimed at a porous water ice substrate (H2O) held at 10 K. Products are analyzed by the temperature-programmed desorption technique. Results. We observe production of HDO and D2O, indicating that water is formed under conditions of the dense interstellar medium from hydrogen and oxygen atoms. This experiment opens up the field of a little explored complex chemistry that could occur on dust grains, believed to be the site where key processes lead to the molecular diversity and complexity observed in the Universe.
Deep Dive into Experimental evidence for water formation on interstellar dust grains by hydrogen and oxygen atoms.
Context. The synthesis of water is one necessary step in the origin and development of life. It is believed that pristine water is formed and grows on the surface of icy dust grains in dark interstellar clouds. Until now, there has been no experimental evidence whether this scenario is feasible or not on an astrophysically relevant template and by hydrogen and oxygen atom reactions. Aims. We present here the first experimental evidence of water synthesis by such a process on a realistic grain surface analogue in dense clouds, i.e., amorphous water ice. Methods. Atomic beams of oxygen and deuterium are aimed at a porous water ice substrate (H2O) held at 10 K. Products are analyzed by the temperature-programmed desorption technique. Results. We observe production of HDO and D2O, indicating that water is formed under conditions of the dense interstellar medium from hydrogen and oxygen atoms. This experiment opens up the field of a little explored complex chemistry that could occur o
Water, the spring of life (Brack 2002), is the most abundant molecule in biological systems, and it is almost certainly of extraterrestrial origin. Water has been detected, in gaseous or solid form, in numerous astrophysical environments such as planets, comets, interstellar clouds and star forming regions where strong maser emission can be also observed (Ehrenfreund et al. 2003;Dartois 2005).
Amorphous water ice was directly detected in dark interstellar clouds through infra-red absorption (Leger et al. 1979). During the formation of stars deep inside molecular clouds, gas and dust become part of the infalling material feeding the central object. Part of this gas and dust grains, covered with icy mantles (mainly composed of water), ends up in the rotating disks surrounding young stars and forms the basic material from which icy planetesimals and later planets, together with comets in the external regions, are formed (van Dishoeck 2004). While the means of delivery of water to Earth remain a subject of debate (Morbidelli et al. 2000), the synthesis of water in the Universe is a fundamental link in establishing our origins. Water molecule formation in the gas phase is not efficient enough to reproduce the observed abundances in dark clouds, especially in its solid form (Parise et al. 2005;Ceccarelli et al. 2007). Therefore water ice must form directly on the cold interstellar grains and not as a condensate after being formed in the gas phase.
A complete review of the processes involved both in the gas and solid phase has been recently published (Tielens 2005). It was suggested many years ago that interstellar dust grains act as catalysts (Oort & van de Hulst 1946;van de Hulst 1946). Starting from simple atoms or molecules such as H, O, C, N, CO, grains are believed to be chemical nanofactories on which more complex molecules are synthesized leading eventually to prebiotic species produced concurrently by surface reactions and by UV photons and cosmic rays irradiation, as already shown long ago (Hagen et al. 1979;Pirronello et al. 1982). The most volatile species may be released in the gas phase upon formation (Garrod et al. 2007), while the refractory ones remain on the grain surface, building up a so called “dirty icy mantle”, and at least partially may be sputtered by the heavy component of cosmic rays (Johnson et al. 1991). Such mantles, having a typical thickness of a hundred monolayers, are mainly composed of water, the most abundant solid phase species in the Universe.
Under dark cloud conditions, except for the very first monolayer that has to grow on bare silicate or carbonaceous grains (Papoular 2005), most water molecules should be subsequently synthesized on a surface mainly composed of water.
Chemical models including water formation on grain surfaces were proposed years ago by (Tielens & Hagen 1982). They suggested that H 2 O formation would be initiated by H-atoms reacting with O, O 2 and O 3 , although the O 3 +H pathway was considered the most effective and O 2 would play more a catalytic role. Recent Monte Carlo simulations (Cuppen and Herbst 2007) show that while the main route to water formation on cosmic dust grains in diffuse and translucent clouds is the reaction H + OH, in dense clouds the principal source of H 2 O is the reaction between H 2 and OH. This study also emphasizes the non-negligible contribution from the H + H 2 O 2 reaction (H 2 O 2 being a product of the H + O 2 pathway) and the unusual high abundance of reactants such as OH and O 3 . Interestingly, another code by (author?) (Parise 2004) proposed a water formation scheme where O 3 molecules react with H-or D-atoms to form OH or OD, and subsequently the reaction H 2 + OH/OD leads to H 2 O/HDO. It should be noted that this scheme was in part also constrained by the observed abundances of deuterated species.
In previous laboratory works, (Hiraoka et al. 1998) succeeded in producing water molecules from the reaction of H-and O-atoms initially trapped in a N 2 O matrix. Very recently (Miyauchi et al. 2008) investigated the reaction between cold H-atoms and an O 2 ice at 10 K and demonstrated the production of H 2 O 2 and H 2 O molecules and estimated the efficiency of the reactions. (Ioppolo et al. 2008) did a similar experiment but with varying O 2 substrate temperatures. They confirmed the production of H 2 O 2 and H 2 O, made an estimate of the reactions efficiency and also drew conclusions upon the temperature dependence of the amount of species produced. These two experiments dealt with the H 2 O production pathway in which O 2 is the species consumed to produce water as shown in preliminary experiments by our group (Momeni et al. 2007;Matar et al. 2008) In the present study, the formation of water is studied for the first time using hydrogen and oxygen atoms interacting on the surface of an amorphous solid water (ASW) ice film, hence under conditions that are much more relevant to the interstellar medium. The aim
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