Chemistry induced by energetic ions in water ice mixed with molecular nitrogen and oxygen

Abstract

Context. Several molecular species have been observed as frozen gases in cold environments such as grains in the interstellar/circumstellar medium or icy objects in the outer solar system. Because N 2 and O 2 are homonuclear, symmetric molecules are not easily observed. It is therefore relevant to find indirect methods to prove their presence from astronomical observations. Aims. Here we investigate one of the possible indirect methods, namely the formation of specific molecules by cosmic ion bombardment of ices in astrophysical environments that contain O 2 and N 2 . The observation of these molecules in astronomical environments could act as a trojan horse to detect the presence of frozen molecular oxygen and/or nitrogen. Methods. We have conducted ion bombardment experiments of frozen O 2 , H 2 O and their mixtures with N 2 at the laboratories of CIMAP-GANIL at Caen (France) and LASp at Catania (Italy). Different ions ( 13 C 2+ , Ar 2+ and H + ) and energies (30-200 keV) have been used. Results. We have found that 13 CO 2 is formed when carbon ions are implanted in ices containing H 2 O and/or O 2 . Ozone and nitrogen oxides (NO, N 2 O, NO 2 ) are formed in the studied ices containing O 2 and N 2 with different relative abundances. Conclusions. We suggest that ozone and nitrogen oxides are present and have to be searched for in some specific environments such as dense clouds in the interstellar medium and the surfaces of Pluto, Charon and Triton. Their observation could demonstrate the presence of molecular oxygen and/or nitrogen. A possible interest for the observations of atmospheres in exo-planetary objects is also discussed.

Our astrochemical heritage

“…Specifically, the enhanced abundances of 10 Be, 7 Li and 21 Ne ( [292]; [102]) can only be explained by spallation reactions of solar energetic particles with O and C atoms of the Solar Nebula. Similarly, other short-lived nuclides, 36 Cl, 53 Mg and 41 Ca, are now explained in terms of irradiation from the early Sun (e.g. [283]; [185]).…”

Solid state chemistry of nitrogen oxides – Part I: surface consumption of NO

“…30 Nitric oxide has been identified in a number of different astrophysical regions, including star forming regions, 30 dark clouds, [31][32][33] hot cores, 34 PDRs, 35 and in the nucleus of the starburst galaxy NGC 253 -an extragalactic source. 36 [38][39][40] So far no evidence has been found that NO forms in non-energetic surface reactions like, for example, N + O. Astrochemical models predict typical NO abundances in the gas-phase, with respect to molecular hydrogen, of f (NO/H 2 ) E 10 À7 -10 À6 . The actual observed abundance 41 is about a factor 10 lower: f (NO/H 2 ) E 10 À8 .…”

N2 Chemistry in Interstellar and Planetary Ices: Radiation-driven Oxidation

“…Among the conclusions drawn by dB15 (page 12) is the following: "Comparing the current results with those of Boduch et al (2012), it seems that ozone is a good indicator for the presence of molecular oxygen, while nitrogen oxides give the best indication of the presence of molecular nitrogen mixed with water." Ozone certainly is made from molecular oxygen, but other sources exist, including CO 2 .…”

“…One can safely predict that the same products will be observed from other irradiated nitrogen-rich ices, such as N 2 +O 3 , NH 3 +O 2 , and so forth. For completeness, an ion-implantation study of Boduch et al (2012) should be mentioned here. It included two IR spectra of N 2 +O 2 after irradiation by 30 keV 13 C 2+ at 15 K, but neither an initial N 2 -to-O 2 ratio nor a radiation dose was given, and so that work is not included in our Table 5, although it too reported NO, NO 2 , and N 2 O formation.…”