Radiolysis of N 2 -rich astrophysical ice by swift oxygen ions: implication for space weathering of outer solar system bodies.

In order to investigate the role of medium mass cosmic rays and energetic solar particles in the processing of N ₂ -rich ice on frozen moons and cold objects in the outer solar system, the bombardment of an N ₂  : H ₂ O : NH ₃  : CO ₂ (98.2 : 1.5 : 0.2 : 0.1) ice mixture at 16 K employing 15.7 MeV ¹⁶ O ⁵⁺ was performed. The changes in the ice chemistry were monitored and quantified by Fourier transformed infrared spectroscopy (FTIR). The results indicate the formation of azide radicals (N ₃ ), and nitrogen oxides, such as NO, NO ₂ , and N ₂ O, as well as the production of CO, HNCO, and OCN ^- . The effective formation and destruction cross-sections are roughly on the order of 10 ^-12 cm ² and 10 ^-13 cm ² , respectively. From laboratory molecular analyses, we estimated the destruction yields for the parent species and the formation yields for the daughter species. For N ₂ , this value was 9.8 × 10 ⁵ molecules per impact of ions, and for the most abundant new species (N ₃ ), it was 1.1 × 10 ⁵ molecules per impact of ions. From these yields, an estimation of how many species are destroyed or formed in a given timescale (10 ⁸ years) in icy bodies in the outer solar system was calculated. This work reinforces the idea that such physicochemical processes triggered by cosmic rays, solar wind, and magnetospheric particles (medium-mass ions) in nitrogen-rich ices may play an important role in the formation of molecules (including pre-biotic species precursors such as amino acids and other "CHON" molecules) in very cold astrophysical environments, such as those in the outer region of the solar system (e.g. Titan, Triton, Pluto, and other KBOs).