Ratios of H/C and N/C in organic refractory residues produced after irradiation with 40 keV H⁺. Grey rhombus give the average elemental ratios of UV residues analysed by means of VHRMS in both positive and negative electron spry ESI modes in the range 50–400 m/z by Danger et al. (2016). The bars refer to the relative standard deviation (RSD) given by Fresneau et al. (2017): 3% for H/C and 6% for N/C values. Top x-axes give the timescale necessary for a frozen surface (50 μm thick) to accumulate the dose given in the bottom x-axes. To calculate timescales, we take into account four cases: bombardment by SEP on a surface located at 40 AU; bombardment by CR for a surface at 40 AU; bombardment by CR on a surface at 85 AU, bombardment by CR on a surface in the Local Interstellar Medium (LISM), which is beyond 100 AU. Timescales do not take into account the time needed for asurface to experience the heating necessary for the production of organic refractory materials.