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tokens: available for the recombination, resulting in the observed slight isotopic gradient with depth. Fig. 9 Left : D/H ratio measured in the labelled residue from a hot region (red), the bulk (green), and a cold region (blue). Black point indicates the D/H estimated from the IR ice mixture spectra prior to irradiation. Right : D/H histogram with dots corresponding to the hot (red), bulk (green), and cold (blue) D/H values. As a result, the total residue appears as a mixture between a poly-HCN component formed in the N 2 -CH 4 layers with some D due to migration and another layer formed within or at the vicinity of the D-rich ice layer. The use of a CD 4 layer was chosen to maximize the range of D/H variation in the residue. Formation of precursors at the direct vicinity of this layer induces the very high D/H ratios observed in this laboratory simulation, which are much larger than the values observed in UCAMMs or interplanetary organic matter in general. Strikingly, the spatial scale of the isotopic anomalies found in the residue is comparable to that observed in UCAMMs or IDPs ( Duprat et al. 2010 ; Messenger 2002 ). This suggests that the micro-scale isotopic heterogeneities observed in these organics may result from irradiation/condensation of thin ices layers with isotopic heterogeneities. In addition, these experiments show that energetic particle irradiation of D-rich ices preserves the bulk isotopic composition of initial ices and produces a residue which is an excellent candidate precursor compatible with the organic matter observed in UCAMMs. 5.1.2 Timescale The interaction of high energy ions from the GCR with the surface of objects orbiting at far heliocentric distances (in the Kuiper Belt or in the Oort cloud region) have been studied for more than a decade (e.g. Cooper et al. 1998 , 2003 ). These swift ions penetrate icy surfaces and deposit energy along their tracks. Ultraviolet photons may also contribute to the deposited energy, but they affect a much lower ice volume, as their penetration depth does not exceed a few to hundreds of nanometers. The dose deposited by cosmic rays at large heliocentric distances can be modelled using a functional form for the ions differential flux ( Shen et al. 2004 ). Including the respective Galactic abundance of elements and integrated over a wide energy range, Shen et al. (2004) give a mean deposited energy of about 6 × 10 −15 eV molecule −1 s −1 in ices, corresponding to an ionization rate of about 3 × 10 −17 s −1 . We irradiated our samples up to a total dose of 19 eV molecule −1 (≈4.0 × 10 23 eV cm −3 ), corresponding to about 100 million years of exposition to the GCR. This time is small compared to the solar system age and thus fully compatible with the astrophysical timescale of irradiation of small bodies after their formation. The irradiation period is therefore not time-constrained. It contrasts with the scenarios restricted to a hypothetical high irradiation episode occurring during