tokens: al. 2007 ), photodesorption (Bertin et al. 2013 ), and cosmic ray induced photodesorption (Hasegawa Herbst 1993 ). These three switches are set to ‘on’, with the photodesorption yield set at 1 × 10 −4 . 2.2 Network The nautilus reaction network used in this study has been expanded to include more complex chemical species and intermediates that lead up to C 2 H 4 O 2 isomers, as well as dimethyl ether. The base network of gaseous reactions is taken from the KIDA network (Wakelam et al. 2012 , http://kida.astrophy.u-bordeaux.fr/ ). The granular reaction network is from the nautilus package, with additional thermal grain-surface reaction pathways leading to C 2 H 4 O 2 isomer precursors from Garrod Herbst ( 2006 ). Used initially in hot core models, these reactions have been included here to provide likely thermal pathways to COMs in competition with radiolysis. Other notable additions include new gas-phase reactions for acetic acid and glycolaldehyde from Skouteris et al. ( 2018 ). The full list of additional reactions is contained in the supplementary material , Tables S1–S3 (available online), with important reactions discussed in Sections 3.1 , 3.3 , and 3.2 . Species with the first letter ‘J’ are those on the surface of the ice mantle, while species with a ‘K’ lie within the bulk of the ice, which, after the simulation has completed running (10 7 yr), contains close to 100 monolayers of ice. Additional binding energies E D for the four principal molecules studied were taken from Garrod ( 2013 ) and are listed in Table 3 . The diffusion barriers ( E b ) for all granular species are 0.4 × E D for ice surface species, and 0.8 × E D for bulk species. 2.3 Radiolysis The method used for radiolysis was first developed by Shingledecker Herbst ( 2018 ). It describes four different results that follow an incoming cosmic ray or secondary electron interacting initially with a target species A, for which we use a ‘’. The subsequent results can be described by the following equations, which represent different paths to ions and suprathermal neutrals, the latter denoted with an asterisk: The first process leads to the ionization of A, while the second process describes the formation of suprathermal fragments B and C with an intermediate suprathermal A formed via dissociative recombination of the A ion. The third process describes the direct formation of an intermediate suprathermal species followed by its fragmentation into non-excited products, while the last process describes the formation of suprathermal A, which does not subsequently fragment. In order to implement both generation of suprathermal species, and fast suprathermal reactions into the rate-equation based model used, we can derive a pseudo-first-order rate coefficient by multiplying a cross-section (σ e ), in cm 2 , with a cosmic ray flux (φ) ,in cm −2 s −1 . In order to obtain a cross-section, we start with a stopping power cross-section S ( E ), which is defined through the loss of kinetic
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