Composition evolution of ethanol, acetaldehyde, and ketene as a function of energy dose in the H⁺ radiolysis experiment of C₂H₂:H₂O mixed ice at 17 K (middle panel). Here, “time” stands for the approximate years required in dense molecular clouds to reach the same amount of energy dose. The shadow area (in background) reflects the composition ratio in previous non-energetic study obtained from Chuang et al. (2020). The selected observational data toward dense cloud Barnard 5 (left panel), protostellar shock region L1157 B1, protostar IRAS 16293-2422A, B, HH 212, B1-c, and S68N, as well as comet C/2014 Q2(Lovejoy) (right panel), are obtained from Taquet et al. (2017), Lefloch et al. (2017), Manigand et al. (2020), Jørgensen et al. (2018), Lee et al. (2019), van Gelder et al. (2020) and Biver & Bockelée-Morvan (2019) respectively, standing for an early and later stage in the star-formation process.