A New "Non-energetic" Route to Complex Organic Molecules in Astrophysical Environments: The C + H$_2$O $\rightarrow$ H$_2$CO Solid-state Reaction

The solid-state reaction C + H$_2$O $\rightarrow$ H$_2$CO was studied experimentally following the codeposition of C atoms and H$_2$O molecules at low temperatures. In spite of the reaction barrier and absence of energetic triggering, the reaction proceeds fast on the experimental timescale pointing to its quantum tunneling mechanism. This route to formaldehyde shows a new "non-energetic" pathway to complex organic and prebiotic molecules in astrophysical environments. Energetic processing of the produced ice by UV irradiation leads mainly to the destruction of H$_2$CO and the formation of CO$_2$ challenging the role of energetic processing in the synthesis of complex organic molecules under astrophysically relevant conditions.