  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.
