  Formamide (NH2CHO) is considered an important prebiotic molecule because of
its potential to form peptide bonds. It was recently detected in the atmosphere
of the HH 212 protostellar disk on the Solar-System scale where planets will
form. Here we have mapped it and its potential parent molecules HNCO and H2CO,
along with other molecules CH3OH and CH3CHO, in the disk atmosphere, studying
its formation mechanism. Interestingly, we find a stratified distribution of
these molecules, with the outer emission radius increasing from ~ 24 au for
NH2CHO and HNCO, to 36 au for CH3CHO, to 40 au for CH3OH, and then to 48 au for
H2CO. More importantly, we find that the increasing order of the outer emission
radius of NH2CHO, CH3OH, and H2CO is consistent with the decreasing order of
their binding energies, supporting that they are thermally desorbed from the
ice mantle on dust grains. We also find that HNCO, which has much lower binding
energy than NH2CHO, has almost the same spatial distribution, kinematics, and
temperature as NH2CHO, and is thus more likely a daughter species of desorbed
NH2CHO. On the other hand, we find that H2CO has a more extended spatial
distribution with different kinematics from NH2CHO, thus questioning whether it
can be the gas-phase parent molecule of NH2CHO.
