spin temperatures ($T_{\rm spin}$) for ammonia of 27 $\pm$ 1 K was derived. The 21P value is consistent with typical values measured in comets (see Figure \[fig:OPR\_NH3\_comets\]). 21P cannot then be distinguished from other comets based on its NH$_2$ OPR (see Figure \[fig:Tspin\_NH3\_comets\]), a possible cosmogonic indicator linked to the formation temperature of the molecule.

  NH$_{2}$ band    NH$_{2}$ OPR    NH$_{3}$ OPR    $T_{\rm spin}$(K)
  --------------- --------------- --------------- -------------------
  (0,7,0)          3.30$\pm$0.13   1.15$\pm$0.07    28$^{+5}/_{-3}$
  (0,8,0)          3.55$\pm$0.08   1.28$\pm$0.04    23$^{+2}/_{-1}$
  (0,9,0)          3.15$\pm$0.10   1.08$\pm$0.05    34$^{+8}/_{-4}$
  Average          3.38$\pm$0.06   1.19$\pm$0.03       27$\pm$1

  : Derived NH$_{2}$ and NH$_{3}$ OPRs in comet 21P[]{data-label="tab_opr"}

We would like to point out that recent laboratory experiments demonstrate that the OPR of water does not keep the memory of its formation temperature [@Hama2011; @Hama2016; @Hama_Watanabe2013]. It is likely that this is also the case for ammonia. The OPRs of cometary volatiles might have been modified by an ortho-para conversion process in the inner coma or other catalyst activities of dust crust surfaces of the nucleus rather than reflected by a formation temperature in the solar nebula 4.6 Gy ago. OPRs might be a diagnostic of the physico-chemical conditions in the inner-most coma or beneath the surface.

Dynamical evolution {#sec_dynamical}
===================

In this section we analyse 