  The cometary materials are thought to be the reservoir of primitive materials
in the Solar System. The recent detection of glycine and CH$_3$NH$_2$ by the
ROSINA mass spectrometer in the coma of 67P/Churyumov-Gerasimenko suggests that
amino acids and their precursors may have been formed in an early evolutionary
phase of the Solar System. We investigate the evolution of cometary interior
considering the evaporation process of water followed by the concentration of
non-volatile species. We develop a Simplified Cometary Concentration Model
(SCCM) to simulate the evaporation and concentration processes on the cometary
surface.We use 67P/Churyumov-Gerasimenko as the benchmark of SCCM. We
investigate the depth of the layer where non-volatile species concentrate after
the numerous passages of perihelion after the formation of the Solar System. As
a result, the SCCM explains the observed production rates of water and
CH$_3$NH$_2$ at 100 comet years. SCCM results suggest that the non-volatile
species would concentrate at depths between 0 and 100cm of comet surface within
10 comet years. Our results also suggest that the non-volatile species would
concentrate several meters beneath the surface before it hit the early Earth.
This specific mass of non-volatile species may provide unique chemical
condition to the volcanic hot spring pools.
