Cometary nuclei are believed to contain important information on the condition of the solar nebula, but there is little observational data available on their interior structure. Our ground-based observations of NASA's Deep Impact event show that comet 9P/Tempel 1 has a surface layer consisting of small (sub-micron sized) carbonaceous grains whose thickness is several tens of cm. This suggests that comet 9P/Tempel 1 contains at several tens of cm of depth material that has not metamorphosed since this comet left the trans-Neptunian region. This further implies that many short-period comets may maintain the components they had upon leaving the trans-Neptunian region at ~ 1 m of depth from the surface even after numerous perihelion passages.