The theory of cooling of cometary dust by a gas outflowing from the nucleus is developed and used for explaining the anomalous distribution of sodium atom emission in the head of Comet Mrkos 1957 d. The results obtained indicate the universal character of the phenomenon of anomalous distribution of metal atom emissions,
including the Na I lines, in the heads of bright comets and the possibility of condensation of a cometary gas (Hg^2O, CO^2 etc.) in the nearnuclear region.