Adiabatic Cooling and Thermal Balance of Gas Expanding in the Coma

This paper examines the temperature fall of coma gas as it expands away from the nucleus. Using only published input values and transparent formulae, the calculation obtains the following headline results: surface_gas_temperature_K = 200; adiabatic_index = 1.333; T_at_10km_K = 68.4. Temperature falls from two hundred kelvin at the surface scale to about sixty-eight kelvin at ten kilometres, about fifteen kelvin at one hundred kilometres, about three kelvin at one thousand kilometres and below one kelvin at ten thousand kilometres, if adiabatic cooling acted alone and collisions persisted. For the temperature fall of coma gas as it expands away from the nucleus, every input is named and every formula is stated, and the deposited script regenerates each quoted value, so the calculation can be audited or rescaled by any reader. Agreement and disagreement with independent published estimates are reported at their actual size, and the conclusions are limited to what the budget arithmetic for water vapour expanding adiabatically from a two-hundred-kelvin surface layer supports.