Probing the Origin of Comets

Observations made with the High Dispersion Spectrograph (HDS) of the Subaru Telescope have, for the first time, allowed astronomers to measure the formation temperature of ammonia ice in a comet. Such data give us direct and crucial evidence of the environment in which the comet was born. In the case of Comet LINEAR (C/1999 S4) that was observed in 2000, its -245C (or -410F) temperature suggested that this comet was formed between the orbits of Saturn and Uranus in the primordial solar system nebula. On the other hand, recent observation of Comet ISON (C/2012 S1) in 2013 revealed its formation temperature to be -260C, lower than that of Comet LINEAR. This suggests we need to reconsider our picture of the environment in which ammonia was formed.