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CO abundance in comets of different dynamical classes ordered by increasing CO/H2O ratio. Dynamically new (DN) comets are those that are making their first passage through the inner solar system from the Oort Cloud. Figure after [71] including data from [34,71,72]. The boundary between CO-rich and CO-normal comets is arbitrarily defined as twice the median value, following [71]. (Online version in colour.)
