Do the nuclear-spin temperatures of cometary molecules reflect the temperature of their birth place?

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The nuclear-spin temperatures of molecules such as H2O have often been observed in cometary comae, because they have been suggested to reflect the temperature of their birth place. However, laboratory investigation to support this hypothesis is not thus far reported, and the real meaning of the observed nucelar-spin temperature in cometary comae remains a topic of debate. Here I studied nuclear-spin temperature of photodesorbed H2O molecules from water ice at 8 K using 532 nm laser photo-stimulated desorption and resonance-enhanced multiphoton ionization (PSD-REMPI). The PSD-REMPI spectra of H2O molecules imply that the nuclear spin temperature is higher than 8 K. On the basis of the present and previous experimental studies, the hypothesis that the nuclear-spin temperatures of cometary molecules reflect the temperature of their birth place appears unlikely.