Water in comets, observations and models
Abstract
Water is believed to be the main constituent of ices in cometary nuclei. Its sublimation is driving cometary activity at short distances from the Sun. It is difficult to observe directly water in comets, however, because of absorption by the earth's atmosphere. Recent breakthroughs --- in comets C/1996 B2 (Hyakutake), C/1995 O1 (Hale-Bopp), 103P/Hartley 2, and C/1999 H1 (Lee) --- were made using new-generation, high-resolution ground-based infrared spectrometers, the Infrared Space Observatory (ISO) and the Submillimeter Wave Astronomy Satellite (SWAS). The deuterated species HDO was also observed by ground-based submillimetre telescopes. The infrared fundamental and combination vibrational bands of H2O are emitted by fluorescence excited by the Sun, which can be modelled using available spectroscopic data bases. Their observation gives us access to the rotational distribution of water and its physical conditions. The ortho-to-para ratio of water is observed to be significantly smaller than 3, its high-temperature equilibrium value, but the meaning of this measurement is still controversial. The rotational lines of water were observed for the first time in comets by ISO and SWAS. The [D]/[H] ratio of cometary water is higher that the ``cosmic" value, but smaller than observed in interstellar molecular clouds. This puts stringent constraints to the formation scenarios of the Solar System.
- Publication:
-
IAU Joint Discussion
- Pub Date:
- 2000
- Bibcode:
- 2000IAUJD...1E..16C