DETECTION OF CO CAMERON BAND EMISSION IN COMET P/HARTLEY-2 (1991 XV) WITH THE HUBBLE-SPACE-TELESCOPE

Ultraviolet (UV) spectra of comet P/Hartley 2 (1991 XV) taken with the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST) in 1991 September reveal several bands of the Cameron system of CO (a(3)II-X(1) Sigma). These bands are most likely due to ''prompt'' emission from CO, and, thus, provide a direct tracer of the CO2 abundance in the nucleus. Since CO2 has no allowed radio transitions, does not fluoresce in the UV or optical, and cannot be observed in the infrared except from above Earth's atmosphere, the detection of Cameron band emission is a potentially important new technique for measuring the relative CO, abundance in comets. Photodissociative excitation of CO2 is probably the largest contributor to the Cameron band emission, but significant contributions from electron impact excitation of CO, electron impact dissociation of CO2, and dissociative recombination of CO2+, are also possible. Using our estimate that photodissociative excitation is responsible for similar to 60% of the total excitation of the Cameron system, we derive Q(CO2) similar to 2.6 x 10(27) molecules s(-1), which implies CO2/H2O similar to 4%. If all of the Cameron band emission is due to photodissociative excitation, then CO2/H2O = 7 +/- 2%. For the largest possible contributions from the other excitation mechanisms considered, the CO, abundance could be as small as similar to 2-3%. We did not detect CO Fourth Positive Group emission in our data and derive an upper limit of CO/H2O less than or equal to 1% (3 sigma) for CO coming directly from the nucleus. Comparison of the relative CO2 and CO abundances in P/Hartley 2 to those in P/Halley (CO2/H2O similar to 3%-4%, CO/H2O similar to 4% for the nucleus source) indicates that selective devolatilization of the nucleus may have occurred for P/Hartley 2. A relatively large CO2/CO ratio (i.e., greater than or similar to 1) seems to be a common property of cometary nuclei. Since gas phase chemistry, in either the solar nebula or the interstellar medium (ISM), appears incapable of producing large relative CO2 abundances, the CO2 in cometary nuclei is probably produced either by UV and/or cosmic ray irradiation of ISM grains prior to the formation of the Solar System, or by condensation fractionation in the solar nebula.