MICROWAVE OBSERVATIONS OF HYDROGEN-SULFIDE AND SEARCHES FOR OTHER SULFUR-COMPOUNDS IN COMETS AUSTIN (1989C1) AND LEVY (1990C)

We detected hydrogen sulfide through observations of its 110-101 rotational line at 2 mm in Comets Austin (1989c1) and Levy (1990c) using the IRAM 30-m radiotelescope. In Comet Levy, the 220-211 line of H2S was also observed; the relative intensities of the two lines suggest a cold rotational distribution for H2S. We also unsuccessfully searched for other sulfur molecular species in Comet Levy: SO2, OCS, H2CS. From these observations, we derive molecular production rates using an out-of-equilibrium excitation model which takes into account collisions and vibrational radiative excitation. The infereed hydrogen sulfide production rate is about 0.002 that of water for both comets. H2S and the parent of the CS radical appear to be the main depositories of sulfur in cometary volatiles. This element seems to be depleted in cometary volatiles with respect to cosmic abundance. The presence in cometary nuclei of H2S, which is a highly volatile molecule, is a clue that comets formed and remained stored at very cold temperatures. © 1991.