Ethane production and release in Comet C/1995 O1 Hale-Bopp

Ethane (C2H6) was detected in Comet C/1995 O1 Hale-Bopp on 13 dates between UT 1996 September 20.3 (R-h = 3.01 AU preperihelion) and 1997 September 25 .7 (R-h = 2.83 AU post-perihelion) using high-resolution infrared spectrosc opy. Production rates and rotational temperatures were measured, and the de rived heliocentric dependence for ethane production was Q = (5.52 +/- 0.20) x 10(28) [R-h((-2.43 +/-0.13))] molecules s(-1). The spatial distribution of C2H6 molecules in the coma was consistent with all ethane being released directly from the nucleus, although the possibility that a small fraction was released as a distributed source cannot be excluded. When our derived p roduction rates for ethane, water, and acetylene (C2H2) are compared, we ob tain an average relative abundance of C2H6/H2O = (6.23 +/- 0.42) x 10(-3), and C2H6/C2H2 = 2.4 +/- 0.7. The high ethane abundance relative to acetylen e in Hale-Bopp suggests its ices were altered by radiation processing and/o r hydrogen-atom addition reactions on the surfaces of ice-mantled grains in the natal cloud. These results are not consistent with ices in Hale-Bopp o riginating in a thermally or chemically equilibrated region of the solar ne bula. (C) 2001 Academic Press.