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.