DEUTERATED WATER IN COMET C-1996 B2 (HYAKUTAKE) AND ITS IMPLICATIONS FOR THE ORIGIN OF COMETS

The close approach to the Earth of Comet C/1996 B2 (Hyakutake) in Marc h 1996 allowed searches for minor volatile species outgassed from the nucleus. We report the detection of deuterated water (HDO) through its 1(01)-0(00) rotational transition at 464.925 GHz using the Caltech Su bmillimeter Observatory. We also present negative results of a sensiti ve search for the J(5-4) line of deuterated hydrogen cyanide (DCN) at 362.046 GHz. Simultaneous observations of two rotational lines of meth anol together with HDO in the same spectrum allow us to determine the average gas temperature within the telescope beam to be 69 +/- 10 K. W e are thus able to constrain the excitation conditions in the inner co ma and determine reliably the HDO production rate as (1.20 +/- 0.28) x 10(26) s(-1) on March 23-24, 1996. Available IR, UV, and radio measur ements led to a water production rate of (2.1 +/- 0.5) x 10(29) s(-1) at the time of our HDO observations. The resulting D/H ratio in cometa ry water is thus (29 +/- 10) x 10(-5), in good agreement with the valu es of(30.8-(+3.8)(5.3)) x 10(-5) (H. Balsiger et al., 1995, J, Geophys . Res. 100, 5827-5834). and (31.6 +/- 3.4) x 10(-5) (P. Eberhard et al ., 1995, Astron. Astrophys. 302, 301-316) determined in Comet P/Halley from in situ ion mass spectra, The inferred 3 sigma upper limit for t he D/H ratio in HCN is 1%. Deuterium abundance is a key parameter for studying the origin and the early evolution of the Solar System and of its individual bodies. Our HDO measurement confirms that, in cometary water, deuterium is enriched by a factor of at least 10 relative to t he protosolar ratio, namely the D/H ratio in H-2 in the primitive Sola r Nebula which formed from the collapse of the protosolar cloud. This indicates that cometary water has preserved a major part of the high D /H ratio acquired in this protosolar cloud through ion-molecule isotop ic exchanges or grain-surface reactions and was not re-equilibrated wi th Hz in the Solar Nebula. However, there are strong presumptions that interstellar grains were partly mixed in the early nebula prior to co metary formation with water reprocessed in the warm inner part of the nebula and transported by turbulent diffusion. Scenarios of formation of comets consistent with these results are discussed. (C) 1998 Academ ic Press.