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High-dispersion (l/dl ~ 25,000) infrared spectra of Comet C/2004 Q2(Machholz) were acquired on Nov. 28-29, 2004, and Jan. 19, 2005 (UT dates) withNIRSPEC at the Keck-2 telescope on Mauna Kea. We detected H2O, CH4, C2H2, C2H6,CO, H2CO, CH3OH, HCN, and NH3 and we conducted a sensitive search for CH3D. Wereport rotational temperatures, production rates, and mixing ratios (withrespect to H2O) at heliocentric distances of 1.49 AU (Nov. 2004) and 1.21 AU(Jan. 2005). We highlight three principal results: (1) The mixing ratios ofparent volatiles measured at 1.49 AU and 1.21 AU agree within confidencelimits, consistent with homogeneous composition in the mean volatile releasefrom the nucleus of C/2004 Q2. Notably, the relative abundance of C2H6/C2H2 issubstantially higher than those measured in other comets, while the mixingratios C2H6/H2O, CH3OH/H2O, and HCN/H2O are similar to those observed incomets, referred to as "organics-normal". (2) The spin temperature of CH4 is >35-38 K, an estimate consistent with the more robust spin temperature found forH2O. (3) We obtained a 3s upper limit of CH3D/CH4 < 0.020 (D/H < 0.005). Thislimit suggests that methane released from the nucleus of C/2004 Q2 is notdominated by a component formed in extremely cold (near 10 K) environments.Formation pathways of both interstellar and nebular origin consistent with themeasured D/H in methane are discussed. Evaluating the relative contributions ofthese pathways requires further modeling of chemistry including both gas-phaseand gas-grain processes in the natal interstellar cloud and in theprotoplanetary disk.

Comet C/2004 Q2 (MACHHOLZ): Parent Volatiles, a Search for Deuterated Methane, and Constraint on the CH4 Spin Temperature