The volatile composition of 81P/Wild 2 from ground-based high-resolution infrared spectroscopy

• We measured abundances of volatile molecules in Comet 81P/Wild 2.
• Abundances of species relative to H2O are typical excepting CH3OH which is depleted.
• Parent volatile chemistry more similar to carbon-chain typical comets.
• No evidence of a change in gas productivity from the 1997 to the 2010 apparition.
• Lines from water hot-band near 2.64 μm detected for first time in a comet.

Volatile abundances in Jupiter-family Comet 81P/Wild 2 were measured on four dates in February and March 2010 using high-dispersion (λΔλ ∼ 2.5 × 104) infrared spectroscopy with NIRSPEC at the W.M. Keck Observatory. H2O was detected on all dates, including measurements on UT March 29 of lines from the ν2 + ν3 − ν2 hot-band not previously targeted in comets. C2H6 and HCN were detected on three dates, and CH3OH was detected on one date. Tentative detections or upper-limits are reported for CH3OH on other dates, as well as for C2H2, H2CO, and NH3. Abundances of all species relative to H2O are in the typical range with the exception of CH3OH, which is depleted compared to other comets. Gas production was significantly higher in late February than in late March. Rotational temperatures were determined for H2O on UT February 22 and 23 and found to be about 30–40 K. The spatial distributions of H2O, C2H6, and CH3OH are all symmetric and similar to the spatial distribution of the dust continuum. H2O abundances from this work are compared to other measurements from both the 1997 and 2010 apparitions. There is no clear evidence of a change in overall gas productivity between the two apparitions within measurement accuracy. Abundances of C2H2, C2H6, HCN and NH3 are consistent with these species being the primary parents of C2, CN, NH and NH2 as measured at optical wavelengths. Although optically classified as carbon-chain depleted, Wild 2 appears more chemically similar in parent volatile chemistry to carbon-chain typical comets; however, we note that in the small sample of Jupiter-family comets measured to date, each comet is chemically distinct.