2012 Annual Science Report
NASA Goddard Space Flight Center Reporting | SEP 2011 – AUG 2012
Composition of Parent Volatiles in Comets
During the period covered by this progress report we observed the Oort cloud comets C/2009 (Garradd) and C/2010 G2 (Hill). We completed our comprehensive study of prebiotic molecules in comet C/2007 N3 (Lulin). We continued our multi-comet surveys of spin temperatures and searches for deuterated species. We conducted spatially-resolved measurements of water rotational temperature, column abundance, and ortho-para ratio in the inner coma of comet 103P/Hartley 2 – the target of NASA the EPOXI fly-by mission. We studied the volatile composition of another Jupiter-family comets – 21P/Giacobini-Zinner. We studied the activity of comet Christensen beyond 3 AU from the Sun.
1. The nature of volatile release in comet 103P/Hartley-2
We acquired high-resolution near-infrared spectra of H2O emission from comet 103P/Hartley-2 using NIRSPEC at Keck 2. Using long-slit spectroscopy we directly probed the inner coma of Hartley 2 via spectrally and spatially resolved measurements of rotational temperature, column abundance, and ortho-para ratio.
Contribution: These measurements are a “window” to conditions in the inner (collisionally-dominated) cometary coma. Our quantitative view of these conditions has been mostly indirect. It depends strongly on model predictions that are constrained by measurements (in situ and remote) taken only at much larger nucleocentric distances (>1000 km) and later extrapolated to infer near-nucleus conditions. However, our spatial studies of rotational temperature, abundance, and ortho-para ratio of water in comet 103P/Hartley 2 provided direct constraints on both molecular excitation and volatile release in the inner coma. These constraints are critical for accurate determination of abundances of cometary volatiles, including prebiotic molecules. The technique of measuring a spatial distribution of temperature was substantially improved compared to our earlier study of Schwassmann-Wachmann 3B (Bonev et al. 2008).
2. Unusual abundances of prebiotic molecules in comet C/2007 N3 (Lulin)
We detected nine parent volatiles (H2O, HCN, CH4, C2H2, C2H6, CH3OH, C2H2, H2CO, and NH3) and two product species (OH and NH2) in Comet C/2007 N3 (Lulin) using high-dispersion infrared spectroscopy.
Contribution: The ortho-para ratio (OPR) of water was 3.17 ± 0.12, corresponding to nuclear spin temperature Tspin > 45 K. The A:E:F ratio of spin states for CH4 corresponds to a Tspin > 60 K. Coupled with the upper limits of HDO/H2O (< 3.6 VSMOW) and CH3D/CH4 < 2.9%, we suggest that Lulin's volatiles formed under warm (> about 50 K) temperatures. In comparison with other comets we found that HCN and H2CO were depleted, C2H6 and CH4, close to current mean among observed comets, while CH3OH was substantially enriched. Such prebiotic volatile chemistry is not common based on previous observations of comets. It illustrates the importance of continuing studies of these molecules in comets (and other space environments) in order to better understand the synthesis of the more complex prebiotic compounds.
PROJECT MEMBERS:Boncho Bonev
RELATED OBJECTIVES:Objective 2.2
Outer Solar System exploration
Sources of prebiotic materials and catalysts