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Project Progress

We opened a new chapter in understanding the chemical diversity among comets. The abundances of molecules released from ices stored in cometary nuclei have been studied predominantly within 2 astronomical units (AU) from the Sun, where sublimation by water – the most abundant volatile in the cometary nucleus – is fully activated. However, at infrared wavelengths, we are now starting to successfully probe the composition of comets that spend their entire orbital period well outside the zone of active water sublimation, by targeting hypervolatiles that can sublimate (or even drive nucleus activity) beyond 2 AU. Comet C/2006 W3 Christensen (perihelion at 3.13 AU) is one such comet, which we observed with high-resolution infrared spectroscopy in 2009-2010. During this reporting period, we extracted the production rates for three “hypervolatiles”: methane (CH₄), ethane (C₂H₆), and carbon monoxide (CO). These molecules are plausible chemical precursors of organic compounds that are of keen interest for understanding the emergence of life. Prebiotic chemical models link C₂H₆ and CH₄ as the precursors of ethylamine and methylamine respectively, while CO is linked to formaldehyde (H₂CO) and to sugars. Each of these three molecules was detected via multiple emission lines (e.g., Figure 1).

Although CO was detected in comets Hale-Bopp at 4.09 AU and in 29P/Schwassmann-Wachmann-1 at 6.26 AU (see the report by L. Paganini), the infrared detections of CH₄ and C₂H₆ at 3.25 AU from the Sun are unique. These species can be sensed ONLY through vibrational fluorescence at infrared wavelengths because pure rotational transitions are forbidden for symmetric species, precluding detections at mm or sub-mm wavelengths. The gas release rate (“production rate”) of carbon monoxide in Christensen significantly exceeded those measured in a number of Oort Cloud comets (C/2004 Q2 Machholz, C/2007 N4 Lulin, etc.) when much closer to the Sun. These drastic differences in CO production are not caused by distinct orbital histories or by the very different gas rotational temperatures – exceeding 70K in comets Q2/Machholz and N4/Lulin versus less than 20K in Christensen. Instead, they likely reflect different histories for ices frozen in the nuclei of comets Machholz, Lulin, and Christensen during the formative stages of our solar system. Such extreme differences in volatile abundances among comets encode the range of physical environments in the solar nebula favorable to synthesis of prebiotic organic matter.

During this reporting period, we successfully integrated our long-term studies of spatially resolved measurements of H₂O rotational temperature and molecular abundance (see report of 2012) with state-of-the-art 3D physical models of the inner cometary atmosphere (“coma”). We published a pioneering study of this type for comet 73P/Schwassmann-Wachmann-3B (Fougere et al. 2013). The synergy of our spectral-spatial measurements with coma kinetic models constrains the gas temperature, density, and outflow, as well as the nature of volatile release – directly from the nucleus versus from a source of icy grains in the coma. It reveals a previously unrecognized source of heating in the inner coma introduced by water released from icy grain mantles. This insight provided a giant leap in our quantitative understanding of the near-nucleus environment – critical for obtaining the most accurate abundances of water and prebiotic molecules.

We published an advanced solar fluorescence emission model for the ν4 band of mono-deuterated methane (CH₃D). The primary application of this model is to constrain the abundance ratio CH₃D/CH₄ in cometary ices, considered a key cosmogonic parameter, but presently unknown. We derived the upper limit for CH₃D/CH₄ in comet C/2007 N3 (Lulin) using our model and illustrated the ability of current infrared spectrometers to stringently test various astrochemical model predictions for CH₃D/CH₄ in the early solar system.