The compositional evolution of C/2012 S1 (ISON) from ground-based high-resolution infrared spectroscopy as part of a worldwide observing campaign

Organic Matter in Cometary Environments

“…It was first securely detected in comet 1P/Halley [ 70 , 71 ] and is routinely sampled in comets with state-of-the-art instruments, through its intrinsically strong pure rotational and ro-vibrational transitions in the mm/sub-mm and in the near-infrared, respectively. HCN is one of the main reservoirs of volatile nitrogen in comets (as opposed to the total nitrogen, which may be locked up in more refractory material such as ammonium salts [ 72 , 73 ]) and a likely parent of the fragment species CN (see Section 3.9.1 ) [ 23 , 74 , 75 ]. Long-slit near-IR observations (e.g., [ 8 ]) and spatially resolved interferometric studies [ 48 ] have indicated that the distribution of HCN is consistent with direct sublimation from the nucleus, (i.e., a parent species, see Figure 4 ).…”

“…The reason for this discrepancy is not understood. Depending on the wavelength of the study, average HCN abundances compared to H O in comets range from 0.1–0.2% [ 67 , 75 ].…”

“…More recent studies of H CO at mm wavelengths in comets indicated production from unknown parent source(s) with a scale length of 7000 km at 1 AU [ 24 , 45 , 87 ], and sub-mm ALMA observations of the inner coma indicate a parent scale length of 1000–5000 km [ 48 , 49 ]. On the other hand, NIR observations, which are most sensitive to the near-nucleus region, have indicated that H CO can also be released directly from the nucleus [ 75 , 88 ], suggesting that both nucleus and coma sources can contribute to H CO production in comets. Regardless of the nature of its release, H CO has abundances ranging from 0.04–1.3% with respect to H O in comets measured to date [ 67 , 75 ].…”

“…On the other hand, NIR observations, which are most sensitive to the near-nucleus region, have indicated that H CO can also be released directly from the nucleus [ 75 , 88 ], suggesting that both nucleus and coma sources can contribute to H CO production in comets. Regardless of the nature of its release, H CO has abundances ranging from 0.04–1.3% with respect to H O in comets measured to date [ 67 , 75 ].…”

“…Methanol (CH OH) is the simplest alcohol and is the starting point for the formation of more complex organic molecules in the interstellar medium (ISM) [ 92 , 93 , 94 ]. It was first detected in comets at mm wavelengths [ 95 ], and has since been measured in many comets at mm/sub-mm and near-infrared wavelengths [ 67 , 75 ]. The heliocentric dependence of CH OH production, combined with long-slit near-infrared studies and spatially resolved ALMA observations, has shown that it is primarily associated with direct nucleus release [ 46 , 50 , 75 ].…”