Highly absorbing refractory carbonaceous matter in cometary comae dust from the perspective of thermal IR spectroscopy

New thermal modeling analyses of Spitzer IRS observed comets including Oort Cloud comets (OCC) show many have weak silicate features and high carbon-to-silicate ratios from the relative mass fractions of modeled comae particles. In comparison, the enigmatic OCC C/1995 O1 (Hale-Bopp) has a low carbon-to-silicate ratio and high contrast silicate features with distinct and strong resonances from Forsterite and Enstatite. We discuss the motivations for considering the cometary carbonaceous material with a diversity of bonding structures that is optically dark (a) as indigenous to comet nuclei and to comet comae and (b) as variable between comets and temporally variable for a handful of comets. We assert that amorphous carbon is a good optical analog for this dark material.

However, amorphous carbon may not be a good chemical analog for optically dark carbonaceous cometary dust constituents. The carbon bonding structures in Stardust samples or cometary Interplanetary Dust Particles (IDPs) are examined with Carbon-XANES methods (Carbon-Xray Absorption Near Edge Spectroscopy). C-XANES studies of IDPs show that many IDPs have organic carbon with bonding structures with C=C (aromatic) and C=O bonds while fewer IDPs have amorphous carbon, and graphitic carbon is found within some Stardust samples (Wirick et al. 2009; De Gregorio et al. 2017). Some IDPs have aliphatic absorption resonances near 3.4 µm (Merouane et al. 2014). Highly absorbing minerals such as FeS (troilite) and Fe_(1-x)S (pyrrhotite), which are in cometary samples, may contribute to the dark matter in cometary comae. Yet, the interpolated FeS optical constants (n,k) available for 3 - 5 µm wavelengths (Pollack et al. 1994) have a k-value ~10x less than amorphous carbon. For comet 67P/C-G, the dust properties of the quiescent coma and the high color temperatures during the outburst at 1.3 AU (VIRTIS-H, Tdust~600K, Bockelee-Morvan et al. 2017, 2018; VIRTIS-M, Tdust~560K, Rinaldi et al. 2018) are well modeled by amorphous carbon but not by mixed particles of FeS and silicates. At this time, we are challenged to link the laboratory studies of cometary refractory organic matter and its meteoritic Insoluble Organic Matter (IOM) analogs with the thermal models that require the refractory carbonaceous matter in cometary comae to be as optically dark as amorphous carbon.