We present optical and near-infrared (NIR) observations of the outbursting, Halley-type comet 12P/Pons-Brooks. Three NIR spectra were obtained during two outbursts in October and November 2023, with the 3-meter Infrared Telescope Facility and the Palomar 200-inch Telescope, respectively. The NIR spectra exhibited absorption features at 1. 5 and 2. 0 μm, consistent with the diagnostic absorption bands of water ice, superimposed on a red dust-scattering continuum. We find that the absorption bands and the red continuum can be well explained by micrometer-sized crystalline ice at 140--170 K, along with sub-micrometer-sized refractory grains (e. g. , amorphous carbon). In addition, an optical spectrum was obtained with the Lijiang 2. 4-meter Telescope during the November 2023 outburst, which exhibited the emission bands of gaseous CN, C₃, C₂ and NH₂. The C₃/CN and C₂/CN ratios suggest that 12P/Pons-Brooks was ''typical'' in C₃ abundance but somewhat depleted in C₂. The specific kinetic energy of the 2023 November outburst is estimated to be 810³ J kg^-1, suggesting a likely triggering mechanism similar to 332P/Ikeya--Murakami and 17P/Holmes, i. e. , the crystallization of amorphous water ice. A refractory-to-ice ratio of 1. 7--3. 2 is derived from the total mass loss of dust and gas, aligning with the lower-end estimates for 67P/Churyumov-Gerasimenko and 1P/Halley. This suggests either a less evolved nucleus or an outburst region enriched in icy materials relative to the bulk nucleus.
Zhao et al. (Fri,) studied this question.