Comet is thought to be one of the most pristine objects in the solar system. Their chemical compositions provide valuable insights of the chemical environment and evolution in the early solar system, especially formation temperatures. However, it is uncertain whether the derived abundances of CO2 and CO relative to H2O in comets are result of their inherent compositions (nature) or have been influenced by external factors (nurture). To address this question, we focus on a comet firstly approaching the inner solar system from the Oort Cloud, known as a dynamically new comet (DNC). Recently, a new method to estimate abundance ratios of H2O:CO2:CO has been established, which combines the "intensity ratio" with the "intrinsic line widths" of three [O I] lines. This method is based on theoretical simulation of the excess kinetic energy of oxygen atoms given by photodissociation reactions of H2O, CO2 and CO in the coma. This model successfully reproduced the result of the short-period comet 21P/Giacobini-Zinner at ∼1 au from the Sun. Here we propose high-resolution optical spectroscopy of the two DNCs, C/2021 S3 (PanSTARRS) and C/2023 A3 (Tsuchinshan-ATLAS), to explore chemical evolution in the early solar system by abundance ratios of H2O:CO2:CO of these comets.
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