We propose to use NIRSPEC at Keck II to measure the deuterium-to-hydrogen ratios (D/H ratios) in both water and methane in comet C/2023 A3 (Tsuchinshan-ATLAS). This comet is classified as a "dynamically new comet" (its original semimajor axis is at least > 10,000 au) on its first passage into the inner solar system directly from the Oort Cloud, and it will be remarkably bright from late September to early November 2024, when it is a good target to investigate minor species in comets, such as deuterated species (e.g., HDO and CH3D). D/H ratios of water and methane in comets are theoretically predicted to be different because the formation chemical pathways of these molecules are different from each other. Furthermore, the sublimation temperature of methane is ∼30 K, while that of the water is ∼150 K (i.e., measuring these ices samples different regions of the proto-planetary disk of the Sun). Thus, the D/H ratios of cometary water and methane provide critical clues to understanding the physical and chemical alterations of icy materials in the proto-planetary disk. We propose high-resolution near-infrared spectroscopic observations of comet Tsuchinshan-ATLAS to measure emission lines of H2O, HDO, CH4, and CH3D simultaneously sampled by a single grating setting.
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