# # Program information file # PROGRAM_ID 2023A081 PROGRAM_TITLE CO and its isotopologues at Venus cloud top: Tracer for atmospheric chemistry and dynamics PROGRAM_INV1 Hideo Sagawa PROGRAM_INV2 Takao Sato PROGRAM_INV3 Shohei Aoki PROGRAM_INV4 PROGRAM_INV5 PROGRAM_SCICAT major planets / satellites PROGRAM_ABSTRACT_BEG Carbon dioxide [CO2] in the Venusian atmosphere is photodissociated into carbon monoxide [CO] and oxygen by solar UV radiation. However, the reaction pathway that restores the dissociated CO to CO2 is still unknown. This well-known issue of 'Chemical stability of CO2 atmosphere' is a long-standing unsolved problem, not only for Venus and Mars atmospheres but also for exoplanetary terrestrial planets. This proposal will tackle this fundamental problem by observing 13CO/12CO isotopic fractionation for the first time at the Venus cloud-top altitude. The isotopic fractionation of CO can be used to evaluate how effective the CO2 photolysis occurs in Venus, and also to potentially constrain the atmospheric circulation in the Venusian upper atmosphere. PROGRAM_ABSTRACT_END