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# Program information file
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PROGRAM_ID           2024B092
PROGRAM_TITLE        HDO/H2O at the cloud-top of Venus from high-resolution near-infrared spectroscopy
PROGRAM_INV1         Takao Sato
PROGRAM_INV2         Hideo Sagawa
PROGRAM_INV3         Shohei Aoki
PROGRAM_INV4          
PROGRAM_INV5          
PROGRAM_SCICAT       major planets / satellites
PROGRAM_ABSTRACT_BEG
Past studies revealed that D/H ratio in the Venusian atmosphere is highly enriched relative to the terrestrial one [Venusian HDO/H2O is ~100-200 times larger than the Earth's]. This very interesting fact suggests that a huge amount of water might have been lost to space, which has provoked much debate about the presence of oceans in ancient Venus. However, when we try to bring this HDO/H2O fractionation into more quantitative discussions, many difficulties immediately exist. For example, the most recent HDO/H2O near the cloud-top was obtained from Venus Express, but it does not necessarily appear to be in agreement with past ground-based observations. If this is a manifestation of spatiotemporal variations in HDO/H2O, it means that there is some process that causes such isotopic fractionation in the middle atmosphere. In the 2030s, DAVINCI+ will provide more detailed in-situ HDO/H2O measurements, but it will be limited to a single location and an instantaneous timing, and the representativeness of the measured values will be subject to caution. To properly understand the results of DAVINCI+'s observations, it is very important to keep conducting more detailed observations of D/H on Venus with currently available ground-based facilities.
We propose a high-resolution near-infrared spectroscopy using iSHELL to determine a reliable HDO/H2O at the Venusian cloud-top and to map its variability in latitude and local time. We request two 4.5-hour daytime observations to obtain at least four slit positions at different latitudes and local time on Venus by putting the 15-arcsec slit on its dayside disk in parallel to the North-South direction of Venus. The requested dates are selected to be satisfied with both a sufficient extent of doppler-shift between Venusian and terrestrial lines and an enough illuminated portion of the Venusian disk.
PROGRAM_ABSTRACT_END