1,2,3,4,5,6,- 1The University of Tokyo, Kashiwa, Japan (kazuto-kashiwakura@g.ecc.u-tokyo.ac.jp)
- 2Tohoku University, Sendai, Japan
- 3NASA Goddard Space Flight Center, Greenbelt, MD, USA
- 4American University, Washington, DC, USA
- 5University of Basilicata, Potenza, Italy
- 6Kyoto Sangyo University, Kyoto, Japan
The D/H ratio in the Martian atmosphere is a key tracer for understanding the present-day water cycle on Mars and its broader connection to long-term atmospheric evolution. Because deuterium is less efficiently lost to space than hydrogen, the atmospheric D/H ratio has long been used to constrain the history of water loss from Mars [e.g., 1]. At the same time, the spatial and seasonal variability of D/H in the lower atmosphere is expected to reflect ongoing water-cycle processes, including exchange with surface and polar reservoirs, formation of water ice clouds, and potential subsurface-atmosphere interactions associated with condensation and sublimation of water. Characterizing the present-day distribution of D/H is therefore important not only for understanding the evolution of Martian water, but also for identifying the physical processes that control the current Martian water cycle.
Previous observations have shown that Martian water vapor is typically enriched in deuterium to about 5 VSMOW, and that its D/H ratio varies substantially in space and time [e.g., 1–5]. Additional infrared observations have shown that HDO/H2O varies with season and altitude, suggesting that the D/H distribution is influenced by water transport, cloud-related fractionation, and exchange with surface and polar reservoirs [3–7]. In particular, Villanueva et al. (2015) [2] reported strong local variability across the Martian disk from ground-based mapping. Their results suggest that water released from the north polar cap may have a representative D/H value of about 7 VSMOW, while some basins and low-lying regions show even higher enrichment and high-altitude regions show much lower values of about 1–3 VSMOW. However, these observed spatial variations cannot be explained solely by simple fractionation associated with phase changes, and their underlying mechanisms remain unclear.
In this study, we investigate the spatial and seasonal variability of D/H in the lower Martian atmosphere, with the objective of clarifying the processes responsible for the observed isotopic variations. We use ground-based high-resolution spectroscopic observations of Mars obtained with iSHELL on the NASA Infrared Telescope Facility (IRTF). IRTF/iSHELL enables spatially resolved high-resolution spectroscopy of the Martian disk, allowing simultaneous retrievals of H2O and HDO. We have obtained disk-resolved Mars observations with IRTF/iSHELL over approximately three years, covering multiple seasons and viewing geometries [8,9]. This long-term data set, combined with disk-resolved two-dimensional mapping, allows us to expand the D/H measurements across multiple times and locations and to assess whether the regional D/H variability suggested by previous studies varies with season and location. These measurements are expected to constrain isotopic fractionation processes and contribute to a more accurate understanding of the present-day Martian water cycle.
As the first step, we are currently analyzing the initial data set acquired in September 2020. This analysis focuses on deriving spatially resolved HDO, H2O, and D/H maps from the iSHELL spectra. We retrieve H2O and HDO abundances by fitting the observed spectra with radiative-transfer calculations using the Planetary Spectrum Generator (PSG), including telluric absorption, solar Fraunhofer lines, and Martian atmospheric absorption. The molecular absorption strengths are scaled relative to the CO2 absorption, which provides a reference for deriving the H2O and HDO abundances. By the time of the presentation, we plan to report the first results from the September 2020 data set, including the disk-resolved D/H map and its latitudinal profile. These results will provide an initial basis for assessing whether lower-atmospheric D/H variability is controlled by seasonal and regional factors in the active Martian water cycle.
[1] Owen et al. 1988, Science, 240, 1767.
[2] Villanueva et al. 2015, Science, 348, 218.
[3] Aoki et al. 2015, Icarus, 260, 7.
[4] Encrenaz et al. 2018, A&A, 612, A112.
[5] Alday et al. 2024, MNRAS, 530, 2919.
[6] Villanueva et al. 2021, Science Advances, 7, eabc8843.
[7] Alday et al. 2021, Nature Astronomy, 5, 943.
[8] Aoki et al. 2024, The Planetary Science Journal, 5, 158.
[9] Faggi et al. 2025, Journal of Geophysical Research: Planets, 130, e2025JE009105.
How to cite: Kashiwakura, K., Aoki, S., Faggi, S., Villanueva, G., Liuzzi, G., Sagawa, H., and Imamura, T.: Spatially resolved measurements of the D/H ratio in the lower Martian atmosphere with IRTF/iSHELL: preliminary results from 2020 observations, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-188, https://doi.org/10.5194/epsc2026-188, 2026.
