arXiv subjects

Ryoya Sakata

Publications and source records attributed to Ryoya Sakata.

3 recordsLinked to original sources

arXiv2026

Oxygen isotope fractionation in the Martian atmosphere induced by CO$_2$ photolysis and O$_3$ formation

The enrichment of heavy isotopes of volatile elements in the Martian atmosphere indicates that Mars lost a large portion of its atmosphere through escape to space. Recent atmospheric measurements by ExoMars Trace Gas Orbiter (TGO) have suggested that the vertical profiles of oxygen isotopic compositions are influenced by chemical reactions involving isotopic fractionation. However, their quantitative impacts have not yet been fully evaluated. In this study, we develop a 1D photochemical model that incorporates oxygen isotopic fractionation associated with CO$_2$ photolysis and O$_3$ formation to investigate the vertical profiles of oxygen isotopic compositions. Our calculations show that CO is depleted in heavy oxygen isotopes relative to CO$_2$, reaching $\delta ^{18}$O $\sim -25$ per mil and $\delta ^{17}$O $\sim -15$ per mil, primarily due to isotopic fractionation during CO$_2$ photolysis. The vertical profiles of oxygen and carbon isotopic compositions are in good agreement between our model and the TGO measurements. O$_3$ is strongly enriched in $^{18}$O and $^{17}$O, reaching $\delta ^{18}$O $\sim 100$ per mil and $\delta ^{17}$O $\sim 50$ per mil as a consequence of the isotopic fractionation during its formation, whereas atomic oxygen is highly depleted in the heavy oxygen isotopes with $\delta ^{18}$O $\lesssim -100$ per mil and $\delta ^{17}$O $\lesssim -50$ per mil so as to compensate for their enrichment in O$_3$. These chemical fractionation processes can deplete the heavy oxygen isotopes in species that escape from the upper atmosphere, and thereby enhance the isotopic fractionation associated with oxygen escape to space. Such fractionated isotopic compositions of escaping oxygen may be detectable by the Martian Moons eXploration (MMX) mission.

astro-ph.EP↗

arXiv2026

Atmospheric Escape Rates from Mars - If it Orbited an Old M-Dwarf Star

Atmospheric escape is an important process that influences the evolution of planetary atmospheres. A variety of physical mechanisms can contribute to escape from an atmosphere, including thermal escape, ion escape, photochemical escape, and sputtering. Here we estimate escape rates via each of these processes for a hypothetical Mars-like exoplanet orbiting Barnard's star (an old, inactive M dwarf star). We place the planet at an orbital distance that receives the same total stellar flux as it does in our solar system. We use the measured stellar extreme ultraviolet (EUV) spectrum and assumptions on the star's magnetic field to determine both the high-energy radiation and the stellar wind environment around the planet. This information is used to model the response of the planet's thermosphere, exosphere and magnetosphere using a variety of models that have been validated against solar system observations. We find overall escape rates that are dominated by thermal processes and elevated by 2-5 orders of magnitude relative to present-day Mars, suggesting that a Mars-like planet orbiting Barnard's star would not retain a significant atmosphere for more than 10's of millions of years. Recently reported planets around Barnard's star should also not have retained significant atmospheres. By extension, Mars-like planets orbiting any M dwarf near the 'Habitable Zone' should not retain atmospheres for extended periods of time.

astro-ph.EP↗

arXiv2025

Exoplanet Atmospheric Escape Observations with the Habitable Worlds Observatory

The Decadal Survey on Astronomy and Astrophysics 2020 highlights the importance of advancing research focused on discovering and characterizing habitable worlds. In line with this priority, our goal is to investigate how planetary systems evolve through atmospheric escape and to develop methods for identifying potentially Earth-like planets. By leveraging the ultraviolet (UV) capabilities of the Habitable Worlds Observatory (HWO), we can use transit spectroscopy to observe atmospheric escape in exoplanets and explore the processes that shape their evolution, assess the ability of small planets to retain their atmospheres, and search for signs of Earth-like atmospheres. To achieve this, we support the development of a UV spectrograph with moderate- to high-resolution capabilities for point-source observations, coverage of key spectral features in the 100-300 nm range, and detectors that can register high count rates reliably. This article is an adaptation of a science case document developed for the Characterizing Exoplanets Steering Committee within HWO's Solar Systems in Context Working Group.

astro-ph.IM↗