Tatsuya Yoshida 교수

Abstract Terrestrial planets currently in the habitable zones around M dwarfs likely experienced a long-term runaway-greenhouse condition because of a slow decline in host-star luminosity in its pre-main-sequence phase. Accordingly, they might have lost significant portions of their atmospheres including water vapor at high concentration by hydrodynamic escape induced by the strong stellar X-ray and extreme ultraviolet (XUV) irradiation. However, the atmospheric escape rates remain highly uncert

Abstract The isotopic signature of atmospheric carbon offers a unique tracer for the history of the Martian atmosphere and the origin of organic matter on Mars. The photolysis of CO 2 is known to induce strong isotopic fractionation of the carbon between CO 2 and CO. However, its effects on the carbon isotopic compositions in the Martian atmosphere remain uncertain. Here, we develop a 1D photochemical model to consider the isotopic fractionation via photolysis of CO 2 , to estimate the vertical

ABSTRACT Recent cosmochemical studies have shown that most of Earth’s building blocks were close to enstatite meteorites in isotopic compositions. This implies the formation of an impact-induced proto-atmosphere enriched in H2 and CH4 on accreting Earth. Such a reduced proto-atmosphere would have been largely lost by hydrodynamic escape, but its flux and time-scale for hydrogen depletion remain highly uncertain. Here we carry out 1D hydrodynamic escape simulations for such an H2–CH4 proto-atmosp

Abstract Radiative cooling by molecules is a crucial process for hydrodynamic escape, as it can efficiently remove the thermal energy driving the outflow, acquired through X-ray and extreme UV absorption. Carbon oxides, such as CO and CO 2 , and their photochemical products are anticipated to serve as vital radiative cooling sources not only in atmospheres dominated by carbon oxides but also in H 2 -rich atmospheres. However, their specific effects on the hydrodynamic escape, especially in H 2 -

Earth is expected to have acquired a reduced proto-atmosphere enriched in H₂ and CH₄ through the accretion of building blocks that contain metallic Fe and/or the gravitational trapping of surrounding nebula gas. Such an early, wet, reduced atmosphere that covers a proto-ocean would then ultimately evolve toward oxidized chemical compositions through photochemical processes that involve reactions with H₂O-derived oxidant radicals and the selective escape of hydrog

On the basis of the hydrodynamic model that the propulsion of flagellated bacteria in a fluid is a consequence of the propagation of helical waves along the length of flagella or flagellar bundles, it is predicted that propulsion must be accompanied by a rotation of bacterial body about the direction of translation (Chwang and Wu, 1971), and that propulsive velocity u is directly proportional to the frequency of bodily rotation fB, the proportional constant being a complicated function of variou

The demographics of sub-Jovian planets around low-mass stars is dominated by populations of sub-Neptunes and super-Earths, distinguished by the presence or absence of envelopes of volatiles with a low molecular weight, that is, H 2 , He, and H 2 O. The current paradigm is that sub-Neptunes on close-in orbits evolve into super-Earths via atmospheric escape driven by high-energy stellar irradiation. We used an integrated hydrodynamic-radiation-chemical network model of the outflow to demonstrate t

The isotopic signature of atmospheric carbon offers a unique tracer for the history of the Martian atmosphere and the origin of organic matter on Mars. Photolysis of CO$_{2}$ is known to induce strong isotopic fractionation of carbon between CO$_{2}$ and CO. However, its effect on the carbon isotopic compositions in the Martian atmosphere remains uncertain. Here we develop a 1-D photochemical model considering isotopic fractionation via photolysis of CO$_{2}$ to estimate the vertical profiles of

The atmospheric D/H ratio on Mars is enhanced by ~5 times the value on Earth, suggesting that large amounts of water have escaped into space. Additionally, water supply processes into the atmosphere, like ablation of interplanetary dust particles (IDPs) and volcanic outgassing, are considered important to satisfy the current isotopic composition. IDPs, containing water as hydrous minerals with a relatively low D/H ratio, ablate at high altitudes and supply water into the upper atmosphere. Nevert

Abstract Die oxidative Addition von Wasser an den Hydridokomplex (I) in Pyridin, die reversibel ablaufen kann, liefert das Produkt (II), das als Tetraphenylborat isoliert wurde.

AbstractGeomorphological evidence suggests that early Mars had oceans and valley networks, which implies that it had a dense atmosphere and an active hydrological cycle. However, the effects of orbital obliquity cycles remain largely unexplored. We performed fully coupled GCM simulations with a 2 bar CO₂ atmosphere and an initial 500 m global ocean, varying obliquity (40°±10°) and H₂ concentration (from 0 to 6%) for 1.2×10⁵ years. The results show that obliquity and H₂ significantly influence th