Hidenori Genda

Enhanced atmospheric loss on protoplanets at the giant impact phase in the presence of oceans.

The atmospheric compositions of Venus and Earth differ significantly, with the venusian atmosphere containing about 50 times as much 36Ar as the atmosphere on Earth. The different effects of the solar wind on planet-forming materials for Earth and Venus have been proposed to account for some of this difference in atmospheric composition, but the cause of the compositional difference has not yet been fully resolved. Here we propose that the absence or presence of an ocean at the surface of a protoplanet during the giant impact phase could have determined its subsequent atmospheric amount and composition. Using numerical simulations, we demonstrate that the presence of an ocean significantly enhances the loss of atmosphere during a giant impact owing to two effects: evaporation of the ocean, and lower shock impedance of the ocean compared to the ground. Protoplanets near Earths orbit are expected to have had oceans, whereas those near Venus’ orbit are not, and we therefore suggest that remnants of the noble-gas rich proto-atmosphere survived on Venus, but not on Earth. Our proposed mechanism explains differences in the atmospheric contents of argon, krypton and xenon on Venus and Earth, but most of the neon must have escaped from both planets’ atmospheres later to yield the observed ratio of neon to argon.

Emergence of two types of terrestrial planet on solidification of magma ocean.

Understanding the origins of the diversity in terrestrial planets is a fundamental goal in Earth and planetary sciences. In the Solar System, Venus has a similar size and bulk composition to those of Earth, but it lacks water. Because a richer variety of exoplanets is expected to be discovered, prediction of their atmospheres and surface environments requires a general framework for planetary evolution. Here we show that terrestrial planets can be divided into two distinct types on the basis of their evolutionary history during solidification from the initially hot molten state expected from the standard formation model. Even if, apart from their orbits, they were identical just after formation, the solidified planets can have different characteristics. A type I planet, which is formed beyond a certain critical distance from the host star, solidifies within several million years. If the planet acquires water during formation, most of this water is retained and forms the earliest oceans. In contrast, on a type II planet, which is formed inside the critical distance, a magma ocean can be sustained for longer, even with a larger initial amount of water. Its duration could be as long as 100 million years if the planet is formed together with a mass of water comparable to the total inventory of the modern Earth. Hydrodynamic escape desiccates type II planets during the slow solidification process. Although Earth is categorized as type I, it is not clear which type Venus is because its orbital distance is close to the critical distance. However, because the dryness of the surface and mantle predicted for type II planets is consistent with the characteristics of Venus, it may be representative of type II planets. Also, future observations may have a chance to detect not only terrestrial exoplanets covered with water ocean but also those covered with magma ocean around a young star.

Constraints on the Mass of a Habitable Planet with Water of Nebular Origin

From an astrobiological point of view, special attention has been paid to the probability of habitable planets in extrasolar systems. The purpose of this study is to constrain a possible range of the mass of a terrestrial planet that can get water. We focus on the process of water production through oxidation of atmospheric hydrogen—the nebular gas having been attracted gravitationally—by oxides available at the planetary surface. For the water production to work well on a planet, a sufficient amount of hydrogen and a temperature high enough to melt the planetary surface are needed. We have simulated the structure of the atmosphere that connects with the protoplanetary nebula for wide ranges of the heat flux, the opacity, and the density of the nebular gas. We have found that both requirements are fulfilled for an Earth-mass planet for wide ranges of the parameters. We have also found that the surface temperature of planets of ≤0.3ME (where ME is Earths mass) is lower than the melting temperature of silicate (~1500 K). On the other hand, a planet of more than several ME becomes a gas giant through runaway accretion of the nebular gas.

FORMATION OF TERRESTRIAL PLANETS FROM PROTOPLANETS UNDER A REALISTIC ACCRETION CONDITION

The final stage of terrestrial planet formation is known as the giant impact stage where protoplanets collide with one another to form planets. So far this stage has been mainly investigated by N-body simulations with an assumption of perfect accretion in which all collisions lead to accretion. However, this assumption breaks for collisions with high velocity and/or a large impact parameter. We derive an accretion condition for protoplanet collisions in terms of impact velocity and angle and masses of colliding bodies, from the results of numerical collision experiments. For the first time, we adopt this realistic accretion condition in N-body simulations of terrestrial planet formation from protoplanets. We compare the results with those with perfect accretion and show how the accretion condition affects terrestrial planet formation. We find that in the realistic accretion model about half of collisions do not lead to accretion. However, the final number, mass, orbital elements, and even growth timescale of planets are barely affected by the accretion condition. For the standard protoplanetary disk model, typically two Earth-sized planets form in the terrestrial planet region over about 108 yr in both realistic and perfect accretion models. We also find that for the realistic accretion model, the spin angular velocity is about 30% smaller than that for the perfect accretion model, which is as large as the critical spin angular velocity for rotational instability. The spin angular velocity and obliquity obey Gaussian and isotropic distributions, respectively, independently of the accretion condition.

Origin of the Ocean on the Earth: Early Evolution of Water D/H in a Hydrogen-rich Atmosphere

Abstract The origin of the Earths ocean has been discussed on the basis of deuterium/hydrogen ratios (D/H) of several sources of water in the Solar System. The average D/H of carbonaceous chondrites (CCs) is known to be close to the current D/H of the Earths ocean, while those of comets and the solar nebula are larger by about a factor of two and smaller by about a factor of seven, respectively, than that of the Earths ocean. Thus, the main source of the Earths ocean has been thought to be CCs or adequate mixing of comets and the solar nebula. However, those conclusions are correct only if D/H of water on the Earth has remained unchanged for the past 4.5 Gyr. In this paper, we investigate evolution of D/H in the ocean in the case that the early Earth had a hydrogen-rich atmosphere, the existence of which is predicted by recent theories of planet formation no matter whether the nebula remains or not. Then we show that D/H in the ocean increases by a factor of 2–9, which is caused by the mass fractionation during atmospheric hydrogen loss, followed by deuterium exchange between hydrogen gas and water vapor during ocean formation. This result suggests that the apparent similarity in D/H of water between CCs and the current Earths ocean does not necessarily support the CCs origin of water and that the apparent discrepancy in D/H is not a good reason for excluding the nebular origin of water.

On the Origin of HD 149026b

The high density of the recently discovered close-in extrasolar planet HD 149026b suggests the presence of a huge core in its interior, which challenges planet formation theory. We first derive constraints on the total mass of heavy elements in the planet and find its preferred value is 50-80 M? . We then explore the possibility of the formation of HD 149026b through subcritical core accretion as envisioned for Uranus and Neptune, and find the subcritical accretion scenario is very unlikely in the case of HD 149026b for at least two reasons: (1) subcritical planets are such that the ratio of their core mass to their total mass is above ~0.7, in contradiction with constraints for all but the most extreme interior models of HD 149026b and (2) high accretion rates and large isolation mass required for the formation of a subcritical >50 M? core are possible only at specific orbital distances in a disk with a surface density of dust equal to at least 30 times that of the minimum-mass solar nebula. These facts point toward two main routes for the formation of HD 149026b: (i) gas accretion limited by a slow viscous inflow in an evaporating disk or (ii) a significant modification of the planetary composition after gas accretion ended. Illustrating the second route, we show that collision between two gas giants leads to a substantial loss of the gas component and thus may make the planet highly enriched in heavy elements. Alternatively, the planet may be supplied with heavy elements by planetesimals through secular perturbations. In both the giant impact and the secular perturbation scenarios, we expect an outer giant planet to be present. Observational studies by imaging, astrometry, and long-term interferometry of this system are needed to better narrow down the ensemble of possibilities.

Merging Criteria for Giant Impacts of Protoplanets

At the final stage of terrestrial planet formation, known as the giant impact stage, a few tens of Mars-sized protoplanets collide with one another to form terrestrial planets. Almost all previous studies on the orbital and accretional evolution of protoplanets in this stage have been based on the assumption of perfect accretion, where two colliding protoplanets always merge. However, recent impact simulations have shown that collisions among protoplanets are not always merging events, that is, two colliding protoplanets sometimes move apart after the collision (hit-and-run collision). As a first step towards studying the effects of such imperfect accretion of protoplanets on terrestrial planet formation, we investigated the merging criteria for collisions of rocky protoplanets. Using the smoothed particle hydrodynamic (SPH) method, we performed more than 1000 simulations of giant impacts with various parameter sets, such as the mass ratio of protoplanets,

WARM DEBRIS DISKS PRODUCED BY GIANT IMPACTS DURING TERRESTRIAL PLANET FORMATION

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Modification of a proto-lunar disk by hydrodynamic escape of silicate vapor

, the total mass of two protoplanets,

Infrared Doppler instrument for the Subaru Telescope (IRD)

M_{\rm T}