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Shock‐induced metamorphism in meteorites informs us about the collisional environment and history of our solar system. Recently, the importance of material strength in impact heating was reported from head‐on impact simulations. Here, we perform three‐dimensional oblique impact simulations and confirm the additional heating due to material strength for oblique impacts. Despite a large difference in the peak pressure at the impact point at a given impact velocity, we find that the heated mass for an oblique impact is nearly the same as that for a head‐on impact. Thus, our results differ from the previous finding that the heated mass decreases as the impact becomes more oblique and show that the additional shear heating is more effective for oblique impacts than for head‐on impacts. This also indicates that material ejected during oblique impact tends to experience lower shock pressures but higher temperatures. We studied heating in oblique impacts using a shock physics codeOblique impacts can produce almost the same amount of heated mass as head‐on impactsVarious shock features could be produced by a single oblique impact event

When planets receive insolation above a certain critical value called the runaway threshold, liquid surface water vaporizes completely, which forms the inner edge of the habitable zone. Because land planets can emit a large amount of radiation from the dry tropics, they have a higher runaway threshold than aqua planets do. Here we systematically investigated the runaway threshold for various surface water distributions using a three‐dimensional dynamic atmosphere model. The runaway threshold for the meridionally uniform surface water distribution increases from the typical value for the aqua planet regime (~130% S0) to one for the land planet regime (~155% S0) as the dry surface area increases, where S0is the present Earth's insolation. Although this result is similar to the previous work considering zonally uniform surface water distributions, the runaway threshold for the land planet regime is quite low compared to that of the previous work. This is because a part of the tropical atmosphere is always wet for the meridionally uniform case. We also considered the surface water distributions determined by the Earth's, Mars's, and Venus's topographies. We found that their runaway thresholds are close to that for the meridionally uniform cases, and the amount of water at the boundary between an aqua planet regime and land planet regime is around 10% of the Earth's ocean. This clearly shows that the runaway threshold is not determined uniquely by the luminosity of the central star, but it has a wide range caused by the surface water distribution of the terrestrial water planet itself. Some of the detected exoplanets are expected to be Earth‐sized rocky planets, probably with liquid water on their surface due to an adequate orbital distance from the central star. The region where liquid water is stable on the planetary surface is called the habitable zone. Previous studies showed that the habitable zone for a planet with a small amount of water (called land planets) is wider than that of a planet with a large amount of water (aqua planets). By using a three‐dimensional model for the atmospheric circulation, we investigated the inner edge of the habitable zone considering the effects of various surface water distributions. As the previous studies have done, we also recognized two climate regimes (aqua planet regime and land planet regime). The insolation that a planet receives at the inner edge of the habitable zone increases with an increase of the dry surface area. We considered the real landform patterns of Earth, Venus, and Mars and found that the boundary between the two climate regimes appears when the total amount of the surface water is around 10% of the Earth's ocean. Our results will give us a strong constraint on the amount of surface water in future observations using the significant difference between climates for an aqua planet and a land planet. Climates for various surface water distributions are calculated via three‐dimensional general circulation modelsThe insolation when a planet enters the runaway greenhouse state depends on the surface water distributionA small difference in the amount of surface water forms a significant difference in climate between an aqua planet and a land planet

Liquid water is one of the most important materials affecting the climate and habitability of a terrestrial planet. Liquid water vaporizes entirely when planets receive insolation above a certain critical value, which is called the runaway greenhouse threshold. This threshold forms the inner most limit of the habitable zone. Here we investigate the effects of the distribution of surface water on the runaway greenhouse threshold for Earth‐sized planets using a three‐dimensional dynamic atmosphere model. We considered a 1 bar atmosphere whose composition is similar to the current Earth's atmosphere with a zonally uniform distribution of surface water. As previous studies have already showed, we also recognized two climate regimes: the land planet regime, which has dry low‐latitude and wet high‐latitude regions, and the aqua planet regime, which is globally wet. We showed that each regime is controlled by the width of the Hadley circulation, the amount of surface water, and the planetary topography. We found that the runaway greenhouse threshold varies continuously with the surface water distribution from about 130% (an aqua planet) to 180% (the extreme case of a land planet) of the present insolation at Earth's orbit. Our results indicate that the inner edge of the habitable zone is not a single sharp boundary, but a border whose location varies depending on planetary surface condition, such as the amount of surface water. Since land planets have wider habitable zones and less cloud cover, land planets would be good targets for future observations investigating planetary habitability. The onset of the runaway greenhouse effect depends strongly on the surface water distributionThe runaway threshold increases as the surface water distribution retreats toward higher latitude outside the Hadley circulationThe lower the water amount on a terrestrial planet, the longer the planet remains in habitable condition

The record of impact‐induced shock heating in meteorites is an important key for understanding the collisional history of the solar system. Material strength is important for impact heating, but the effect of impact angle and impact velocity on shear heating remains poorly understood. Here, we report three‐dimensional oblique impact simulations, which confirm the enhanced heating due to material strength and explore the effects of impact angle and impact velocity. We find that oblique impacts with an impact angle that is steeper than 45‐degrees produce a similar amount of heated mass as vertical impacts. On the other hand, grazing impacts produce less heated mass and smaller heated regions compared to impacts at steeper angles. We derive an empirical formula of the heated mass, as a function of the impact angle and velocity. This formula can be used to estimate the impact conditions (velocity and angle) that occurred and caused Ar loss in the meteoritic parent bodies. Furthermore, our results indicate that grazing impacts at higher impact velocities could generate a similar amount of heated material as vertical impacts at lower velocities. As the heated material produced by grazing impacts has experienced lower pressure than the material heated by vertical impacts, our results imply that grazing impacts may produce weakly shock‐heated meteorites. Meteorites are extraterrestrial materials that have been delivered to the Earth from asteroids. The materials in meteorites can record information about their formation and subsequent evolution. Thus, they are excellent sources of information used to explore the history of the solar system. One such feature recorded is evidence of shock: high pressures and temperatures caused by collisions between asteroids. Previous work investigating impacts found that material strength is a key factor in determining the amount of impact heating, especially in low speed collisions like those expected to occur in the main asteroid belt. In this work, we explore various oblique incidence impacts to study the effects of material strength by using a shock physics model. We confirm that material strength plays a key role in oblique impacts, just as in head‐on impacts. Our results show that head‐on and 45‐degree impacts can generate nearly the same amount of heated mass in total. However, more oblique impacts with a shallower angle produce less heated mass than other steeper‐angle impacts (i.e., head‐on and 45‐degree impacts). We also find that low speed vertical impacts and high speed grazing impacts can produce the same amount of material in asteroids that have experienced a given temperature. We examined the dependence of impact heating on impact angle and velocity using a shock physics codeThe amount of heated mass is similar among >45° impacts, while it is smaller for shallower impactsWe derived an empirical formula for the cumulative heated mass over 1000 K during oblique impacts We examined the dependence of impact heating on impact angle and velocity using a shock physics code The amount of heated mass is similar among >45° impacts, while it is smaller for shallower impacts We derived an empirical formula for the cumulative heated mass over 1000 K during oblique impacts

The climates of terrestrial planets with a small amount of water on their surface, called land planets, are significantly different from the climates of planets having a large amount of surface water. Land planets have a higher runaway greenhouse threshold than aqua planets, which extends the inner edge of the habitable zone inward. Land planets also have the advantage of avoiding global freezing due to drier tropics, leading to a lower planetary albedo. In this study, we systematically investigate the complete freezing limit for various surface water distributions using a three‐dimensional dynamic atmospheric model. As in a previous study, we found that a land planet climate has dry tropics that result in less snow and fewer clouds. The complete freezing limit decreases from that for aqua planets (92% S0, where S0is Earth's present insolation) to that for land planets (77% S0) with an increasing dry area. Values for the complete freezing limit for zonally uniform surface water distributions are consistently lower than those for meridionally uniform surface water distribution. This is because the surface water distribution in the tropics in the meridionally uniform cases causes ice‐albedo feedback until a planet lapses into the complete freezing state. For a surface water distribution using the topographies of the terrestrial planets, the complete freezing limit has values near those for the meridionally uniform cases. Our results indicate that the water distribution is important for the onset of a global ice‐covered state for Earth‐like exoplanets. Some exoplanets are thought to be Earth‐like rocky planets within the habitable zone, where liquid water is stable on the planetary surface. Land planets with a small amount of surface water have the advantage of maintaining liquid water on their surface. We investigated the complete freezing limit using a three‐dimensional general circulation model assuming various surface water distributions. The insolations at the complete freezing limit gradually decrease from that for a water‐rich planet to that for a dry planet with an increasing dry area. Our results showed that the amount of water significantly affects the initiation of a global ice‐covered state for Earth‐like exoplanets. The onset of a globally ice‐covered state becomes lower as the area of dry region increasesAt lower insolation, land planets are warmer than aqua planets because of less cloud and snow which lead to lower planetary albedoThe surface water distribution is one of the keys to determining the complete freezing limit The onset of a globally ice‐covered state becomes lower as the area of dry region increases At lower insolation, land planets are warmer than aqua planets because of less cloud and snow which lead to lower planetary albedo The surface water distribution is one of the keys to determining the complete freezing limit