Inner Edge of Habitable Zones for Earth‐Sized Planets With Various Surface Water Distributions

Abstract

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 S0 is 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.

Venusian Habitable Climate Scenarios: Modeling Venus Through Time and Applications to Slowly Rotating Venus‐Like Exoplanets

“…However, the clouds also differ to some degree because of water availability. Contrary to the work of Abe et al () and Kodama et al (), the Arid‐Venus cases all have higher surface temperatures than their counterparts. This is because those previous works used modern Earth's rotation rate, whereas the cloud processes on these slower rotating worlds better regulate the climate, more so the more water that is available for cloud formation.…”

“… Arid‐Venus: This planet has modern Venus topography but only contains 20 cm of water in the subsurface soil layers, soil consisting of 100% sand, and no surface standing water at the start of the simulation. The atmosphere is initialized with zero water vapor and an isothermal temperature profile at 300 K. This initial condition is similar to that of Kodama et al () and Abe et al () who attempt to limit the amount of water vapor in the atmosphere (a strong greenhouse gas) and subsequently push the inner edge of the habitable zone farther inward. However, Kodama et al () and Abe et al () use modern Earth's rotation rate for all their experiments. 10m‐Venus: Uses modern Venus topography and places a 10 m liquid water equivalent layer in the lowest lying topographic areas.…”

Venusian Habitable Climate Scenarios: Modeling Venus through time and applications to slowly rotating Venus-Like Exoplanets

“…However, the clouds also differ to some degree because of water availability. Contrary to the work of Abe et al (2011); Kodama et al (2019) the Arid-Venus cases all have higher surface temperatures than their counterparts. This is because those previous works used modern Earth's rotation rate, whereas the cloud processes on these slower rotating worlds better regulate the climate, more so the more water that is available for data, all of which correspond to 2.9 Ga.…”

What is the Oxygen Isotope Composition of Venus? The Scientific Case for Sample Return from Earth’s “Sister” Planet

“…There is a growing recognition that developing realistic models for the long-term climatic and geological evolution of Venus will help to improve our understanding of why the Earth has remained habitable for most its existence (e.g. Driscoll and Bercovici 2013;Kodama et al 2019), despite a progressive increase in solar energy input; an issue generally referred to as the "faint young Sun paradox" (Feulner 2012). The luminosity of the Sun will continue to increase in the future, with stark, albeit very long-term, implications for Earth's habitability (Wolf and Toon 2014).…”