The Onset of a Globally Ice-covered State for a Land Planet

TL;DR

This study uses a 3D atmospheric model to explore how surface water distribution affects the likelihood of a planet entering a globally ice-covered state, revealing that drier land planets can withstand higher insolation before freezing over.

Contribution

It systematically investigates the complete freezing limit for various water distributions on land planets, highlighting the importance of water distribution in planetary climate stability.

Findings

Complete freezing limit decreases with increasing dry area.

Meridionally uniform water distribution leads to earlier freezing.

Topography-based water distribution yields limits similar to meridionally uniform cases.

Abstract

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 distribution 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 S0 is 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.