Abrupt climate transition of icy worlds from snowball to moist or runaway greenhouse

TL;DR

This study uses climate models to show that icy planets and moons with low greenhouse gases are unlikely to become habitable after ice melts, often transitioning directly to uninhabitable greenhouse states due to high stellar fluxes.

Contribution

It demonstrates through simulations that certain icy worlds cannot achieve habitability and may skip the habitable phase, transitioning directly to moist or runaway greenhouse states.

Findings

High stellar fluxes needed to melt ice cause water loss.

Icy bodies often bypass habitable states, going straight to greenhouse conditions.

Water escape occurs at fluxes exceeding moist and runaway greenhouse limits.

Abstract

Ongoing and future space missions aim to identify potentially habitable planets in our Solar System and beyond. Planetary habitability is determined not only by a planet's current stellar insolation and atmospheric properties, but also by the evolutionary history of its climate. It has been suggested that icy planets and moons become habitable after their initial ice shield melts as their host stars brighten. Here we show from global climate model simulations that a habitable state is not achieved in the climatic evolution of those icy planets and moons that possess an inactive carbonate-silicate cycle and low concentrations of greenhouse gases. Examples for such planetary bodies are the icy moons Europa and Enceladus, and certain icy exoplanets orbiting G and F stars. We find that the stellar fluxes that are required to overcome a planet's initial snowball state are so large that they lead to significant water loss and preclude a habitable planet. Specifically, they exceed the moist greenhouse limit, at which water vapour accumulates at high altitudes where it can readily escape, or the runaway greenhouse limit, at which the strength of the greenhouse increases until the oceans boil away. We suggest that some icy planetary bodies may transition directly to a moist or runaway greenhouse without passing through a habitable Earth-like state.