No Snowball on Habitable Tidally Locked Planets with a Dynamic Ocean

Abstract

Terrestrial planets orbiting within the habitable zones of M-stars are likely to become tidally locked in a 1:1 spin:orbit configuration and are prime targets for future characterization efforts. An issue of importance for the potential habitability of terrestrial planets is whether they could experience snowball events (periods of global glaciation). Previous work using an intermediate complexity atmospheric Global Climate Model (GCM) with no ocean heat transport suggested that tidally locked planets would smoothly transition to a snowball, in contrast with Earth, which has bifurcations and hysteresis in climate state associated with global glaciation. In this paper, we use a coupled ocean-atmosphere GCM (ROCKE-3D) to model tidally locked planets with no continents. We chose this configuration in order to consider a case that we expect to have high ocean heat transport. We show that including ocean heat transport does not reintroduce the snowball bifurcation. An implication of this result is that a tidally locked planet in the habitable zone is unlikely to be found in a snowball state for a geologically significant period of time.

Detectability of biosignatures on LHS 1140 b

“…The mixing ratios of the species are constant over height. We consider pressures leading to surface temperatures between 220 K (approximated limit of open water with ocean heat transport in climates of tidally locked exoplanets around M dwarf stars, see Hu & Yang 2014;Checlair et al 2017Checlair et al , 2019 and 395 K (see Clarke 2004;McKay 2014). Further we limit our Notes.…”

An Energy Balance Model for Rapidly and Synchronously Rotating Terrestrial Planets

“…M-dwarf stars like TRAPPIST-1 emit a greater fraction of radiation at longer wavelengths compared to the sun, which leads to a reduction in ice-albedo feedback for planets around such stars (Shields et al 2013). The reduction of this ice-albedo feedback leads to the loss of bistability in the climate system, which has been demonstrated using calculations with EBMs and GCMs (Checlair et al 2017(Checlair et al , 2019. Calculations with the tidally-locked configuration of HEXTOR likewise do not show bistability between frozen and ice-free states analogous to the rapidly rotating cases shown in Fig.…”

“…A similar result was observed by Checlair et al (2017), who observed a small amount of hysteresis using a GCM of intermediate complexity when examining synchronously rotating planets with a cold start versus a warm start. Such hysteresis is likely due to limitations of the GCM, and this hysteresis indeed vanished in subsequent experiments by Checlair et al (2019) with a more complex GCM that included a dynamic ocean. Similarly, this hysteresis in HEXTOR for synchronously rotating planets is likely a model artefact, rather than a physical result, due to the limitations of the diffusive energy transport parameterization.…”

An Energy Balance Model for Rapidly and Synchronously Rotating Terrestrial Planets

“…A similar result was observed by Checlair et al (2017), who observed a small amount of hysteresis using a GCM of intermediate complexity when examining synchronously rotating planets with a cold start versus a warm start. Such hysteresis is likely due to limitations of the GCM, and this hysteresis indeed vanished in subsequent experiments by Checlair et al (2019) with a more complex GCM that included a dynamic ocean. Similarly, this hysteresis in HEXTOR for synchronously rotating planets is likely a model artifact, rather than a physical result, due to the limitations of the parameterization of diffusive energy transport.…”