  The TRAPPIST-1, Proxima Centauri, and LHS 1140 systems are the most exciting
prospects for future follow-up observations of potentially inhabited planets.
All orbit nearby M-stars and are likely tidally locked in 1:1 spin-orbit
states, which motivates the consideration of the effects that tidal locking
might have on planetary habitability. On Earth, periods of global glaciation
(snowballs) may have been essential for habitability and remote signs of life
(biosignatures) because they are correlated with increases in the complexity of
life and in the atmospheric oxygen concentration. In this paper we investigate
the snowball bifurcation (sudden onset of global glaciation) on tidally locked
planets using both an energy balance model and an intermediate-complexity
global climate model. We show that tidally locked planets are unlikely to
exhibit a snowball bifurcation as a direct result of the spatial pattern of
insolation they receive. Instead they will smoothly transition from partial to
complete ice coverage and back. A major implication of this work is that
tidally locked planets with an active carbon cycle should not be found in a
snowball state. Moreover, this work implies that tidally locked planets near
the outer edge of the habitable zone with low CO2 outgassing fluxes will
equilibrate with a small unglaciated substellar region rather than cycling
between warm and snowball states. More work is needed to determine how the lack
of a snowball bifurcation might affect the development of life on a tidally
locked planet.
