Some icy planets and moons may transition abruptly from icehouse to hothouse as their host stars brighten, without passing through a habitable phase, reports a paper published online in Nature Geoscience this week. It had been thought that such icy worlds - including Europa and Enceladus in our Solar System or icy exoplanets - could become habitable after their ice cover melts.
Stars such as the Sun brighten over time. So a planet that starts out cold and icy around a young dim star, such as early Earth, may eventually warm and transition to a stable habitable state as the star brightens. Earth has been geologically active throughout its history and the volcanic emission of greenhouse gases and weathering processes have acted to warm and stabilize the climate. But many small icy planets and moons lack these processes and climatic evolution depends solely on the energy received from the host star.
Jun Yang and colleagues used a global climate model to simulate the climatic evolution of icy planets. They find that, without atmospheric greenhouse gases to help warm the planet, the energy required to melt the planet is high - so high that once the ice cover melts, the planet transitions abruptly to a greenhouse state where the oceans vaporize and substantial water is lost. The result is a planet that cannot support water-based life as we know it.
In an accompanying News & Views article, Andy Ingersoll writes: “…that making an icy planet habitable is not as simple as melting its ice: many icy bodies swing from too cold to too hot, bypassing just right.”