Planets locked in perpetual winter provide a great background for adventure stories, from the ice world Hoth in The Empire Strikes Back, to Gethen in Ursula K Le Guin's The Left Hand of Darkness, to the dystopian vision of Earth in Snowpiercer. Even Game of Thrones plays with the trope with the advance of the frosty White Walkers and their climatic influence over the fantasy continent of Westeros.
But ice worlds aren't only a product of fiction—these frozen orbs are apparently abundant in the universe as well. Within our own solar system, Jupiter's moon Europa and Saturn's moon Enceladus stand out as tantalizing examples, and many "iceball planets" have been discovered orbiting stars located thousands of light years from Earth.
Scientists have speculated that such worlds may be potentially habitable, especially when their host stars brighten and expand at the end of their lifetimes, a process that could gradually transform ice-covered places into lush ocean environments.
But new research published Monday in Nature Geoscience throws a wrench in these dreams of melting utopias, by presenting evidence that many ice worlds would quickly spiral from too cold to too hot before life has a chance to take hold.
Led by Jun Yang, an assistant professor of Atmospheric and Oceanic Sciences at Peking University, the authors used the research software 3D Community Atmospheric Model (CAM3) to simulate the climatic shifts of snowball worlds orbiting different types of stars.
Numerous factors influence these global dynamics, including a planet's size, mass, reflectivity, and distance from its star. The presence or absence of a carbonate-silicate cycle, in which sedimentary forces create carbonate minerals out of silicate rocks that are then transformed back into silicate by metamorphic processes, is a particularly significant feature. Planets with active carbonate-silicate cycles, like Earth, gradually release greenhouse gases such as carbon, which can seed hospitable climates like the one we enjoy on our planet (for the time being).
Worlds that lack active carbonate-silicate cycles are likely to be completely frozen over until their host stars brighten. One might imagine that as stellar radiation increases, these frigid worlds would finally thaw and release gases that could form an atmosphere, and conditions conducive to life.
But Yang's team found that starlight has to reach a high threshold before it can disrupt an icy planet's equilibrium. Once that tipping point is hit, ice on these worlds is so rapidly melted that any oceans could be vaporized before a stable atmosphere can form. This could produce a runaway greenhouse gas effect like the one seen on Venus.
"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," the team wrote in the paper. "We suggest that some icy planetary bodies may transition directly to a moist or runaway greenhouse without passing through a habitable Earth-like state."
Worlds in our own solar neighborhood will likely fall victim to this fate. "Europa and Enceladus will have no habitable period," Yang and his colleagues predict. "They will transit to a moist or runaway greenhouse state when the Sun becomes a red giant in six to seven billion years, at which time the stellar flux at the location of Europa will reach the snowball-melting threshold."
So much for using the Sun's red giant phase to remodel Europa into a balmy ocean paradise. Still, it's useful to recognize the crucial role that Earth's carbonate-silicate cycle has played in producing a stable environment for life on our home planet. This makes it a particularly valuable process to look out for when assessing whether frozen worlds are good targets for exploration, or if they are just one stellar threshold away from boiling into hellscapes.
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