Moons Like Europa May Offer No Refuge to Humanity as Sun Brightens

Joel Hruska

Astronomers like to think on the big scale, whether it’s measuring the brightness of galaxies millions of light years away, or modeling how our own sun and solar system formed billions of years ago. Right now, the sun is roughly middle-aged. As it ages, its luminosity increases, which results in more solar radiation reaching Earth.

While this effect is not visible at human timescales, it has tremendous implications for our planet in the long term. Current estimates suggest that most plant life will die roughly 600 million years from now, with total life extinction in roughly one billion years. New research suggests that humans looking for an idyllic spot to park in the far future (assuming we make it that long, obviously) won’t find it on moons like Europa, despite the fact that we currently think Europa is one of the most promising places to find life.

First, a few basic details. As the sun’s luminosity increases, every planet and moon will receive commensurately more solar energy, with the current rate of change measured at 1 percent increased solar radiation every 100 million years. By the 1-billion-year mark, Earth is expected to be uninhabitable, but the sun won’t exit the main sequence for another 4.5 to 5 billion years. Once it does, it’ll expand into a red giant, swallowing Mercury and Venus and making Earth the first planet from the sun.

Well, that looks… cozy. Image by Fsgregs, courtesy of Wikipedia

Given these facts, and the current lack of anything resembling a warp drive, stargate, or hyperdrive, researchers have done some work on where we might live in the distant future, assuming we haven’t decided to evolve into gaseous clouds or something. Icy moons like Europa seem like viable options, given that we already know they possess huge liquid reservoirs. According to a new research study reported by Ars Technica, icy moons may never have a habitable period at all. The scientists took climate models developed to map and forecast changes in Earth’s climate, then plugged in variables that would match a currently frozen planet or moon and how it would change as the sun’s luminosity increased. These models have been used to successfully model Earth’s icy past, so they should be a reasonably accurate look at what could happen in the future. The model they used, CAM 3.0, is open-source and available for download.

Disappointing Findings

The researchers’ test case was an icy world that didn’t receive enough sunlight to melt the ice, with an atmosphere that completely lacked any greenhouse gases. The resulting iceball had a uniform ice sheet, broken by a trench around the equator where the ice sometimes sublimated into a gas. In the researcher’s own words:

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.

Europa could wind up looking a lot more like Venus than Earth, at least during this period of its evolution.

Because icy planets reflect a very high degree of incoming sunlight, it takes a significant increase in solar output to begin to melt the ice. Once the ice started melting, however, it melted extremely quickly, releasing huge amounts of water vapor. The albedo — a measure of how much light is reflected from an object — of the planet drops dramatically as the ice melts, which results in more ice melting, which lowers the albedo, which… you get the picture.

Now, these clouds of water vapor would form an atmosphere, but the H₂O molecules in the upper atmosphere would be split by the sun’s increased output. Both hydrogen and oxygen would escape into space, and the weaker the surface gravity of the planet/moon, the faster this happens. It might take a billion years or so to complete the process, but the runaway greenhouse effect on the planet would turn it into a hellscape, with temperatures of up to 1,000C. In other words, don’t plan your honeymoon getaway on Europa in a billion years.