Stars like the sun are getting brighter over their history, a trend that has significant implications for the habitability of Earth and other bodies, both in our solar system and beyond. An icy world at the very edge of the habitable zone can turn into a temperate paradise with enough time.
Or it could turn out to be downright Venus-style hell if a new study proves right. The study’s authors ran a full planetary climate model on a planet covered in ice. They discover that, below a level of incident light sufficient to melt the ice, the planet reaches a greenhouse state that would cause it to lose all its water to space and possibly go straight to a runaway greenhouse.
Ironically, the only thing that has saved Earth from a runaway greenhouse is the presence of greenhouse gases in the atmosphere.
Modeling a cool box
Although there is a lot of talk about exoplanets in a habitable zone, their location in that zone doesn’t say much about whether they are actually habitable. Habitability is characterized by the right temperature for liquid water to exist on the Earth’s surface. But that temperature is not only determined by the distance of the planet from its parent star. Instead, it depends on things like how much light the surface reflects back into space, details about the clouds, and how much of the atmosphere is made up of greenhouse gases.
We have tools to understand these things: they’re called climate models. Unfortunately, most of those are set up to look at Earth, and they don’t allow researchers to input arbitrary values for things like Earth’s gravity. But with the explosion of exoplanet discoveries, that’s starting to change. Here the team of researchers were able to use a model called CAM 3.0 (you can download the CAM 3.1 source code if you want to take a look). CAM allowed them to try different conditions, and researchers had successfully used it to model times in Earth’s past when the water was completely frozen.
For the starting conditions of their model runs, the researchers created a watery world that was frozen solid without enough incident light to melt it and an atmosphere free of greenhouse gases. These conditions corresponded to an icy world with a groove around the equator. This happened because there was enough light there to sublimate the ice into a gas, which then condensed again closer to the poles.
The team then gradually accelerated the light coming from the planet’s star and tried out different star types – smaller, redder stars produce light that is more easily absorbed by ice, which made a difference. But in all cases, the world remained frozen until more light came in than Earth receives. That’s because the ice itself is so effective at bouncing light back into space that increasing its intensity didn’t make much of a difference.
When the ice finally started to melt, however, it happened quite quickly. The reflectivity of the planet’s surface dropped dramatically, amplifying the warming. This released a lot of water vapor into the atmosphere and that powerful greenhouse gas warmed the planet even further. In most cases, this places the planets in what is called a “humid greenhouse.” Here, water vapor reaches the upper atmosphere, where the star’s higher-energy light splits the water into oxygen and hydrogen, which can then escape into space. The planet would lose all of its water, though it could take more than a billion years to do so.
Smaller planets make things worse, because there is less gravity to hold the atmosphere. But for certain stars (G and F classes), it doesn’t matter how big the planet is. The model shows that they would all go to a completely out of control greenhouse, where they would reach surface temperatures of more than 1000 degrees Celsius and boil away all their surface water.
So why didn’t this happen to Earth? After all, it has been through several global ice ages, periods known as “snowball Earths.” The main factor appears to be carbon dioxide. Its role in warming the planet means the snowball would begin to melt with less sunlight coming in. It also keeps much of Earth’s heat in the lower atmosphere, which cools the altitudes where water vapor would be lost to space.
On Earth, the loss of ice would allow for some warming due to the feedback, but the newly exposed rock would undergo weathering, reacting with the carbon dioxide in the atmosphere and pulling it out again. This has the general effect of tempering warming before any kind of greenhouse climate develops.
Eventually, the Sun will naturally age into a red giant, after which the Earth will lose all of its water. But if you think this is a great opportunity to look for beachfront real estate on one of the water bodies of the outer solar system, the authors suggest you think again. Because of their low gravity, they would quickly lose all their water: “Europa and Enceladus will have no habitable period. They will transition to a humid or runaway greenhouse state when the sun becomes a red giant in 6 to 7 billion years, at when the stellar flux at the location of Europa will reach the snowball melting threshold.”
This of course also has consequences for the habitability elsewhere. For exoplanets with low concentrations of greenhouse gases, entering a snowball state would mean they stay there even at starlight levels that keep Earth at moderate temperatures. And in most cases, abandoning it again would mean there would only be a small window of time (in astronomical terms, at least) before they lost all that water.
Natural Geosciences2017. DOI: 10.1038/NGEO2994 (About DOIs).