We often think about how thin Earth’s atmosphere is, imagining our planet as an apple, with the atmosphere no thicker than the skin of the fruit. That vast blue sky can seem all but infinite, but the great bulk of it is within sixteen kilometers of the surface, always thinning as we climb toward space. Now a presentation by graduate student Laura Schaefer (Harvard-Smithsonian Center for Astrophysics) at the 225th meeting of the American Astronomical Society in Seattle points out that, like the atmosphere, water is also a tiny fraction of what makes up our planet.

A small enough fraction, in fact, that although water does cover seventy percent of the Earth’s surface, it makes up only about a tenth of one percent of the overall bulk of a world that is predominantly rock and iron. Dimitar Sasselov (CfA), co-author of the paper on this work, thinks of Earth’s oceans as a film as thin as fog on a bathroom mirror. But we’ve seen recently that water isn’t strictly a surface phenomenon. The Earth’s mantle, in fact, holds several oceans of water pulled underground by plate tectonics and subduction of the ocean seafloor.

What Schaefer presented at the AAS is a report on her computer simulations of the planet-wide recycling that keeps Earth’s oceans from disappearing. Volcanic outgassing from the mantle, primarily at the mid-ocean ridges, keeps water returning to the surface even as subduction returns water to the mantle. The cycle maintains the oceans over aeons. The question for the researchers was whether similar cycles occur on super-Earths, and how long it would take an ocean to form after the cooling of a planet’s crust during its formation period.

The results are encouraging for those hoping to find stable oceans on super-Earths. Planets two to four times Earth’s mass turn out to be better at maintaining their oceans than Earth itself. Super-Earth oceans can persist for ten billion years unless destroyed by a red giant primary star as it nears the end of its life. The largest planet in these simulations — five times Earth’s mass — took a billion years to develop its ocean in the first place, however, the result of a thicker crust and lithosphere and the resultant delay in volcanic outgassing.


Image: This artist’s depiction shows a gas giant planet rising over the horizon of an alien waterworld. New research shows that oceans on super-Earths, once established, can last for billions of years. Credit: David A. Aguilar (CfA).

We have nothing to compare the timeframe of life’s development on Earth with, having no data on life elsewhere. But if we took our model as the norm, says this CfA news release, we would be wise to look for life on older super-Earths, those perhaps a billion years older than the Earth, given the lag time in getting those oceans into play. Sasselov notes:

“It takes time to develop the chemical processes for life on a global scale, and time for life to change a planet’s atmosphere. So, it takes time for life to become detectable.”

My own guess is that once we do develop the ability to study exoplanet atmospheres on the level of Earth-sized worlds, we’ll run into surprises on this front as well, depending on how typical the experience of getting life started on Earth really was. In any case, screening for older planets as the best targets for complex life seems like a rational procedure, but especially with super-Earths for whom surface water may be a slow-developing resource.

The paper is Schaefer and Sasselov, “Persistence of oceans on Earth-like planets,” American Astronomical Society, AAS Meeting #225, #406.04 (abstract).