The Earth is about 29 percent land and 71 percent oceans. How important is that mix for habitability? What does it tell us about the habitability of exoplanets?
There are few places on earth where life has no foothold. Multiple factors contribute to the overall habitability of our planet: abundant liquid water, plate tectonics, bulk composition, proximity to the sun, the magnetosphere, etc.
What role does the relationship between oceans and land play?
Our understanding of habitability is pretty crude at this point, though it’s based on evidence. We rely on the habitable zone around stars to locate potentially habitable exoplanets. It is a factor that can be easily determined from a great distance and is based on the potential for liquid water on planets.
We’re still drawing a bigger, more detailed picture of habitability, and we know that things like plate tectonics, bulk composition, a magnetosphere, atmospheric composition and pressure, and other factors play a role in habitability.
But what about the ratio of oceans to land on a planet?
A new study examines that ratio in detail. The study is”Land faction diversity on Earth-sized planets and implications for their habitability“The article has been submitted to the journal Astrobiology and is available on the pre-print site arxiv.org. It has not yet been peer-reviewed.
The authors are Dennis Höning and Tilman Spohn. Höning is from the Potsdam Institute for Climate Impact Research in Germany, where he focuses on the interface between planetary physics and Earth system science.
Spohn is Executive Director of the International Space Science Institute in Bern, Switzerland. Spohn was also the principal investigator for the InSight landers “mole” instrument, the Package heat flux and physical properties (HP3.)
Plate tectonics and related factors are at the root of the problem. Plate tectonics is the movement of the continental plates on the Earth’s surface as they travel across the mantle.
Plate tectonics is still an active area of research, and even with all we’ve learned, there’s still a lot that scientists don’t know.
One of the critical factors in plate tectonics is the “conveyor belt” principle. It says that as plates are pushed back into the mantle at convergent plate boundaries, new oceanic crust is created at divergent boundaries, called seafloor spreading. As a result, Earth’s land-ocean ratio remains consistent.
Because that ratio remains consistent, other factors also remain consistent. And if those factors stimulate the biosphere, it’s good for habitability. One of those things is nutrients.
Exposed land is subject to weathering, which moves nutrients around the world. The continental plates of the Earth are biologically rich areas. One reason is that all the nutrients that run off the continents end up on the shelves. So the continents and their shelves contain most of the Earth’s biomass, while there is much less in the deep ocean.
Heat is another factor in plate tectonics and habitability. The continents act as a blanket over the mantle, helping the Earth to retain heat. But that blanket effect is tempered by the depletion of radioactive elements in the mantle.
Radioactive decay of elements such as uranium in the mantle creates heat that is trapped by the blanketing effect of the continents.
At the same time, crustal renewal through tectonics brings more of these elements to the crust, where their heat is dissipated more efficiently.
Earth’s carbon cycle is also critical to sustaining life. That cycle is influenced by plate tectonics and also by the land-ocean relationship. The weathering of continents removes carbon from the atmosphere, roughly in equilibrium with the carbon ejected from the mantle by volcanoes.
Then there is the water content in the mantle. More water in the mantle lowers the viscosity of the mantle, defined as resistance to flow. Jacket water content is part of a jacket temperature feedback loop. As more water enters the mantle, it flows more easily. That increases convection, releasing more heat from the mantle.
As the paper explains, all of these factors are interrelated, usually in feedback loops.
All of these factors and others combine on Earth to create robust habitability. If the Earth’s land-to-water ratio were more land, then the climate would be much drier and large parts of the continents could be cold, dry deserts, and the biosphere would not be large enough to support an oxygen-rich atmosphere to produce.
Conversely, if there was much more water, there may be a nutrient deficiency due to continental weathering. That lack of nutrients also prevents a large enough biosphere needed to produce the oxygen-rich atmosphere needed for complex life and a richer biosphere.
There is an extraordinary amount of detail in Earth’s tectonics and it is impossible to model it all. Especially since scientists have not yet reached a consensus on many details. Much of it is hidden from researchers. They don’t have enough evidence yet to draw firm conclusions.
This study relied on scientific modeling to understand how planets have different land-to-ocean ratios.
Höning and Spohn modeled the three main processes that create the land-ocean relationship: growth of the continental crust, exchange of water between the reservoirs on and above the surface (oceans, atmosphere) and in the mantle, and cooling by mantle convection.
Of the paper:
“These processes are linked through mantle convection and plate tectonics to:
- subduction zone related melting and volcanism, and continental erosion controlling the growth of the continents
- outgassing of mantle water by volcanism and regasification by subduction that regulates the water budget
- heat transfer by mantle convection that controls thermal evolution.”
The authors reached one base conclusion. “…the distribution of continental cover on Earth-like planets is determined by the respective strengths of positive and negative feedback in continental growth and by the relationship between thermal blanket and radioactive isotope depletion in continental crust growth,” she to write.
“Uncertainty in these parameter values represents the main uncertainty in the model.”
These feedback loops will be present on any planet with tectonic activity and water. The relative strength of these loops is difficult to quantify. There are probably a bewildering number of factors at play in exoplanet population.
No researcher can model every single factor, but this research boils down to the feedback loops between all factors and whether they are positive or negative.
Strong negative feedback “…would lead to an evolution largely independent of the planet’s initial conditions and early history, which would imply a single stable current value of the continental surface,” they argue to conclude.
However, strong positive feedback loops produce different results. “However, for strong positive feedbacks, the outcome of evolution may be very different depending on starting conditions and early history,” they say. to write.
The question is, do these same feedback loops form exoplanets? Can exoplanets with plate tectonics also achieve a balance between land and ocean cover? Will a planet roughly the size of Earth and with a similar heat budget ultimately be comparable to Earth, with its stability that supports life?
First of all, the research shows that both land planets and ocean planets are possible, which should come as no surprise. And of course we know that mixed planets like Earth are possible.
In a previous paper, the same pair of authors concluded that terrestrial planets are the most likely outcome. The next most likely outcome is ocean planets.
The authors point out that there are of course uncertainties in all this work and a lack of data. Still, their work sheds light on the mechanisms that create different land-to-ocean ratios on planets.
“Our discussion aims to provide a better qualitative understanding of the feedback processes; we admit that we have no data for a detailed understanding of quantitative differences,” they say. to write.
Other researchers have also dealt with this problem. A Study from 2015 looked at planets around M dwarfs, the most common type of star in the Milky Way, and where we’re likely to find the most exoplanets.
That study found “… a similar bimodal distribution of the resulting land area, with most planets either having their surfaces completely covered by water or with significantly less surface water than Earth,” the authors to write.
However, that study looked at other factors and did not just focus on continental growth.
What does this study mean for the Earth? How can we answer the question in the headline, “What is the best mix of oceans to land on for a habitable planet?”
As anthropocentric or terracentric as it may sound, we could live off the answer.