Earth is about 29% land and 71% oceans. How significant is that mix for habitability? What does it tell us about exoplanet habitability?
There are very few places on Earth where life doesn’t have a foothold. Multiple factors contribute to our planet’s overall habitability: abundant liquid water, plate tectonics, bulk composition, proximity to the Sun, the magnetosphere, etc.
What role does the ratio of oceans to land play?
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Our understanding of habitability is pretty crude at this point, though it is based on evidence. We rely on the habitable zone around stars to locate potentially habitable exoplanets. It’s a factor that’s easy to ascertain 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 a planet’s ratio of oceans to land?
A new study examines that ratio in detail. The study is “Land Fraction Diversity on Earth-like Planets and Implications for their Habitability” The paper’s been submitted to the journal Astrobiology and is available on the pre-print site arxiv.org. It hasn’t been peer-reviewed yet.
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 sciences. Spohn is the Executive Director of the International Space Science Institute in Bern, Switzerland. Spohn was also the principal investigator for the InSight lander’s “mole” instrument, the Heat Flow and Physical Properties Package (HP3.)
Plate tectonics and related factors are at the root of the issue. Plate tectonics is the movement of the continental plates on the surface of the Earth as they ride along on top of 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 subducted back into the mantle at converging plate boundaries, new oceanic crust is created at divergent boundaries, called sea-floor spreading. The result is that Earth’s land-to-ocean ratio remains consistent.
With that ratio staying consistent, other factors remain consistent, too. And if those factors encourage the biosphere, that’s good for habitability. One of those things is nutrients.
Exposed land is subject to weathering, which moves nutrients around the globe. Earth’s continental shelves are biologically rich areas. One reason is that all the nutrient run-off from the continents ends up on the shelves. So the continents and their shelves contain most of Earth’s biomass, while there’s 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 Earth retain warmth. But that blanket effect is moderated by the depletion of radioactive elements in the mantle. Radioactive decay of elements like uranium in the mantle creates heat that’s trapped by the continents’ blanket effect. At the same time, crust renewal through tectonics brings more of these elements to the crust, where their heat is more efficiently shed.
Earth’s carbon cycle is critical to sustaining life, too. That cycle is affected by plate tectonics and also by the land-to-ocean ratio. The weathering of continents removes carbon from the atmosphere roughly in equilibrium with the carbon emitted from the mantle by volcanoes.
Then there’s the water content in the mantle. More water in the mantle lowers the mantle’s viscosity, defined as resistance to flow. Mantle water content is part of a feedback loop with mantle temperature. As more water enters the mantle, it flows more easily. That increases convection, which releases more heat from the mantle.
As the paper explains, all of these factors are related, usually in feedback loops.
All of these factors and others combine on Earth to create robust habitability. If Earth’s ratio of land to water were biased toward more land, then the climate would be much dryer, and large portions of the continents could be cold, dry deserts, and the biosphere might not be large enough to produce an oxygen-rich atmosphere.
Conversely, if there was much more water, there may be a lack of nutrients from continental weathering. That lack of nutrients also prohibits a large enough biosphere needed to produce the oxygen-rich atmosphere necessary for complex life and a richer biosphere.
There’s an extraordinary amount of detail in Earth’s tectonics, and it’s impossible to model it all. Especially since scientists haven’t reached a consensus on many of the details. Much of it’s hidden from researchers. They don’t have enough evidence to make solid conclusions yet.
This study relied on scientific modelling to understand how planets have different land-to-ocean ratios. Höning and Spohn modelled the three main processes that create the land-to-ocean ratio: growth of continental crust, exchange of water between the reservoirs on and above the surface (oceans, atmosphere) and in the mantle, and cooling by mantle convection.
From the paper:
“These processes are linked through mantle convection and plate tectonics with:
- subduction zone-related melting and volcanism, and continental erosion governing the growth of the continents
- mantle water degassing through volcanism and regassing through subduction governing the water budget
- heat transfer through mantle convection governing the thermal evolution.”
The authors reached one foundational conclusion. “… the spread of continental coverage on Earth-like planets is determined by the respective strengths of positive and negative feedback in continental growth and by the relationship between thermal blanketing and depletion of radioactive isotopes upon the growth of the continental crust,” they 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 hard to quantify. There are likely a bewildering number of factors at play throughout the exoplanet population.
No researchers can model every single factor, but this research comes down to the feedback loops between all the factors and whether they’re positive or negative. Strong negative feedback “… would lead to an evolution largely independent of the starting conditions and the early history of the planet, which would imply a single stable present-day value of the continental surface area,” they conclude.
Strong positive feedback loops create different results, though. “For strong positive feedback, however, the outcome of the evolution may be quite different depending on starting conditions and the early history,” they write.
The question is, do these same feedback loops shape exoplanets? Can exoplanets with plate tectonics also reach an equilibrium between land and ocean coverage? Will a planet roughly Earth-sized and with a similar heat budget end up similar to Earth, with its life-enabling stability?
First of all, the research shows that land planets and ocean planets are both possible, which shouldn’t come as a surprise. And, of course, we know that mixed planets like Earth are possible.
In a previous paper, the same pair of authors concluded that land planets are the most likely outcome. The next most likely outcome is ocean planets.
The authors point out that there are uncertainties in all of this work, of course, and that there’s a lack of data. Still, their work sheds light on the mechanisms that create different ratios of land to ocean on planets. “Our discussion aims to provide a better qualitative understanding of the feedback processes;
we admit to lacking data for a detailed understanding of quantitative differences,” they write.
Other researchers have tackled this issue, too. A 2015 study 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 emerged land area, with the most planets either having their surface entirely covered with water or with significantly less surface water than Earth,” the authors write. That study, however, looked at other factors and wasn’t focused solely on continental growth.
What does this study mean for Earth? How can we answer the question in the headline: “What’s the Best Mix of Oceans to Land for a Habitable Planet?”
As anthropocentric or terracentric as it might sound, we could be living on the answer.
- New Research: Land Fraction Diversity on Earth-like Planets and Implications for their Habitability
- Universe Today: Earthlike Worlds With Oceans and Continents Could be Orbiting red Dwarfs, Detectable by James Webb
- Universe Today: Exoplanets Will Need Both Continents and Oceans to Form Complex Life
2 Replies to “What’s the Best Mix of Oceans to Land for a Habitable Planet?”
A very interesting paper, and I should say that it references some of my own work (with Rodrick Wallace). One of the tenants of science is that there should be a predictive quality (or at least it’s better if there is); and the work can be validated by other unrelated work. In this instance, the model, covering continental growth and feedback, produces near identical results as Lingam and Loeb’s 2019 papers; and my work on landscape and biological diversity.
Lingam and Loeb’s work modeled biological diversity, linked to nutrient cycles and observation of land area; while our work took crude models of continental growth and related these to available land area and species diversity.
That each of these models and that of Höning and Spohn, reach the same conclusion tends to reinforce the idea that the evolution of continental (land) area is critical in determining the evolution of complex, diverse biospheres. The model does not address the likelihood that life is or is not abundant, per se, but does contribute to our understanding of biological diversity on those planets that permit the development of life in the first place.
It is in principle easy to use the two end members of a dry planet and a massive water world with deep water and ices constraining nutrient mixing to find that the optimum lies quite close to Earth. And additionally on Earth plate tectonic speed up nutrient recycling and stabilize the climate. But the analysis of feedback cycles is interesting.
@ Stevenson: I’m not sure the two papers of Höning and Spohn respectively Lingam and Loeb can be independently tested since the previous rely on the latter to model the net primary productivity part. But the latter is interesting since they quantify the naive “just so” model and find a 1:1 ocean-to-land ratio is near optimal and – if complex organisms analogous to eukaryote based vascular plants have evolved – have two orders of magnitude more primary productivity on land. Plate tectonics was not considered in the basic Lingam and Loeb model, what I can see.
I’m not so sure about the applicability of the Stevenson and Wallace paper in Astrobiology. Again, the naive model above has by default more geographical diversity correlated with optimum net primary productivity. But that doesn’t translate easily to ecological diversity among prokaryotes, who are not as diverse as eukaryotes. The problem with eukaryotes is that while life evolved early and diversify rapidly so seems an easy enough process, eukaryotes evolved once and late on Earth. And vascular plants even later, indicating that it is likely very rare and possibly unique. [Disclaimer: I first found Wallace work on unsuccessfully criticising population genomics. That fringe industry bias me – as much as the H&S and L&L papers are biasing any proposed tests – for starters, it seems so futile. While studying population genomics is awesome! But I hope I got the science here correct anyway.]
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