The Earth is approximately 29% land and 71% ocean. How important is this mix for habitability? What does it tell us about the habitability of exoplanets?
There are very few places on Earth where life has not taken hold. Multiple factors contribute to the overall habitability of our planet: abundant liquid water, plate tectonics, volume composition, proximity to the Sun, the magnetosphere, etc.
What role does the relationship of the oceans to the land play?
Our understanding of habitability is quite crude at this point, although it is evidence-based. We rely on the habitable zone around stars to locate potentially habitable exoplanets. This is an easy factor to determine at great distances and based on the liquid water potential on the planets.
We’re always 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 part in habitability.
But what about the relationship of the oceans to the land of a planet?
A new study examines this ratio in detail. The study is “Land Fraction Diversity on Earth-like Planets and Implications for their Habitability” The article has been submitted to the journal Astrobiology and is available on the preprint 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 It has not yet been peer reviewed.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 the executive director of the International Institute of Space Science in Bern, Switzerland. Spohn was also the principal investigator of the InSight lander’s “mole” instrument, the Heat Flow and Physical Properties Package (HP3.)
Plate tectonics and related factors are causing the problem. Plate tectonics is the movement of continental plates on the Earth’s surface as they overlap the mantle.
Plate tectonics is still an active area of research, and even with all that 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. He says that when plates are pushed back into the mantle at convergent plate boundaries, new oceanic crust is created at divergent boundaries, called seabed spreading. The result is that the Earth’s land-ocean ratio remains constant.
This ratio remaining constant, other factors also remain consistent. And if these factors favor the biosphere, it is good for habitability. One of those things is nutrients.
Exposed land is prone to weathering, which moves nutrients around the world. The Earth’s continental shelves are biologically rich areas. One reason is that all the nutrient runoff from the continents ends up on the shelves. Thus, the continents and their shelves contain most of the terrestrial 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 Earth retain heat. But this covering effect is moderated by the depletion of radioactive elements in the mantle.
The radioactive decay of elements like uranium in the mantle creates heat that is trapped by the continental blanket effect.
At the same time, tectonic turnover of the crust brings more of these elements to the crust, where their heat is more efficiently removed.
The Earth’s carbon cycle is also essential for sustaining life. This cycle is affected by plate tectonics and also by the land-ocean relationship. The weathering of the continents removes carbon from the atmosphere roughly in equilibrium with the carbon emitted from the mantle by volcanoes.
Then there is the water content in the mantle. More water in the mantle lowers mantle viscosity, defined as resistance to flow. Mantle water content is part of a feedback loop with mantle temperature. The more water penetrates the mantle, the more easily it drains. This increases convection, which releases more heat from the mantle.
As the document explains, all of these factors are linked, usually in feedback loops.
All of these factors and more combine on Earth to create robust habitability. If Earth’s land to water ratio were skewed towards more land, then the climate would be much drier, and large parts of the continents might be cold, dry deserts, and the biosphere might not be large enough to produce a oxygen-rich atmosphere.
Conversely, if there was much more water, there may be a lack of nutrients due to continental weathering. This lack of nutrients also prohibits a biosphere large enough to produce the oxygen-rich atmosphere necessary for complex life and a richer biosphere.
There is an extraordinary amount of detail in Earth’s tectonics, and it’s impossible to model everything. Especially since scientists have not reached consensus on many details. Much of it is hidden from researchers. They don’t yet have enough evidence to draw solid conclusions.
This study used scientific modeling to understand how planets have different land-ocean relationships.
Höning and Spohn modeled the three main processes that create the land-ocean relationship: growth of the continental crust, exchange of water between reservoirs on the surface and above (oceans, atmosphere) and in the mantle, and cooling by mantle convection.
“These processes are linked by mantle convection and plate tectonics with:
- melting and volcanism related to the subduction zone, and continental erosion governing the growth of continents
- degassing of mantle water by volcanism and regassing by subduction governing the water balance
- heat transfer by mantle convection governing the thermal evolution.”
The authors have reached a fundamental conclusion. “…the spread of continental cover on Earth-like planets is determined by the respective strengths of positive and negative feedbacks in continental growth and by the relationship between thermal cover and the depletion of radioactive isotopes during continental growth. continental crust,” they write.
“The uncertainty in these parameter values represents the main uncertainty of 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 factor, but this research boils down to the feedback loops between all the factors and whether they are positive or negative.
A strong negative feedback “…would lead to an evolution largely independent of starting conditions and ancient planetary history, implying a single stable present value of land area,” they conclude.
Strong positive feedback loops, however, create different outcomes. “For strong positive feedback, however, the evolutionary outcome can be quite different depending on starting conditions and early history,” they write.
The question is: do these same feedback loops shape exoplanets? Can plate-tectonic exoplanets also reach a balance between land and ocean cover? Will a planet roughly the size of Earth and with a similar thermal budget end up looking like Earth, with its vital stability?
First, the research shows that terrestrial planets and oceanic planets are both 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 uncertainties in all of this work, of course, and that there is a lack of data. Yet their work sheds light on the mechanisms that create different land/ocean relationships on the planets.
“Our discussion aims to provide a better qualitative understanding of feedback processes; we admit to lacking data for a detailed understanding of quantitative differences,” they write.
Other researchers have also looked into this problem. 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.
This study found “…a similar bimodal distribution of land surface area, with most planets either having their surfaces completely covered in water or with significantly less surface water than Earth,” the authors write.
This study, however, looked at other factors and was not solely focused on continental growth.
What does this study mean for the Earth? How can we answer the title question: “What is the best mix of oceans to land on a habitable planet?”
As anthropocentric or terracentric as it sounds, we could live off the answer.
This article was originally published by Universe Today. Read the original article.
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