(ORDO NEWS) — The earth is approximately 29% land and 71% oceans. How important is this combination for habitability? What does this tell us about the habitability of exoplanets?
There are very few places on Earth where life cannot take root. The general habitability of our planet is influenced by many factors: the abundance of liquid water, plate tectonics, volumetric composition, proximity to the Sun, magnetosphere, etc.
What role does the ratio of oceans to land play?
At the moment, our understanding of habitability is rather crude, although based on evidence. We rely on the habitable zone around stars to find potentially habitable exoplanets. This is a factor that is easy to ascertain from a great distance and is based on the possibility of liquid water on planets.
We are still painting a broader and more detailed picture of habitability, and we know that things like plate tectonics, bulk composition, the magnetosphere, atmospheric composition and pressure, and other factors play a role in habitability.
But what about the ratio of oceans to land on the planet?
A new study looks at this ratio in detail. The study is titled “Diversity of Earth fractions on Earth-like planets and implications for their habitability.” Article submitted to the journal Astrobiology and available on the preprint site arxiv.org. The review has not yet taken place.
The authors are Dennis Hoening and Tilman Spon. Hoening is from the Potsdam Institute for Climate Impact Research in Germany, where he focuses on the interaction between planetary physics and earth system sciences.
Spon is Executive Director of the International Space Science Institute in Bern, Switzerland. Spon was also the principal investigator for the InSight lander’s mole instrument, the Heat Flow and Physical Properties (HP3) package.
Plate tectonics and related factors are at the heart of the problem. Plate tectonics is the movement of continental plates across the Earth’s surface as they move across the surface of the mantle.
Plate tectonics is still an active area of research, and even with everything we’ve learned, there’s still a lot that scientists don’t know.
One of the most important factors in plate tectonics is the “conveyor belt” principle. It says that as the plates submerge back into the mantle at convergent plate boundaries, new oceanic crust is created at the divergent boundaries, which is called seafloor spreading. As a result, the ratio of land to ocean on Earth remains constant.
Since this ratio remains constant, other factors also remain constant. And if these 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 shelves of the Earth 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 biomass of the Earth, while in the depths of the ocean it is much less.
Heat is another factor in plate tectonics and habitability. The continents act like a blanket over the mantle, helping the Earth to keep warm. But this overall effect is mitigated by the depletion of radioactive elements in the mantle.
The radioactive decay of elements such as uranium in the mantle creates heat, which is retained by the effect of a continuous layer of continents.
At the same time, the renewal of the earth’s crust as a result of tectonics brings more of these elements into the earth’s crust, where their heat is removed more efficiently.
The Earth’s carbon cycle is also critical to sustaining life. This cycle is influenced by plate tectonics, as well as the ratio of land and ocean. Continental weathering removes carbon from the atmosphere in roughly equilibrium with the carbon expelled from the mantle by volcanoes.
There is still water content in the mantle. More water in the mantle reduces the viscosity of the mantle, defined as the resistance to flow.
The water content of the mantle is part of a feedback loop with mantle temperature. The more water enters the mantle, the easier it flows. This increases convection, which releases more heat from the mantle.
As explained in the paper, all these factors are connected, usually in feedback loops.
All these and other factors are combined with each other. on Earth to create secure habitability. If the ratio of land to water on Earth were shifted 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 create an oxygen-rich atmosphere.
Conversely, if there were much more water, there might be a shortage of nutrients due to continental weathering. This lack of nutrients also prevents a biosphere large enough to create the oxygen-rich atmosphere needed for complex life and a richer biosphere.
Earth’s tectonics are incredibly detailed and impossible to model. all. Moreover, scientists have not come to a consensus on many details. Much of this is hidden from researchers. They don’t have enough evidence yet to draw firm conclusions.
This study is based on scientific modeling to understand how planets have different land-to-ocean ratios.
Hoening and Spon modeled three main processes that create the land-ocean relationship: the growth of continental crust, the exchange of water between reservoirs on and above the surface (oceans, atmosphere) and in the mantle, and cooling by mantle convection.
“These processes are linked through mantle convection and plate tectonics to:
- Melting and volcanism associated with the subduction zone, and continental erosion driving the growth of continents
- Degassing of mantle water as a result of volcanism and regassing as a result of subduction, which determines the water balance
- Heat transfer by mantle convection, which determines thermal evolution.”
The authors came to one fundamental conclusion.“…the spread of continental cover on planets like Earth is determined by the respective strengths of positive and negative feedback in continental growth and the relationship between thermal cover and radioactive isotope depletion during continental crust growth,” they write.
“The uncertainty in the values of these parameters represents the underlying 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 is probably a staggering number of factors involved in the exoplanet population.
No researcher can model each factor in isolation, but this study comes down to feedback loops between all factors and whether they are positive or negative.
Strong negative feedback “…would lead to an evolution largely independent of the initial conditions and early history of the planet, implying the only stable modern value of continental surface area,” they conclude.
However, strong positive feedback leads to different results. “However, for strong positive feedback, the outcome of evolution can vary greatly depending on initial conditions and early history,” they write.
The question is, do exoplanets form the same feedback loops? Could exoplanets with plate tectonics also achieve an equilibrium between land and ocean coverage? Will the planet become roughly the size of the Earth and with the same heat balance, similar to the Earth with its habitable stability?
First, research shows that both terrestrial planets and oceanic planets are possible, which should come as no surprise. And, of course, we know that mixed planets like Earth are possible.
In a previous article, the same pair of authors concluded that terrestrial planets are the most likely outcome. The next most likely outcome is the ocean planets.
The authors note that in all this work, of course, there are inaccuracies and that there is not enough data. However, 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 feedback processes; we acknowledge that we do not have enough data to understand quantitative differences in detail,” they write.
Other researchers have also dealt with this issue. A 2015 study looked at planets around M dwarfs, the most common type of star in the Milky Way, where we are likely to find the most exoplanets.
This study found “…a similar bimodal distribution of land area that emerged, with most planets either having a surface entirely covered by water or having significantly less surface water than Earth,” the authors write.
However, this study looked at other factors and did not focus solely on continental growth.
What does this study mean for the Earth? How can we answer the question in the title, “What is the best combination of oceans to land on a habitable planet?”
As anthropocentric or terra-centric as it may sound, we could live on the answer.
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