Earth’s surface is approximately 29 percent land and 71% oceans. How important is this mixture for habitability? What does this tell us about exoplanets’ habitability?
It is rare that life does not have a home in any of the places on Earth. Numerous factors are responsible for our planet’s overall habitability, including abundant liquid water and plate tectonics. Bulk composition, proximity to Sun, magnetosphere, and bulk composition.
What role does the ratio oceans to land play in this equation?
Although our understanding of habitability is not perfect, it is based upon evidence. To locate potential habitable exoplanets, we rely on the habitable zones around stars. This is an easy-to-reach factor, based on the potential of liquid water on other planets.
We are still trying to draw a larger, more detailed picture about habitability. We know that factors like plate tectonics and bulk composition, as well as a magnetosphere and atmospheric composition and pressure play a part in habitability.
What about the ratio of land to oceans on a planet?
A new study analyzes this ratio in depth. The study is called “The Land Fraction Diversity of Earth-like Planets: Implications for Their Habitability“The paper was submitted to the journal AstrobiologyThe pre-print version is available at arxiv.org. It has not been peer-reviewed yet.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 Executive Director of the International Space Science Institute (Bern, Switzerland). Spohn was also the principal investigator of the InSight lander’sInstrument called “mole”. Package on Heat Flow & Physical Properties (HP3.)
The root cause of the problem is plate tectonics, and other related factors. Plate tectonics refers to the movement of continental plates on the Earth’s surface as they travel along the mantle.
The research in plate tectonics remains a hot area of inquiry. Even with all that we know, scientists still have a lot to learn.
The “conveyor belt principle” is a critical factor in plate tectonics. According to this principle, plates are subducted into the mantle at converging boundaries. New oceanic crust is formed at divergent boundaries. This is called sea-floor spreading. It is now that Earth’s land to ocean ratio is stable.
This ratio will ensure that other factors are consistent. Habitability is possible if these factors are positive for the biosphere. One of these things is nutrition.
Weathering can cause soil to become exposed, which in turn moves nutrients around the world. Earth’s continental shelves contain biologically rich areas. The reason is that all of the nutrients run-off from continents end up on the shelves. The continents and their shelves are home to most of Earth’s biomass. However, the depths of the ocean contain much less.
Another factor that influences plate tectonics is heat. The continents act like a blanket on the mantle and help Earth retain heat. The mantle’s radioactive elements are gradually depleted, reducing the blanket effect.
Radioactive decay of elements such as uranium within the mantle results in heat that is trapped by the blanket effect of the continents.
The crust is also being renewed by tectonics, which brings more of these elements to its crust. Their heat is then more efficiently shed.
The carbon cycle of Earth is crucial for the survival of life. The land-to-ocean ratio and plate tectonics both affect this cycle. The carbon removed from the atmosphere by the weathering of continents is roughly equal to the carbon emitted by volcanoes from the mantle.
The mantle’s water content is another factor. The mantle has more water, which lowers its viscosity (resistance to flow). A feedback loop is formed between the mantle temperature, which in turn affects the mantle water contents. The mantle will flow more easily if there is more water. Convection increases, which results in more heat being released from the mantle.
The paper explains that all these factors are interrelated, often in feedback loops.
These factors, along with others, combine to make Earth habitable. Earth’s land-to-water ratio would favor more land. Large swathes of the planets could become dry and deserted. The biosphere may not be large enough for an oxygen-rich atmosphere.
If there were less water, it could indicate a deficiency in nutrients due to continental weathering. This lack of nutrients can also prevent a large enough biosphere from producing the oxygen-rich atmosphere required for complex life.
The complexity of Earth’s Earth tectonics is amazing. It’s almost impossible to model it all. It’s not easy to model all of it, especially since scientists aren’t able agree on many details. Many of the details are kept secret from researchers. There isn’t enough evidence yet to draw firm conclusions.
This study was based on scientific modeling that analyzed how different planets had different land-to–ocean ratios.
Höning and Spohn modeled 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 by mantle conection and platetectonics with:
- Subduction zone-related melting, volcanism, continental erosion that governs the growth of the continents
- The mantle water budget is governed by subduction and volcanism.
- Mantle convection regulates the heat transfer process and is responsible for the thermal evolution.
One foundational principle was reached by the authors. 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 regarding these parameter values represents main uncertainty in the modeling.”
These feedback loops will exist on any planet that has tectonic activity or water. It is difficult to determine the relative strength of these feedback loops. The exoplanet population is likely to have a number of factors.
Researchers cannot model every factor. However, this research focuses on the feedback loops that exist between the various factors and their impact on the overall outcome.
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 Final conclusion.
But, positive feedback loops that are strong and provide positive feedback can have different outcomes. “For strong positive feedback however, the evolution of the evolution could be quite different depending upon the beginning conditions and the early history.” Write.
These feedback loops can also shape exoplanets, but the question is: Exoplanets that have plate tectonics can also achieve an equilibrium between ocean and land coverage. A planet approximately Earth-sized, with a similar heat bill, and its life-enabling stability, could end up being similar to Earth.
First, it is clear that both ocean and land planets are possible. This shouldn’t be surprising. Of course, mixed planets, such as Earth, are possible.
In a previous paperAccording to the same authors, land planets are most likely. Ocean planets are the next most likely outcome.
The authors acknowledge that there are uncertainties and a dearth of data. However, their research sheds light onto the mechanisms that cause different ratios of ocean to land on planets.
“Our discussion aims at providing a better qualitative understanding the feedback processes; however, we confess to not having sufficient data for a detailed understanding quantitative differences.” Write.
This issue has also been addressed by other researchers. A 2015 studyWe examined planets around M.dwarfs as they are the most common type in the Milky Way. This is where we expect to find the most exoplanets.
This 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.
However, the study did not focus on only continental growth.
What does this study have to do with Earth? How do we answer the headline question: “What is the best mix of oceans and land for a habitable planet?”
It doesn’t matter if it sounds anthropocentric or even terracentric, we might be living on the truth.
This article was first published by Universe Today. Please read the Original article.
