by The interiors of rocky planets and moons are usually quite hot compared to their surfaces. This heat, which can be caused by a number of sources — such as tidal stretching and compression, initial accretion of the planet, and radioactive decay of heavy elements — can drive large convective currents of rocky material within the bodies. this, like the. circular motion of boiling water in a pot.
The solid surface of the Earth, or lithosphere, extends more than a hundred miles into the planet. It is broken into large chunks, and the convective currents of molten rock below exert pressure on these chunks, causing them to erupt and slide apart and break apart. This process, known as plate tectonics, is responsible for earthquakes, volcanoes, ocean ridges and huge mountain ranges on the Earth’s surface.
We know that there are other rocky bodies, for example Venus, which is also geologically active. But do they also have plate tectonics? And how common is plate tectonics in other bodies in the universe?
There are two main types of convection regimes observed in the solar system, according to Cédric Gillmann, a planetary scientist at the Institute of Geophysics at ETH Zürich. Although the Earth has plate tectonics, some other bodies — for example Mars, mercury and Earth’s Moon — have a stagnant cover.
“In short, a stagnant cover represents a static surface, while plate tectonics has a mobile surface divided into plates,” said Gillmann. “Plate tectonics is a specific type of mobile plate regime, which is the regime of mantle dynamics in which the mantle (lithosphere) participates in the convection cell.
Therefore, it is characterized by high surface mobility compared to other regimes. The surface has a lateral velocity (in short, it moves),” Gillmann told Space.com in an email.
Related: The world’s oldest crystals indicate the age of plate tectonics
Plate tectonics is unique in that the entire surface is divided into shells, or plates with sharp boundaries. Ar World, these boundaries are called subduction zones, where plates are forced under each other; changing belts, where the plates slide over each other; and different zones, where they go away from each other. When we look at maps of the Earth, it is easy to see the geological features associated with these boundaries, with most of the world’s volcanic activity located along them.
In contrast, Gillmann said, a stagnant mantle has a largely static lithosphere. Although convection currents may exist in the mantle of a planet or moon beneath its lithosphere, these currents do not provide enough stress to break or pull the lithosphere along in a stagnant program.
“The mantle can get quite hot because the mantle provides good insulation between the interior and the atmosphere, and convection can be vigorous – but in a stagnant mantle, the lithosphere is decoupled from the mantle,” a Gillmann said.
“A stagnant cover is generally seen as an end member where all the planets eventually come together as they lose internal heat (and the lithosphere grows), but it is thought that some hot planets could sustain high activity and remain under a stagnant cover ,” he said. .
What about other bodies in the solar system?
Mercury and Earth’s moon are examples of stagnant terminal envelopes, where the interior has cooled to the point where there is no convection in their interior, since they do not have enough material to sustain their internal heat. It was thought that Mars was the same, said Gillmann, but opinions on recent volcanic activity and the discovery of a possible submerged or molten layer at the core-mantle boundary suggests a warm interior.
“The reign of Venus is uncertain,” said Gillmann. Its surface is dominated by volcanic features, with 80% of its surface covered with basalt (cooled lava)and has many volcanoes and the longest lava flows in the solar system.
“It also has a highly deformed surface, which indicates that it is tectonically active, or has been recently,” Gillmann said. Most of the surface of Venus is thought to be quite young – between 200 million and 1 billion years old, compared to the surface of Mars which is about 4 billion years old.
Two possible scenarios could explain the geological activity of Venus: episodic cover, in which the dynamics change from mobile to stagnant cover from time to time, or the plutonic-squishy-lid regime, where the surface is mostly static but magma can enter the lithosphere, at creating a stretchable, deformable surface.
Venus, Earth and Mars all started out as bodies of the same size made from a roughly similar region of matter in the early solar system. But various other factors, some unknown, caused the planets to develop geological differences.
“Mars is probably dry and small, which contributes to its current state, although there could be more,” Gillmann said. “Venus is the great unknown. We have very little precise information about the interior of Venus, its structure and composition. Everything we see seems to correspond to a Venus that is not very different from Earth (size, mass, density, probably composition), but differences could have important consequences,” he said.
Despite their similar building blocks, Earth and Venus have taken two very different geological paths. Surface temperatures may have played a role, allowing Earth to have liquid water on its surface, which may favor plate tectonics. In the case of Venus, it may have been more difficult to break up, or allowed to heal more easily, if it had a softer and more ductile lithosphere due to high temperatures. Water probably also played a role in the mantle, as it affects the viscosity and melting temperature of the mantle, Gillmann explained.
“But we have no certainty about water in the interior of Venus; it is hypothesized to be very dry, and there may be reasons for that, looking at its very early history,” said Gillmann. “But nothing proves it yet. If it goes well, the upcoming missions could help answer that.”
Plate tectonics in exoplanets
As far as determining which exoplanet have plate tectonics, scientists still have a way to go.
There are two main ways we can figure out what might be happening inside an inner exoplanet. One is to detect the planet’s magnetic field, which can tell scientists if materials are moving around inside, “although we’re still not sure how that relates entirely to plate tectonics,” which Gillmann said.
The second method is to analyze the atmosphere of the exoplanet. We know that plate tectonics affects the carbon cycle on Earthso planets with an abundance of carbon dioxide may have a form of plate tectonics, while planets with a huge abundance of CO2 may lack plate tectonics.
Plate tectonics and complex life
Related stories:
—If Venus had Earth-like plate tectonics in the past, did it also have life?
—Do extraterrestrial auroras occur on other planets?
—What color is the sunset on another planet?
Some researchers have argued that Plate tectonics played a critical role in the emergence of complex life on Earthsuggesting that advanced civilizations could only be possible with complex geological regimes that included surface continents and oceans.
This raises the question of whether the presence of plate tectonics could help narrow our search for complex life elsewhere in the cosmos.
“In the case of complex life, I have no strong opinion, although a case can be made that plate tectonics puts stress on species that advance evolution and stabilize surface conditions and avoid large, violent changes that could be cause total extinction,” Gillmann said. “As a result, it could well be one of the necessary ingredients for this complex life recipe.”
If this is the case, and we can convincingly identify plate tectonics on an exoplanet, it would make such a planet a prime candidate for survey.
