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, the initial accretion of the planet, and the radioactive decay of heavy elements – can drive large convective currents of rocky material in these bodies, similar to the circular movement of water boiling in a pot.
The solid surface of the earth, or lithosphere, extends more than a hundred miles into the planet. It has broken into large pieces and the convective currents of molten rock beneath it exert pressure on these pieces, causing them to push against each other, slide past and separate from each other. This process, called plate tectonics, is responsible for earthquakes, volcanoes, ocean ridges and vast mountain ranges on the Earth’s surface.
We know that other rocky bodies, such as Venus, are also geologically active. But do they also have plate tectonics? And how often does plate tectonics occur in other bodies in the universe?
Two main types of convective regimes have been observed in the solar system, according to Cédric Gillmann, a planetary scientist at the Institute for Geophysics at ETH Zurich. While Earth has plate tectonics, there are other bodies – such as Mars, Mercury And Earth’s moon – have a so-called stationary lid.
“Basically, a stationary lid indicates a static surface, while plate tectonics has a mobile surface divided into plates,” Gillmann said. “Plate tectonics is a specific type of mobile lid regime, that is, a mantle dynamics regime where the lid (lithosphere) participates in the convective cell.
As such, it is characterized by high surface mobility compared to other regimes. The surface has a lateral velocity (in short, it’s moving),” Gillmann told Space.com in an email.
Related: Earth’s oldest crystals reveal the age of plate tectonics
Plate tectonics is unique because the entire surface is divided into shells, or plates with sharp boundaries. SoilThese boundaries are known as subduction zones, where plates are forced under each other; transform zones, where the plates slide past each other; and divergent zones, where they move away from each other. When looking at maps of the Earth, it is easy to see the geological features associated with these boundaries, with most of Earth’s volcanic activity occurring along these boundaries.
In contrast, Gillmann said a stationary lid has a largely static lithosphere. Although there may be convection currents in the mantle of a planet or moon beneath the lithosphere, these currents do not provide enough tension to break the lithosphere or drag it into a stationary cover.
“The mantle can get quite hot because the lid provides good insulation between the interior and the atmosphere, and the convection can be intense – but in a stationary lid the lithosphere is decoupled from the mantle,” Gillmann said.
“Standing lid is generally thought of as the end member that all planets eventually converge towards as they lose their internal heat (and the lithosphere grows), but it is thought that some hot planets may still maintain high activity and remain in a stationary lid can stay,” he added. .
What about other bodies in the solar system?
Mercury and Earth’s moon are examples of terminal stagnant lids, where the insides have cooled to the point where there is no longer convection because they do not have enough material to retain their internal heat. Mars was thought to be the same, Gillmann said, but observations of relatively recent volcanic activity and the discovery of a possible mushy or molten layer at the core-mantle boundary suggests a warm interior.
“The regime of Venus is uncertain,” Gillmann said. The surface is dominated by volcanic features 80% of the surface covered with basalt (cooled lava)and it has many volcanoes and the longest lava flows in the solar system.
“It also has a strongly deformed surface, indicating that it is tectonically active, or has been recently so,” Gillmann added. Most of Venus’s surface is thought to be quite young: 200 million to 1 billion years old, compared to the roughly 4 billion year old surface of Mars.
Two possible scenarios could explain Venus’s geological activity: an episodic lid, where the dynamics change from a mobile lid to a stationary lid from time to time, or the plutonic-squishy lid regime, where the surface is largely static but magma can penetrate the lithosphere, creating a ductile, deformable surface arises.
Venus, Earth and Mars all started out as bodies of relatively equal size formed from a roughly similar region of materials in the early solar system. But several other factors, some unknown, caused these planets to develop geological differences.
“Mars is probably dry and small, which contributes to its current state, although there may be more to it,” Gillmann said. “Venus is the greatest unknown. We have very little precise information about the interior of Venus, its structure and composition. Everything we see seems consistent with the fact that Venus is not very different from Earth (size, mass, density, probably composition), but is small.” differences could prove to have important consequences,” he added.
Despite their similar building blocks, Earth and Venus followed two very different geological paths. Surface temperatures may have played a role, causing Earth to have liquid water on its surface. which could promote plate tectonics. In the case of Venus, having a softer, more ductile lithosphere due to high temperatures could also have made it harder to break apart, or made it easier to heal. Water in the mantle likely played a role as well, as it affects the mantle’s viscosity and melting temperature, Gillmann explained.
“But we have no certainty about the water in Venus’s interior; it is thought to be very dry, and there may be reasons for that, given its very early history,” Gillmann said. “But nothing proves it yet. If all goes well, the upcoming missions could help answer that.”
Plate tectonics in exoplanets
As for determining whether exoplanets If we have plate tectonics, scientists still have a way to go.
There are two main ways we can infer what might be happening in the interior of an exoplanet. One is detecting the planet’s magnetic field, which can tell scientists whether materials are moving inland, “although we’re still not sure how that fully relates to plate tectonics,” Gillmann said.
The second method is to analyze the exoplanet’s atmosphere. We know that Plate tectonics influences the carbon cycle on Earthso planets with a regulated amount of carbon dioxide may have some form of plate tectonics, while planets with a huge amount of CO2 may not have plate tectonics.
Plate tectonics and complex life
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—What color is the sunset on other planets?
Some researchers have argued that Plate tectonics played a crucial role in the emergence of complex life on EarthThis suggests that advanced civilizations may only be possible with complex geological regimes that include surface continents and oceans.
This raises the question of whether the presence of plate tectonics could limit our search for complex life elsewhere in the cosmos.
“For complex life, I don’t have a strong opinion, although a case can be made that plate tectonics puts pressure on species that pushes evolution forward while stabilizing surface conditions and avoiding major brutal changes that could lead to total extinction,” Gillmann said. “As a result, it may well be one of the necessary ingredients for the complex recipe of life.”
If this is the case, and we can convincingly identify plate tectonics on an exoplanet, such a planet would become an excellent candidate for research.