The interiors of rocky planets and moons tend to be pretty hot compared with 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 convection currents of rocky materials inside these bodies, similar to the circular motion of water boiling in a pot.
Earth's solid surface, or lithosphere, extends more than a hundred miles into the planet. It's broken into large chunks, and the convection currents of molten rock below exert pressure on these chunks, causing them to push up against, slide by and separate from one another. This process, called plate tectonics, is responsible for earthquakes, volcanoes, ocean ridges and vast mountain ranges on Earth's surface.
We know that other rocky bodies, such as Venus, are geologically active, too. 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 that have been observed in the solar system, according to Cédric Gillmann, a planetary scientist at the Institute of Geophysics at ETH Zürich. Whereas Earth has plate tectonics, some other bodies — such as Mars, Mercury and Earth's moon — have what is called a stagnant lid.
"In short, a stagnant 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) takes part in the convection cell.
As such, 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: Earth's oldest crystals reveal age of plate tectonics
Plate tectonics is unique in that the entire surface is divided into shells, or plates that have sharp boundaries. On Earth, these boundaries are known as subduction zones, where plates are forced underneath each other; transform zones, where the plates slip past each other; and divergent zones, where they move away from each other. When we look at maps of Earth, it is easy to see the geological features associated with these boundaries, with most of the volcanic activity on Earth being located along them.
By contrast, Gillmann said, a stagnant lid has a mostly static lithosphere. While there may be convection currents in the mantle of a planet or moon below its lithosphere, these currents do not provide enough stress to break or drag the lithosphere along with them in a stagnant lid.
"The mantle can become quite hot because the lid provides a good insulation between the interior and the atmosphere, and convection can be vigorous — but in a stagnant lid, the lithosphere is decoupled from the mantle," Gillmann said.
"Stagnant lid is generally seen as the end-member toward which all planets eventually converge when they lose internal heat (and lithosphere grows), but it is thought that some hot planets could still sustain high activity and remain in stagnant lid," he added.
What about other bodies in the solar system?
Mercury and Earth's moon are examples of terminal stagnant lids, where the interiors have cooled to the point where there is no convection in their interiors, since they do not have enough material to sustain 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 suggest a warm interior.
"Venus' regime is uncertain," Gillmann said. Its surface is dominated by volcanic features, with 80% of its surface covered in basalt (cooled lava), and it has many volcanoes and the longest lava flows in the solar system.
"It also has a much deformed surface, indicating it is tectonically active, or has been recently," Gillmann added. Most of Venus' surface is thought to be quite young — 200 million to 1 billion years old, compared with Mars' roughly 4 billion-year-old surface.
Two possible scenarios could explain Venus' geologic activity: an episodic lid, where the dynamics change from a mobile to a stagnant lid from time to time, or the plutonic-squishy-lid regime, where the surface is mostly static but magma can enter the lithosphere, creating a ductile, deformable surface.
Venus, Earth and Mars all started off as relatively similarly sized bodies that formed from a roughly similar region of materials in the early solar system. But various 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 major unknown. We have very little precise information about the interior of Venus, its structure and composition. All we see seems consistent with Venus not being very different from Earth (size, mass, density, probably composition), but small differences may prove to have important consequences," he added.
Despite their similar building blocks, Earth and Venus took two very different geological paths. Surface temperatures may have played a role, allowing Earth to have liquid water on its surface, which could favor plate tectonics. In Venus' case, having a softer, more ductile lithosphere due to high temperatures also could have made it more difficult to break up, or allowed it to heal more easily. Water in the mantle probably also played a role, as it affects the mantle's viscosity and melting temperature, Gillmann explained.
"But we have no certainty regarding water in the interior of Venus; it has been hypothesized to be very dry, and there may be reasons for that, looking at its very early history," Gillmann said. "But nothing proves it yet. If all goes well, the upcoming missions could help get an answer to that."
Plate tectonics in exoplanets
As far as determining whether exoplanets have plate tectonics, scientists still have a way to go.
There are two main ways we can deduce what might be happening in an exoplanet's interior. One is to detect the planet's magnetic field, which can tell scientists if there are materials moving around in its interior, "although we are still uncertain how that relates fully 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 Earth, so planets with regulated carbon dioxide abundances might have a form of plate tectonics, while planets with massive CO2 abundances may lack plate tectonics.
Plate tectonics and complex life
Some researchers have argued that plate tectonics played a crucial role in the emergence of complex life on Earth, suggesting that advanced civilizations may be possible only with complex geological regimes that include surface continents and oceans.
That raises the question of whether the presence of plate tectonics could help narrow our search for complex life elsewhere in the cosmos.
"For complex life, I have no strong opinion, although a case can be made that plate tectonics imposes a stress on species that pushes evolution forward while stabilizing surface conditions and avoiding large brutal changes that could lead to total extinction," Gillmann said. "As a result, it could well be one of the required ingredients to the 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.
