How is Earth the only known planet with active plate tectonics?
Earth is special in that it has two things that other terrestrial planets do not have: an abundance of internal heat, from when our planet was molten rock, and liquid water. To understand why our planet is unique in this respect, let’s first look at Earth versus Mars.
Earth is relatively large for a rocky planet. Its mass has allowed it to retain its internal heat for billions of years. Heat causes the Earth’s surface to warp and plays a key role in ensuring that the Earth’s outer surface layer, called the lithosphere, does not become too cold and therefore too rigid to move. But Mars is smaller than our planet. Because of this, the Red Planet cooled at a much faster rate. Mars’ lithosphere has become very rigid — too rigid to break into plates.
Heating isn’t the only thing at play when it comes to plate tectonics. Venus is about the same size as Earth, so theoretically one would think that it is also likely to have moving plates. But this is not the case. While heating is enough to ward off a rigid lithosphere, it’s not enough to move Earth’s plates. This is where liquid water comes into the equation. On Earth, the interior water lubricates the tectonic plates, allowing them to flow and slide past each other, but there is no water inside Venus.
To be clear, tectonic deformation is currently occurring in the outer layers of Venus and Mars, and has already taken place on Mercury. However, since the outer layers of these planets are not divided into plates, we consider these planets to be single-plate planets. Thus, Venus and Mars still experience tectonics, but not plate tectonics.
However, Earth may not be the only body in the solar system to experience plate tectonics. Jupiter’s icy moon Europa is covered in a cold, brittle shell of ice that is said to float above a warmer, more fluid layer of ice. Like Earth’s plates, when two plates of this cold ice collide, one of these plates is able to slide under the other inside Europa. Scientists have also observed evidence of water rising on this moon’s surface, much like magma erupts from vents on Earth.
Ocean Worlds Planetary Scientist, Goddard Space Flight Center, Greenbelt, Maryland