To date, Earth is the only known planet with active tectonic plates – only drifting continents on it. But the surface of Venus next door is not completely solid and immutable either, as a study has now confirmed. In particular, the deep, thin parts of its crust move slightly against each other like ice floating in a polar sea ice pack. It can be observed in stable, undistorted regions of Venus’ surface, which are surrounded by regions of expansion and narrow fissures, the researchers report. The surface of the young hot Earth would have looked the same.
On our planet, large-scale circulating currents generate forces in the Earth’s mantle that also keep the Earth’s crust moving. In the context of plate tectonics, mantle material rises at mid-ocean edges and produces new crust material; And at the boundary with the continental plates, this crust is pushed down again and merging. This cycle ensures continental drift and creates, among other things, mountain ranges, volcanic chains and deep-sea trenches. To date, the Earth is unique among the known planets: no other celestial body has been found with similar active tectonics. Our neighboring planet Mars seems too cold to develop a strong enough convection, but Venus can be too hot: its crust was considered too soft and pliable to form solid plates.
Fracture areas and stretch joints
“In contrast to the mosaic of mobile tectonic plates that characterize Earth, a common assumption is that Venus has a globally continuous lithosphere,” Paul Byrne of North Carolina State University in Raleigh and colleagues explain. “Venus has shown all the signs of a monoplanetary planet for at least 0.5 to a billion years.” However, there are also signs of changes on the surface of Venus: in some places, long fault zones and trenches indicate a possible expansion of the crust, in others there are cracks and edges indicative of compression. Evidence of lateral displacement of parts of the Earth’s crust has already been observed. These geological structures are especially common in the lowlands of Venus – regions of the Earth’s crust that are particularly small and thin, the researchers report. However, it is not yet clear to what extent these are merely local phenomena and what mechanisms could be behind them.
To get more clarity, Byrne and his team used radar data from NASA’s Venus Magellan probe to specifically examine the geological structure of Venus’ lowlands. In doing so, they focused their attention primarily on a specific formation, which they call campi – Latin for “fields”. “We define a campus as a low-level area that appears smooth on radar and that is at least 50% surrounded by tectonic structures,” the scientists explain. “These plains, surrounded by rifts, range in size on average from 100 to 1,000 kilometers.” Using radar images, the team determined how many such universities are on Venus and what conclusions they made about the movement of the Earth’s crust and the evolution of the planet.
Mantle currents move crustal masses
In their assessments, researchers identified 58 campuses on Venus, most of them on shallow plains. It can be concluded from the surrounding faults that these cortical units remained stable by themselves, but moved against each other. “They show signs of being rotated and/or skewed laterally relative to each other – similar to blocks smashing into each other in ice packs,” Byrne and colleagues said. “Our observations indicate that each of the identified blocks has undergone some lateral movement over time.” To find out the cause of these movements, scientists reconstructed what happened with the help of a geophysical model. They tested whether the supposed currents of Venus’ mantle generate enough stresses in the crust to explain the cracks and motions observed.
It turns out that the deformation of the surface of Venus is also caused by internal processes – albeit somewhat different from Earth. “On Earth, the movement of tectonic plates is driven by convection,” Byrne explains. A different kind of this seems to have an effect on Venus. Although no large mountain ranges or giant subduction zones were created there, their more subtle changes at the surface are also due to internal currents. That this occurs on Venus on a global scale has not yet been proven. “It remains unclear how long these distortions have occurred or whether they will persist. “But some masses of the rising crust formed and deformed in the small lava plains, so the lithosphere must have been fragmented after those plains formed,” Byrne says. “This gives us reason to believe that geologically these blocks have moved only recently – and may continue to do so to this day.”
According to the research team, this new form of tectonics could provide additional insights into not only the geology and evolution of Venus, but those of other planets as well. Rocky exoplanets with similar temperature conditions to Venus could have similar tectonic conditions. And last but not least, our Earth may have gone through such a phase of icy-floating tectonics in its early, hot days before plate tectonic movement began.
Coyle: Paul Byrne (North Carolina State University, Raleigh) et al, Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2025919118
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