So far, the earth is the only known planet with active plate tectonics – only on it are there drifting continents. But the surface of our neighboring planet Venus is not completely rigid and unchangeable either, as a study has now confirmed. In particular, the deep, thin parts of their crust move slightly against each other like ice floes in the pack ice of the polar sea. This can be seen in stable, hardly deformed areas of the Venus surface, which are surrounded by narrow expansion and fracture zones, as researchers report. The surface of the young, hot earth could have looked similar.
On our planet, large-scale circulating currents in the Earth’s mantle generate forces that also keep the overlying crust in motion. As part of plate tectonics, mantle material rises on the mid-ocean ridges and creates new crust material; at the borders with the continental plates, this crust is pushed down again and subducted. This cycle causes the continental drift and creates, among other things, mountain ranges, volcanic chains and deep-sea trenches. So far, the earth is unique among the known planets: no other celestial body has been found to have comparable active tectonics. Our neighboring planet Mars seems too cold to develop sufficiently strong mantle convection, but Venus could be too hot: its crust was considered too soft and flexible to form solid plates.
Fracture zones and expansion joints
“In contrast to the mosaic of mobile tectonic plates that characterize the earth, the common assumption is that Venus has a globally continuous lithosphere,” explain Paul Byrne of North Carolina State University in Raleigh and his colleagues. “Venus has been showing all signs of a single-plate planet for at least 0.5 to a billion years.” Nevertheless, there are also signs of changes on the surface of Venus: in some places elongated fracture zones and trenches indicate a possible expansion of the crust there, in other places there are faults and ridges that indicate a compression. Evidence for a lateral displacement of crustal parts has already been observed. These geological structures are particularly common in the lowlands of Venus – the crustal regions that are considered to be particularly young and thin, as the researchers report. So far, however, it was unclear to what extent these are only local phenomena and what mechanisms could be behind them.
To gain more clarity, Byrne and his team used radar data from NASA’s Venus probe Magellan to specifically examine the geological structure of the Venus lowlands. In doing so, they focused their attention primarily on a certain formation, which they call Campi – Latin for “fields”. “We define a campus as a low-lying region of levels that appear smooth on the radar, at least 50 percent of which are surrounded by tectonic structures,” the scientists explain. “These plains surrounded by faults are on average 100 to 1000 kilometers in size.” Using the radar images, the team determined how many such campuses there are on Venus and what conclusions they allow about the crustal movement and development of the planet.
Mantle currents move crust clods
In their evaluations, the researchers identified 58 such campuses on Venus, most of them in shallow plains. From the surrounding faults it can be concluded that these crustal units remained stable in themselves, but moved against each other. “They show signs that they have rotated and / or shifted sideways relative to one another – similar to clods bumping into each other in pack ice,” said Byrne and his colleagues. “Our observations suggest that each of the identified blocks has undergone some lateral movements over time.” To find out what caused these movements, the scientists reconstructed what happened with the help of a geophysical model. They tested whether the currents postulated for the Venus mantle in the mantle generate enough stresses in the crust to explain the cracks and movements observed.
It turned out that the deformation of the surface of Venus is also driven by internal processes – albeit somewhat differently than on Earth. “On Earth, plate tectonics is driven by convection,” explains Byrne. “A variant of this also seems to have an impact on Venus. Although no large mountain ranges or gigantic subduction zones are created there, their more subtle changes to the surface are also due to internal mantle currents. That this takes place on Venus on a global scale has not yet been proven. ”It is still unclear in which time periods these deformations took place or whether they may still continue. “But some of the bumping crust blocks formed and deformed in young lava plains, so the lithosphere must have been fragmented after those plains were formed,” says Byrne. “This gives us reason to believe that geologically speaking, these blocks have only recently moved – they may even continue to do so to this day.”
According to the research team, this newly identified form of tectonics could provide further insights not only into the geology and evolution of Venus, but also into those of other planets. Extrasolar rock planets with similar temperature conditions as Venus could also have similar tectonics. And last but not least, our earth in its hot early days may have gone through such a phase of ice floe tectonics before its plate tectonics began.
Source: Paul Byrne (North Carolina State University, Raleigh) et al., Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.2025919118