Thick lithosphere casts doubt on plate tectonics in Venus’ recent geological past

At a time between 300 million and 1 billion years ago, a large cosmic object struck the planet Venus, leaving a crater more than 170 kilometers in diameter. A team of researchers from Brown University used the ancient impact scar to investigate the possibility that Venus once had terrestrial plate tectonics.

For a study published in Natural Astronomy, the researchers used computer models to recreate the impact that Mead crater, Venus’ largest impact basin, carved out. Mead is surrounded by two rocky bugs – rocky ripples that are frozen in time after the impact of the sink. The models showed that Venus’ lithosphere – its rocky outer cover – had to be quite thick, much thicker than that of Earth, in order to have those rings where they are in relation to the central crater. This finding suggests that a tectonic regime such as Earth’s, where continental plates float like floats on top of a slowly rippling mantle, probably did not occur on Venus during the Mead impact.

“It tells us that Venus probably had a stationary lid during the impact,” says Evan Bjonnes, a graduate student at Brown and lead author of the study. “Unlike Earth, which has an active cover with moving plates, it appears that Venus was a planet with one plate for at least as far back as this impact.”

Bjonnes says the findings offer a contradiction with recent research suggesting that plate tectonics could have been a possibility in Venus’ relatively recent past. On earth, evidence of plate tectonics can be found around the world. There are large cracks called subduction zones, where parts of the crusty rock are driven into the subsoil. Meanwhile, new crusts form at ridges in the middle of the ocean, winding mountain ranges where lava flows from deep in the earth to the surface and hardens into rock. Data from orbiting planes revealed rifts and ridges on Venus that look a bit like tectonic features. But Venus is shrouded in its thick atmosphere, which makes it difficult to make definitive interpretations of fine surface features.

This new study is another way of approaching demand by using the Mead impact to investigate the properties of the lithosphere. Mead is a basin with more rings, similar to the large Oriental bowl on the moon. Brandon Johnson, a former professor at Brown who is now at the University of Purdue, published in 2016 a detailed study of the rings of Orientale. The work showed that the final position of the rings is strongly linked to the thermal gradient of the crust – the rate at which the rock temperature rises. with depth. The thermal gradient affects the way the rocks deform and break apart after an impact, which in turn helps to determine where the basin ends up.

Bjonnes used the technique that Johnson, who is also co-author of this new research, used to study Mead. The work showed that Venus’ crust had a relatively low heat gradient to keep Mead’s rings where they are. That low gradient – which means a relatively gradual increase in temperature and depth – indicates a fairly thick Venus lithosphere.

“You can think of it as a lake freezing in winter,” Bjonnes said. “The water at the surface first reaches the freezing point, while the water at the depths is a little warmer. When the deeper water cools to similar temperatures as the surface, you get a thicker ice sheet.”

The calculations indicate that the gradient is much lower and the lithosphere is much thicker than you would expect for an active cover planet. This would mean that Venus was without plate tectonics a billion years ago, the earliest point at which scientists think the impact of Mead occurred.

Alexander Evans, an assistant professor at Brown and co-author of the study, said that one compelling aspect of Mead’s findings is the consistency with other features on Venus. Several other craters that the researchers looked at were evenly similar to Mead, and the estimate of the heat gradient corresponds to the thermal profile needed to support Maxwell Montes, Venus’ highest mountain.

“I think the finding further underscores the unique place that the earth and its system of global plate tectonics have among our planetary neighbors,” Evans said.