A thick lithosphere casts doubt on plate tectonics in the recent geological history of Venus

PROVIDENCE, RI [Brown University] – At some point between 300 million and 1 billion years ago, a huge cosmic object broke into the planet Venus, leaving a crater larger than 170 miles in diameter. A team of Brown University researchers has used that ancient impact scarf to study the possibility that Venus once had an Earth-like plate tectonic.

For a study published in Astronomy of nature, the researchers used computer models to recreate the effect that carved out the Mead crater, the largest impact basin in Venus. Mead is surrounded by two rocky ridges – frozen rocky ripples in time after the impact in a bass shape. The models showed that for these rings to be where they are with respect to the central crater, the Venus lithosphere – the outer rocky shell – must be very thick, much thicker than the Earth. That finding suggests that a tectonic regime such as Earth, where continental plates would move like rafts at the top of a slow-moving column, did not appear to have occurred on Venus at the time of Mead’s conquest.

“This tells us that Venus may have been what we would call a deadly peak at the time of impact,” said Evan Bjonnes, a Brown graduate student and lead author of the study. “Unlike Earth, which has an active roof with moving plates, Venus appears to have been a single-plate planet for at least as far back as this impact.”

Bjonnes says the findings offer an overview of recent research suggesting that plate tectonics may have been possible in Venus’ recent past. On Earth, plate tectonics are found throughout the world. There are large movements called movement zones where ores of crustal rock are moved down into the subsurface. At the same time, new crusts are formed at the ridges of the mid-ocean, vibrant mountains where lava from deep inside the Earth flows to the surface and hardens into rock. Data from an orbital spacecraft have revealed motions and ridges on Venus that look somewhat similar to tectonic features. But Venus is illuminated by the thick atmosphere, making it difficult to make definitive interpretations of delicate surface features.

This new study is a different approach to the question, using Mead’s influence to study the features of the lithosphere. Mead is a multi-ringed basin similar to the large Orientale basin on the Moon. Brandon Johnson, a former Brown professor and now at Purdue University, published a detailed study of Orientale rings in 2016. That work showed that the final position of the rings is strongly related to the thermal gradient of the shell. – the rate at which the temperature of the rock increases with depth. The thermal gradient affects the way the rocks deform and disintegrate after impact, which in turn helps determine where the bass rings come from. end.

Bjonnes modified the approach used by Johnson, who is also a coauthor of this new research, to study Mead. The work for Mead rings showed that where they are, the Venus bark must have had a low thermal gradient. That low gradient – resulting in a gradual relative increase in temperature with depth – makes Venusian lithosphere very dense.

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

The calculations suggest that the gradient is much lower, and the lithosphere much thicker, than you would expect for a fully functioning planet. That would mean that Venus has been without plate tectonics for as far back as a billion years ago, the earliest stage at which scientists believe Mead’s influence occurred.

Alexander Evans, an assistant professor at Brown and co – author of a study, said one strong aspect of the results from Mead was their consistency with other features of Venus. There were several other ring filters that the proportional researchers looked at like Mead, and the thermal gradient estimates are consistent with the thermal profile needed to support Maxwell Montes, the mountain of height in Venus.

“I think the discovery illuminates the special place of the Earth, and its system of global plate tectonics, among our planetary neighbors,” Evans said.

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