World news – The thick lithosphere raises doubts about the plate tectonics in the geologically recent past of Venus

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January 28, 2021

from Brown University

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

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For a study published in Nature Astronomy, the researchers used computer models to recreate the effects of Mead Crater, Venus’ largest impact basin. Mead is surrounded by two cliff-like faults – rocky waves that are frozen after hitting the basin formation. The models showed that the lithosphere of Venus – its rocky outer shell – must have been quite thick and far thicker than that of Earth for these rings to be where they are in relation to the central crater. This finding suggests that a tectonic regime like that of Earth, in which tectonic plates float like rafts on a slowly churning mantle, probably did not occur on Venus at the time of the Mead impact.

« This tells us that Venus likely had something called a stagnant lid at the time of impact, « said Evan Bjonnes, Brown graduate student and lead author of the study. « Unlike Earth, which has an active lid with moving plates, Venus appears to be at least as far back as this impact. »

According to Bjonnes, the results offer a counterpoint to recent research that suggests the Plate tectonics could have been a possibility in Venus’ relatively recent past. There are indications of plate tectonics on Earth worldwide. There are huge cracks, so-called subduction zones, in which plumes of crustal rock are driven into the subsurface. Meanwhile, a new crust is forming on mid-ocean ridges, winding mountain ranges in which lava flows from the interior of the earth to the surface and hardens into rock. Orbital spacecraft data has discovered cracks and ridges on Venus that look a bit like tectonic features. However, Venus is surrounded by its dense atmosphere, making it difficult to definitively interpret final surface features.

This new study is a different approach to the question that uses the Mead influence to determine the properties of the lithosphere to investigate. Mead is a multi-ring basin that resembles the giant Oriental basin on the moon. Brandon Johnson, a former Brown professor who is now at Purdue University, published a detailed study of Orientales rings in 2016. This work showed that the final position of the rings depends heavily on the thermal gradient of the crust – the rate at which rock temperature increases with depth. The thermal gradient affects the way in which the rocks deform and break apart after impact, which in turn helps determine where the basin rings end.

Bjonnes adapted the technique from Johnson, who is also co-author of this one new research is on to examine Mead. The work showed that in order for Mead’s rings to be where they are, the crust of Venus must have a relatively low thermal gradient. This low gradient – which means a comparatively gradual increase in temperature with depth – suggests a fairly thick Venusian lithosphere.

« You can think of it as a lake that freezes in winter, » said Bjonnes. « The water on the surface reaches freezing point first, while the water below is a little warmer. When the deeper water cools to temperatures similar to the surface, you get a thicker ice cover. »

The calculations suggest that the gradient is much lower and the lithosphere is much thicker than would be expected for a planet with an active cover. That would mean that Venus was without plate tectonics a billion years ago, the earliest time scientists believe the Mead Impact occurred.

Alexander Evans, Assistant Professor at Brown and co-author of the study, said that a compelling aspect of Mead’s findings was their correspondence with other features on Venus. Several other ring craters the researchers looked at were proportionally similar to Mead, and the thermal gradient estimates are consistent with the thermal profile required to support Maxwell Montes, the tallest mountain on Venus.

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

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