“Ice floe” tectonics reveal the geological secrets of Venus

A new analysis of the surface of Venus shows signs of tectonic movement in the form of crustal blocks that have jostled against each other like shattered pieces of ice floe. The movement of these blocks could indicate that Venus is still geologically active, and give scientists insight into both exoplanet tectonics and early tectonic activity on Earth.

“We have identified a previously unrecognized tectonic deformation pattern on Venus, which is driven by inner movement just as it is on Earth,” says Paul Byrne, associate professor of planetary science at North Carolina State University and main author and co-correspondent of the work. “Although different from the tectonics that we are currently seeing on Earth, it is still evidence of the inner movement expressed on the surface of the planet.”

The discovery is significant because Venus has long been believed to have an immobile solid outer shell, or lithosphere, much like Mars or Earth’s Moon. In contrast, the Earth’s lithosphere is divided into tectonic plates, which slide against each other, apart and under each other, above a warm, weaker mantle layer.

Byrne and an international group of researchers used radar images from NASA’s Magellan mission to map the surface of Venus. Examining the vast Venusian plains that make up most of the planet’s surface, they saw areas where large boulders of the lithosphere appear to have moved: separate, come closer, spin, and slide over each other. like broken ice on a frozen lake.

The team created a computer model of this deformation and found that the slow motion of the planet’s interior may explain the style of tectonics seen on the surface.

“These observations tell us that inner movement causes deformation of the surface on Venus, much like what happens on Earth,” Byrne explains. “Plate tectonics on Earth are driven by convection in the mantle. The mantle is hot or cold in different places, it moves, and part of this movement is transferred to the Earth’s surface in the form of plate movement.

“A variation on this theme also seems to play out on Venus. This is not plate tectonics like on Earth – there are no huge mountain ranges being created here, or giant subduction systems – but it is evidence of due deformation. to the inner mantle flow, which has never been demonstrated on a global scale before.

The deformation associated with these crustal blocks could also indicate that Venus is still geologically active.

“We know that a large part of Venus has resurfaced volcanically over time, so parts of the planet could be very young, geologically speaking,” Byrne explains. “But several of the jostling blocks formed and deformed these young lava plains, which means that the lithosphere fragmented after the establishment of these plains. This gives us reason to believe that some of these blocks may have moved geologically very recently – perhaps even until today. “

Researchers are optimistic that the newly recognized “ice floe” model of Venus may offer clues to understanding tectonic deformation on planets outside of our solar system, as well as on a much younger Earth.

“The thickness of a planet’s lithosphere depends primarily on its temperature, both inside and on the surface,” Byrne explains. “The heat flux from within young Earth was up to three times greater than it is now, so its lithosphere may have been similar to what we see on Venus today: no thick enough to form plaques that subduct, but thick enough to fragment into blocks that pushed, pulled, and shoved.

NASA and the European Space Agency recently approved three new space missions to Venus that will acquire observations of the planet’s surface at a much higher resolution than Magellan. “It’s great to see a renewed interest in exploring Venus, and I’m especially excited that these missions can test our key discovery that the planet’s lowlands have fragmented into jostling crustal blocks,” Byrne said.

The work appears in Proceedings of the National Academy of Sciences. Sean Solomon of Columbia University is the co-corresponding author. Richard Ghail of the University of London, Surrey; AM Celâl Şengör from Istanbul Technical University; Peter James of Baylor University; and Christian Klimczak from the University of Georgia also contributed.

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Note to editors: A summary follows.

“A globally fragmented and mobile lithosphere on Venus”

DO I: 10.1073 / pnas.2025919118

Authors: Paul K. Byrne, North Carolina State University; Richard C. Ghail, University of London, Surrey; AM Celâl Şengör, Istanbul Technical University; Peter B. James, Baylor University; Christian Klimczak, University of Georgia; Sean C. Solomon, Columbia University

Posted: June 21, 2021 to Proceedings of the National Academy of Sciences

Abstract:
Venus is believed to have a globally continuous lithosphere, unlike the mosaic of moving tectonic plates that characterizes Earth. Still, the surface of Venus has been largely deformed, and convection of the underlying mantle, perhaps acting in concert with a low-resistance lower crust, has been suggested as the source of some horizontal deformations of the surface. The extent of surface mobility on Venus driven by mantle convection, however, and the style and scale of its tectonic expression have not been clear. We report a globally distributed set of crustal boulders in the Venus Lowlands that show evidence of rotation and / or lateral displacement with respect to each other, like an ice pack compass. At least part of this deformation on Venus is subsequent to the placement of locally younger plain materials. Lithospheric stresses calculated from interior viscous flow models compatible with gravity and long-wavelength topography are sufficient to cause brittle fracture in the upper crust of Venus in all areas where these blocks are present, confirming that interior convective motion can provide a mechanism to cause deformation at the surface. The limited but widespread lithospheric mobility of Venus, in marked contrast to the tectonic styles indicative of a static lithosphere on Mercury, Moon, and Mars, may offer parallels to interior-surface coupling on early Earth, when heat flow overall was considerably higher, and the lithosphere generally thinner than today.


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