“Ancient Scar” –Massive Impact Crater Casts Doubt that Venus had Earthlike Continental Plates

 

NASA Venus

 

 

A new finding about Venus highlights the unique place that Earth, and its system of global plate tectonics, has among its planetary neighbors. Research by NASA n 2016 and Yale astronomers in 2020 suggested that Venus may have had a shallow liquid-water ocean and habitable surface temperatures for up to 2 billion years of its early history, with vast oceans and a thin atmosphere, hinting that our sister planet may have been the solar system’s first habitable planet.

New research by a team of Brown University researchers explored the possibility that Venus once had Earth-like plate tectonics, using an event that occurred at some point between 300 million and 1 billion years ago, when a large cosmic object smashed into the planet, leaving a crater more than 170 miles in diameter.

Mead Crater Story Reveals a Unique Earth

The researchers used computer models to recreate the impact that carved out Mead crater, Venus’s largest impact basin (mage below). Mead is surrounded by two clifflike faults — rocky ripples frozen in time after the basin-forming impact. The models showed that for those rings to be where they are in relation to the central crater, Venus’s lithosphere — its rocky outer shell — must have been quite thick, far thicker than that of Earth. That finding suggests that a tectonic regime like Earth’s, where continental plates drift like rafts atop a slowly churning mantle, was likely not happening on Venus at the time of the Mead impact.

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Venus was a One-Plate Planet

“This tells us that Venus likely had what we’d call a stagnant lid at the time of the impact,” said Evan Bjonnes, a doctoral candidate at Brown and study’s lead author. “Unlike Earth, which has an active lid with moving plates, Venus appears to have been a one-plate planet for at least as far back as this impact.”

Bjonnes says the findings offer a counterpoint to recent research suggesting that plate tectonics may have been a possibility in Venus’s relatively recent past. On Earth, evidence of plate tectonics can be found all over the globe. There are huge rifts called subduction zones where swaths of crustal rock are driven down into the subsurface. Meanwhile, new crust is formed at mid-ocean ridges, sinuous mountain ranges where lava from deep inside the Earth flows to the surface and hardens into rock. Data from orbital spacecraft have revealed rifts and ridges on Venus that look a bit like tectonic features. But Venus is shrouded by its thick atmosphere, making it hard to make definitive interpretations of fine surface features.

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This new study is a different way of approaching the question, using the Mead impact to probe characteristics of the lithosphere. Mead is a multi-ring basin similar to the huge Orientale basin on the Moon. Brandon Johnson, a former Brown professor who is now at Purdue University, published a detailed study of Orientale’s rings in 2016. That work showed that the final position of the rings is strongly tied to the crust’s thermal gradient — the rate at which rock temperature increases with depth. The thermal gradient influences the way in which the rocks deform and break apart following an impact, which in turn helps to determine where the basin rings end up.

Mead Impact Reveals a Thick Lithosphere

Bjonnes adapted the technique used by Johnson, who is also a coauthor on this new research, to study Mead. The work showed that for Mead’s rings to be where they are, Venus’s crust must have had a relatively low thermal gradient. That low gradient — meaning a comparatively gradual increase in temperature with depth — suggests a fairly thick Venusian lithosphere.

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

The calculations suggest that the gradient is far lower, and the lithosphere much thicker, than what you’d expect for an active-lid planet. That would mean that Venus has been without plate tectonics for as far back as a billion years ago, the earliest point at which scientists think the Mead impact occurred.

Alexander Evans, an assistant professor at Brown and study co-author, said that one compelling aspect of the findings from Mead is their consistency with other features on Venus. Several other ringed craters that the researchers looked at were proportionally similar to Mead, and the thermal gradient estimates are consistent with the thermal profile needed to support Maxwell Montes, Venus’s tallest mountain.

Jackie Faherty, astrophysicist, Senior Scientist with AMNH via Brown University. Jackie was formerly a NASA Hubble Fellow at the Carnegie Institution for Science.

 

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