At some point between 300 million and 1 billion years, a large cosmic object collapsed into the planet Venus, making the crater more than 170 miles in diameter. A team of researchers from Brown University has used that ancient impact trail to explore the possibility that Venus once had Earth-like plate tectonics.
For a study published in Nature astronomy, The researchers used computer models to influence the carving on the largest impact basin of the Mead crater, Venus. In time after the impact of the formation of the basin, the frozen rock is surrounded by two rocks. The models showed that for the rings where they are in relation to the central crater, the lithosphere of Venus — its rocky outer cover — would have been much thicker, much thicker than Earth. This suggests that a tectonic regime like Earth, where continental plates flow like rafts, was not occurring on Venus at the time of the Mead Effect, possibly after a gradual churning.
“It tells us that Venus was likely what we would call a stationary lid at the time of impact,” said Evan Beyonce, a graduate student at Brown and lead author of the study. “Unlike Earth, which has an active lid with moving plates, Venus appears as a one-plate planet, at least to this effect.”
Bjonnes states that the findings offer a counterpoint to recent research suggesting that plate tectonics may be a possibility in the relatively recent times of Venus. On Earth, evidence of plate tectonics can be found worldwide. There are huge cracks called subduction zones where swaths of crustal rock are driven into subsurface. Meanwhile, new crusts form in mid-oceanic ridges, sinuous mountain ranges where lava flows from deep within the earth to the surface and hardens into rock. Data from the orbital spacecraft have shown cracks and ridges on Venus that look like tectonic features. But Venus has been shaken by its dense atmosphere, making it difficult to definitively explain the finest features of the surface.
This new study is a different way to approach the question, using the Mead effect to examine the characteristics of the lithosphere. Mead is a multi-ring basin similar to the giant Oriental Basin on the Moon. Brandon Johnson, a former Brown professor 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 thermal gradient of the crust – with increasing depth in rock temperature. The thermal gradient affects the way in which rocks erode and break, following an impact that in turn helps determine where the basin rings.
Beyonce adapted the technique used by Johnson, who is also a sub-legislator on this new research to study Mead. The work revealed that the Mead rings where they are, the Venus’s crust should have had a relatively low heat shield. That low gradient — meaning a comparative gradual increase in temperature with depth — suggests a significantly thicker Venusian lithosphere.
“You can think of it like a freezing lake in winter,” Beyonce said. “The water at the surface reaches the freezing point first, while at depth the water is slightly warmer. When this deep water cools to the same temperature as the surface, you get a thick ice sheet.”
Calculations show that the gradient is very low, and the lithosphere is very thick, which is what you’d expect of an active-lid planet. This would mean that Venus has been without plate tectonics until a billion years ago, the earliest point at which scientists think the Mead effect occurred.
Brown’s assistant professor and study co-author Alexander Evans said that a compelling aspect of Mead’s findings is his continuity with other characteristics on Venus. The researchers observed that several other ring craters were proportionally similar to the mead, and the thermal gradient estimates corresponded to the thermal profile needed to support Maxwell Montes, the highest mountain in Venus.
“I think further exploration uncovers the unique place that the Earth, and its system of global plate tectonics, is between our planetary neighbors,” Evans said.
The Origin of Archaean Crates: Learning from Venus
E. Beyoncé et al., Estimating Venusian thermal conditions using multirin basin morphology, Nature astronomy (2021). DOI: 10.1038 / s41550-020-01289-6
Provided by Brown University
Citation: Thick lithosphere suspected of plate tectonics in Venus’s geological period (2021, 28 January), recaptured from https://phys.org/news/2021-01-thick-lithosphere-plate-tectonics-venus.html on 29 January 2021 Received.
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