Much of what we know about Venus comes from data that the Magellan probe, launched by NASA in 1989, collected over four years in orbit around the planet. In that period, the spacecraft was able to map 98% of the planet’s surface to a resolution of around 150 m. This data revealed that about 80% of the surface of Venus was covered with volcanic flow and marked by evidence of tectonic movement and surface winds.
Looking over these data from over 20 years ago, scientists discovered that the surface of Venus is not as static as previously thought. Based on new maps constructed from existing data, they revealed that low-lying plains on the surface of Venus are surrounded by ridges and faults, which may be a result of tectonic forces similar to those that cause the formation of mountains on Earth. This suggested that Venus exhibits active tectonics. Scientists also observed telltale signs of edges rubbing against each other along the boundaries of these plains, further building the case for Venusian tectonics.
The crust of a planet can range from being a stagnant lid, in which the entire crust is made up of one piece, to a mobile lid, similar to the plates on Earth. The tectonics on Venus fall somewhere between the two extremes: it doesn’t display stagnant tectonics like the Moon or Mars nor do distinct pieces of the Venusian crust move.
Researchers also observed tectonic similarities between two regions in Venus, named Nuwa and Lada Campus, with continental interiors that resembled the Sichuan and Tarim basins in China. In a talk during the recently concluded Lunar and Planetary Science Conference in Texas, researchers likened tectonics on Venus to a “sea of jostling pack ice” akin to “drifting icebergs” on Earth.
They surmised that the high surface temperature on Venus – breaching 450 °C – would have rendered the rocks on Venus “kind of gooey”, making it easier for them to move around. They added that convection in the mantle, like in Earth, could also have helped move these blocks around.
Experimental verification
Another team of researchers set out to build models to reproduce features observed on the surface of Venus. Resorting to a physical model involving a heating plate, water and silica nanoparticles, the team was able to create a medium with viscosity similar to semi-molten rock. The researchers used the heating plate to create convection currents like those observed in the mantle. They could then study how the system evolved and understand how certain features on Venus came about.
One such feature that could be explained using this model was the corona.
A corona approximately 200 km in diameter in the southern hemisphere of Venus. Credit: NASA/JPL
Coronae are large, circular, pancake-shaped domes found only on Venus. These features are hundreds of kilometres across, with an elevated, bulging centre dominated by volcanic rocks, and deep ridges and trenches marking the edges.
The model revealed that coronae are caused by mantle plumes – a phenomenon that leads to local volcanic activity on Earth, such as in Yellowstone and Hawaii. When it happens on our planet, these plumes manifest as upwellings of hot, molten rocks burning up through the crust. They can at times force tectonic plates apart.
Since the surface of Venus is more pliable than Earth’s, the effect of a mantle plume is different. According to the model, the plume exploits existing fractures on the crust to push molten rocks onto the surface. The flexible crust sags under the weight of the material, causing a depression at the centre, and fractures around the edges that are formed. As more material piles on, the centre may either sink and melt into the mantle or solidify as is, leading to the formation of coronae.
For scientists, understanding Venus’s tectonics has opened up the possibility that planets go through different phases of plate tectonics, and provides a clue about how the planet could have evolved.