As we've explored the Solar System, some items we're familiar with from Earth's geology have kept appearing in new places. Glaciers, volcanoes, and geysers have all been found on other planets and moons. With all that's familiar, it's easy to forget that one of the defining features of Earth's geology—plate tectonics—is notably absent. There are some hints of it on the icy crust of Europa, but it would have to be powered by a different mechanism there.
If there was an obvious candidate for hosting plates, it would be Venus, similar in size and composition to the Earth and home to active volcanoes. But most of Venus' surface appears to have been there for hundreds of millions of years with no sign of the tectonic recycling we have on Earth. New research, however, suggests that some of Venus' crust does get recycled, just through a radically different process—one that may have been active early in Earth's history.
While we've been able to map Venus' surface, the planet's thick atmosphere has limited what we know about its surface, and we've not had the sort of repeated imaging that can reveal active geology. Even though we know its surface is littered with volcanoes, for example, we're not currently certain whether any of them are active. But crater counts suggest that most of the material on the surface is hundreds of millions of years old and had been put in place by massive eruptions.
All of that would suggest an absence of plate tectonics, which regularly recycles large portions of the Earth's surface. But there are also some features like trenches and rifts that suggest something tectonic might be going on.
Modeling Venus
For many planetary scientists, the solution in these cases is to build computer models and see whether they can reproduce any of the features we see on Venus. But, according to the scientists behind the new research, computational models just aren't currently up to the task of doing a full, three-dimensional simulation of something as complicated as the mantle and crust of Venus. So, they went a bit old fashioned and created a physical model.
The model is basically a heating plate, some water, and finely ground sand. You can create a suspension of silica nanoparticles in water and, by altering the concentration, alter physical properties like the viscosity to match that of semi-molten rock. Put that solution on a heating plate, and it will start convecting, as hot areas become less dense and rise to the surface. That's analogous to the circulation of the rock in the mantle, driven by heat from Venus' core.
Leave the top of this system open, and a crust will form naturally as water evaporates off. The authors could watch the evolution of the system using cameras and even gently pull off the crust to analyze what happens.
And what happens in some cases is the production of a reasonable analog of some of Venus' most distinctive features, called coronae. As the paper describing the work puts it, "Coronae are volcanotectonic features unique to Venus." More helpfully, they're largely circular features, hundreds to thousands of kilometers across, with an elevated bulge at the center. The center is dominated by volcanic rocks, while the edges of the coronae are defined by ridges and a deep trench. The trench is reminiscent of areas where tectonic plates are being subducted on Earth, though nothing about the rest of the feature is at all similar.
Bust the crust
How do these features form? In the model, they're the result of something we've seen on Earth: mantle plumes. These upwellings of hot, molten rock sometimes reach the surface on Earth. Here, they tend to create local volcanic activity—think Hawaii, Iceland, and Yellowstone. At some points in the past, they've also created huge outpourings of molten rock like the Siberian and Deccan traps.
But on Earth, the crust is relatively thick and rigid. On Venus, surface temperatures average over 450 degrees Celsius, resulting in a thin and flexible crust. So when a mantle plume hits the surface, the results could be very different. The model suggests that they are.
As on Earth, when the model plume reaches the surface, it exploits fractures to begin belching hot material onto the surface. But in the model, the thin, flexible crust sags under the weight of the new material. This expands the initial fault, allowing more hot material to pile up on the surface, deflecting the crust around it downward and deeper into the mantle. Ultimately, the ruptured crust will pick up water and melt within the mantle.
On the surface, you get a corona, a massive central bulge of volcanic material pushed up by the plume. Surrounding it on all sides are the equivalent of a subduction zone, where the prior crust is being shoved back into the mantle, creating the sort of deep trenches we see on Earth.
The authors go on to look at two specific coronae, Artemis and Quetzalpetlatl, and show that they have many of the features found in the model. Obviously, not all of these features are identical (Artemis, for example, seems to be an agglomerate of five individual volcanic rises), but the general outlines all line up.
The end result is a form of tectonics distinct to Venus—at least at the present. As the authors note, the temperature conditions within Venus at the present look a lot like what the Earth probably did for much of its early history. Thus, there could have been a period of time when the Earth experienced a Venus-style form of tectonics before shifting over to its present plate system.
And that's rather informative regarding our past. There's ongoing debate within the geosciences community regarding when it was that plate tectonics started on Earth. If some form of subduction could take place prior to the formation of solid, moving plates, then it would create evidence that looked like it came from plate tectonics, but didn't. So while the results are directly relevant to Venus, they could inform arguments about our own planet's past, as well.
Quelle: ars TECHNICA