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SYFY WIRE Bad Astronomy

New evidence that parts of Venus’ crust may be squishy

By Phil Plait
A large crustal block on Venus (smooth dark area), bigger than Alaska, shows evidence of tectonic motion (grooves and ridges surrounding it), indicating Venus may yet be active today. Credit: Paul Byrne, based on original NASA/JPL imagery.

File this under "Venus is still doing stuff we didn't know about until recently": Blocks of the crust on the more-than-broiling-hot planet show that they've moved relative to one another, both sliding past and rotating, like plates of ice on a frozen lake. This may be due to movement of the mantle underneath them, which in turn indicates that our evil twin planet may yet be geologically active. Like, now.

Venus is similar to Earth in some ways; it's about the same size and mass, but at the same time it also has a tremendously thick atmosphere that, via the runaway greenhouse effect, is hot enough to melt lead.

Global radar view of Venus by the Magellan and Pioneer Venus Orbiter showing the topology of the surface. Credit: NASA/JPL-Caltech

Another difference is that it doesn't seem to have plate tectonics like Earth does. Our crust is broken up into many individual units that can slide past each other, under or above each other, or rotate around. All this is driven by the mantle, the solid layer of hot dense rock underneath (it's a common misconception that the mantle is liquid; it's not. It's fluid, which is different. It can flow, but still be solid).

Venus, though, doesn't have large-scale features like our plates. That has led scientists to think that it's continuous, a one-piece shell around the planet like chocolate around a piece of candy.

However, a team of planetary scientists has been examining data from Magellan, a NASA mission that mapped the surface of Venus in the 1990s via radar that penetrates the planet's opaque atmosphere, and found evidence that maybe not all is as it seems.

There is some evidence of horizontal strain in some places in the crust of Venus, like what happens when two features are moving in opposite directions and rubbing against one another, and that maybe this was due to motion in the mantle. But these are scattered and on small scales, maybe a few dozen kilometers long.

Lavinia Planitia is a 1,100-km-wide lowland area on Venus, which is apparently divided into a series of crustal blocks (smooth purple regions) surrounded by ridges and grooves (yellow) indicating tectonic movement.

The new work looks at the lowlands (as opposed to mountainous regions), flattish plains that cover roughly 90% of the planet. These areas do have some up-and-down topography to them, but the scientists looked at individual flat and smooth areas they call campi (singular campus, Latin for field), likely due to volcanic and sedimentary action.

One in particular is nearly 2 million square kilometers, larger than Alaska. It's bounded by a series of ridges and grooves, indicating extensional (stretching), transpressional (like shear but at an angle, where two blocks hit each other not quite head-on), and rotational movement.

A large crustal block on Venus (smooth dark area), bigger than Alaska, shows evidence of tectonic motion (grooves and ridges surrounding it), indicating Venus may yet be active today. Credit: Paul Byrne, based on original NASA/JPL imagery.

That's the largest block they examined but they found many others that were smaller, including areas where many of these campi were together, apparently interacting with one another. Features like these have been identified before, but the difference here is that the new work shows that these intersecting regions of grooves and ridges actually delimit the crustal blocks inside their boundaries. Importantly, they also show that convection (hot material rising and cooler stuff sinking) in the mantle of Venus can move them in the ways seen, similar to what's happening beneath Earth's surface.

Idunn Mons, a volcano on Venus, may still be active; this topographic map from the Magellan probe has been overlayed by thermal data from Venus Express to show the top of the peak is still hot. Credit: NASA/JPL-Caltech/ESA

That would seem to indicate that the crust of Venus isn't as monolithic as thought. It doesn't directly contradict that idea, either; if the crust acts like a lid over the entire surface it might be squishy in some places, and that could be due to the mantle heating the crust above it in those places.

The thing is, these areas showing all this motion are young, much younger than a lot of other regions on Venus. This all means Venus could still be tectonically active today. Now.

It might be that Venus is in transition, going from "mobile-lid" tectonics (where pieces of the crust can move around) to "stagnant-lid," where it acts as all one big motionless unit. It's thought that the crust of Venus has been one piece for the past billion years or so, but perhaps that's not the case, and was more mobile in the past.

A Magellan spacecraft radar image of Fotla Corona, a volcanic feature on Venus about 150 kilometers wide. Note the circular feature in the middle and the concentric cracks. Credit: NASA/JPL/Magellan probe

It's hard to interpret the surface of this hellish planet. For example, it's covered in volcanoes, with 1,600 (!!) large ones identified. It doesn't have many craters, which could mean the surface has been repaved in the recent past. And there's other evidence it's tectonically and/or volcanically active in the very recent past: the Idunn Mons volcano appears to have a hot top, as if magma is pooled there; several hot spots have been found that could indicate young volcanic vents, and features called coronae —nested circular cracks in the crust, some over 150 kilometers across, that look like where a small rock pings a windshield at high speed — may be where upwelling magma pushes up on the crust and then subsides, leaving a depression in the surface.

This evidence is all circumstantial, with no smoking plume (or caldera) to point at that Venus is active now… but together they are fairly provocative. If this new results holds up, it will show Venus as the only other planet in the solar system known to have crustal plates like this*.

Understanding Venus is helpful in understanding the planet beneath our own feet. They're so similar, yet took such different paths. Why? And we're finding Earth-sized (or Venus-sized if you prefer) exoplanets orbiting other stars as well; if we are to hope to figure out how they behave, we'll have to better know our immediate neighbors as well.

Happily, NASA has given the go-ahead to two missions to Venus; one to study its atmosphere (which has recently made itself very interesting) and the other to look at the surface and the tectonics that may be powering its motion. We have more questions than answer about Venus — that's true of everything, really — but more answers will be coming soon.

*Some icy moons show features like these, where water or gooey nitrogen ice (in the case of Pluto) under the surface may create segmented crustal blocks above them; these are similar to what's seen on Venus and Earth but have different materials and timescales operating.