- Venus has hundreds of impact craters on its surface, but seems to lack larger impact basins. Why?
- A new study of giant concentric rings in a region of uniquely wrinkled and deformed terrain provides some clues. Researchers think two asteroids impacted this region, one right after the other. The impacts, in lava, created the wrinkled terrain and concentric rings.
- The concentric rings are similar to others on some of Jupiter’s icy moons, such as Callisto and Europa.
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Venus has many craters
Beneath its thick, cloudy atmosphere, Venus has hundreds of craters on its surface. But unlike the moon, Mars and Mercury, Venus lacks giant impact craters, or basins. But Venus also lacks many of the processes that erase larger craters on Earth, so there should still be traces of them, if they existed. On October 28, 2024, researchers in the U.S. and Spain said they’ve identified signs of at least two major impact events that went unrecognized … until now. They were hiding in plain sight, as it were. The clues lie in a region of wrinkled and deformed terrain called tesserae.
The researchers published their peer-reviewed findings in JGR Planets on October 28, 2024.
No giant craters on Venus?
The rocky planets (other than Earth) and some moons display huge impact craters on their surfaces. That’s apart from the thousands of smaller ones. On our planet, larger impact scars have been erased over billions of years by wind, water, plate tectonics and widespread life.
Venus is a bit of an oddity, however. It has around 900 known craters on its hot, desiccated surface. The largest known is Mead crater, at 174 miles (280 km) across. But larger impact basins have been difficult to identify. They should still be there, if they ever existed. On Venus, there is no erosion from water, plate tectonics or surface life. Winds are powerfully strong higher up in the atmosphere, but slow down also completely at the surface to only about 3 miles (5 km) per hour.
All the inner rocky planets were bombarded by asteroids in the early solar system, so where are the remains of them on Venus? It turns out they may be there after all, just “hiding in plain sight.”
New research: Back-to-back impacts on a younger Venus with a thin outer shell, or lithosphere, created conditions in which giant impact events look nothing like those on the Moon and Mars, but rather the Valhalla crater on Callisto. ? ??
Read why: pic.twitter.com/ACZmUTpXP4
— Planetary Science Institute (PSI) (@planetarysci) October 28, 2024
Concentric rings and tesserae
The researchers focused on a region called Haasttse-baad Tessera. It is an example of tessera terrain, which is heavily deformed, wrinkled and corrugated. Haasttse-baad Tessera also contains a set of huge concentric rings, about 900 miles (1,450 km) across. The new study suggests these rings are actually an ancient impact structure. And it resulted from not just one, but two asteroid – or bolide – impacts, which occurred back-to back. They don’t look like typical craters or impact basins, so that could explain why scientists didn’t recognize them at first.
Vicki Hansen at the Planetary Science Institute in Tucson, Arizona, said:
If this is really an impact structure it would be Venus’ oldest and largest, giving us a rare glimpse into Venus’ past and informing early planet processes. And perhaps even more important, it shows us that not all impact structures look alike. Impact structures result from a bolide – a body of unspecified composition – that collides with a target planet. The nature of the bolide is important, but so too is the nature of the target.
Venus’ ‘pea soup’
So, how did the unique impact feature form? The answer lies in the tesserae. In tesserae regions, a thin and strong layer of material forms over a weaker layer. As Hansen described it:
Think of pea soup with a scum forming on top.
On Venus, the “pea soup” would have been lava. Ancient Venus had a thinner lithosphere – the rigid, rocky outer layer consisting of the crust and the solid outermost layer of the upper mantle – than it does now, only about 6 miles (10 km) thick. An impacting asteroid could punch right through that thin layer and enter the mantle, creating a sea of lava. When the lava cooled, it formed the tesserae. The researchers estimated this occurred 1.5 to 4 billion years ago.
Buoyant material in the mantle
In addition, some tesserae on Venus form on top of plateaus. The asteroid impacts leave behind solid but buoyant material in the mantle called residuum. Residuum is weathered rock that is not transported by erosion. The lava sea is beneath the residuum and lifts it up to form the tessera terrain. As Hansen explained it:
This is where it gets fun. When you have vast amounts of partial melt in the mantle that rushes to the surface, what gets left behind is something called residuum. Solid residuum is much stronger than the adjacent mantle, which did not experience partial melting. What may be surprising is that the solid residuum is also lower density than all the mantle around it. So, it’s stronger, but it’s also buoyant. You basically have an air mattress sitting in the mantle beneath your lava pond, and it’s just going to rise up and raise that tessera terrain.
In the case of Haasttse-baad Tessera, however, the residuum was moved by mantle connection. That meant that Haasttse-baad Tessera remained at the same elevation as the planet’s surface instead of being taller in height.
Formation of the concentric rings
The lava helped to form the concentric rings as well. The study suggested they formed when the two asteroids hit Venus in the same area back-to-back. The first impact created the lava sea, which formed the tessera terrain. Then, the second impact hit the lava sea, creating the concentric rings. Scientists think the process was similar to other giant concentric ring structures such as the craters Valhalla on Callisto and Tyre on Europa. They formed when impacts hit a thin, strong layer – the icy crust – on top of a weaker layer, either ocean water or slushy water. That would result in the concentric “ripples” or rings.
The appearance of this probable impact basin was unexpected for scientists. It was a surprise that – on blistering hot Venus – impact basins would similar to those on icy ocean moons. But now scientists know it was due to the thinness of the lithosphere. As Hansen noted:
Who would have thought flat low-lying tessera terrain or a big plateau is what an impact crater could look like on Venus? We had been looking for big holes in the ground, but for that to happen, you need a thick lithosphere, and early Venus didn’t have that. Mars had a thick lithosphere. The moon had a thick lithosphere. Earth likely had a thin lithosphere when it was young too, but its record has been greatly modified or erased by erosion and plate tectonics.
Bottom line: Where are the giant craters on Venus? Huge concentric rings in the Haasttse-baad Tessera region may be the result of two back-to-back asteroid impacts.
Source: Haasttse-baad Tessera Ring Complex: A Valhalla-Type Impact Structure on Venus?
Via Planetary Science Institute
Read more: ‘Continents’ on Venus similar to those of early Earth?
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