Researchers, including Vicki Hansen from the Planetary Science Institute, have studied Venus’ Haastte-baad Tessera region to explain why Venus lacks large impact basins. The region, known for its ancient tessera terrain, shows evidence of two massive, back-to-back impact events.
The unique concentric rings and the tessera’s structure suggest that Venus’ early geological conditions led to impact features that differ significantly from traditional impact craters on the Moon, Mars, or Venus.
These distinctive formations offer new insights into how impacts shaped Venus’ surface.
Hansen 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 us of early planet processes. And perhaps even more importantly, it shows us that not all impact structures look alike. Impact structures result from a bolide – an unspecified composition body–colliding with a target planet. The nature of the bolide is important, but so too is the nature of the target.”
Oldest terrain on Venus has layering that is consistent with volcanic activity
Tesserae on Venus are highly deformed terrains formed by a strong, thin material layer over a weak, convecting layer, resembling the movement of boiling water. These formations likely resulted from volcanic activity rather than any “pea soup” analogy.
Researchers, including Vicki Hansen, suggest that in Venus’ early, hotter history, its lithosphere was only about 6 miles thick (compared to the current 70 miles). When a large bolide struck this thin lithosphere, it would have penetrated the mantle, releasing a massive lava flow that eventually cooled and formed the tesserae.
This process is believed to have occurred between 1.5 and 4 billion years ago.
An added mystery is that tesserae are sometimes found atop plateaus on Venus—the formation of the huge volume of lava results in a solution.
The study discovered the secret to Venus’s youthful appearance
Hansen said, “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 also has a lower density than all the mantle around it. So, it’s stronger, but it’s also buoyant. You have an air mattress sitting in the mantle beneath your lava pond, which will rise up and raise that tessera terrain.”
Convection beneath Venus’ lithosphere can move material, affecting tesserae’s elevation. If the material underneath remains in place, the tessera terrain stays elevated; if mantle convection sweeps it away, the tesserae settle to the same level as the rest of the surface. This explains why Haastte-baad Tessera remains at a higher elevation than other regions.
The team also focused on the unique ring structures surrounding the tesserae, which aren’t found anywhere else on Venus. These rings resemble features on other icy moons, such as Callisto’s Valhalla crater and Europa’s Tyre crater. On these moons, the rings are thought to have formed from impacts on a thin, strong outer layer (ice) overlying a weaker, fluid-like layer (liquid water or slush).
Venus’s squishy surface shows signs of geothermal activity
Similarly, the team suggests that Venus’ tessera terrain and its rings formed from impacts on a thin, strong material layer (lava) over a weaker, convecting layer, creating a structure similar to those on Callisto and Europa.
Co-author Evan Bjonnes, from the Lunar and Planetary Institute and Lawrence Livermore National Laboratory, is the first to model how Valhalla-like ring structures could form under Venus’ conditions. The team’s research suggests that two large bolides hit Venus rapidly, creating the ringed tessera terrain.
The first impact formed a lava pond, which later cooled into tesserae. The second impact struck the lava pond, generating the distinctive ring structures.
While the idea of back-to-back impacts might seem rare, evidence from ancient South African rocks suggests similar events happened on Earth around 3.5 billion years ago.
Furthermore, the Moon and Mars show signs of massive impacts, which were more common during the first 2.5 billion years of the Solar System. These planets were affected by such impacts, which formed large basins due to the thick lithospheres at the time, in contrast to Venus’ thinner early lithosphere.
Hansen said, “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, but its record has been greatly modified or erased by erosion and plate tectonics.”
The team is further planning to run more models varying the size of the bolide, the thickness of the lava pond, and the thickness of tessera-forming scum.
Journal Reference:
- I. López, E. Bjonnes, V. L. Hansen. Haasttse-baad Tessera Ring Complex: A Valhalla-Type Impact Structure on Venus? JGR Planets. DOI: 10.1029/2023JE008256