In a study focused on Venus' Haastte-baad Tessera, a geological feature covering over 900 miles, a research team led by Planetary Science Institute's Vicki Hansen explored this area's unusual concentric rings and suggested they mark two consecutive impact events.

"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," Hansen said. "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' tessera terrain, regions of intensely wrinkled, layered material, forms atop a flowing substrate, creating a unique topography. Hansen explained it by comparing the process to "pea soup with a scum forming on top." Though in Venus' case, this layer formed atop vast lava plains. Venus' hotter, thinner lithosphere in its youth allowed impacts to break through, exposing molten mantle material that later solidified into tessera terrain.

Researchers date this event between 1.5 and 4 billion years ago, though another layer of mystery arose with tessera terrain found on elevated plateaus. According to Hansen, the process behind these plateaus involves solid residuum, a denser layer beneath the mantle, which forms when molten material rises to the surface. "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," Hansen said.

The ring structures of the Haastte-baad Tessera resemble features on other celestial bodies, such as Callisto's Valhalla crater and Europa's Tyre crater. Co-author Evan Bjonnes of the Lunar and Planetary Institute modeled these structures under Venusian conditions, confirming the necessity of back-to-back impacts. The researchers suggest that one impact created the lava pond that formed tesserae, while a second impact generated the concentric ring structure.

This process, though rare, has Earthly parallels. Hansen cited ancient South African rocks indicating a similar dual-impact event on Earth about 3.5 billion years ago. In contrast, the thick lithospheres of Mars and the Moon preserved their early craters as more traditional basins. On Venus, however, the thin lithosphere may have reshaped impact scars into features more akin to flat tessera terrain or elevated plateaus.

"Who would have thought flat low-lying tessera terrain or a big plateau is what an impact crater could look like on Venus?" Hansen said. "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."

The study's results point to two large bolides, each about 45 miles wide, that hit Venus billions of years ago, leaving distinct marks on the planet's surface. Next, the team plans to model how different bolide sizes and lava pond characteristics might have affected Venus' geological features.

Research Report:Haasttse-baad Tessera Ring Complex: A Valhalla-Type Impact Structure on Venus?

Related Links
Planetary Science Institute
Venus Express News and Venusian Science