4.11.2024

A synthetic aperture radar image of Haastte-baad Tessera, cut by a set of unique concentric rings that record a newly recognized type of impact crater on Venus, previously only identified on the icy moons of Jupiter.
Credit: NASA

TUCSON, Ariz. – The Moon and Mars are pocked with giant impact craters acquired very long ago, while there appears to be a dearth of them on Earth and Venus.

Time may have healed many of Earth’s largest impact scars with its wind, water, life and plate tectonics, but Venus lacks these destructive processes. What’s more, Venus is home to some of the most pristine impact craters in the Solar System. So why the lack of huge impact basins? 

After geologically mapping a region on Venus called Haastte-baad Tessera and modeling how its unique features could have formed, a small group of researchers, including Planetary Science Institute Senior Scientist Vicki Hansen, think they have an answer.

Haastte-baad Tessera hosts one of the oldest surfaces on Venus, called tessera terrain, cut by a unique set of concentric rings over 900 miles at its widest.

The team concluded that the tessera and its rings record two huge back-to-back impact events. The rings and its host tessera look nothing like traditional-looking craters on the Moon and Mars, and even Venus; this is because Venus’ early conditions led to impact structures that significantly differ from classic impact craters.

“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.”

The paper was published in the Journal of Geophysical Research: Planets.

A primordial puzzle

Tesserae are regions of heavily deformed terrain on Venus characterized by their wrinkled and corrugated terrain, which forms when a relatively thin but strong layer of material forms over a weak layer able to vigorously flow and convect, like boiling water.

“Think of pea soup with a scum forming on top,” Hansen said.

Venus’ tessera terrain didn’t form on pea soup, but rather a huge pond of lava. So from where might that lava have come?

While today’s Venus boasts an outer shell 70 miles thick, called the lithosphere, young Venus was much hotter and likely had a lithosphere about 6 miles thick. If a thin lithosphere is hit by a large bolide, the bolide will punch right through the thin lithosphere and into the mantle below, releasing a huge sea of lava to the surface that eventually cools into tesserae, Hansen said. The team’s mapping suggests this occurred 1.5 to 4 billion years ago.

An added mystery, however, is that tesserae can sometimes lie atop plateaus.

The formation of the huge volume of lava results in a solution.

“This is where it gets fun,” she said. “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.”

But convection beneath the lithosphere can sometimes move material. If the residuum remains in place, the tessera remains high; if the residuum is swept away by mantle convection, the overlying tessera will be at the same elevation as the rest of the planet’s surface, Hansen said. This is the case for Haasttse-baad Tessera.

Then, the team had to account for the ring structures, which aren’t seen anywhere else on Venus. They agreed that the rings are reminiscent of the Valhalla crater on Callisto and the Tyre crater on Europa. These are thought to have formed by an impact on a target of thin strong layer atop a weak, fluid-like layer, very similar to the configuration necessary to form tesserae. On Callisto and Europa, that means a strong thin layer of ice above an ocean or slushy liquid.

Co-author Evan Bjonnes of the Lunar and Planetary Institute and Lawrence Livermore National Laboratory, is the first person to ever model how these Valhalla-like structures could have formed under Venus conditions.

Ultimately, the research team found that the formation of the ringed structure on Venus would have required two large bolides hitting Venus back-to-back. The first created the lava pond to form tessera terrain and the second bolide impacted the lava pond, forming the unique ring structure.

Back-to-back bolides might seem too serendipitous, but there’s evidence in ancient rocks preserved in South Africa that this occurred about 3.5 billion years ago on Earth, Hansen said. Even on the Moon and Mars, there’s evidence of many of these enormous impacts. In those cases, the large bolides, not uncommon during the first 2.5 billion years of our Solar System history, struck planetary bodies with thick lithospheres, forming the huge impact basins we still see today

“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 bolides used in the model were relatively large – a bolide about 45 miles wide formed the ring structures on the earlier tessera-terrain lava pond. In the case of Earth, the two large bolides that hit about 3.5 billion years ago were likely up to 45 and 33 miles across, respectively. Hansen and her team next want to run more models varying the size of the bolide, the thickness of the lava pond, and the thickness of tessera-forming scum.

Quelle: Planetary Science Institute