Mysterious crown-like features on Venus may finally have an explanation

The mystery behind Venus' giant, crown-shaped geological features, known as coronae, may finally have an explanation: A "glass ceiling" in Venus' mantle is trapping heat and driving slow, shifting currents that might lead to the formation of the crown-like surface features, scientists propose in a new study.

"On Venus, there is a pattern that is telling us something," Madeleine Kerr, a doctoral candidate at the University of San Diego's Scripps Institution of Oceanography and the study's lead author, said in a statement. "We think what we found is the key to unlocking the mystery of the origin of these coronae."

Venus and Earth are considered "twin" planets because they have roughly the same size, bulk density and distance from the sun. However, the planets' surfaces indicate that they diverged on their evolutionary paths, leading to two substantially different worlds. One such difference is these coronae, which are unique to Venus.

Related: Earth's 'evil twin' Venus may have mirrored our planet more than expected

Scientists have mapped more than 700 coronae across Venus' surface, and they span a wide range of sizes and features. Yet their origin is still a puzzle, given that Venus is covered in a single, continuous crust — unlike Earth, which has shifting tectonic plates.

Some hypotheses link the formation of Venus' larger coronae — those bigger than 310 miles (500 kilometers) in diameter — to mantle plumes and tectonic processes such as subduction and the delamination of denser parts of the planet's crust, the research team wrote in the study, published Sept. 16 in the journal PNAS. The smaller coronae — those with a mean diameter of about 124 miles (200 km) — on the other hand, can be attributed to smaller hot upwellings in the mantle, like blobs of wax rising in a lava lamp.

However, these theories have been difficult to substantiate.

"The current state of knowledge of the planet Venus is analogous to the 1960's pre-plate tectonic era because we currently lack an equivalent unifying theory capable of linking how heat transfer from the planet's interior gets manifested into the tectonics and magmatic features observed on Venus' surface," David Stegman, a professor of geosciences at the University of San Diego's Scripps Institution of Oceanography and one of the study's authors, explained in the statement.

Now, Stegman and his colleagues believe they may have found a crucial piece of the puzzle.

Cold material that sinks from the surface and hot material that rises from deeper inside both encounter a barrier at a depth of about 370 miles (600 km) — what the team calls a "glass ceiling." Most rising hot plumes aren't strong enough to break through this barrier, so they get deflected and spread sideways underneath it. Only the largest plumes can penetrate all the way to the surface, where they form huge volcanic rises. The material trapped beneath this ceiling stays warm, but it doesn't melt — so it acts like a hidden reservoir of heat in the mantle.

"This layer of warm fluid trapped between 600 to 740 km [370 to 460 miles] depth provides a global source of smaller-scale thermal instabilities," the researchers wrote in the study. "These plumes have a wide range of sizes since they do not necessarily obey classical boundary layer theory."

Using computational models, the team showed how these small-scale plumes beneath Venus' crust could form naturally. A cold "drip" of rock from the base of Venus' stagnant crust cools and becomes denser, and it eventually sinks into the hotter mantle below. This event then sets off a chain reaction that pushes up several pockets of hot rock.

In past studies, scientists had to start their geodynamic models with these hot blobs already in place below the lithosphere — the planet's rigid outer layer — to simulate how coronae and volcanoes form. However, this research has taken it a step further by showing a plausible natural origin for those initial conditions.

As these secondary plumes rise, melt and eventually sink again — interacting with Venus' mantle along the way — they could provide the variety of crown-shaped coronae seen across the planet's surface, the researchers proposed. The models suggest that this mechanism works when the mantle is 250 to 400 kelvins hotter than Earth's mantle, but it's still unclear how long such a state could last.

The scientists cautioned that more work is needed. Future studies should model plume dynamics in 3D, account for melting both inside and on the surface, include different mantle compositions, and track changes over Venus' entire history, they said. These steps will help to reveal how Venus' interior heat and movements shape the planet's coronae, volcanoes and overall surface.