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A newly identified tectonic "regime" may rewrite our understanding of how rocky worlds evolve, scientists report in a new study.

The findings may help to explain why Earth became geologically vibrant while Venus remained stagnant and scorching, with possible implications for our understanding of what makes a planet habitable.

When researchers used advanced geodynamic simulations to map diverse planetary tectonic regimes — distinct patterns that describe how a planet's outer shell deforms and releases heat under different conditions — they discovered a missing link they've dubbed the "episodic-squishy lid."

This striking new framework offers a fresh perspective on how planets shift between active and inactive states, thus reshaping scientific assumptions about planetary evolution and habitability, the team said in a statement explaining the study.

Tectonic regimes influence a planet's geological activity, internal evolution, magnetic field, atmosphere and even its potential to support life. The episodic-squishy lid builds on the traditional divide between plate tectonics or mobile lid regimes (like modern Earth) and stagnant-lid behavior (like Mars). It describes a state in which a planet's lithosphere cycles between relatively quiet periods and sudden bursts of tectonic motion. Unlike a classic stagnant lid, this regime permits intermittent weakening driven by intrusive magmatism and regional delamination, temporarily softening the crust before it stiffens again.

This on-again, off-again behavior could be a missing link in Earth's early evolution, the researchers said. The models suggest that Earth may have passed through a squishy-lid phase that gradually primed its lithosphere for full plate tectonics as the planet cooled.

The findings also help to clarify the "memory effect" — the idea that a planet's tectonic behavior is shaped by its past — by showing that as a planet's lithosphere weakens over time, as Earth's did, the transitions between tectonic states become far more predictable.

By mapping all six tectonic regimes under different physical conditions for the first time, the team constructed a comprehensive diagram revealing likely transition pathways as a planet cools.

"Geological records suggest that tectonic activity on early Earth aligns with the characteristics of our newly identified regime," study co-author Guochun Zhao, a geologist at the Chinese Academy of Sciences, said in the statement. "As Earth gradually cooled, its lithosphere became more prone to fracturing under specific physical mechanisms, eventually leading to today's plate tectonics. This provides a key piece of the puzzle in explaining how Earth became a habitable planet."

The episodic-squishy lid may also shed light on Venus's long-standing mysteries. Although Venus is roughly the same size as Earth, it lacks clear evidence of plate tectonics, instead displaying volcanically reshaped terrain and distinctive features called coronae. The new simulations reproduce Venus-like patterns by placing the planet in an episodic or plutonic squishy-lid regime, where magmatism and mantle plumes periodically weaken the surface without generating true plates.

"Our models intimately link mantle convection with magmatic activity," study co-author Maxim Ballmer, an associate professor of geodynamics at University College London, said in the statement. "This allows us to view the long geological history of Earth and the current state of Venus within a unified theoretical framework, and it provides a crucial theoretical basis for the search for potentially habitable Earth analogs and super-Earths outside our solar system."

Because tectonics govern how water and carbon dioxide circulate through a planet's interior and atmosphere, understanding how lithospheres weaken and transition between regimes could help scientists assess which distant worlds might support stable climates, or even life, and guide decisions on observational targets for future missions.

The findings were published Nov. 24 in the journal Nature Communications.