Why Earth developed plate tectonics while Venus, a planet of almost identical size, did not is among the oldest open questions in comparative planetology. An international team based in the Department of Earth and Planetary Sciences at HKU used numerical models to classify planetary tectonic regimes quantitatively for the first time, distinguishing six styles and identifying a new one they call the episodic-squishy lid, which alternates between two modes of activity.
The models also clarify a confounding factor. Planetary tectonic state exhibits hysteresis, a memory effect in which the current regime depends on the body's history rather than only on its present conditions. The team found this history dependence can nonetheless be predictable along an evolutionary path where the lithosphere weakens over time, as appears to have happened on Earth, which makes the problem tractable rather than merely contingent.
Venus provides an observational anchor. Its circular coronae, structures over 1,000 kilometres across, match the conditions of either the plutonic-squishy lid or the episodic-squishy lid regime in the classification. That gives planetary geologists a quantitative framework for interpreting features that have been described for decades without being firmly tied to an underlying tectonic mode. The study was published in Nature Communications.
The practical value extends to mission planning and to exoplanet work. A regime classification grounded in physical parameters offers a basis for deciding what future Venus missions should measure, and for assessing which super-Earths might sustain the kind of tectonic recycling thought to support habitability. Earth's own tectonic style looks less like a default and more like one outcome among several.