Evolution of terrestrial planetary bodies and implications for habitability

The terrestrial planetary bodies of our solar system—Mercury, Venus, Earth, and Mars—share a common origin through nebular accretion and early magma ocean differentiation, yet they diverged significantly in geological evolution, tectonic regimes, and habitability. Differences include distance from Sun, size, mechanism of internal cooling, degassing record, and resultant surficial conditions. Mercury, Mars, and Earth's Moon preserve largely mafic crust formed early in their evolution in a stagnant lid setting. Venus and Earth with their larger size record a long history of tectonic activity. Venus's mafic crust underwent large-scale resurfacing in the last billion years, likely in a stagnant lid setting but with potential areas of squishy lid behavior. The Earth preserves a long-lived plate tectonic regime in which young mafic crust beneath oceans is continually generated and recycled, whereas felsic crust forms emergent continents and spans much of the planet's history. The Earth is also characterized by a persistent magnetic field and a complex biosphere. Variations in tectonic modes between the terrestrial planets impact volatile exchange, magmatic outgassing, nutrient recycling and, in the case of Earth, provision of ecological niches. Other planets experienced transient habitability or remained uninhabitable, largely due to early cessation of tectonic and magnetic activity or atmospheric loss. Life may emerge under stagnant lid conditions, but sustained habitability and biological diversification require continued geological activity and crustal emergence. Insights from the terrestrial planets inform the search for habitable exoplanets, highlighting the intertwined roles of planetary interiors, surface processes, and atmosphere-crust interactions in shaping life-supporting environments.