A boring billion for mountains: Earth's crust has changed over time as supercontinents formed and broke apart. Tied into this cycle are the building and erosion of high mountains, which are tied to collisions between tectonic plates. Tang et al. use europium anomalies in zircons to estimate the mean thickness of crust over Earth's history. This proxy shows that mountain building has not always been as active as it is today or as it was very early in Earth's history. Mountain building, and the subsequent erosion, was less intense for about a billion years, roughly correlated with a so-called “boring billion” period of biological evolution. Science , this issue p. 728

Saturn’s moon Enceladus has a potentially habitable subsurface water ocean that contains canonical building blocks of life (organic and inorganic carbon, ammonia, possibly hydrogen sulfide) and chemical energy (disequilibria for methanogenesis). However, its habitability could be strongly affected by the unknown availability of phosphorus (P). Here, we perform thermodynamic and kinetic modeling that simulates P geochemistry based on recent insights into the geochemistry of the ocean–seafloor system on Enceladus. We find that aqueous P should predominantly exist as orthophosphate (e.g., HPO₄²⁻), and total dissolved inorganic P could reach 10⁻⁷to 10⁻²mol/kg H₂O, generally increasing with lower pH and higher dissolved CO₂, but also depending upon dissolved ammonia and silica. Levels are much higher than <10⁻¹⁰mol/kg H₂O from previous estimates and close to or higher than ∼10⁻⁶mol/kg H₂O in modern Earth seawater. The high P concentration is primarily ascribed to a high (bi)carbonate concentration, which decreases the concentrations of multivalent cations via carbonate mineral formation, allowing phosphate to accumulate. Kinetic modeling of phosphate mineral dissolution suggests that geologically rapid release of P from seafloor weathering of a chondritic rocky core could supply millimoles of total dissolved P per kilogram of H₂O within 10⁵y, much less than the likely age of Enceladus’s ocean (10⁸to 10⁹y). These results provide further evidence of habitable ocean conditions and show that any oceanic life would not be inhibited by low P availability.