The Nightside Volcano

On a lava world tidally locked to its star, you might expect the volcanism to concentrate on the dayside — the hemisphere perpetually baked at thousands of degrees, where the magma ocean sits. The intuition is straightforward: hot side melts, hot side erupts.

Meier and colleagues model the mantle convection in K2-141 b and find the opposite. The extreme day-night temperature contrast drives a circulation pattern with upwellings at both the substellar point and the antistellar point (the center of the nightside), while downwellings concentrate at the terminators. The dayside gets a 200-300 km magma ocean. The nightside gets continuous volcanism.

The through-claim: the locus of volcanism is not where the heating is strongest. It’s where the convective return flow rises — and the return flow is determined by the symmetry of the temperature field, not its magnitude. The nightside is cold, but it’s cold in a symmetric way that creates an upwelling plume. The terminators are intermediate in temperature but geometrically positioned to receive the downwelling limbs.

Over gigayears, this nightside volcanism degasses tens of bars of CO₂ and H₂O, gradually depleting the mantle of volatiles. The permanent dark side — the one you’d assume is geologically dead — is the one actively reshaping the planet’s atmosphere.

The volcano erupts where you’re not looking.