Mantle Convection and Nightside Volcanism on Lava World K2-141 b

Abstract

Ultra-short period lava worlds offer a unique window into the coupled evolution of planetary interior and atmospheres under extreme irradiation. In this study, we investigate the mantle dynamics, nightside volcanism, and volatile outgassing on lava world K2-141 b ($1.54 R_{\oplus}$, $5.31 M_{\oplus}$) using two-dimensional convection models with tracer-based volatile tracking. Our simulations explore a range of interior configurations, including models with and without plastic yielding, basal versus mixed heating, core cooling, and melt intrusion. In models without plastic yielding (i.e. with a strong lithosphere), we find that mantle upwellings form at the substellar and antistellar points, while downwellings form near the day-night terminators at the boundary between the magma ocean and cold, solid nightside. These downwellings facilitate the recycling of crustal material, representing a form of asymmetric, single-lid tectonics. The resulting magma ocean thickness varies from 200 to 300 km depending on the model parameters, corresponding to about 2-3% of the planet's radius. Continuous nightside volcanism produces a basaltic crust and gradually depletes the mantle of volatiles. We find that over a billion years, volcanic eruptions can outgas tens of bars of CO$_{2}$ and H$_{2}$O. We show that even relatively large volcanic eruptions on the nightside produce thermal emission signals of no more than 1 ppm, remaining below the current detectability threshold in thermal phase curves. However, for most models, outgassing rates are increased near the day-night terminators and future studies should assess whether such localised outgassing could lead to atmospheric signatures in transmission spectroscopy.

Figures (15)

• Figure 1: Reference density profile for lava world K2-141 b including olivine (ol), pyroxene-garnet (px-gt), and melt (same for molten olivine and pyroxene-garnet). The purple line shows the combined reference density for pyrolite.
• Figure 2: Snapshots of mantle temperature (left column), and basalt fraction (right column) at $1$, $3$, and $5$ Gyr for MixCC (mixed heating with decreasing CMB temperature and no plastic yielding). The top panel shows a zoom-in of the mantle temperature and basalt fraction at $5\,$Gyr. The model evolves towards a degree-2 convection pattern with stable upwellings near the substellar and antistellar points and downwellings near the day-night terminators. A basaltic crust gradually forms across the nightside, while the dayside remains molten due to sustained high stellar irradiation. The magma ocean is shallow with a thickness around $300$ km. Videos for this model run are available online and on Zenodo ZenodoMeier2025.
• Figure 3: Mean mantle temperature evolution for the different models of K2-141 b. Models MixCC and MixIntr (both with mixed heating) reach a thermal steady state after approximately $2-3$ Gyr, with similar final mantle temperatures ($3470$ K). Model MBasal (basal heating) cools more gradually and reaches a lower steady state temperature ($3300$ K). Model MPlastic, which includes plastic yielding, exhibits the most significant cooling due to efficient heat loss driven by strong downwellings. Models with decreasing CMB temperature from core cooling (MixCC and MPlastic) are expected to cool further over time, as the core continues to lose heat and thermal buoyancy at the CMB diminishes.
• Figure 4: Surface heat flux evolution on the (a) dayside (in W/m$^2$) and (b) nightside (in mW/m$^2$) of K2-141 b for the different mantle convection models. MixCC, MBasal, and MixIntr show similar trends and stabilise after around $2$ Gyr, while MPlastic shows higher and more variable heat fluxes due to enhanced downwellings and therefore more efficient mantle cooling.
• Figure 5: Snapshots of mantle temperature and melt fraction (top row) and basalt fraction (bottom row) at $6.5$ Gyr for each interior model of K2-141 b. All models show a shallow magma ocean on the dayside (left hemisphere) and a solid nightside (right hemisphere). Models without plastic yielding (MixCC, MBasal, MixIntr), exhibit upwellings at the substellar and antistellar points, with downwellings forming at the day-night terminators. MPlastic, which includes plastic yielding (lithospheric weakening), displays a more asymmetric pattern, with downwellings forming on the nightside and upwellings rising preferentially on the dayside. Videos for these model runs are available online and on Zenodo ZenodoMeier2025.