Distinguishing Oceans of Water from Magma on Mini-Neptune K2-18b

Abstract

Mildly irradiated mini-Neptunes have densities potentially consistent with them hosting substantial liquid-water oceans (“Hycean” planets). The presence of CO2 and simultaneous absence of ammonia (NH3) in their atmospheres has been proposed as a fingerprint of such worlds. JWST observations of K2-18b, the archetypal Hycean, have found the presence of CO2 and the depletion of NH3 to <100 ppm; hence, it has been inferred that this planet may host liquid-water oceans. In contrast, climate modeling suggests that many of these mini-Neptunes, including K2-18b, may likely be too hot to host liquid water. We propose a solution to this discrepancy between observation and climate modeling by investigating the effect of a magma ocean on the atmospheric chemistry of mini-Neptunes. We demonstrate that atmospheric NH3 depletion is a natural consequence of the high solubility of nitrogen species in magma at reducing conditions; precisely the conditions prevailing where a thick hydrogen envelope is in communication with a molten planetary surface. The magma ocean model reproduces the present JWST spectrum of K2-18b to ≲3σ, suggesting this is as credible an explanation for current observations as the planet hosting a liquid-water ocean. Spectral areas that could be used to rule out the magma ocean model include the >4 μm region, where CO2 and CO features dominate: magma ocean models suggest a systematically lower CO2/CO ratio than estimated from free-chemistry retrieval, indicating that deeper observations of this spectral region may be able to distinguish between oceans of liquid water and magma on mini-Neptunes.

JWST Observations of K2-18b Can Be Explained by a Gas-rich Mini-Neptune with No Habitable Surface

“…Hu (2021) predicted stratospheric NH 3 could be photochemically depleted to undetectable concentrations (<1 ppm) on a gas-rich K2-18b if tropospheric mixing is slow (10 3 cm 2 s −1 ). Also, nitrogen could dissolve into a magma ocean at the base of a thick H 2 -rich envelope, preventing a large observable NH 3 abundance in the upper atmosphere (Shorttle et al 2024).…”

The Geochemical Potential for Metabolic Processes on the Sub-Neptune Exoplanet K2-18b

“…Much more work needs to be done on K2-18b. Now that excitement has built up, it can be anticipated that alternative models will emerge to challenge the hycean interpretation (e.g., Shorttle et al 2024;Wogan et al 2024). A next step will be to decide whether the hycean interpretation is reasonable, or if K2-18b might be a hycean world in disguise.…”

“…The exception would be if the observational data are more uncertain than what was found (Madhusudhan et al 2023a). Another interesting idea that was recently proposed is that NH 3 could dissolve in a reduced silicate magma ocean (Shorttle et al 2024), potentially explaining why NH 3 was not detected by JWST. However, this scenario may also be inconsistent with the lack of CO detection (Madhusudhan et al 2023a).…”

Why Observations at Mid-infrared Wavelengths Partially Mitigate M Dwarf Star Host Stellar Activity Contamination in Exoplanet Transmission Spectroscopy

“…Critically, stars lack these reduced gases, doubly ensuring there will not be any contamination from starspots or faculae. The lack of NH 3 in the presence of reduced gases (CH 4 and also PH 3 ) for an H 2 -dominated atmosphere may be able to indicate the presence of mini-Neptune water oceans (Hu et al 2021;Yu et al 2021) or instead surface magma oceans (Shorttle et al 2024), owing to the extremely high solubility of NH 3 in both water (e.g., Huang et al 2022 and references therein) and magma (e.g., Dasgupta et al 2022;Suer et al 2023 and references therein).…”