Distinguishing oceans of water from magma on mini-Neptune K2-18b (2401.05864v3)

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 modelling 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 modelling 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 < 3 sigma, 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 >4um 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.

• The paper employs JWST spectral data and climate simulations to compare water ocean and magma ocean models on mini-Neptune K2-18b.
• It finds that the observed ammonia depletion, explained by its solubility in both water and magma, supports the magma ocean hypothesis within a 3σ confidence level.
• The paper emphasizes that targeted 4 µm spectroscopic observations are crucial for definitively characterizing the exoplanet’s surface and advancing habitability theories.

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

The paper titled "Distinguishing oceans of water from magma on mini-Neptune K2-18b" provides an extensive investigation into the atmospheric and potentially oceanic conditions of exoplanet K2-18b. Situated in the intriguing mini-Neptune category, K2-18b poses unique challenges for planetary characterization due to its intermediate density and substantial hydrogen-rich atmosphere. The investigation leverages recent observations from the James Webb Space Telescope (JWST) to discern whether the exoplanet might host liquid water oceans or, alternatively, be covered by a hot magma ocean.

The paper begins by examining the possibility of K2-18b being a member of the 'Hycean' planets, characterized by hydrogen-dominated atmospheres possibly overlaying vast liquid water oceans. Such a hypothesis has garnered support primarily due to the detected presence of \ce{CO2} in the atmosphere, alongside an apparent depletion of ammonia (\ce{NH3}) below 100 ppm. This fingerprint aligns with expectations for a Hycean world, where ammonia solubility in liquid water could explain its atmospheric scarcity.

However, the research highlights a significant disparity when considering the thermal structures predicted by climate models. These models, considering the solar irradiation K2-18b experiences, suggest conditions too warm for stable liquid water. Instead, the authors propose a magma ocean scenario as a plausible reconciliation. In this model, the interaction between a deep, potentially molten silicate surface and the hydrogen-rich envelope leads to reduced ammonia presence due to its high solubility in magma under reducing conditions. This complex interaction mirrors the observed spectrum with notable accuracy, approximately within 3σ, indicating it as a valid competing hypothesis to liquid water oceans.

The paper meticulously models a spectrum of scenarios for K2-18b's interior and atmospheric conditions, exploring variables such as magma ocean mass, planetary volatile inventories, and oxygen fugacity in magma. These factors dramatically influence nitrogen and carbon atmospheric concentrations, impacting the presence of spectrally prominent molecules like \ce{NH3}, \ce{CO2}, and \ce{CO}. Interestingly, typical indicators such as a high \ce{CO2}/\ce{CO} ratio common in water-world scenarios may not readily distinguish between a water or magma ocean, necessitating refined observational techniques and deeper spectral analysis in the 4 µm wavelength region.

The authors argue the need for continued observations, targeting more diagnostic spectral features that can discriminate between the water and magma ocean models. Practically, distinguishing between these planetary characteristics informs our understanding of exoplanet formation, potential habitability, and the diversity of planetary systems beyond our solar reach. Theoretically, it challenges existing concepts about the habitability potential and thermal evolution of sub-Neptune-type planets.

Identifying definitive spectroscopic signatures remains crucial for unambiguously determining the nature of K2-18b's surface and atmospheric composition. Moving forward, this paper sets the stage for future investigations, emphasizing the delicate interplay between volcanic and atmospheric processes in defining observable exoplanet characteristics. As our observational capabilities improve, so too does our potential to unravel the complex history and habitability of planets like K2-18b, which dwell on the fringes of what we currently consider possible for planetary life.