Exoplanets & Exomoons

Distinguishing Oceans Of Water From Magma On Mini-Neptune K2-18b

By Keith Cowing
Status Report
January 12, 2024
Filed under , , , , , , , , , , , ,
Distinguishing Oceans Of Water From Magma On Mini-Neptune K2-18b
Total atmospheric surface pressure (bar) and volume mixing ratio of NH3, and CO2 output from the atmospheremagma ocean system over the parameter space which we have sampled, outlined in table 1. This corresponds to the output of step 2 in the schematic presented in figure 1. Points are colour-coded by the log of the planetary hydrogen budget, in units part-per-million of the planetary mass. Histograms of the distribution of NH3 mixing ratios (top left), CO2 mixing ratios (top right) and total surface pressures (lower right) from across the model runs are shown alongside. — astro-ph.EP

Mildly rradiated 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.

Oliver Shorttle, Sean Jordan, Harrison Nicholls, Tim Lichtenberg, Dan J. Bower

Comments: 13 pages, 5 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2401.05864 [astro-ph.EP] (or arXiv:2401.05864v1 [astro-ph.EP] for this version)
Submission history
From: Oliver Shorttle [via Oliver Shorttle as proxy]
[v1] Thu, 11 Jan 2024 12:14:23 UTC (623 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻