Habitability of Exoplanet Waterworlds (1801.00748v2)

Abstract: Many habitable zone exoplanets are expected to form with water mass fractions higher than that of the Earth. For rocky exoplanets with 10-1000x Earth's H2O but without H2, we model the multi-Gyr evolution of ocean temperature and chemistry, taking into account C partitioning, high-pressure ice phases, and atmosphere-lithosphere exchange. Within our model, for Sun-like stars, we find that: (1)~the duration of habitable surface water is strongly affected by ocean chemistry; (2)~possible ocean pH spans a wide range; (3)~surprisingly, many waterworlds retain habitable surface water for >1 Gyr, and (contrary to previous claims) this longevity does not necessarily involve geochemical cycling. The key to this cycle-independent planetary habitability is that C exchange between the convecting mantle and the water ocean is curtailed by seafloor pressure on waterworlds, so the planet is stuck with the ocean mass and ocean cations that it acquires during the first 1% of its history. In our model, the sum of positive charges leached from the planetary crust by early water-rock interactions is - coincidentally - often within an order of magnitude of the early-acquired atmosphere+ocean inorganic C inventory overlaps. As a result, pCO2 is frequently in the "sweet spot" (0.2-20 bar) for which the range of semimajor axis that permits surface liquid water is about as wide as it can be. Because the width of the HZ in semimajor axis defines (for Sun-like stars) the maximum possible time span of surface habitability, this effect allows for Gyr of habitability as the star brightens. We illustrate our findings by using the output of an ensemble of N-body simulations as input to our waterworld evolution code. Thus (for the first time in an end-to-end calculation) we show that chance variation of initial conditions, with no need for geochemical cycling, can yield multi-Gyr surface habitability on waterworlds.

Habitability of Exoplanet Waterworlds

The paper by Kite and Ford explores the habitability conditions of exoplanetary waterworlds—rocky planets with significantly higher water content than Earth, within the range of 10 to 1000 times Earth's H$_2$O mass fraction. This research addresses the evolving dynamics of ocean temperature and chemistry over gigayear timescales under the constraints of lacking geological recycling processes, specifically in environments absent of gaseous hydrogen presences like H$_2$.

The authors present a model that primarily considers carbon partitioning, high-pressure ice phases, and atmosphere-lithosphere interactions, focusing on planets orbiting Sun-like stars. The main objectives are to shed light on the potential longevity and maintenance of surface water in these exotic planetary settings and to evaluate whether such environments can sustain life-sustaining conditions over prolonged periods without the stabilizing influence of biogeochemical cycles.

Key Findings

1. Ocean Chemistry as a Critical Factor: The paper underscores the crucial role of ocean chemistry in determining the duration of surface liquid water. Unlike Earth-like planets where geochemical cycles involving rock weathering and volcano-tectonic processes regulate climate, waterworlds studied here have minimal to no relief from volcanic or tectonic resurfacing.
2. Wide Potential Range of Ocean pH: The possible range of ocean pH varies substantially. The pH levels significantly influence the carbon storage capacity of the ocean and, consequently, the partial pressure of atmospheric CO$_2$ ($p_{\mathrm{CO2}}$). At higher pH, carbon tends to be stored more in ocean as bicarbonates and carbonates, reducing atmospheric CO$_2$ and aiding in maintaining cooler, potentially habitable conditions.
3. Multi-Gyr Habitable Living Conditions: Interestingly, the models suggest that many waterworlds could retain surface liquid water conducive to habitability for over a billion years. This contradicts earlier assumptions that such longevity would necessitate active geochemical cycles. The curtailment of carbon exchange between the mantle and the ocean due to high seafloor pressures effectively traps the initial conditions set during the early life of the planet.
4. Optimal Conditions for Prolonged Habitability: For these waterworlds, $p_{\mathrm{CO2}}$ often aligns within a "sweet spot" ranging from 0.2 to 20 bars. This range enables a broad semimajor axis spread for habitable zones around Sun-like stars, maximizing the habitable lifespan as stars transition and increase in main-sequence brightness.

Theoretical and Practical Implications

Kite and Ford's paper provides insights into understanding habitability beyond the context of Earth analogs by reframing the classical requirement for geochemical and tectonic cycles. Their findings advocate that life potential on exoplanetary waterworlds could significantly diverge from the Earth-centric view, emphasizing the importance of initial chemical boundary conditions over active geological processes.

The research implies that with a potentially large population of small-radius exoplanets detected in habitable zones, waterworlds might contribute significantly to the pool of habitable zones across the galaxy. This alternative habitability paradigm opens new avenues for characterizing exoplanetary atmospheres and surfaces using observational data from missions like the James Webb Space Telescope and future exoplanet missions.

Despite uncertainties, such as initial volatile inventories and atmospheric compositions of these worlds, the model puts forward a framework that, with rigorous observational follow-up, could validate or refine our understanding of life-supporting conditions in extreme aqueous exoplanet environments.

Future Developments

Moving forward, this line of research calls for enhanced models integrating atmospheric chemistry dynamics, improved experimental data for solubility limits at high pressures, and a broader exploration of photosynthetic and chemosynthetic life potentials under these conditions. Future missions targeting atmospheric characterization of inferred waterworlds are expected to be crucial in testing the predictions made by Kite and Ford, marking a pivotal step in expanding the frontiers of astrobiology.