Land planets in a ROCKE-3D GCM perturbed parameter ensemble: fractional habitability

Land is required not only to provide life a stable surface but also to support a stable carbonate-silicate cycle. Modeling studies have surmised that land planets may support a wider and longer continuous habitable zone than aqua planets, both at the moist greenhouse limit and at the outer edge. The habitability of a planet is not well characterized by its mean state but by its diversity of climate regimes that distribute heat and water, if present, over its surface heterogeneously, producing environmental niches that will differ in suitability for life. A planet’s climate is subject to the interaction of many planetary features that affect its circulation patterns including: parent star, orbital dynamics, atmospheric composition, surface composition. Given such a diverse parameter space, if a habitable planet harbors water, how might it distribute over the planet and where will the liquid water be for life?

Because most exoplanet climate modeling studies thus far have biased the literature sampling of the parameter space to aqua planets, Earth continents, the Sun, M stars, and extremes of atmospheric CO₂ content, in this study we filled the gaps through a perturbed parameter ensemble (PPE) in the NASA Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics general circulation model (ROCKE-3D GCM). We simulated the climates of idealized planets that are all land with flat topography. We conducted a Latin hypercube sampling from the following ten variables in 110 experiments: stellar temperature and spectra from observed G, K, and M stars; irradiance spanning from that of TRAPPIST1-e to early Venus; rotation period including tidally locked, 3:2 spin-orbit, and 1-128 Earth days; obliquity from 0 to 90°; surface pressure from 0.5-10 bar; CO₂ content (in N₂/CO₂ atmospheres) from 0 to 10 bar; surface albedo 0.11-0.3; surface roughness from the GCM minimum of 0.005 to 0.7, the average of bare soils on Earth; two soil textures having water holding capacity at the minimum, sand, and optimum for life, silt loam; and initial soil water content spanning the field capacities of the two soil textures, which give 0.008-0.026% of the Earth’s ocean. Thus, these were fairly dry planets, but with water free to circulate and accumulate in different climate zones and in the atmosphere.

We evaluated the planets’ “climatological period” to quantify mean climate states and, through multivariate statistical analysis, identify continuous, non-linear relations between the parameters and several metrics of habitability, including surface water cover, aridity, soil relative extractable water, and the planetary water phase budget. From these we derived classifications of rocky land planet climatologies, such as eyeball, billiard ball, and Fabergé egg planets. We identified parameter thresholds for transitions to slushball, clement, and steam planet states. 80% of the variance in surface temperature may be explained by stellar type, irradiance, and greenhouse gas content, with the solar day length, surface pressure, and obliquity only weakly significant. Available liquid water for life by mass and areal extent are then correlated with temperature, with obliquity up to about 60° weakly promoting surface wetness. We look for surface water metric relationships to those parameters and climate features that might be observed by exoplanet missions, including stellar type, irradiance, planetary albedo, cloud cover, and upper atmosphere water vapor. This framework offers a modeled context to quantify the uncertainty in habitability given many parameters that cannot be observed. This ensemble of idealized land planets lays foundations for further filling the parameter space with more diverse specifications of the planet size, and compositions of the surface and atmosphere for understanding the climatologies of land planets in particular.