Early (In)Habitability Among Exoplanets: A 1D Parameterized Approach Linking the Mantle-Tectonics-Atmospherics System

With the plethora of terrestrial-like exoplanets recently discovered from the Kepler mission [1], ranging from ~ 0.5 to ~ 2 Earth radius (E_R), it is natural to consider how many of these bodies may have had an atmosphere that allowed for stable liquid water at the surface. Although many discovered terrestrial exoplanets fall within or on the edges of the classically defined habitable zone [2], many do not. Further, the distance and flux from the host star alone may not be a sufficient metric to assess habitability. The habitability of a planetary body is significantly influenced by linked atmospheric and interior processes, such as mantle convection, tectonic mode, geochemical evolution, melting and outgassing, as well as atmospheric development, physics, and chemistry [3-6].

With the early atmospheric development of a planet inherently linked to its interior evolution, the thermal and chemical interior evolution of a rocky planetary body is integral to elucidating its surface and atmospheric evolution. Here we apply parameterized models of thermal-chemical mantle convection with atmospheric moist static energy balance and single column models, to understand the linked behavior and evolution of a planet, its interior, atmosphere, and surface temperatures. From Earth-like mobile lids to Mars-like stagnant lids and heat-pipe planets, we investigate the role of differing tectonic states in creating a habitable surface environment. We consider planetary radii ranging from that of Mercury to super-Earths (~0.4–2.5 E_R). Internal structures, mineralogies, radiogenic abundances, and volatile inventories are constrained from coupled hydrostatic equilibrium models and estimates of Earth’s starting compositions from chondritic material [7, 8]. Additionally, we examine the influence that initial atmospheric composition and stellar properties have on habitability.

Our preliminary results indicate that melt production, and therefore atmospheric generation, is enhanced in the first billion years of planetary evolution for mobile lids before tapering off to lower levels. Stagnant lids by contrast show enhanced melt production and atmospheric generation rates at > 1 Gyr and until cessation of convection. Enhanced melt production rates are achieved for stagnant lids < 1 E_R and for mobile lids ≥ 1 E_R as compared to their respective tectonic counterparts. For mobile lids > ~1.5 E_R, melt production largely ceases by 2-3 Gyr due to efficient heat loss.

These results suggest that mobile lids generate atmospheres early in punctuated melting events (within the first few 100 million years, but less than 1 billion years), whereas stagnant lids generate atmospheres across longer timescales through stable melting over billions of years. For small Earth-like terrestrial planets (< 1 E_R) stagnant lids produce denser atmospheres than their mobile lid counterparts. Larger Earth-like mobile lid terrestrial planets (> 1 E_R) produce thicker melt generated atmospheres than stagnant lid counterparts. In aggregate these results suggest that mobile lids are more efficient at generating secondary atmospheres through outgassing for planets ≥ ~1 E_R, and stagnant lids are more efficient for planets < 1 E_R, suggesting there exists an optimal planetary size / tectonic state phase space for both atmosphere generation and timing.

Our preliminary results provide insight into a more comprehensive understanding of processes that are likely to foster the presence of stable liquid water at the surface of a planet. We will discuss properties of the atmospheres produced from outgassing and the constraints for the timing of potential habitability for each tectonic state, planetary size, and insolation rate.

References: [1] Rogers (2015) Astrophys. J., 801, 1; [2] Choudhuri, (2012). Astrophysics for Physicists, Cambridge University Press; [3] Foley, B. J. (2015), Astrophys. J., 812; [4] Tosi et al. (2016), Geophys. Res. Abstr., 18; [5] Lenardic et al. (2016b), J. Geophys. Res. Planets, 121; [6] Weller and Kiefer (2019) J. Geophys. Res. Planets. [7] McDonough and Sun, (1995), Geology, 120; [7] Driscoll and Olson (2011), Icarus, 213