CO2 ocean bistability on terrestrial exoplanets

Journal of Geophysical Research: Planets American Geophysical Union 127:10 (2022) e2022JE007456

Authors:

Robert J Graham, Tim Lichtenberg, Raymond T Pierrehumbert

Abstract:

Cycling of carbon dioxide between the atmosphere and interior of rocky planets can stabilize global climate and enable planetary surface temperatures above freezing over geologic time. However, variations in global carbon budget and unstable feedback cycles between planetary sub-systems may destabilize the climate of rocky exoplanets toward regimes unknown in the Solar System. Here, we perform clear-sky atmospheric radiative transfer and surface weathering simulations to probe the stability of climate equilibria for rocky, ocean-bearing exoplanets at instellations relevant for planetary systems in the outer regions of the circumstellar habitable zone. Our simulations suggest that planets orbiting G- and F-type stars (but not M-type stars) may display bistability between an Earth-like climate state with efficient carbon sequestration and an alternative stable climate equilibrium where CO2 condenses at the surface and forms a blanket of either clathrate hydrate or liquid CO2. At increasing instellation and with ineffective weathering, the latter state oscillates between cool, surface CO2-condensing and hot, non-condensing climates. CO2 bistable climates may emerge early in planetary history and remain stable for billions of years. The carbon dioxide-condensing climates follow an opposite trend in pCO2 versus instellation compared to the weathering-stabilized planet population, suggesting the possibility of observational discrimination between these distinct climate categories.

K2 and Spitzer phase curves of the rocky ultra-short-period planet K2-141 b hint at a tenuous rock vapor atmosphere

Astronomy and Astrophysics EDP Sciences 664 (2022) A79

Authors:

S Zieba, M Zilinskas, L Kreidberg, Tg Nguyen, Y Miguel, Nb Cowan, R Pierrehumbert, L Carone, L Dang, M Hammond, T Louden, R Lupu, L Malavolta, Kb Stevenson

Abstract:

K2-141 b is a transiting, small (1.5 R⊕) ultra-short-period (USP) planet discovered by the Kepler space telescope orbiting a K-dwarf host star every 6.7 h. The planet's high surface temperature of more than 2000 K makes it an excellent target for thermal emission observations. Here we present 65 h of continuous photometric observations of K2-141 b collected with Spitzer's Infrared Array Camera (IRAC) Channel 2 at 4.5 μm spanning ten full orbits of the planet. We measured an infrared eclipse depth of ppm and a peak to trough amplitude variation of ppm. The best fit model to the Spitzer data shows no significant thermal hotspot offset, in contrast to the previously observed offset for the well-studied USP planet 55 Cnc e. We also jointly analyzed the new Spitzer observations with the photometry collected by Kepler during two separate K2 campaigns. We modeled the planetary emission with a range of toy models that include a reflective and a thermal contribution. With a two-temperature model, we measured a dayside temperature of Tp,d = 2049 362-359 K and a night-side temperature that is consistent with zero (Tp,n < 1712 K at 2σ). Models with a steep dayside temperature gradient provide a better fit to the data than a uniform dayside temperature (ΔBIC = 22.2). We also found evidence for a nonzero geometric albedo Ag = 0.2820.070-0.078. We also compared the data to a physically motivated, pseudo-2D rock vapor model and a 1D turbulent boundary layer model. Both models fit the data well. Notably, we found that the optical eclipse depth can be explained by thermal emission from a hot inversion layer, rather than reflected light. A thermal inversion may also be responsible for the deep optical eclipse observed for another USP, Kepler-10 b. Finally, we significantly improved the ephemerides for K2-141 b and c, which will facilitate further follow-up observations of this interesting system with state-of-the-art observatories such as James Webb Space Telescope.

A mini-chemical scheme with net reactions for 3D general circulation models. I. Thermochemical kinetics

Astronomy and Astrophysics EDP Sciences 664 (2022) A82

Authors:

S-M Tsai, Ekh Lee, R Pierrehumbert

Abstract:

Context. Growing evidence has indicated that the global composition distribution plays an indisputable role in interpreting observational data. Three-dimensional general circulation models (GCMs) with a reliable treatment of chemistry and clouds are particularly crucial in preparing for upcoming observations. In attempts to achieve 3D chemistry-climate modeling, the challenge mainly lies in the expensive computing power required for treating a large number of chemical species and reactions.
Aims. Motivated by the need for a robust and computationally efficient chemical scheme, we devise a mini-chemical network with a minimal number of species and reactions for H2-dominated atmospheres.
Methods. We apply a novel technique to simplify the chemical network from a full kinetics model, VULCAN, by replacing a large number of intermediate reactions with net reactions. The number of chemical species is cut down from 67 to 12, with the major species of thermal and observational importance retained, including H2O, CH4, CO, CO2, C2H2, NH3, and HCN. The size of the total reactions is also greatly reduced, from ~800 to 20. We validated the mini-chemical scheme by verifying the temporal evolution and benchmarking the predicted compositions in four exoplanet atmospheres (GJ 1214b, GJ 436b, HD 189733b, and HD 209458b) against the full kinetics of VULCAN.
Results. The mini-network reproduces the chemical timescales and composition distributions of the full kinetics well within an order of magnitude for the major species in the pressure range of 1 bar–0.1 mbar across various metallicities and carbon-to-oxygen (C/O) ratios.
Conclusions. We have developed and validated a mini-chemical scheme using net reactions to significantly simplify a large chemical network. The small scale of the mini-chemical scheme permits simple use and fast computation, which is optimal for implementation in a 3D GCM or a retrieval framework. We focus on the thermochemical kinetics of net reactions in this paper and address photochemistry in a follow-up paper.

Top-of-the-atmosphere and Vertical Cloud Structure of a Fast-rotating Late T Dwarf

The Astronomical Journal American Astronomical Society 164:2 (2022) 65

Authors:

Elena Manjavacas, Theodora Karalidi, Xianyu Tan, Johanna M Vos, Ben WP Lew, Beth A Biller, Natalia Oliveros-Gómez

Large Interferometer For Exoplanets (LIFE)

Astronomy & Astrophysics EDP Sciences 664 (2022) a21

Authors:

SP Quanz, M Ottiger, E Fontanet, J Kammerer, F Menti, F Dannert, A Gheorghe, O Absil, VS Airapetian, E Alei, R Allart, D Angerhausen, S Blumenthal, LA Buchhave, J Cabrera, Ó Carrión-González, G Chauvin, WC Danchi, C Dandumont, D Defrére, C Dorn, D Ehrenreich, S Ertel, M Fridlund, A García Muñoz, C Gascón, JH Girard, A Glauser, JL Grenfell, G Guidi, J Hagelberg, R Helled, MJ Ireland, M Janson, RK Kopparapu, J Korth, T Kozakis, S Kraus, A Léger, L Leedjärv, T Lichtenberg, J Lillo-Box, H Linz, R Liseau, J Loicq, V Mahendra, F Malbet, J Mathew, B Mennesson, MR Meyer, L Mishra, K Molaverdikhani, L Noack, AV Oza, E Pallé, H Parviainen, A Quirrenbach, H Rauer, I Ribas, M Rice, A Romagnolo, S Rugheimer, EW Schwieterman, E Serabyn, S Sharma, KG Stassun, J Szulágyi, HS Wang, F Wunderlich, MC Wyatt