Thermodynamic and Energetic Limits on Continental Silicate Weathering Strongly Impact the Climate and Habitability of Wet, Rocky Worlds

Abstract

The "liquid water habitable zone" (HZ) concept is predicated on the ability of the silicate weathering feedback to stabilize climate across a wide range of instellations. However, representations of silicate weathering used in current estimates of the effective outer edge of the HZ accounts for neither the thermodynamic limit on the concentration of weathering products in runoff set by clay precipitation nor the energetic limit on precipitation set by planetary instellation. We find that when the thermodynamic limit is included in an idealized coupled climate/ weathering model, the steady-state planetary climate loses sensitivity to silicate dissolution kinetics, becoming sensitive to temperature primarily through the effect of temperature on runoff and to pCO 2 through an effect on solute concentration mediated by pH. This increases sensitivity to land fraction, CO 2 outgassing, and geological factors such as soil age and lithology, all of which are found to have a profound effect on the position of the effective outer edge of the HZ. The interplay between runoff sensitivity and the energetic limit on precipitation leads to novel warm states in the outer reaches of the HZ, owing to the decoupling of temperature and precipitation. We discuss strategies for detecting the signature of the silicate weathering feedback through exoplanet observations in light of insights derived from the revised picture of weathering.Unified Astronomy Thesaurus concepts: Habitable planets (695); Habitable zone (696); Exoplanet atmospheres (487); Exoplanet evolution (491)

Carbonate-silicate cycle predictions of Earth-like planetary climates and testing the habitable zone concept

“…As colored dots, Figure 1 shows habitable, steady-state solutions. Our model predicts that atmospheric CO 2 abundances should broadly increase and narrow in their spread with orbital distance in the HZ (Figure 1), consistent with other models of CO 2 in the HZ [29,38]. As justified next in Section 2.2, the scatter is about a nominal linear trend between incident flux, S, and log(pCO 2 ), which is different from a non-linear trend in models that assume a constant surface temperature in the HZ from negative feedbacks [32,34] but do not actually model the carbonate-silicate feedbacks.…”

“…Planets very different from the modern Earth, such as waterworlds without a carbonate–silicate weathering cycle 57 or CH 4 -rich worlds 58 , 59 , could introduce additional uncertainty in an observed relationship between S and pCO 2 in the HZ. Despite such uncertainties, future missions should measure the relationship between S and pCO 2 in the HZ, or possibly a sharp transition in pCO 2 at the inner edge of the HZ due to loss of surface water and subsequent shutoff of surface weathering 38 , 60 . A more complex model than presented here is necessary to predict such a jump in pCO 2 at the inner edge of the HZ.…”

Lithologic Controls on Silicate Weathering Regimes of Temperate Planets

“…Previous studies applying the fluid transportcontrolled model to continental weathering limit the lithology to monomineralic (e.g., oligoclase feldspar, Graham & Pierrehumbert 2020) or one rock type (e.g., granite, Maher & Chamberlain 2014; Winnick & Maher 2018). However, the chemistry of common silicate rocks ranges from ultramafic to felsic, with increasing SiO 2 content.…”

“…In this study, we evaluate the contribution of silicate minerals and rocks on the weathering component of the carbon cycle. An important feature of the carbon cycle on Earth is the negative feedback of silicate weathering (e.g., Walker et al 1981;Berner et al 1983;Kump et al 2000;Sleep & Zahnle 2001;Abbot et al 2012;Foley 2015;Krissansen-Totton & Catling 2017;Graham & Pierrehumbert 2020). This feedback buffers the climate against changes in stellar luminosity and impacts the extent of the habitable zone (e.g., Kasting et al 1993;Kopparapu et al 2013).…”

“…Observations that regional weathering fluxes on Earth depend on runoff (e.g., Riebe et al 2004) in addition to reaction kinetics suggest that global weathering fluxes are a mixture of kinetically-and thermodynamically-limited regimes. The fluid-transport approach (Steefel et al 2005;Maher 2010;Maher & Chamberlain 2014;Li et al 2017) allows the modeling of both thermodynamic and kinetic limits of weathering on temperate planets (Winnick & Maher 2018;Graham & Pierrehumbert 2020).…”