Raymond Pierrehumbert FRS

Abstract:

Many super‐Earths are on very short orbits around their host star and, therefore, more likely to be tidally locked. Because this locking can lead to a strong contrast between the dayside and nightside surface temperatures, these super‐Earths could exhibit mantle convection patterns and tectonics that could differ significantly from those observed in the present‐day solar system. The presence of an atmosphere, however, would allow transport of heat from the dayside toward the nightside and thereby reduce the surface temperature contrast between the two hemispheres. On rocky planets, atmospheric and geodynamic regimes are closely linked, which directly connects the question of atmospheric thickness to the potential interior dynamics of the planet. Here, we study the interior dynamics of super‐Earth GJ 486b ( R = 1.34 $R=1.34$ R ⊕ ${R}_{\oplus }$ , M = 3.0 $M=3.0$ M ⊕ ${M}_{\oplus }$ , T eq ≈ 700 ${\mathrm{T}}_{\text{eq}}\approx 700$ K), which is one of the most suitable M‐dwarf super‐Earth candidates for retaining an atmosphere produced by degassing from the mantle and magma ocean. We investigate how the geodynamic regime of GJ 486b is influenced by different surface temperature contrasts by varying possible atmospheric circulation regimes. We also investigate how the strength of the lithosphere affects the convection pattern. We find that hemispheric tectonics, the surface expression of degree‐1 convection with downwellings forming on one hemisphere and upwelling material rising on the opposite hemisphere, is a consequence of the strong lithosphere rather than surface temperature contrast. Anchored hemispheric tectonics, where downwellings und upwellings have a preferred (day/night) hemisphere, is favored for strong temperature contrasts between the dayside and nightside and higher surface temperatures.

Abstract:

With sizable volatile envelopes but smaller radii than the solar system ice giants, sub-Neptunes have been revealed as one of the most common types of planet in the galaxy. While the spectroscopic characterization of larger sub-Neptunes (2.5–4 R ⊕) has revealed hydrogen-dominated atmospheres, smaller sub-Neptunes (1.6–2.5 R ⊕) could either host thin, rapidly evaporating, hydrogen-rich atmospheres or be stable, metal-rich “water worlds” with high mean molecular weight atmospheres and a fundamentally different formation and evolutionary history. Here, we present the 0.6–2.8 μm JWST/NIRISS/SOSS transmission spectrum of GJ 9827 d, the smallest (1.98 R ⊕) warm (T eq,A=0.3 ∼ 620 K) sub-Neptune where atmospheric absorbers have been detected to date. Our two transit observations with NIRISS/SOSS, combined with the existing HST/WFC3 spectrum, enable us to break the clouds–metallicity degeneracy. We detect water in a highly metal-enriched “steam world” atmosphere (O/H of ∼4 by mass and H2O found to be the background gas with a volume mixing ratio of >31%). We further show that these results are robust to stellar contamination through the transit light source effect. We do not detect escaping metastable He, which, combined with previous nondetections of escaping He and H, supports the steam atmosphere scenario. In water-rich atmospheres, hydrogen loss driven by water photolysis happens predominantly in the ionized form, which eludes observational constraints. We also detect several flares in the NIRISS/SOSS light curves with far-UV energies of the order of 1030 erg, highlighting the active nature of the star. Further atmospheric characterization of GJ 9827 d probing carbon or sulfur species could reveal the origin of its high metal enrichment.

Abstract:

Implicit in the definition of the classical circumstellar habitable zone (HZ) is the hypothesis that the carbonate-silicate cycle can maintain clement climates on exoplanets with land and surface water across a range of instellations by adjusting atmospheric CO2 partial pressure (pCO2). This hypothesis is made by analogy to the Earth system, but it is an open question whether silicate weathering can stabilize climate on planets in the outer reaches of the HZ, where instellations are lower than those received by even the Archean Earth and CO2 is thought likely to dominate atmospheres. Since weathering products are carried from land to ocean by the action of water, silicate weathering is intimately coupled to the hydrologic cycle, which intensifies with hotter temperatures under Earth-like conditions. Here, we use global climate model simulations to demonstrate that the hydrologic cycle responds counterintuitively to changes in climate on planets with CO2-H2O atmospheres at low instellations and high pCO2, with global evaporation and precipitation decreasing as pCO2 and temperatures increase at a given instellation. Within the Maher & Chamberlain (or MAC) weathering formulation, weathering then decreases with increasing pCO2 for a range of instellations and pCO2 typical of the outer reaches of the HZ, resulting in an unstable carbon cycle that may lead to either runaway CO2 accumulation or depletion of CO2 to colder (possibly snowball) conditions. While the behavior of the system has not been completely mapped out, the results suggest that silicate weathering could fail to maintain habitable conditions in the outer reaches of the nominal HZ.

Abstract:

A significant share of anthropogenic global warming comes from livestock production. There is debate about whether there can be any role for livestock in a climatically sustainable future; the debate is particularly heated for cows and sheep, largely due to the methane they burp out. However, short-lived gases like methane affect climate in a fundamentally different way than long-lived gases like carbon dioxide. Consequently, climate stabilization does not require zeroing-out cattle herds. But this doesn't mean we can eat our beef and have it (a tolerable climate) too-livestock still contribute to global warming. Preventing or limiting future growth in livestock-related emissions can represent a sensible part of the portfolio of responses to the climate crisis, particularly when carbon dioxide emissions are not on track to reach net zero sufficiently quickly.