Chemistry of Earth's Earliest Atmosphere

Schaefer

et al. 2020

ApJ

Planets with 2 R ⊕ < R < 3 R ⊕ and orbital period <100 d are abundant; these sub-Neptune exoplanets are not well understood. For example, Kepler sub-Neptunes are likely to have deep magma oceans in contact with their atmospheres, but little is known about the effect of the magma on the atmosphere. Here we study this effect using a basic model, assuming that volatiles equilibrate with magma at T ∼ 3000 K. For our Fe-Mg-Si-O-H model system, we find that chemical reactions between the magma and the atmosphere and dissolution of volatiles into the magma are both important. Thus, magma matters. For H, most moles go into the magma, so the mass target for both H 2 accretion and H 2 loss models is weightier than is usually assumed. The known span of magma oxidation states can produce sub-Neptunes that have identical radius but with total volatile masses varying by 20-fold. Thus, planet radius is a proxy for atmospheric composition but not for total volatile content. This redox diversity degeneracy can be broken by measurements of atmosphere mean molecular weight. We emphasise H 2 supply by nebula gas, but also consider solid-derived H 2 O. We find that adding H 2 O to Fe probably cannot make enough H 2 to explain sub-Neptune radii because >10 3 -km thick outgassed atmospheres have high mean molecular weight. The hypothesis of magma-atmosphere equilibration links observables such as atmosphere H 2 O/H 2 ratio to magma FeO content and planet formation processes. Our model's accuracy is limited by the lack of experiments (lab and/or numerical) that are specific to sub-Neptunes; we advocate for such experiments.

Section: Materials Propertiesmentioning

confidence: 97%

“…At higher temperatures, they could be in error (e.g. Fegley & Schaefer 2014;Fegley et al 2016;Sossi & Fegley 2018). For example, we neglect Tdependence of H 2 O solubility.…”

Section: Materials Propertiesmentioning

confidence: 99%

Atmosphere Origins for Exoplanet Sub-Neptunes

Kite

Schaefer

et al. 2020

ApJ

“…Melosh 1990;Cameron and Benz 1991;Canup 2004;Nakajima and Stevenson, 2014;Lock and Stewart 2017). Such temperatures not only far exceed those present in the solar nebula (<2000 K for any reasonable nebular pressure), but they lead to extensive melting, as they surpass the low-pressure peridotite liquidus (≈ 2000 K) and cause vaporisation of silicate material (e.g., see the discussion in section 6.3.5 of Fegley and Schaefer 2014). Computational models that minimise the Gibbs free energy of both liquid and vapour species for major elements (Fegley and Cameron 1987;Schaefer and Fegley 2004a;Visscher and Fegley 2013) show that the composition of the gas evolved from molten peridotite (>3000 K) is dominated by SiO vapour, though Na predominates between 1800 -3000 K, yielding oxygen fugacities 10 7 × more oxidising than that of the nebular gas (Visscher and Fegley, 2013;Fig.…”

Section: Planetary Bodiesmentioning

confidence: 99%

Thermodynamics of Element Volatility and its Application to Planetary Processes

Sossi

Reviews in Mineralogy and Geochemistry

2018

3) Evaporation on small planetary bodies (section 4.0.) 4) Evaporation during giant impacts (section 4.0.)To constrain the conditions under which these processes occur necessitates thermodynamic data of gaseous-and condensed species. Therefore, a review of our knowledge and lack thereof, is also presented (section 2.0.). Much of our information on thermodynamic quantities of gaseous species comes from spectrometric measurements, undertaken from the 1950s onwards, largely on simple (binary and ternary) systems (e.g., see Brewer 1953;Margrave 1967;Lamoreaux et al. 1987, and section 2.0.). This information is now readily available in compilations of thermodynamic data such as JANAF (Chase 1998) or IVTAN (Glushko et al. 1999). However, the application of these data to more complex, natural geological systems is less straightforward.Instead, for lack of any other comprehensive volatility scale, 50% nebular condensation temperatures, Tc, (Grossman and Larimer 1974;Lodders 2003), which describe the temperature at which half of the abundance of an element is condensed in the solar nebula, are frequently used to discuss elemental volatility in planetary environments. However, these Tc are strictly relevant only at the pressure and temperature conditions of a solar-composition protoplanetary accretion disk (e.g., the solar nebula). Namely, at low total pressures (PT ≲10 -2 bar), solar or close to solar metallicity 2 (i.e., PT ~ PH2 + PH + PHe), oxygen fugacities ~ 6 log bar units below the Iron -Wüstite (IW) buffer (due to the low H2O/H2 ratios, 5 × 10 -4 ; Rubin et al. 1988;Grossman et al., 2008). Boss (1998) and Woolum and Cassen (1999), to give two examples, used astronomical observations to model midplane temperatures in protoplanetary accretion disks in the planet-forming region and found temperatures <1600 K.Petrological observations of Calcium, Al-rich inclusions (CAI) with the highly fractionated group II rare-earth element (REE) pattern 3 (Boynton 1975) indicate they formed by fractional condensation (Boynton 1975;Davis and Grossman 1979) at temperatures between 1676 K, 1463 K, 1296 K at total pressures (PT) of 10 -3 bar, 10 -6 bar, and 10 -9 bar, respectively (Kornacki and Fegley 1986). At total pressures above ≈10 -2 bar, liquids condense in place of solids (Ebel, 2004) and departures from solar composition (e.g., by changes in metallicity or H-loss on a planetary body) lead to condensation temperatures different to those in a solar-composition system (Larimer and Bartholomay 1979; Lodders and Fegley 1999; Schaefer and Fegley 2010). Thus, the complexity of evaporation and condensation reactions are such that, at conditions outside this range, the relevant gas species, their equilibrium partial pressures and mechanisms driving their escape diverge greatly. Potentially powerful tracers of the conditions under which these gas-liquid/solid processes occur are the moderately volatile elements (MVEs), defined as those that condense from a solar composition nebular condensation and evaporation of silicates under post-n...

“…Assuming that the observed insensitivity of solubility to temperature and composition extends to the relevant temperatures and compositions, such variations, while not negligible, represent refinements to the picture here described. As in the analogous terrestrial magma ocean and atmosphere system (Fegley and Schaefer, 2014), experimental determination of water solubility in ultramafic silicate melts at the temperatures of relevance will help to clarify the role of temperature and compositional dependence on equilibrium H dissolution in the proto-lunar disk.…”

Section: Trace Elements -Hmentioning

confidence: 99%

Speciation and dissolution of hydrogen in the proto-lunar disk

Pahlevan

Karato

Earth and Planetary Science Letters

2016

Despite very high temperatures accompanying lunar origin, indigenous water in the form of OH has been unambiguously observed in Apollo samples in recent years. Such observations have prompted questions about the abundance and distribution of lunar hydrogen. Here, we investigate the related question of the origin of lunar H: is the hydrogen observed a remnant of a much larger initial inventory that was inherited from a "wet" Earth but partly depleted during the process of origin, or was primordial hydrogen quantitatively lost from the lunar material, with water being delivered to lunar reservoirs via subsequent impacts after the origins sequence? Motivated by recent results pointing to a limited extent of hydrogen escape from the gravity field of the Earth during lunar origin, we apply a newly developed thermodynamic model of liquid-vapor silicates to the proto-lunar disk to interrogate the behavior of H as a trace element in the energetic aftermath of the giant impact. We find that:(1) pre-existing H-bearing molecules are rapidly dissociated at the temperatures considered (3,100-4,200 K) and vaporized hydrogen predominantly exists as OH(v), H(v) and MgOH(v) for nearly the full range of thermal states encountered in the proto-lunar disk, (2) despite such a diversity in the vapor speciation -which reduces the water fugacity and favors hydrogen exsolution from co-existing liquids -the equilibration of the vapor atmosphere with the disk liquid results in significant dissolution of H into proto-lunar magmas, and (3) equilibrium H isotopic fractionation in this setting is limited to < 10 per mil and the "terrestrial" character of lunar D/H recently inferred should extend to such a precision if liquid-vapor equilibration in the proto-lunar disk is the process that gave rise to lunar hydrogen. Taken together, these results implicate dissolution as the process responsible for establishing lunar H abundances.