arXiv subjects

Bruce Fegley

Publications and source records attributed to Bruce Fegley.

5 recordsLinked to original sources

arXiv2024

Condensation and the Volatility Trend of the Earth

This article describes condensation of the elements and use of condensation temperatures (Tcond) to interpret the volatility trend of the Earth. Major points are: (1) a listing of updated 50% Tcond for all natural elements and Pu at 1e-2 to 1e-8 bar pressure for solar composition matter. (2) The Tcond are mainly controlled by the Gibbs energy of condensation reactions and also by the Gibbs energy of ideal mixing if elements (compounds) condense in a solution. The Gibbs energy change of non-ideal solution (activity coefficients not equal to 1) is a secondary effect. (3) The theoretically correct relationship between Tcond and fraction condensed is derived from mass balance and chemical thermodynamic considerations. (4) The maximum amount of element condensed per 1/T, is at the inflection point in the logistic (sigmoid) curve for an element, which is also at (or close to) the 50% Tcond. (5) Plots of normalized elemental abundances versus 50% Tcond (volatility trends) are qualitative indicators of elemental fractionations due to volatility. (6) Volatility trend plots for average elemental abundances in CM, CO, CV, CR, H, L, LL, EH, EL chondrites show different trends for moderately and highly volatile elements, which may be linear, curved, a step function, or plateau. Comparison of three abundance sets for CM and CV chondrites shows trends depend on which elements are plotted, which data sources are used, and which temperature range is considered. (7) Proposed mechanisms for volatile element depletion in carbonaceous chondrites and the Earth are reviewed. (8) Possible implications of volatile element abundances in the bulk silicate Earth are discussed.

astro-ph.EP↗

arXiv2024

Sulfur in the Giant Planets, their Moons, and Extrasolar Gas Giant Planets

We review sulfur chemistry of the gas giant planets and their moons where sulfur compounds are observed. The major S-bearing gas in the upper atmospheres of the giant planets is H2S and is removed from their observable atmospheres by condensation into cloud layers (NH4SH on all four planets and additionally H2S ice on Uranus and Neptune). Any remaining H2S at higher altitudes is destroyed photochemically. Among the moons Io is the world dominated by sulfur. We summarize the sulfur cycle on Io and how pyrovolcanism is spreading sulfur across the Jovian system. Implantation of sulfur into icy surfaces of the other Galilean moons via magnetospheric transfer and radiolysis are major processes affecting the sulfur chemistry on their icy surfaces. On the icy worlds, we are literally looking at the top of the icebergs. Subsurface liquid salty bodies reveal themselves through cryovolcanism on Europa, Ganymede, and Enceladus, where salt deposits are indicated. Subsurface oceans are suspected on several other moons. We summarize the sulfur cycle for the icy Galilean moons. The occurrence of sulfates can be explained by salt exchange reactions of radiolytically produced H2SO4 with brine salts (carbonates and halides), or from a subsurface ocean that has become acidified by uptake of H2SO4 leaked from ice. In the primordial oceans of the moons that accreted with high ice rock ratios, sulfur is expected as sulfide and bisulfide anions and H2S in aqueous solution. Cosmochemical constraints suggest that pyrrhotite, tochilinite and green rusts could be important sulfide bearing compounds found with hydrous silicates such as serpentine, and magnetite on the sea floors. In N-C-rich worlds such as Titan, sulfides such as NH4SH and possibly thiazyl compounds could be important, and sulfates are unstable. Nothing is known about the sulfur chemistry on the Uranian and Neptunian moons.

astro-ph.EP↗

arXiv2016

Speciation and Dissolution of Hydrogen in the Proto-Lunar Disk

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 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.

astro-ph.EP↗

arXiv2014

The Atmospheres of Earth-like Planets after Giant Impact Events

It is now understood that the accretion of terrestrial planets naturally involves giant collisions, the moon-forming impact being a well known example. In the aftermath of such collisions the surface of the surviving planet is very hot and potentially detectable. Here we explore the atmospheric chemistry, photochemistry, and spectral signatures of post-giant-impact terrestrial planets enveloped by thick atmospheres consisting predominantly of CO2, and H2O. The atmospheric chemistry and structure are computed self-consistently for atmospheres in equilibrium with hot surfaces with composition reflecting either the bulk silicate Earth (which includes the crust, mantle, atmosphere and oceans) or Earth's continental crust. We account for all major molecular and atomic opacity sources including collision-induced absorption. We find that these atmospheres are dominated by H2O and CO2, while the formation of CH4, and NH3 is quenched due to short dynamical timescales. Other important constituents are HF, HCl, NaCl, and SO2. These are apparent in the emerging spectra, and can be indicative that an impact has occurred. The use of comprehensive opacities results in spectra that are a factor of 2 lower in surface brightness in the spectral windows than predicted by previous models. The estimated luminosities show that the hottest post-giant-impact planets will be detectable with near-infrared coronagraphs on the planned 30m-class telescopes. The 1-4um region will be most favorable for such detections, offering bright features and better contrast between the planet and a potential debris disk. We derive cooling timescales on the order of 10^5-10^6 Myrs, based on the modeled effective temperatures. This leads to the possibility of discovering tens of such planets in future surveys.

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arXiv2011

The orbital phases and secondary transit of Kepler-10b - A physical interpretation based on the Lava-ocean planet model -

The Kepler mission has made an important observation, the first detection of photons from a terrestrial planet by observing its phase curve (Kepler-10b). This opens a new field in exoplanet science: the possibility to get information about the atmosphere and surface of rocky planets, objects of prime interest. In this letter, we apply the Lava-ocean model to interpret the observed phase curve. The model, a planet with no atmosphere and a surface partially made of molten rocks, has been proposed for planets of the class of CoRoT-7b, i.e. rocky planets very close to their star (at few stellar radii). Kepler-10b is a typical member of this family. It predicts that the light from the planet has an important emission component in addition to the reflected one, even in the Kepler spectral band. Assuming an isotropical reflection of light by the planetary surface (Lambertian-like approximation), we find that a Bond albedo of \sim50% can account for the observed amplitude of the phase curve, as opposed to a first attempt where an unusually high value was found. We propose a physical process to explain this still large value of the albedo. The overall interpretation can be tested in the future with instruments as JWST or EChO. Our model predicts a spectral dependence that is clearly distinguishable from that of purely reflected light, and from that of a planet at a uniform temperature.

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