The Sonora Substellar Atmosphere Models. II. Cholla: A Grid of Cloud-free, Solar Metallicity Models in Chemical Disequilibrium for the JWST Era

Exoplanet and brown dwarf atmospheres commonly show signs of disequilibrium chemistry. In the James Webb Space Telescope (JWST) era, high-resolution spectra of directly imaged exoplanets will allow the characterization of their atmospheres in more detail, and allow systematic tests for the presence of chemical species that deviate from thermochemical equilibrium in these atmospheres. Constraining the presence of disequilibrium chemistry in these atmospheres as a function of parameters such as their effective temperature and surface gravity will allow us to place better constraints on the physics governing these atmospheres. This paper is part of a series of works presenting the Sonora grid of atmosphere models. In this paper, we present a grid of cloud-free, solar metallicity atmospheres for brown dwarfs and wide-separation giant planets with key molecular species such as CH4, H2O, CO, and NH3 in disequilibrium. Our grid covers atmospheres with T eff ∈ [500 K, 1300 K], log g ∈ [3.0, 5.5] (cgs) and an eddy diffusion parameter of log K zz = 2 , 4 and 7 (cgs). We study the effect of different parameters within the grid on the temperature and composition profiles of our atmospheres. We discuss their effect on the near-infrared colors of our model atmospheres and the detectability of CH4, H2O, CO, and NH3 using the JWST. We compare our models against existing MKO and Spitzer observations of brown dwarfs and verify the importance of disequilibrium chemistry for T dwarf atmospheres. Finally, we discuss how our models can help constrain the vertical structure and chemical composition of these atmospheres.

Thermochemical Equilibrium Modeling Indicates That Hg Minerals Are Unlikely to Be the Source of the Emissivity Signal on the Highlands of Venus

Several of the highlands of Venus exhibit unexpectedly low radar emissivity compared to that of the lowlands. The source has been hypothesized to be a mineral with a high dielectric constant. Recently HgTe (coloradoite) has been suggested to explain the low emissivity signal; however, little research has been completed to verify its stability on Venus. In this project, we used a Gibbs free energy minimization software to investigate whether HgTe, as well as HgS and HgSe, can form at simulated highland conditions. According to our calculations, approximately 1.3 wt% of mercury in the crust needs to be outgassed in order for HgS to be stable at 4 km in altitude. In addition, approximately 250 ppb of tellurium in the crust needs to be outgassed for HgTe to precipitate at the same altitude. The required mercury abundance for HgSe to be stable at this altitude is less, approximately 0.6 wt%; however, this is significantly larger than the 10–90 ppb generally present in basaltic rocks on Earth. Therefore, Hg-bearing minerals are likely not the source of the low radar emissivity signal.

Using thermochemical code EXPLO5 to predict the performance parameters of explosives

Thanks to the development of more powerful computers and efficient numerical techniques, numerical modelling has become a compulsory tool in solving various problems in the field of energetic materials. In cases where measuring techniques are still unable to measure a given parameter, numerical modelling may be the only option of obtaining a value. In addition, numerical modelling helps us to better understand some phenomena, particularly in understanding the influence of input parameters on output results, as well as saving time and money. The thermochemical equilibrium code EXPLO5 is such a tool which enables theoretical prediction of performance of high explosives, propellants and pyrotechnic compositions. The code is used by more than 80 research laboratories worldwide.

Unveiling shrouded oceans on temperate sub-Neptunes via transit signatures of solubility equilibria vs. gas thermochemistry

The recent discovery and initial characterization of sub-Neptune-sized exoplanets that receive stellar irradiance of approximately Earth's raised the prospect of finding habitable planets in the coming decade. Some of these temperate planets may support liquid water oceans, if they do not have massive H2/He envelopes and are thus not too hot at the bottom of the envelopes. For planets larger than Earth, and especially planets in the 1.7-3.5 R_Earth population, the mass of the H2/He envelope is typically not sufficiently constrained to assess the potential habitability. Here we show that the solubility equilibria vs. thermochemistry of carbon and nitrogen gases results in observable discriminators between small H2 atmospheres vs. massive ones. On temperate sub-Neptunes, the condition to form a liquid-water ocean and that to achieve the thermochemical equilibrium are mutually exclusive. The dominant carbon and nitrogen gases are typically CH4 and NH3 due to thermochemical recycling in a massive atmosphere of a temperate planet, and those in a small atmosphere overlying a liquid-water ocean are most likely CO2 and N2, followed by CO and CH4 produced photochemically. NH3 is depleted in the small atmosphere by dissolution into the liquid-water ocean. These gases lead to distinctive features in the planet's transmission spectrum, and a moderate number of repeated transit observations with the James Webb Space Telescope should readily tell apart a small atmosphere vs. a massive one via these spectral features on planets like K2-18 b. This method thus provides a way to use near-term facilities to constrain the atmospheric mass and habitability of temperate sub-Neptune exoplanets.

Thermodynamically Equilibrium Compositions of the Products Formed During the Filtration Combustion of the Metal-Containing Mixtures

Thermodynamic calculations for describing the compositions of the products formed in conditions of the filtration combustion of the metal-containing mixtures were carried out. The analysis of the equilibrium compositions of the products was carried out using the TERRA high-temperature thermochemical equilibrium calculation program. According to the results of calculations, the metals were divided into two groups. First one forms both the condensed and gaseous phases and in the second one ‒ metals that are only in the condensed phase. In case of the presence of metal compounds in the gas phase, as a rule, these are the following compounds: metals, oxides, hydroxides, hydrides, sulfides and metal sulfates. Metals of the second group cannot be subjected to mass transfer under conditions of the filtration combustion wave and will remain in solid combustion products (in ash).

Atmospheric chemistry on Uranus and Neptune

Comparatively little is known about atmospheric chemistry on Uranus and Neptune, because remote spectral observations of these cold, distant ‘Ice Giants’ are challenging, and each planet has only been visited by a single spacecraft during brief flybys in the 1980s. Thermochemical equilibrium is expected to control the composition in the deeper, hotter regions of the atmosphere on both planets, but disequilibrium chemical processes such as transport-induced quenching and photochemistry alter the composition in the upper atmospheric regions that can be probed remotely. Surprising disparities in the abundance of disequilibrium chemical products between the two planets point to significant differences in atmospheric transport. The atmospheric composition of Uranus and Neptune can provide critical clues for unravelling details of planet formation and evolution, but only if it is fully understood how and why atmospheric constituents vary in a three-dimensional sense and how material coming in from outside the planet affects observed abundances. Future mission planning should take into account the key outstanding questions that remain unanswered about atmospheric chemistry on Uranus and Neptune, particularly those questions that pertain to planet formation and evolution, and those that address the complex, coupled atmospheric processes that operate on Ice Giants within our solar system and beyond. This article is part of a discussion meeting issue ‘Future exploration of ice giant systems’.

Investigation of the Formation of Coherent Ash Residues during Fluidized Bed Gasification of Wheat Straw Lignin

Thermal conversion of ash-rich fuels in fluidized bed systems is often associated with extensive operation problems caused by the high amount of reactive inorganics. This paper investigates the behavior of wheat straw lignin—a potential renewable fuel for dual fluidized bed gasification. The formation of coherent ash residues and its impact on the operation performance has been investigated and was supported by thermochemical equilibrium calculations in FactSage 7.3. The formation of those ash residues, and their subsequent accumulation on the surface of the fluidized bed, causes temperature and pressure fluctuations, which negatively influence the steady-state operation of the fluidized bed process. This paper presents a detailed characterization of the coherent ash residues, which consists mostly of silica and partially molten alkali silicates. Furthermore, the paper gives insights into the formation of these ash residues, dependent on the fuel pretreatment (pelletizing) of the wheat straw lignin, which increases their stability compared to the utilization of non-pelletized fuel.

Hydrogen Production From Petroleum Hydrocarbons

The authors develop a small and simple steam-reforming reactor in a home-use size for such various heavy-hydrocarbons fuels as n-octane, n-decane, n-tetradecane, and n-hexadecane in addition to n-dodecane and measure the inside-temperature profile and the molar fractions of main-gas components such as H2, CH4, CO, and CO2. This reactor is designed only for laboratory-test use, not for a commercial product. As a result, the authors successfully achieve suitable inside-temperature profiles, namely, temperature almost linearly increases in the downstream direction along a reactor, under two conditions such as 600–950 K at the upstream end of the catalyst-layer bed in the reactor and less than 1070 K everywhere in the reactor. And, the authors reveal the effects of the liquid-hourly space velocity (LHSV) upon the molar fractions, a conversion ratio and reforming efficiencies for various heavy-hydrocarbons fuels. All the molar fractions, which agree well with thermochemical-equilibrium theory, are approximately independent of LHSV. The conversion ratio is about 90% for LHSV ≤ 0.6 h−1 and monotonically decreases with increasing LHSV for LHSV > 0.6 h−1. Then, each reforming efficiency always attains the maximum for LHSV ≈ 0.6 h−1 being independent of fuels. This suggests the common upper limit of LHSV for practically suitable operation.