The atmosphere of Venus remains mysterious, with many outstanding chemical connundra. These include the unexpected presence of ∼10 ppm O
in the cloud layers, an unknown composition of large particles ...in the lower cloud layers, and hard to explain measured vertical abundance profiles of SO
and H
O. We propose a hypothesis for the chemistry in the clouds that largely addresses all of the above anomalies. We include ammonia (NH
), a key component that has been tentatively detected both by the Venera 8 and Pioneer Venus probes. NH
dissolves in some of the sulfuric acid cloud droplets, effectively neutralizing the acid and trapping dissolved SO
as ammonium sulfite salts. This trapping of SO
in the clouds, together with the release of SO
below the clouds as the droplets settle out to higher temperatures, explains the vertical SO
abundance anomaly. A consequence of the presence of NH
is that some Venus cloud droplets must be semisolid ammonium salt slurries, with a pH of ∼1, which matches Earth acidophile environments, rather than concentrated sulfuric acid. The source of NH
is unknown but could involve biological production; if so, then the most energy-efficient NH
-producing reaction also creates O
explaining the detection of O
in the cloud layers. Our model therefore predicts that the clouds are more habitable than previously thought, and may be inhabited. Unlike prior atmospheric models, ours does not require forced chemical constraints to match the data. Our hypothesis, guided by existing observations, can be tested by new Venus in situ measurements.
We present an open-source and validated chemical kinetics code for studying hot exoplanetary atmospheres, which we name VULCAN. It is constructed for gaseous chemistry from 500 to 2500 K, using a ...reduced C-H-O chemical network with about 300 reactions. It uses eddy diffusion to mimic atmospheric dynamics and excludes photochemistry. We have provided a full description of the rate coefficients and thermodynamic data used. We validate VULCAN by reproducing chemical equilibrium and by comparing its output versus the disequilibrium-chemistry calculations of Moses et al. and Rimmer & Helling. It reproduces the models of HD 189733b and HD 209458b by Moses et al., which employ a network with nearly 1600 reactions. We also use VULCAN to examine the theoretical trends produced when the temperature-pressure profile and carbon-to-oxygen ratio are varied. Assisted by a sensitivity test designed to identify the key reactions responsible for producing a specific molecule, we revisit the quenching approximation and find that it is accurate for methane but breaks down for acetylene, because the disequilibrium abundance of acetylene is not directly determined by transport-induced quenching, but is rather indirectly controlled by the disequilibrium abundance of methane. Therefore we suggest that the quenching approximation should be used with caution and must always be checked against a chemical kinetics calculation. A one-dimensional model atmosphere with 100 layers, computed using VULCAN, typically takes several minutes to complete. VULCAN is part of the Exoclimes Simulation Platform (ESP; exoclime.net) and publicly available at https://github.com/exoclime/VULCAN.
To constrain the formation history of an exoplanet, we need to know its chemical composition
. With an equilibrium temperature of about 4,050 kelvin
, the exoplanet KELT-9b (also known as HD 195689b) ...is an archetype of the class of ultrahot Jupiters that straddle the transition between stars and gas-giant exoplanets and are therefore useful for studying atmospheric chemistry. At these high temperatures, iron and several other transition metals are not sequestered in molecules or cloud particles and exist solely in their atomic forms
. However, despite being the most abundant transition metal in nature, iron has not hitherto been detected directly in an exoplanet because it is highly refractory. The high temperatures of KELT-9b imply that its atmosphere is a tightly constrained chemical system that is expected to be nearly in chemical equilibrium
and cloud-free
, and it has been predicted that spectral lines of iron should be detectable in the visible range of wavelengths
. Here we report observations of neutral and singly ionized atomic iron (Fe and Fe
) and singly ionized atomic titanium (Ti
) in the atmosphere of KELT-9b. We identify these species using cross-correlation analysis
of high-resolution spectra obtained as the exoplanet passed in front of its host star. Similar detections of metals in other ultrahot Jupiters will provide constraints for planetary formation theories.
The Peculiar Atmospheric Chemistry of KELT-9b Kitzmann, Daniel; Heng, Kevin; Rimmer, Paul B. ...
Astrophysical journal/The Astrophysical journal,
08/2018, Letnik:
863, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The atmospheric temperatures of the ultra-hot Jupiter KELT-9b straddle the transition between gas giants and stars, and therefore between two traditionally distinct regimes of atmospheric chemistry. ...Previous theoretical studies assume the atmosphere of KELT-9b to be in chemical equilibrium. Despite the high ultraviolet flux from KELT-9, we show using photochemical kinetic calculations that the observable atmosphere of KELT-9b is predicted to be close to chemical equilibrium, which greatly simplifies any theoretical interpretation of its spectra. It also makes the atmosphere of KELT-9b, which is expected to be cloud-free, a tightly constrained chemical system that lends itself to a clean set of theoretical predictions. Due to the lower pressures probed in transmission (compared to emission) spectroscopy, we predict the abundance of water to vary by several orders of magnitude across the atmospheric limb depending on temperature, which makes water a sensitive thermometer. Carbon monoxide is predicted to be the dominant molecule under a wide range of scenarios, rendering it a robust diagnostic of the metallicity when analyzed in tandem with water. All of the other usual suspects (acetylene, ammonia, carbon dioxide, hydrogen cyanide, methane) are predicted to be subdominant at solar metallicity, while atomic oxygen, iron, and magnesium are predicted to have relative abundances as high as 1 part in 10,000. Neutral atomic iron is predicted to be seen through a forest of optical and near-infrared lines, which makes KELT-9b suitable for high-resolution ground-based spectroscopy with HARPS-N or CARMENES. We summarize future observational prospects of characterizing the atmosphere of KELT-9b.
There are two dominant and contrasting classes of origin of life scenarios: those predicting that life emerged in submarine hydrothermal systems, where chemical disequilibrium can provide an energy ...source for nascent life; and those predicting that life emerged within subaerial environments, where UV catalysis of reactions may occur to form the building blocks of life. Here, we describe a prebiotically plausible environment that draws on the strengths of both scenarios: surface hydrothermal vents. We show how key feedstock molecules for prebiotic chemistry can be produced in abundance in shallow and surficial hydrothermal systems. We calculate the chemistry of volcanic gases feeding these vents over a range of pressures and basalt C/N/O contents. If ultra-reducing carbon-rich nitrogen-rich gases interact with subsurface water at a volcanic vent they result in 10 - 3 ⁻ 1 M concentrations of diacetylene (C₄H₂), acetylene (C₂H₂), cyanoacetylene (HC₃N), hydrogen cyanide (HCN), bisulfite (likely in the form of salts containing HSO₃
), hydrogen sulfide (HS
) and soluble iron in vent water. One key feedstock molecule, cyanamide (CH₂N₂), is not formed in significant quantities within this scenario, suggesting that it may need to be delivered exogenously, or formed from hydrogen cyanide either via organometallic compounds, or by some as yet-unknown chemical synthesis. Given the likely ubiquity of surface hydrothermal vents on young, hot, terrestrial planets, these results identify a prebiotically plausible local geochemical environment, which is also amenable to future lab-based simulation.
Lightning and charge processes in brown dwarf and exoplanet atmospheres Helling, Christiane; Rimmer, Paul B
Philosophical transactions - Royal Society. Mathematical, Physical and engineering sciences/Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences,
09/2019, Letnik:
377, Številka:
2154
Journal Article
Recenzirano
Odprti dostop
The study of the composition of brown dwarf atmospheres helped to understand their formation and evolution. Similarly, the study of exoplanet atmospheres is expected to constrain their formation and ...evolutionary states. We use results from three-dimensional simulations, kinetic cloud formation and kinetic ion-neutral chemistry to investigate ionization processes that will affect their atmosphere chemistry: the dayside of super-hot Jupiters is dominated by atomic hydrogen, and not H
O. Such planetary atmospheres exhibit a substantial degree of thermal ionization and clouds only form on the nightside where lightning leaves chemical tracers (e.g. HCN) for possibly long enough to be detectable. External radiation may cause exoplanets to be enshrouded in a shell of highly ionized, H
-forming gas and a weather-driven aurora may emerge. Brown dwarfs enable us to study the role of electron beams for the emergence of an extrasolar, weather system-driven aurora-like chemistry, and the effect of strong magnetic fields on cold atmospheric gases. Electron beams trigger the formation of H
in the upper atmosphere of a brown dwarf (e.g. LSR-J1835), which may react with it to form hydronium, H
O
, as a longer lived chemical tracer. Brown dwarfs and super-hot gas giants may be excellent candidates to search for H
O
as an H
product. This article is part of a discussion meeting issue 'Advances in hydrogen molecular ions: H
, H
and beyond'.
We perform a study of stellar flares for the 24,809 stars observed with 2 minute cadence during the first two months of the TESS mission. Flares may erode exoplanets' atmospheres and impact their ...habitability, but might also trigger the genesis of life around small stars. TESS provides a new sample of bright dwarf stars in our galactic neighborhood, collecting data for thousands of M dwarfs that might host habitable exoplanets. Here, we use an automated search for flares accompanied by visual inspection. Then, our public allesfitter code robustly selects the appropriate model for potentially complex flares via Bayesian evidence. We identify 1228 flaring stars, 673 of which are M dwarfs. Among 8695 flares in total, the largest superflare increased the stellar brightness by a factor of 16.1. Bolometric flare energies range from 1031.0 to 1036.9 erg, with a median of 1033.1 erg. Furthermore, we study the flare rate and energy as a function of stellar type and rotation period. We solidify past findings that fast rotating M dwarfs are the most likely to flare and that their flare amplitude is independent of the rotation period. Finally, we link our results to criteria for prebiotic chemistry, atmospheric loss through coronal mass ejections, and ozone sterilization. Four of our flaring M dwarfs host exoplanet candidates alerted on by TESS, for which we discuss how these effects can impact life. With upcoming TESS data releases, our flare analysis can be expanded to almost all bright small stars, aiding in defining criteria for exoplanet habitability.
Life in the clouds of Venus, if present in sufficiently high abundance, must be affecting the atmospheric chemistry. It has been proposed that abundant Venusian life could obtain energy from its ...environment using three possible sulfur energy-metabolisms. These metabolisms raise the possibility of Venus's enigmatic cloud-layer SO
-depletion being caused by life. We here couple each proposed energy-metabolism to a photochemical-kinetics code and self-consistently predict the composition of Venus's atmosphere under the scenario that life produces the observed SO
-depletion. Using this photo-bio-chemical kinetics code, we show that all three metabolisms can produce SO
-depletions, but do so by violating other observational constraints on Venus's atmospheric chemistry. We calculate the maximum possible biomass density of sulfur-metabolising life in the clouds, before violating observational constraints, to be ~10
- 10
mg m
. The methods employed are equally applicable to aerial biospheres on Venus-like exoplanets, planets that are optimally poised for atmospheric characterisation in the near future.
Abstract
We report the detection of an atmosphere on a rocky exoplanet, GJ 1132 b, which is similar to Earth in terms of size and density. The atmospheric transmission spectrum was detected using ...Hubble WFC3 measurements and shows spectral signatures of aerosol scattering, HCN, and CH
4
in a low mean molecular weight atmosphere. We model the atmospheric loss process and conclude that GJ 1132 b likely lost the original H/He envelope, suggesting that the atmosphere that we detect has been reestablished. We explore the possibility of H
2
mantle degassing, previously identified as a possibility for this planet by theoretical studies, and find that outgassing from ultra-reduced magma could produce the observed atmosphere. In this way we use the observed exoplanet transmission spectrum to gain insights into magma composition for a terrestrial planet. The detection of an atmosphere on this rocky planet raises the possibility that the numerous powerfully irradiated super-Earth planets, believed to be the evaporated cores of sub-Neptunes, may, under favorable circumstances, host detectable atmospheres.
ABSTRACT
We present and validate a new network of atmospheric thermochemical and photochemical sulfur reactions. We use a 1D chemical kinetics model to investigate these reactions as part of a ...broader HCNO chemical network in a series of hot and warm Jupiters. We find that temperatures approaching $1400\, \mathrm{K}$ are favourable for the production of H2S and HS around $\mathrm{10^{-3}\, bar}$ at mixing ratios of around 10−5, an atmospheric level where detection by transit spectroscopy may be possible. At $\mathrm{10^{-3}\, bar}$ and at lower temperatures, down to $1000\, \mathrm{K}$, mixing ratios of S2 can be up to 10−5, at the expense of H2S and HS, which are depleted down to a mixing ratio of 10−7. We also investigate how the inclusion of sulfur can manifest in an atmosphere indirectly, by its effect on the abundance of non-sulfur-bearing species. We find that in a model of the atmosphere of HD 209458 b, the inclusion of sulfur can lower the abundance of NH3, CH4, and HCN by up to two orders of magnitude around $\mathrm{10^{-3}\, bar}$. In the atmosphere of the warm Jupiter 51 Eri b, we additionally find the inclusion of sulfur depletes the peak abundance of CO2 by a factor of 5, qualitatively consistent with prior models. We note that many of the reactions used in the network have poorly determined rate constants, especially at higher temperatures. To obtain an accurate idea of the impact of sulfur chemistry in hot and warm Jupiter atmospheres, experimental measurements of these reaction rates must take place.
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