UV radiation can induce photochemical processes in exoplanet atmospheres and produce haze particles. Recent observations suggest that haze and/or cloud layers could be present in the upper ...atmospheres of exoplanets. Haze particles play an important role in planetary atmospheres and may provide a source of organic material to the surface that may impact the origin or evolution of life. However, very little information is known about photochemical processes in cool, high-metallicity exoplanetary atmospheres. Previously, we investigated haze formation and particle size distribution in laboratory atmosphere simulation experiments using AC plasma as the energy source. Here, we use UV photons to initiate the chemistry rather than the AC plasma, as photochemistry driven by UV radiation is important for understanding exoplanet atmospheres. We present photochemical haze formation in current UV experiments; we investigated a range of atmospheric metallicities (100×, 1000×, and 10000× solar metallicity) at three temperatures (300, 400, and 600 K). We find that photochemical hazes are generated in all simulated atmospheres with temperature-dependent production rates: the particles produced in each metallicity group decrease as the temperature increases. The images taken with atomic force microscopy show the particle size (15-190 nm) varies with temperature and metallicity. Our laboratory experimental results provide new insight into the formation and properties of photochemical haze, which could guide exoplanet atmosphere modeling and help to analyze and interpret current and future observations of exoplanets.
Aerosol growth in Titan’s ionosphere Lavvas, Panayotis; Yelle, Roger V.; Koskinen, Tommi ...
Proceedings of the National Academy of Sciences - PNAS,
02/2013, Letnik:
110, Številka:
8
Journal Article
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Photochemically produced aerosols are common among the atmospheres of our solar system and beyond. Observations and models have shown that photochemical aerosols have direct consequences on ...atmospheric properties as well as important astrobiological ramifications, but the mechanisms involved in their formation remain unclear. Here we show that the formation of aerosols in Titan’s upper atmosphere is directly related to ion processes, and we provide a complete interpretation of observed mass spectra by the Cassini instruments from small to large masses. Because all planetary atmospheres possess ionospheres, we anticipate that the mechanisms identified here will be efficient in other environments as well, modulated by the chemical complexity of each atmosphere.
Super-Earths and mini-Neptunes are the most abundant types of planets among the ∼3500 confirmed exoplanets, and are expected to exhibit a wide variety of atmospheric compositions. Recent transmission ...spectra of super-Earths and mini-Neptunes have demonstrated the possibility that exoplanets have haze/cloud layers at high altitudes in their atmospheres. However, the compositions, size distributions, and optical properties of these particles in exoplanet atmospheres are poorly understood. Here, we present the results of experimental laboratory investigations of photochemical haze formation within a range of planetary atmospheric conditions, as well as observations of the color and size of produced haze particles. We find that atmospheric temperature and metallicity strongly affect particle color and size, thus altering the particles' optical properties (e.g., absorptivity, scattering, etc.); on a larger scale, this affects the atmospheric and surface temperature of the exoplanets, and their potential habitability. Our results provide constraints on haze formation and particle properties that can serve as critical inputs for exoplanet atmosphere modeling, and guide future observations of super-Earths and mini-Neptunes with the Transiting Exoplanet Survey Satellite, the James Webb Space Telescope, and the Wide-Field Infrared Survey Telescope.
Photochemistry induced by stellar UV flux should produce haze particles in exoplanet atmospheres. Recent observations indicate that haze and/or cloud layers exist in the atmospheres of exoplanets. ...However, photochemical processes in exoplanetary atmospheres remain largely unknown. We performed laboratory experiments with the PHAZER chamber to simulate haze formation in a range of exoplanet atmospheres (hydrogen-rich, water-rich, and carbon dioxide-rich at 300, 400, and 600 K), and observed the gas phase compositional change (the destruction of the initial gas and the formation of new gas species) during these experiments with mass spectrometer. The mass spectra reveal that distinct chemical processes happen in the experiments as a function of different initial gas mixture and different energy sources (plasma or UV photons). We find that organic gas products and O2 are photochemically generated in the experiments, demonstrating that photochemical production is one of the abiotic sources for these potential biosignatures. Multiple simulated atmospheres produce organics and O2 simultaneously, which suggests that even the copresence of organics and O2 could be a false positive biosignature. From the gas phase composition changes, we identify potential precursors (C2H2, HCN, CH2NH, HCHO, etc.) for haze formation, among which complex reactions can take place and produce larger molecules. Our laboratory results indicate that complex atmospheric photochemistry can happen in diverse exoplanet atmospheres and lead to the formation of new gas products and haze particles, including compounds (O2 and organics) that could be falsely identified as biosignatures.
Reactions of State-Selected Atomic Oxygen Ions O+(4S, 2D, 2P) with Methane Cunha de Miranda, Barbara; Romanzin, Claire; Chefdeville, Simon ...
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory,
06/2015, Letnik:
119, Številka:
23
Journal Article
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An experimental study has been carried out on the reactions of state selected O+(4S, 2D, 2P) ions with methane with the aims of characterizing the effects of both the parent ion internal energy and ...collision energy on the reaction dynamics and determining the fate of oxygen species in complex media, in particular the Titan ionosphere. Absolute cross sections and product velocity distributions have been determined for the reactions of 16O+ or 18O+ ions with CH4 or CD4 from thermal to 5 eV collision energies by using the guided ion beam (GIB) technique. Dissociative photoionization of O2 with vacuum ultraviolet (VUV) synchrotron radiation delivered by the DESIRS beamline at the SOLEIL storage ring and the threshold photoion photoelectron coincidence (TPEPICO) technique are used for the preparation of purely state-selected O+(4S, 2D, 2P) ions. A complete inversion of the product branching ratio between CH4 + and CH3 + ions in favor of the latter is observed for excitation of O+ ions from the 4S ground state to either the 2D or the 2P metastable state. CH4 + and CH3 + ions, which are by far the major products for the reaction of ground state and excited states, are strongly backward scattered in the center of mass frame relative to O+ parent ions. For the reaction of O+(4S), CH3 + production also rises with increasing collision energy but with much less efficiency than with O+ excitation. We found that a mechanism of dissociative charge transfer, mediated by an initial charge transfer step, can account very well for all the observations, indicating that CH3 + production is associated with the formation of H and O atoms (CH3 + + H + O) rather than with OH formation by an hydride transfer process (CH3 + + OH). Therefore, as the CH4 + production by charge transfer is also associated with O atoms, the fate of oxygen species in these reactions is essentially the O production, except for the reaction of O+(4S), which also produces appreciable amounts of H2O+ ions but only at very low collision energy. The production of O atoms and the nature of the states in which they are formed are discussed for the reactions of O+ ions with CH4 and N2.
The organic matter in carbonaceous chondrites is of two kinds: one is called Insoluble Organic Matter, made of extremely large molecules that cannot be named with the usual nomenclature; one can be ...extracted by laboratory solvents and analyzed as a molecular mixture. Both are of debated origin. Retracing their natural histories requires putting strong constraints on their possible place of birth and their life time in space environments. It cannot be excluded they were formed in an interstellar medium before accretion on the chondritic parent bodies. As ultraviolet rays are the most common in the star forming regions and during the accretion phase of solar system, we propose to test the resilience of the natural organic matter of the Murchison meteorite against photolysis. The meteoritical soluble molecules were extracted by maceration and artificially exposed to a Lα photon dose commensurate to the one expected in molecular clouds and disks. The gaseous photolysis products were analyzed on the fly whereas the solid state mixture was solubilized again after irradiation for Orbitrap High Resolution Mass Spectrometry monitoring. We found that ultraviolet photons do modify the molecular mixture, removing H2 and small carbon bearing species, shifting the mass distribution toward lower masses and increasing the number of cycles and double bonds in the molecules structure. A noteworthy effect of the irradiation is its selective preservation of species with a double bond equivalent consistent with aromatic rings in their structure. This is explained by the higher stability of such compounds. As the pristine Murchison extract lacks those features, we estimate it has not undergone significant irradiation after its synthesis. The extract we used for experiment being water insoluble, we assume its reactivity in hydrothermal condition would have been limited and have had no effect on the irradiation fingerprints. As a result the soluble fraction of Murchison was whether formed where the UV photon flux was negligible or it has been accreted quickly and shielded from photolysis in a parent body.
The classical Kuiper Belt Object (KBO) Arrokoth was surveyed by the New Horizons spacecraft on 1st January 2019, revealing a small bilobed object with a red surface, whose spectral slope lies in the ...average of the whole KBOs population. This red color has been assigned to reddish organic materials, either inherited from the protosolar disk during accretion, or formed through radiolytic processes in the surface due to exposure to solar or interstellar photons, Solar Wind, Solar Energetic Particles or Galactic Cosmic Rays. We report here a study investigating the radiolytic scenario, based on numerical calculations and experimental simulations run with swift heavy ions (74.8 MeV 136Xe19+ and 33.06 MeV 58Ni9+), and low-energy 105 keV 18O6+ ions on CH3OH ice, the only molecule identified at Arrokoth’s surface. Calculations show that sputtering is essentially controlled by Solar Wind (H and He), and that the sputtering rate depends on the nature of the material: erosion thickness over 4.55 Gyr are a few micrometers for amorphous carbon (as an analog of red organics) and a ∼240μm to around ∼10 mm for H2O and CO ice, respectively. Chemistry within the subsurface is essentially controlled by Galactic Cosmic rays (H and He), which penetrate deep down to several tens of meters and deliver an electronic dose higher than 1 eV.atom−1 in the first meter. The electronic and elastic doses delivered by Solar Wind ions are limited to the first 10s nm of the top surface, but Solar Energetic Particles deliver high electronic doses in the first 100μm of the surface (up to 200 eV.atom−1). Experimental simulations show that irradiating methanol ice with a dose consistent with that in planetary conditions, results in the formation of reddish organic materials made of aliphatic, conjugated and unconjugated olefinic, acetylinic, carbonyl and hydroxyl groups. A similarity with irradiated simple polymers (e.g. polyethyleneglycol) and materials formed through cold plasma experiments (tholins) is observed. There is little dependence with the nature and energy of the ion. The residue recovered at room temperature was analyzed with High Resolution Mass Spectrometry (Orbitrap), revealing a complex composition with around 6596 chemical formulas and likely several tens of thousands of molecules. Altogether, these analyses support active polymerization mechanisms similar to those observed in irradiated polymers, as bond-breaking, cross-linking or formation of olefinic bonds through recombination of radicals in adjacent carbon atoms. Considering both sputtering and radiolysis, as well as material ablation due to dust bombardment reported in literature, a scenario is taking shape as the production of reddish organics deep in the subsurface, and the settling of an organic crust at the top surface through volatiles removal. The presence of methanol and absence of water, inconsistent with sputtering fractionation, remains unexplained.
•The origin of Arrokoth red surface through methanol ice radiolysis was investigated.•Doses and destruction yields of methanol were calculated for Solar and Cosmic-rays ions.•Experiments show that abundant red organics are synthesized in the first meter of the subsurface.•These organics are composed of tens of thousands of molecules and cover a broad range of weight.•A radiolytic origin of Arrokoth’s red surface is plausible, provided the lack of major ablation event.
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