Abstract
Methane is typically thought to be formed in the solid state on top of cold interstellar icy grain mantles via the successive atomic hydrogenation of a carbon atom. In the current work we ...investigate the role of molecular hydrogen in the CH
4
reaction network. We make use of an ultrahigh vacuum cryogenic setup combining an atomic carbon atom beam with atomic and/or molecular beams of hydrogen and deuterium on a water ice. These experiments lead to the formation of methane isotopologues detected in situ through reflection absorption infrared spectroscopy. Most notably, CH
4
is experimentally formed by combining C atoms with only H
2
on amorphous solid water, albeit more slowly than in experiments where H atoms are also present. Furthermore, CH
2
D
2
is detected in an experiment involving C atoms with H
2
and D
2
on H
2
O ice. CD
4
, however, is only formed when D atoms are present in the experiment. These findings have been rationalized by means of computational and theoretical chemical insights. This leads to the following conclusions: (a) the reaction C + H
2
→ CH
2
takes place, although it is not barrierless for all binding sites on water, (b) the reaction CH + H
2
→ CH
3
is barrierless, but has not yet been included in astrochemical models, (c) the reactions CH
2
+ H
2
→ CH
3
+ H and CH
3
+ H
2
→ CH
4
+ H can take place only via a tunneling mechanism, and (d) molecular hydrogen possibly plays a more important role in the solid-state formation of methane than assumed so far.
Abstract
We present a comprehensive analysis of the chemical composition of the Jupiter-family comet and potential spacecraft target 46P/Wirtanen, in the near-IR wavelength range. We used iSHELL at ...the NASA Infrared Telescope Facility to observe the comet on 11 pre-, near-, and postperihelion dates in 2018 December and 2019 January and February during its historic apparition. We report rotational temperatures, production rates, and mixing ratios with respect to H
2
O and C
2
H
6
or 3
σ
upper limits of the primary volatiles H
2
O, HCN, CH
4
, C
2
H
6
, CH
3
OH, H
2
CO, NH
3
, CO, C
2
H
2
, and HC
3
N. We also discuss the spatial outgassing of the primary volatiles, to understand their sources and the spatial associations between them. The spatial profiles of H
2
O in 46P/Wirtanen suggest the presence of extended H
2
O outgassing sources in the coma, similar to the EPOXI target comet 103P/Hartley 2. 46P/Wirtanen is among the few known hyperactive comets, and we note that its composition and outgassing behavior are similar to those of other hyperactive comets in many ways. We note that the analyzed parent volatiles showed different variations (relative mixing ratios) during the apparition. We compared the chemical composition of 46P/Wirtanen with the mean abundances in Jupiter-family comets and the comet population as measured with ground-based near-IR facilities to date. The molecular abundances in 46P/Wirtanen suggest that although they were changing, the variations were small compared to the range in the comet population, with CH
3
OH showing notably more variation as compared to the other molecules.
We present a study of the reactions of the meteoritic mineral schreibersite (Fe,Ni)
3
P, focusing primarily on surface chemistry and prebiotic phosphorylation. In this work, a synthetic analogue of ...the mineral was synthesized by mixing stoichiometric proportions of elemental iron, nickel and phosphorus and heating in a tube furnace at 820 °C for approximately 235 hours under argon or under vacuum, a modification of the method of Skála and Drábek (2002). Once synthesized, the schreibersite was characterized to confirm the identity of the product as well as to elucidate the oxidation processes affecting the surface. In addition to characterization of the solid product, this schreibersite was reacted with water or with organic solutes in a choline chloride-urea deep eutectic mixture, to constrain potential prebiotic products. Major inorganic solutes produced by reaction of water include orthophosphate, phosphite, pyrophosphate and hypophosphate consistent with prior work on Fe
3
P corrosion. Additionally, schreibersite corrodes in water and dries down to form a deep eutectic solution, generating phosphorylated products, in this case phosphocholine, using this synthesized schreibersite.
We demonstrate a synthesis of the meteoritic mineral schreibersite (Fe,Ni)
3
P, study its surface chemistry, and show prebiotic phosphorylation.
A number of recent experimental studies have shown that solid-state complex organic molecules (COMs) can form under conditions that are relevant to the CO freeze-out stage in dense clouds. In this ...work, we show that alcohols can be formed well before the CO freeze-out stage (i.e., during the very early stage of the H2O-rich ice phase). This joint experimental and computational investigation shows that isomers n-propanol and isopropanol (H3CCH2CH2OH and H3CCHOHCH3) and n-propenol and isopropenol (H3CCHCHOH and H3CCOHCH2) can be formed in radical-addition reactions starting from propyne (H3CCCH) + OH at the low temperature of 10 K, where H3CCCH is one of the simplest representatives of stable carbon chains already identified in the interstellar medium (ISM). The resulting average abundance ratio of 1:1 for n-propanol:isopropanol is aligned with the conclusions from the computational work that the geometric orientation of strongly interacting species is influential to the extent of which “mechanism” is participating and that an assortment of geometries leads to an averaged-out effect. Three isomers of propanediol are also tentatively identified in the experiments. It is also shown that propene and propane (H3CCHCH2 and H3CCH2CH3) are formed from the hydrogenation of H3CCCH. This experimental finding falls in line with the lower activation barrier of hydrogenation of a CC bond in comparison to a CC bond. Reactants and products are probed by temperature-programmed desorption–quadrupole mass spectrometry (TPD-QMS) and reflection absorption infrared spectroscopy (RAIRS). Product relative abundances are determined from TPD-QMS data. Computationally derived activation barriers give additional insight into what types of reactions and mechanisms are more likely to occur in the laboratory and in the ISM. Our findings not only suggest that the alcohols studied here share common chemical pathways and therefore can show up simultaneously in astronomical surveys but also that their extended counterparts that derive from polyynes containing H3C–(CC) n –H structures may exist in the ISM. Such larger species, such as fatty alcohols, are the possible constituents of simple lipids that primitive cell membranes on the early Earth are thought to be partially composed of.
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Gas-phase and solid-state chemistry in low-temperature interstellar clouds and cores leads to a D/H enhancement in interstellar ices, which is eventually inherited by comets, meteorites, and even ...planetary satellites. Hence, the D/H ratio has been widely used as a tracer for the origins of extraterrestrial chemistry. However, the D/H ratio can also be influenced by cosmic rays, which are ubiquitous and can penetrate even dense interstellar molecular cores. The effects of such high-energy radiation on deuterium fractionation have not been studied in a quantitative manner. In this study, we present rate constants for radiation-induced D-to-H exchange for fully deuterated small (1–2 C) hydrocarbons embedded in H2O ice at 20 K and H-to-D exchange for the protiated forms of these molecules in D2O ice at 20 K. We observed larger rate constants for H-to-D exchange in the D2O ice versus D-to-H exchange in H2O ice, which we have attributed to the greater bond strength of C–D versus C–H. We find that the H-to-D exchange rate constants are smaller for protiated methane than ethane, in agreement with bond energies from the literature. We are unable to obtain rate constants for the unsaturated and reactive hydrocarbons ethylene and acetylene. Interpretation of the rate constants suggest that D/H exchange products are formed in abundance alongside radiolysis products. We discuss how our quantitative and qualitative data can be used to interpret the D/H ratios of aliphatic compounds observed throughout space.
We report new computational and experimental evidence of an efficient and astrochemically relevant formation route to formaldehyde (H2CO). This simplest carbonylic compound is central to the ...formation of complex organics in cold interstellar clouds and is generally regarded to be formed by the hydrogenation of solid-state carbon monoxide. We demonstrate H2CO formation via the reaction of carbon atoms with amorphous solid water. Crucial to our proposed mechanism is a concerted proton transfer catalyzed by the water hydrogen bonding network. Consequently, the reactions 3C + H2O → 3HCOH and 1HCOH → 1H2CO can take place with low or without barriers, contrary to the high-barrier traditional internal hydrogen migration. These low barriers (or the absence thereof) explain the very small kinetic isotope effect in our experiments when comparing the formation of H2CO to D2CO. Our results reconcile the disagreement found in the literature on the reaction route C + H2O → H2CO.
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Abstract
Simple and complex organic molecules (COMs) are observed along different phases of star and planet formation and have been successfully identified in prestellar environments such as dark and ...translucent clouds. Yet the picture of organic molecule formation at those earliest stages of star formation is not complete and an important reason is the lack of specific laboratory experiments that simulate carbon atom addition reactions on icy surfaces of interstellar grains. Here we present experiments in which CO molecules as well as C and H atoms are codeposited with H
2
O molecules on a 10 K surface mimicking the ongoing formation of an “H
2
O-rich” ice mantle. To simulate the effect of impacting C atoms and resulting surface reactions with ice components, a specialized C-atom beam source is used, implemented on SURFRESIDE
3
, an ultra-high vacuum cryogenic setup. Formation of ketene (CH
2
CO) in the solid state is observed in situ by means of reflection absorption IR spectroscopy. C
18
O and D isotope labeled experiments are performed to further validate the formation of ketene. Data analysis supports that CH
2
CO is formed through C-atom addition to a CO molecule, followed by successive hydrogenation transferring the formed :CCO into ketene. Efficient formation of ketene is in line with the absence of an activation barrier in C+CO reaction reported in the literature. We also discuss and provide experimental evidence for the formation of acetaldehyde (CH
3
CHO) and possible formation of ethanol (CH
3
CH
2
OH), two COM derivatives of CH
2
CO hydrogenation. The underlying reaction network is presented and the astrochemical implications of the derived pathways are discussed.
Uracil is one of the four RNA nucleobases and a component of meteoritic organics. If delivered to the early Earth, uracil could have been involved in the origins of the first RNA-based life, and so ...this molecule could be a biomarker on other worlds. Therefore, it is important to understand uracil's survival to ionizing radiation in extraterrestrial environments. Here we present a study of the radiolytic destruction kinetics of uracil and mixtures of uracil diluted in H
O or CO
ice. All samples were irradiated by protons with an energy of 0.9 MeV, and experiments were performed at 20 and 150 K to determine destruction rate constants at temperatures relevant to interstellar and Solar System environments. We show that uracil is destroyed much faster when H
O ice or CO
ice is present than when these two ices are absent. Moreover, destruction is faster for CO
-dominated ices than for H
O-dominated ones and, to a lesser extent, at 150 K compared with 20 K. Extrapolation of our laboratory results to astronomical timescales shows that uracil will be preserved in ices with half-lives of up to ∼10
years on cold planetary bodies such as comets or Pluto. An important implication of our results is that for extraterrestrial environments, the application of laboratory data measured for the radiation-induced destruction of pure (neat) uracil samples can greatly underestimate the molecule's rate of destruction and significantly overestimate its lifetime, which can lead to errors of over 1000%.
Context.
Methoxymethanol (CH
3
OCH
2
OH) has been identified through gas-phase signatures in both high- and low-mass star-forming regions. Like several other C-, O-, and H-containing complex organic ...molecules (COMs), this molecule is expected to form upon hydrogen addition and abstraction reactions in CO-rich ice through radical recombination of CO hydrogenation products.
Aims.
The goal of this work is to experimentally and theoretically investigate the most likely solid-state methoxymethanol reaction channel – the recombination of CH
2
OH and CH
3
O radicals – for dark interstellar cloud conditions and to compare the formation efficiency with that of other species that were shown to form along the CO-hydrogenation line. We also investigate an alternative hydrogenation channel starting from methyl formate.
Methods.
Hydrogen atoms and CO or H
2
CO molecules were co-deposited on top of predeposited H
2
O ice to mimic the conditions associated with the beginning of “rapid” CO freeze-out. The formation of simple species was monitored in situ using infrared spectroscopy. Quadrupole mass spectrometry was used to analyze the gas-phase COM composition following a temperature-programmed desorption. Monte Carlo simulations were used for an astrochemical model comparing the methoxymethanol formation efficiency with that of other COMs.
Results.
The laboratory identification of methoxymethanol is found to be challenging, in part because of diagnostic limitations, but possibly also because of low formation efficiencies. Nevertheless, unambiguous detection of newly formed methoxymethanol has been possible in both CO+H and H
2
CO+H experiments. The resulting abundance of methoxymethanol with respect to CH
3
OH is about 0.05, which is about six times lower than the value observed toward NGC 6334I and about three times lower than the value reported for IRAS 16293B. Astrochemical simulations predict a similar value for the methoxymethanol abundance with respect to CH
3
OH, with values ranging between 0.03 and 0.06.
Conclusions.
We find that methoxymethanol is formed by co-deposition of CO and H
2
CO with H atoms through the recombination of CH
2
OH and CH
3
O radicals. In both the experimental and modeling studies, it is found that the efficiency of this channel alone is not sufficient to explain the observed abundance of methoxymethanol with respect to methanol. The rate of a proposed alternative channel, the direct hydrogenation of methyl formate, is found to be even less efficient. These results suggest that our knowledge of the reaction network is incomplete or involving alternative solid-state or gas-phase formation mechanisms.
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