Laboratory Studies on the Irradiation of Solid Ethane Analog Ices and Implications to Titan's Chemistry

Abstract

Pure ethane ices (C 2 H 6 ) were irradiated at 10, 30, and 50 K under contamination-free, ultrahigh vacuum conditions with energetic electrons generated in the track of galactic cosmic-ray (GCR) particles to simulate the interaction of GCRs with ethane ices in the outer solar system. The chemical processing of the samples was monitored by a Fourier transform infrared spectrometer and a quadrupole mass spectrometer during the irradiation phase and subsequent warm-up phases on line and in situ in order to extract qualitative (products) and quantitative (rate constants and yields) information on the newly synthesized molecules. Six hydrocarbons, methane (CH 4 ), acetylene (C 2 H 2 ), ethylene (C 2 H 4 ), and the ethyl radical (C 2 H 5 ), together with n-butane (C 4 H 10 ) and butene (C 4 H 8 ), were found to form at the radiation dose reaching 1.4 eV per molecule. The column densities of these species were quantified in the irradiated ices at each temperature, permitting us to elucidate the temperature and phase-dependent production rates of individual molecules. A kinetic reaction scheme was developed to fit column densities of those species produced during irradiation of amorphous/crystalline ethane held at 10, 30, or 50 K. In general, the yield of the newly formed molecules dropped consistently for all species as the temperature was raised from 10 K to 50 K. Second, the yield in the amorphous samples was found to be systematically higher than in the crystalline samples at constant temperature. A closer look at the branching ratios indicates that ethane decomposes predominantly to ethylene and molecular hydrogen, which may compete with the formation of n-butane inside the ethane matrix. Among the higher molecular products, n-butane dominates. Of particular relevance to the atmosphere of Saturn's moon Titan is the radiation-induced methane production from ethane-an alternative source of replenishing methane into the atmosphere. Finally, we discuss to what extent the n-butane could be the source of "higher organics" on Titan's surface thus resembling a crucial sink of condensed ethane molecules.

COMPLEX HYDROCARBON CHEMISTRY IN INTERSTELLAR AND SOLAR SYSTEM ICES REVEALED: A COMBINED INFRARED SPECTROSCOPY AND REFLECTRON TIME-OF-FLIGHT MASS SPECTROMETRY ANALYSIS OF ETHANE (C2H6) AND D6-ETHANE (C2D6) ICES EXPOSED TO IONIZING RADIATION

“…Fundamentals corresponding to methane (CH 4 ), acetylene (C 2 H 2 ), ethylene (C 2 H 4 ), the ethyl radical (C 2 H 5 ), 1-butene (C 4 H 8 ), and n-butane (C 4 H 10 ) were assigned in both the ethane (Table 1(a), Figure 1) and D6-ethane ices (Appendix A.1). The detection of 1-butene and n-butane, but not their isomers, is in agreement with previous studies (Jackson et al 1966;Hudson et al 2009;Kim et al 2010) and reaffirms the proposed reaction mechanism of two ethyl radicals forming nbutane followed by dehydrogenation to 1-butene (Section 4). Figure 2 shows that the absorptions in the spectrum from 5000-3350 cm −1 and 2800-500 cm −1 are easily assigned to the reactants and aforementioned products formed within the ethane ice upon irradiation.…”

“…The implications of these observations are that ethane bond breaking occurs between carbon-hydrogen bondsbut not carbon-carbon bonds (Section 4). These findings correlate well with results fromprevious studies examining the processing of ethane at astrophysically relevant temperatures; Kim et al (2010) identified the largest molecule formed in the ices as n-butane via recombination of two ethyl radicals using FTIR spectroscopy.…”

“…According to Mumma et al (1996), ethane detected in comet C/1996 B2 Hyakutake is likely of interstellar origin based upon its relative abundance to water and can be traced back to the energetic processing of methane ice (Boogert et al 2015) in interstellar clouds, as shown in laboratory experiments simulating the irradiation of pure methane ice (Gerakines et al 1996;Kaiser & Roessler 1998;Bennett et al 2006;Jones & Kaiser 2013), and inmixtures of methane with water (Moore & Hudson 1998) by ionizing radiation. Titan has been a main focus for the study of ethane (Kim et al 2010). This moon of Saturn has an atmosphere consisting primarily of nitrogen (98.4%) and methane (1.4%) (Niemann et al 2005).…”

“…Ethane ices were also processed with 15 keV N + ions, which rose to the infrared detection of the nitrile functional group (-C ≡ N) in hydrogen cyanide (HCN) (Strazzulla et al 2002). Finally, the study of solid ethane exposed to 5 keV electrons detected molecules as large as nbutane (Kim et al 2010), where the energetic electrons simulate secondary electrons generated in the track of galactic cosmic rays (GCRs) (Charnley et al 2001). Combined, these aforementioned studies exploited a broad set of analytical techniques ranging from offline and ex situ mass spectrometry and gas chromatography to online and in situ infrared spectroscopy (FTIR) along with quadrupole mass spectrometry coupled with electron impact ionization (EI-QMS).…”

A study of interstellar aldehydes and enols as tracers of a cosmic ray-driven nonequilibrium synthesis of complex organic molecules

“…In an analogous reaction sequence, the propanal (C 2 H 5 CHO)-(E)/(Z)-1-propenol [CH 3 CHCH(OH)] tautomer could be synthesized (reactions 8-12). It should be noted that previous experiments demonstrated that the interaction of energetic electrons with ethane led to a carbon-hydrogen bond rupture, but not carbon-carbon bond cleavage (34). Recall that although acetaldehyde and propanal may have some thermal contributions due to their signals, it is important to point out that the enol isomers did not show any increase in intensity during TPD, which reinforces that the enol isomers are formed solely via nonequilibrium pathways during the irradiation phase of the experiment.…”