LABORATORY STUDIES ON THE IRRADIATION OF SOLID ETHANE ANALOG ICES AND IMPLICATIONS TO TITAN'S CHEMISTRY
- Department of Chemistry, University of Hawaii at Manoa, Honolulu, HI 96822 (United States)
- Department of Physics and Astronomy, Northern Arizona University, Flagstaff, AZ 86011 (United States)
Pure ethane ices (C{sub 2}H{sub 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{sub 4}), acetylene (C{sub 2}H{sub 2}), ethylene (C{sub 2}H{sub 4}), and the ethyl radical (C{sub 2}H{sub 5}), together with n-butane (C{sub 4}H{sub 10}) and butene (C{sub 4}H{sub 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.
- OSTI ID:
- 21394334
- Journal Information:
- Astrophysical Journal, Vol. 711, Issue 2; Other Information: DOI: 10.1088/0004-637X/711/2/744; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
COSMOLOGY AND ASTRONOMY
2-METHYLPROPANE
2-METHYLPROPENE
ACETYLENE
BRANCHING RATIO
BUTANE
BUTENES
COMETS
COSMIC RADIATION
ETHANE
ETHYL RADICALS
ETHYLENE
FOURIER TRANSFORM SPECTROMETERS
HYDROGEN
ICE
INFRARED SPECTROMETERS
MASS SPECTROMETERS
METHANE
MOON
PLANETARY ATMOSPHERES
REACTION KINETICS
SATURN PLANET
SOLAR SYSTEM
TAIL ELECTRONS
ALKANES
ALKENES
ALKYL RADICALS
ALKYNES
ATMOSPHERES
DIMENSIONLESS NUMBERS
ELECTRONS
ELEMENTARY PARTICLES
ELEMENTS
FERMIONS
HYDROCARBONS
IONIZING RADIATIONS
KINETICS
LEPTONS
MEASURING INSTRUMENTS
NONMETALS
ORGANIC COMPOUNDS
PLANETS
RADIATIONS
RADICALS
SATELLITES
SPECTROMETERS