A discussion will be made on methanol on Enceladus' surface and the implicati... more A discussion will be made on methanol on Enceladus' surface and the implications for habitability.
The aromatic benzene molecule (C6H6)—a central building block of polycyclic aromatic hydrocarbon ... more The aromatic benzene molecule (C6H6)—a central building block of polycyclic aromatic hydrocarbon molecules—is of crucial importance for the understanding of the organic chemistry of Saturn's largest moon, Titan. Here, we show via laboratory experiments and electronic structure calculations that the benzene molecule can be formed on Titan's surface in situ via non-equilibrium chemistry by cosmic-ray processing of low-temperature acetylene (C2H2) ices. The actual yield of benzene depends strongly on the surface coverage. We suggest that the cosmic-ray-mediated chemistry on Titan's surface could be the dominant source of benzene, i.e., a factor of at least two orders of magnitude higher compared to previously modeled precipitation rates, in those regions of the surface which have a high surface coverage of acetylene.
ABSTRACT D. E. Shemansky1, X. Zhang2, M-C. Liang3, and Y. L. Yung2 1 SET/PSSD, California, USA ; ... more ABSTRACT D. E. Shemansky1, X. Zhang2, M-C. Liang3, and Y. L. Yung2 1 SET/PSSD, California, USA ; 2 CIT California, USA ; 3 Res. Ctr. Env. Change, Taipei, Taiwan The strong solar reflection spectrum in the FUV in Cassini UVIS observations with line of sight below the solid limb allow the examination of spectral properties of the scattering medium. At wavelengths longward of 1525 A the spectrum is characterized mainly by solar emission structure. Absorption features in acetylene , the main known absorber in this region, are weak. The spectrum therefore shows the properties of a Rayleigh scattering continuum in a wavelength range of 1920. - 1525.1 A, below which the solar continuum and line emissions are cut-off by absorption in the medium. The observed emission is evidently multiply scattered. At a line of sight altitude of 1040 km, analysis assuming Mie scattering gives an estimated particle radius of 76 A. The evident strong absorber at lower altitude giving the sharp extinction of the emission spectrum below 1525 A needs investigation. The PAH naphthalene, with an ionization potential of 1526.9 A is the nearest candidate identified to date. The spectral properties of the scattering medium in the 1920. - 1525.1 A region will be described.
The large variability of H2O and SO2in the atmosphere of Venus above the cloud tops is puzzling, ... more The large variability of H2O and SO2in the atmosphere of Venus above the cloud tops is puzzling, especially since there is little evidence for their variability in the lower atmosphere. We note three important related facts: (1) The abundances of H2O and SO2in the deep atmosphere are of the same order of magnitude ~100 ppmv, (2) there is a rapid decrease in H2O and SO2 just above the cloud tops, resulting in sharp vertical gradients in their vertical profiles, and (3) the primary removal mechanism for H2O and SO2above the cloud tops is formation of H2SO4 aerosols. In this work we examine the possibility that H2O and SO2could mutually regulate each other in 1-D and 2-D models.
We analyzed middle/upper tropospheric CO2 data for 2003 retrieved from infrared spectra obtained ... more We analyzed middle/upper tropospheric CO2 data for 2003 retrieved from infrared spectra obtained by the Atmospheric Infrared Sounder (AIRS) aboard the NASA EOS-Aqua satellite. Contrary to the conventional view, we have found substantial spatiotemporal variability in the middle tropospheric CO2. This relatively large variability is supported by in situ aircraft observations of CO2 from INTEX-NA and COBRA. It is, however, largely missing in global simulations of CO2 with three-dimensional chemistry and transport models (CTMs). The distribution of the middle tropospheric CO2 appears to be strongly influenced by largescale circulations such as the middle-latitude jet streams and by synoptic weather systems, particularly in summer. Contributions from large surface sources, especially in the Southern Hemisphere, are evident in the AIRS CO2 data. The stratosphere-troposphere exchange associated with a Northern Hemisphere Stratospheric Sudden Warming (SSW) event in April 2003 resulted in an increase of 5 ppmv in AIRS CO2 concentrations and a decrease of 40 ppbv in AIRS O3 at 300 hPa. The AIRS middle tropospheric CO2 data thus provide unique insights into the underlying dynamic processes.
The isotopic composition of long-lived trace gases provides a window into atmospheric transport a... more The isotopic composition of long-lived trace gases provides a window into atmospheric transport and chemistry. Carbon dioxide is a particularly powerful tracer, because its abundance remains >100 ppmv in the mesosphere. Current models consider O3 as the main source of O(1D) in the mesosphere, but we note that the photolysis of 16O17O and 16O18O by solar Lyman-α radiation yields O(1D) 10-100 times more enriched in 17O and 18O than that from ozone photodissociation. We therefore incorporate both photochemical sources into stratospheric and mesospheric chemical transport models that quantitatively predict the unusual enhancement of 17O in CO2 from the middle atmosphere. New laboratory and atmospheric measurements are proposed to test our model and validate the use of CO2 isotopic fractionation as a tracer of atmospheric chemical and dynamical processes. Once fully understood the `anomalous' oxygen signature in CO2 can be used in turn to study biogeochemical cycles, in particular to constrain the gross carbon fluxes between the atmosphere and terrestrial biosphere.
In order to recognize signatures of life on extrasolar planets, a suite of computer models called... more In order to recognize signatures of life on extrasolar planets, a suite of computer models called the VPL is being developed that will allow the simulation of a broad range of planetary environments both with and without life, and determination of the spectral signature of these environments. These tools represent a significant innovation for the understanding of extrasolar planetary atmospheres and astronomical biosignatures. We present the overall VPL architecture and a validation of the chemistry/climate/radiative transfer part of the model. To validate the model, we use simulations using a present day Martian model atmosphere in conjunction with a Mie scattering dust column over surface H2O ice. In this way, we attempt to understand the degree to which greenhouse effects produced by the dust particles in the Martian atmosphere can allow for temperatures warm enough to permit the formation of liquid surface water, which is required for the presence of life. Perturbing the parameters of the model will show if there is an optimal amount of dust that can produce a greenhouse effect for periods long enough to facilitate the presence of life on Mars.
Journal of Quantitative Spectroscopy and Radiative Transfer, Jun 1, 1997
Due to larger multiple scattering effects in the troposphere compared to that in the stratosphere... more Due to larger multiple scattering effects in the troposphere compared to that in the stratosphere, the optical path of tropospheric ozone is markedly enhanced (as compared with that of stratospheric ozone) in the Huggins bands from 310 to 345 nm. Model study of the direct and diffuse solar fluxes on the ground shows differences between tropospheric and stratospheric ozone. The
Analysis of the Cassini Ultraviolet Imaging Spectrograph (UVIS) stellar and solar occultations at... more Analysis of the Cassini Ultraviolet Imaging Spectrograph (UVIS) stellar and solar occultations at Titan to date include 12 species: N2 (nitrogen), CH4 (methane), C2H2 (acetylene), C2H4 (ethylene), C2H6 (ethane), C4H2 (diacetylene), C6H6 (benzene), C6N2 (dicyanodiacetylene), C2N 2 (cyanogen), HCN (hydrogen cyanide), HC3N (cyanoacetylene), and aerosols distinguished by a structureless continuum extinction (absorption plus scattering) of photons in the EUV. The introduction of aerosol particles, retaining the same refractive index properties as tholin with radius ~125 Å and using Mie theory, provides a satisfactory fit to the spectra. The derived vertical profile of aerosol density shows distinct structure, implying a reactive generation process reaching altitudes more than 1000 km above the surface. A photochemical model presented here provides a reference basis for examining the chemical and physical processes leading to the distinctive atmospheric opacity at Titan. We find that dicyanodiacetylene is condensable at ~650 km, where the atmospheric temperature minimum is located. This species is the simplest molecule identified to be condensable. Observations are needed to confirm the existence and production rates of dicyanodiacetylene.
Recent (2005) Spitzer Infrared Spectrometer (IRS) data of Neptune between 5 and 20 um contain a w... more Recent (2005) Spitzer Infrared Spectrometer (IRS) data of Neptune between 5 and 20 um contain a wealth of information about the chemical composition and temperature structure of its cold atmospheres. Emission features in its spectrum arise from many hydrocarbons, including ethane, acetylene and methane, and they appear to be superimposed on top of a collision-induced H2 continuum. As the derivation of all other parameters depends on the assumed temperature profile, special efforts were taken to ensure that the stratospheric profile between 1 bar and 0.3 mbar matched the H2 continuum and the H2 S(1) quadrupole feature at 17 um. Additionally we matched the methane v4 feature at 7.7 um in order to constrain the stratospheric temperature profile above the 0.3-mbar level as well as the methane stratospheric volume mixing ratios (VMRs). After the determination of the temperature profile and methane VMRs, the VMRs for several species were then determined through fitting their corresponding features in the spectrum, initially by scaling existing photochemical models (see Mahmud et al., this conference). These species include methylacetylene, diacetylene, benzene, acetylene, ethane, methyl radical, ethylene and carbon dioxide which are all derived from methane photochemistry. I would like to acknowledge the NASA USRP program for supporting this work.
A discussion will be made on methanol on Enceladus' surface and the implicati... more A discussion will be made on methanol on Enceladus' surface and the implications for habitability.
The aromatic benzene molecule (C6H6)—a central building block of polycyclic aromatic hydrocarbon ... more The aromatic benzene molecule (C6H6)—a central building block of polycyclic aromatic hydrocarbon molecules—is of crucial importance for the understanding of the organic chemistry of Saturn's largest moon, Titan. Here, we show via laboratory experiments and electronic structure calculations that the benzene molecule can be formed on Titan's surface in situ via non-equilibrium chemistry by cosmic-ray processing of low-temperature acetylene (C2H2) ices. The actual yield of benzene depends strongly on the surface coverage. We suggest that the cosmic-ray-mediated chemistry on Titan's surface could be the dominant source of benzene, i.e., a factor of at least two orders of magnitude higher compared to previously modeled precipitation rates, in those regions of the surface which have a high surface coverage of acetylene.
ABSTRACT D. E. Shemansky1, X. Zhang2, M-C. Liang3, and Y. L. Yung2 1 SET/PSSD, California, USA ; ... more ABSTRACT D. E. Shemansky1, X. Zhang2, M-C. Liang3, and Y. L. Yung2 1 SET/PSSD, California, USA ; 2 CIT California, USA ; 3 Res. Ctr. Env. Change, Taipei, Taiwan The strong solar reflection spectrum in the FUV in Cassini UVIS observations with line of sight below the solid limb allow the examination of spectral properties of the scattering medium. At wavelengths longward of 1525 A the spectrum is characterized mainly by solar emission structure. Absorption features in acetylene , the main known absorber in this region, are weak. The spectrum therefore shows the properties of a Rayleigh scattering continuum in a wavelength range of 1920. - 1525.1 A, below which the solar continuum and line emissions are cut-off by absorption in the medium. The observed emission is evidently multiply scattered. At a line of sight altitude of 1040 km, analysis assuming Mie scattering gives an estimated particle radius of 76 A. The evident strong absorber at lower altitude giving the sharp extinction of the emission spectrum below 1525 A needs investigation. The PAH naphthalene, with an ionization potential of 1526.9 A is the nearest candidate identified to date. The spectral properties of the scattering medium in the 1920. - 1525.1 A region will be described.
The large variability of H2O and SO2in the atmosphere of Venus above the cloud tops is puzzling, ... more The large variability of H2O and SO2in the atmosphere of Venus above the cloud tops is puzzling, especially since there is little evidence for their variability in the lower atmosphere. We note three important related facts: (1) The abundances of H2O and SO2in the deep atmosphere are of the same order of magnitude ~100 ppmv, (2) there is a rapid decrease in H2O and SO2 just above the cloud tops, resulting in sharp vertical gradients in their vertical profiles, and (3) the primary removal mechanism for H2O and SO2above the cloud tops is formation of H2SO4 aerosols. In this work we examine the possibility that H2O and SO2could mutually regulate each other in 1-D and 2-D models.
We analyzed middle/upper tropospheric CO2 data for 2003 retrieved from infrared spectra obtained ... more We analyzed middle/upper tropospheric CO2 data for 2003 retrieved from infrared spectra obtained by the Atmospheric Infrared Sounder (AIRS) aboard the NASA EOS-Aqua satellite. Contrary to the conventional view, we have found substantial spatiotemporal variability in the middle tropospheric CO2. This relatively large variability is supported by in situ aircraft observations of CO2 from INTEX-NA and COBRA. It is, however, largely missing in global simulations of CO2 with three-dimensional chemistry and transport models (CTMs). The distribution of the middle tropospheric CO2 appears to be strongly influenced by largescale circulations such as the middle-latitude jet streams and by synoptic weather systems, particularly in summer. Contributions from large surface sources, especially in the Southern Hemisphere, are evident in the AIRS CO2 data. The stratosphere-troposphere exchange associated with a Northern Hemisphere Stratospheric Sudden Warming (SSW) event in April 2003 resulted in an increase of 5 ppmv in AIRS CO2 concentrations and a decrease of 40 ppbv in AIRS O3 at 300 hPa. The AIRS middle tropospheric CO2 data thus provide unique insights into the underlying dynamic processes.
The isotopic composition of long-lived trace gases provides a window into atmospheric transport a... more The isotopic composition of long-lived trace gases provides a window into atmospheric transport and chemistry. Carbon dioxide is a particularly powerful tracer, because its abundance remains >100 ppmv in the mesosphere. Current models consider O3 as the main source of O(1D) in the mesosphere, but we note that the photolysis of 16O17O and 16O18O by solar Lyman-α radiation yields O(1D) 10-100 times more enriched in 17O and 18O than that from ozone photodissociation. We therefore incorporate both photochemical sources into stratospheric and mesospheric chemical transport models that quantitatively predict the unusual enhancement of 17O in CO2 from the middle atmosphere. New laboratory and atmospheric measurements are proposed to test our model and validate the use of CO2 isotopic fractionation as a tracer of atmospheric chemical and dynamical processes. Once fully understood the `anomalous' oxygen signature in CO2 can be used in turn to study biogeochemical cycles, in particular to constrain the gross carbon fluxes between the atmosphere and terrestrial biosphere.
In order to recognize signatures of life on extrasolar planets, a suite of computer models called... more In order to recognize signatures of life on extrasolar planets, a suite of computer models called the VPL is being developed that will allow the simulation of a broad range of planetary environments both with and without life, and determination of the spectral signature of these environments. These tools represent a significant innovation for the understanding of extrasolar planetary atmospheres and astronomical biosignatures. We present the overall VPL architecture and a validation of the chemistry/climate/radiative transfer part of the model. To validate the model, we use simulations using a present day Martian model atmosphere in conjunction with a Mie scattering dust column over surface H2O ice. In this way, we attempt to understand the degree to which greenhouse effects produced by the dust particles in the Martian atmosphere can allow for temperatures warm enough to permit the formation of liquid surface water, which is required for the presence of life. Perturbing the parameters of the model will show if there is an optimal amount of dust that can produce a greenhouse effect for periods long enough to facilitate the presence of life on Mars.
Journal of Quantitative Spectroscopy and Radiative Transfer, Jun 1, 1997
Due to larger multiple scattering effects in the troposphere compared to that in the stratosphere... more Due to larger multiple scattering effects in the troposphere compared to that in the stratosphere, the optical path of tropospheric ozone is markedly enhanced (as compared with that of stratospheric ozone) in the Huggins bands from 310 to 345 nm. Model study of the direct and diffuse solar fluxes on the ground shows differences between tropospheric and stratospheric ozone. The
Analysis of the Cassini Ultraviolet Imaging Spectrograph (UVIS) stellar and solar occultations at... more Analysis of the Cassini Ultraviolet Imaging Spectrograph (UVIS) stellar and solar occultations at Titan to date include 12 species: N2 (nitrogen), CH4 (methane), C2H2 (acetylene), C2H4 (ethylene), C2H6 (ethane), C4H2 (diacetylene), C6H6 (benzene), C6N2 (dicyanodiacetylene), C2N 2 (cyanogen), HCN (hydrogen cyanide), HC3N (cyanoacetylene), and aerosols distinguished by a structureless continuum extinction (absorption plus scattering) of photons in the EUV. The introduction of aerosol particles, retaining the same refractive index properties as tholin with radius ~125 Å and using Mie theory, provides a satisfactory fit to the spectra. The derived vertical profile of aerosol density shows distinct structure, implying a reactive generation process reaching altitudes more than 1000 km above the surface. A photochemical model presented here provides a reference basis for examining the chemical and physical processes leading to the distinctive atmospheric opacity at Titan. We find that dicyanodiacetylene is condensable at ~650 km, where the atmospheric temperature minimum is located. This species is the simplest molecule identified to be condensable. Observations are needed to confirm the existence and production rates of dicyanodiacetylene.
Recent (2005) Spitzer Infrared Spectrometer (IRS) data of Neptune between 5 and 20 um contain a w... more Recent (2005) Spitzer Infrared Spectrometer (IRS) data of Neptune between 5 and 20 um contain a wealth of information about the chemical composition and temperature structure of its cold atmospheres. Emission features in its spectrum arise from many hydrocarbons, including ethane, acetylene and methane, and they appear to be superimposed on top of a collision-induced H2 continuum. As the derivation of all other parameters depends on the assumed temperature profile, special efforts were taken to ensure that the stratospheric profile between 1 bar and 0.3 mbar matched the H2 continuum and the H2 S(1) quadrupole feature at 17 um. Additionally we matched the methane v4 feature at 7.7 um in order to constrain the stratospheric temperature profile above the 0.3-mbar level as well as the methane stratospheric volume mixing ratios (VMRs). After the determination of the temperature profile and methane VMRs, the VMRs for several species were then determined through fitting their corresponding features in the spectrum, initially by scaling existing photochemical models (see Mahmud et al., this conference). These species include methylacetylene, diacetylene, benzene, acetylene, ethane, methyl radical, ethylene and carbon dioxide which are all derived from methane photochemistry. I would like to acknowledge the NASA USRP program for supporting this work.
Uploads