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Ionization: A key chemical pathway in ices under radiation
environment?
Murthy S. Gudipati
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109
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• Radiation, Ice and Organic Impurities • Ionization - A Quick Tour• Highlights of Our Laboratory Discoveries • Solar System Ices under Radiation Environment • Implications to Solar System Icy Surfaces • Implications to Present and Future Missions
Contents
AcknowledgmentsCollaboration:
Louis Allamandola (NASA Ames); John F. Cooper (NASA Goddard); Robert E. Johnson (U. Virginia)
Funds: NASA PG&G; PA; DDA; CDAP; Astrobiology; JPL
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Radiation, Ice and
Organic Impurities
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Interaction of Radiation and Ice(dissociation & ionization – storage of energy & highly reactive species)
Quanta of Energy deposited in ices to Quanta of Energy deposited in ices to heat, dissociate, and ionize molecules heat, dissociate, and ionize molecules
in these icesin these icesPhotons (< 20 eV) : a few eventsPhotons (< 20 eV) : a few events
Particles (MeV): 100 to 1000 eventsParticles (MeV): 100 to 1000 events
Photons - few Photons - few micronsmicrons
Cosmic Rays ~ Cosmic Rays ~ metermeter
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Building Blocks of Life created / destroyed in Ice
Dworkin et al. Proc. Nat. Acad. Sci. 98, 815 (2001); Bernstein et al. Nature 416, 401 (2002); Muñoz-Caro et al. Nature 416, 403 (2002)
Amino Acids from VUV-Irradiated NASA Ames Group (H2O:CH3OH:NH3:HCN = 20:2:1:1)Leiden Group (H2O:CH3OH:NH3:CO:CO2 = 2:1:1:1:1)
Laser Desorbed
Mass Spectra
Micelles from VUV-Irradiated H2O:CH3OH:CO:NH3 Ice Residue (100:50:1:1) [NASA Ames Group]
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Can Life Evolve and Sustain in Solar System Ices?
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Ionization - A Quick Tour
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Ionization vs. Dissociation & Excitation
High-Energy electron and ion bombardment also results in
core-ionization, Auger process
etc.
Ionization of Impurities in Ices has not been considered in
Astrophysical Sciences until recently!
(Electron)
(Positive Ion)
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Ionization vs. Ionic Salts
Ionization:Ionization:
• Forced removal of an electron from a molecule or solid (Workfunction)
• Electrons are generated/trapped in ice
Ionic Salts:Ionic Salts:
• Electron is strongly bound to the negative ion of the salt
• Hole is similar in both the cases
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Ionization Energy in the Gas-Phase
Organic Molecules
Inorganic Atoms and Molecules are ionized at lower energies than organics
Small organics (methane, ethane) ~ 12 eV
Vast majority of organic molecules < 8 eV
Large organics ~ 5 eV
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Highlights of Our Laboratory Discoveries
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First Direct Evidence for Ionization of PAHs in Ices
Gudipati & Allamandola, ApJ Letters 615 (2004) L177
4.0 eV4.0 eV
• High ionization yieldsHigh ionization yields
• Electron injection into the Electron injection into the iceice
• Low ionization energy Low ionization energy (2 eV less from gas-(2 eV less from gas-phase)phase)
• Stability of ions up to 120 Stability of ions up to 120 KK
• Low-energy multiple Low-energy multiple ionizationsionizations
• Electron mobilityElectron mobility
Polycyclic Aromatic Hydrocarbons (PAHs) - Abundant in Space
HighlightsHighlights
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Organic Chemistry in Ices after Ionization
Multiple oxidation and hydrogenation reactions Multiple oxidation and hydrogenation reactions result in Ketones, Alcohols, Ethers, …result in Ketones, Alcohols, Ethers, …
Bernstein, Sandford, Allamandola, Seb Gillette, Clemett, Zare, Science 283, 1135 (1999)
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Solar System Ices under
Radiation Environment
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Solar Radiation Reaching Ices on Earth and Mars
Mars Surface & Upper Atmosphere Earth Surface & Upper Atmosphere
< 6 eV< 4 eV
IEICE = (IEgas - 2.0) eV
Atoms & molecules with IEgas < 6 eV on Earth &
IEgas < 8 eV on Mars are ionized when imbedded in ice surface
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Outer Solar System Ices
Jovian and Saturnian Icy Bodies:
• keV to MeV electrons & ions• Hard UV Solar radiation penetration
due to lack of atmosphere (except Titan)
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Example: Europa under Jovian Magnetospheric Radiation
Jupiter’s magnetospheric electrons bombard exclusively trailing hemisphere of Europa, creating a possible charge gradient on Europa’s hemispheres.
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Ionization in IcesBiological & Environmental Importance
In ices some of these atoms In ices some of these atoms and molecules may be and molecules may be
ionized with ~300 nm UV-B ionized with ~300 nm UV-B sunlightsunlight
Chl
orop
hyl
l
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Implications to Solar System Icy Surfaces
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Outer Solar System Ices containing Impurities
Chart: W. M. Grundy, M. W. Buie, J. A. Stansberry, J. R. Spencer, and B. Chart: W. M. Grundy, M. W. Buie, J. A. Stansberry, J. R. Spencer, and B. Schmitt, Schmitt, Icarus Icarus 142142, 536–549 (1999), 536–549 (1999)
Increase in Increase in Chemical Chemical ReactivityReactivity
Ions & Ions & Radicals Radicals stored in stored in frozen frozen IcesIces
Complex & Complex & neutral neutral
Molecules Molecules created created
and and destroyed destroyed
continuouslcontinuouslyy
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Organics in Martian and Polar Ices
Organics in Martian Ices:
• Under ultraviolet (UV < 200 nm) radiation, hydrocarbons in Martian ice surfaces can be quickly oxidized. As a result alcohols and other oxygen-rich organics can be produced, which degrade into small molecules, or finally perhaps to CO2.
Organics in Polar Ices on Earth:
• Anthropogenic PAHs are detected to be drifting to the polar regions and deposit onto the polar ices. UV (< 300 nm) radiation can ionize large PAHs in polar ice surfaces leading to the formation of oxidized
PAhs. Indeed oxidation products of PAHs are seen in polar ices. (Grannas, A. M., et al., 2007. An overview of snow photochemistry: evidence, mechanisms and
impacts. Atmospheric Chemistry and Physics. 7, 4329-4373).
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Implications to Present and Future Missions
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Missions
Mars (Phoenix, MSL, etc.) Larger organics may not survive on the ice surface.
Small oxidized organics such as methanol (CH3OH), formaldehide (CH2O) should be detectable.
Europa (Jovian Ices) High surface and near-surface ionization of organics in high radiation environment.
Near-surface electron and ion currents.
Enceladus, Saturn’s Rings (Saturnian Ices)
Storage of ionized organics. Charge localization? Coloration of Ices?
Trans Saturnian Ices (Uranus, Neptune, Pluto, KBOs)
Cosmic rays induced interrior ionization of organics.
Interrior charge localization?