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]

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?