# Presentation: 174326

## 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*

**Notes:**
Some part of the talk will be like “terms and conditions at radio commercials” - quick brush through

## 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

**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

**Acknowledgments**Collaboration: Louis Allamandola (NASA Ames); John F. Cooper (NASA Goddard); Robert E. Johnson (U. Virginia)Funds: NASA PG&G; PA; DDA; CDAP; Astrobiology; JPL

## Radiation, Ice 
and 
Organic Impurities

**and **
**Organic Impurities**

## Interaction of Radiation and Ice(dissociation & ionization – storage of energy & highly reactive species)

*Quanta of Energy deposited in ices to heat, dissociate, and ionize molecules in these ices*
- Photons (< 20 eV) : a few events
- Particles (MeV): 100 to 1000 events

- Energy Storage
- Charge Storage
- Chemistry

**Photons - few microns****Cosmic Rays ~ meter**

## Building Blocks of Life created / destroyed in Ice

**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]

**Notes:**
Lead desorbed from the surface at high laser powers used to see whether anything organic at high-mass has formed or not

## Can Life Evolve and Sustain in Solar System Ices?

## Ionization - A Quick Tour

## Ionization vs. Dissociation & Excitation

**Ionization vs. Dissociation & Excitation**

- High-Energy electron and ion bombardment _x0010_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)

## Ionization vs. Ionic Salts

**Ionization:**
- Forced removal of an electron from a molecule or solid (Workfunction)
- Electrons are generated/trapped in ice
**Ionic Salts:**
- Electron is strongly bound to the negative ion of the salt
- Hole is similar in both the cases

## 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
 |

## Highlights of Our Laboratory Discoveries

## First Direct Evidence for Ionization of PAHs in Ices

- Gudipati & Allamandola, ApJ Letters 615 (2004) L177

**4.0 eV**

- High ionization yields
- Electron injection into the ice
- Low ionization energy (2 eV less from gas-phase)
- Stability of ions up to 120 K
- Low-energy multiple ionizations
- Electron mobility

- Polycyclic Aromatic Hydrocarbons (PAHs) - Abundant in Space

**Highlights**

## Organic Chemistry in Ices after Ionization

***Multiple oxidation and hydrogenation reactions result in Ketones, Alcohols, Ethers, ...***

- Bernstein, Sandford, Allamandola, Seb Gillette, Clemett, Zare, *Science ***283**, 1135 (1999)

## Solar System Ices under Radiation Environment 

## 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 **IE****gas**** < 6 eV on Earth & ****IE****gas**** < 8 eV on Mars** are ionized when imbedded in ice surface

## 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)

## 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.

## Ionization in IcesBiological & Environmental Importance 

**In ices some of these atoms and molecules may be ionized with ~300 nm UV-B  sunlight**

- Chlorophyll

## Implications to Solar System Icy Surfaces

## Outer Solar System Ices containing Impurities

- Chart: W. M. Grundy, M. W. Buie, J. A. Stansberry, J. R. Spencer, and B. Schmitt, Icarus **142**, 536–549 (1999)

- Increase in Chemical Reactivity

- Ions & Radicals stored in frozen Ices

- Complex & neutral Molecules created and destroyed continuously

## 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).

## Implications to Present and Future Missions

## 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? |