National Aeronautics and Space Administration Logo
Follow this link to skip to the main content NASA Banner
Solar System Exploration
Science & Technology
Laboratory & Modeling Studies to Quantify Electron Penetration into and Charging of Icy Surfaces of Moons of Giant Planets

Can Astrobiologically Important Organics Survive and Evolve in the Near-surface Ices Under an Extreme Radiation Environment?

Electron Flux Comparison

<P>At Europa = ~1.8 X 108 cm-2 s-1<br>
In the Lab = ~ 3 X 1013 cm-2 s-1<br>
The lab's low-energy (0.1 - 2 keV) electron flux is about 105 times higher than at Europa, but the primary electrons reaching Europa's surface have much higher energy (>100 keV - MeV).  As a result, secondary electron flux within Europa's icy surface could be 100-1000 times higher than its primary electron flux. Hence, our lab simulations are comparable to processes on Europa's surface.
Electron Flux Comparison

At Europa = ~1.8 X 108 cm-2 s-1
In the Lab = ~ 3 X 1013 cm-2 s-1
The lab's low-energy (0.1 - 2 keV) electron flux is about 105 times higher than at Europa, but the primary electrons reaching Europa's surface have much higher energy (>100 keV - MeV). As a result, secondary electron flux within Europa's icy surface could be 100-1000 times higher than its primary electron flux. Hence, our lab simulations are comparable to processes on Europa's surface.

Project Objective:

  • To quantitatively determine penetration depths for electrons that cause radiation damage to organics in icy surfaces.
  • To understand the phenomenon of ice-surface charging due to magnetospheric electron and ion bombardment of Jovian and Saturnian icy moons.
  • Develop and assess spectroscopic identification methods for remote sensing organics on icy moons of giant planets.

Research by Murthy S. Gudipati (3227), Irene Li (3227/IGPP, UCLA), Antti Lignell (3227), Krishan Khurana (IGPP, UCLA)

Potential future outer planet missions need a comprehensive understanding of radiation processing of icy surfaces in order to define instrument needs.

In order to better understand surface properties of icy bodies including Europa, Enceladus and Ganymede among others, scientists at JPL demonstrated the use of polycyclic aromatic hydrocarbons (PAHs) to study quantitative electron penetration into ice.

Studies presented here were conducted at JPL's "Ice Spectroscopy Lab (ISL)" of Dr. Murthy Gudipati, on ices kept at 30 K (on a 4 K cryogenic system). The electron gun is mounted on a translation stage (left), sample entry is towards the viewer, and on the right are the fiber optics for UV transmission (absorption) spectroscopy.

Demonstration of a New Idea to quantify penetration depths of electrons into ices

Graph depicting polycyclic aromatic hydrocarbons penetration into ice
Scientists used polycyclic aromatic hydrocarbons (PAHs) to study quantitative electron penetration into ice.
  • PAHs imbedded in thin ice layer are used as detectors, as they degrade upon electron irradiation.
  • UV-VIS spectroscopy is used to monitor remaining PAH concentration.
  • First Demonstration using low-energy (5 eV - 2 keV) electron into planetary ice analogs.

Observations:

Single Layered Ices

  • Pyrene (PAH) depletion is linear with respect to electron energy
  • Threshold electron energy to observe measurable degradation of PAH molecules in ice is ~100 eV
  • No temperature dependence (5 K or 100 K)

Graphic showing Pyrene Depletion compared with Electron Energy
Laboratory data (observed) and modeled (solid) depletion of PAH at various ice depths due to reactions with electrons.

Double Layered Ices

  • Two distinct linear segments in electron penetration
  • At higher energies, the linear curve compares well with the slope of the 'pure' pyrene/ice film (dashed orange line)
  • As water layer increases, this correlation shifts to higher energies

Implications

So far there have not been direct quantitative electron penetration measurements (energy vs. depth) on ices. All the models use data derived from other targets such as silicon. Our experiments show that electrons penetrate deeper than predicted by existing models. The scientists plan to extend these experiments to higher electron energy region and compare with the existing models. Organics in ices are easily destroyed even with low-energy electrons, implying that it may be difficult to detect organics on highly irradiated surfaces and subsurface probing may be inevitable.

Significance to Solar System Exploration

The proposed Europa Jupiter System Mission (EJSM), with its focus on Europa and Ganymede, need the understanding of radiation processing of icy surfaces in order to define instrument needs. This research not only strongly helps these goals, but also in the data analysis of present and future flagship missions to icy bodies.


Last Updated: 24 January 2011

Science Features
Astrobiology
Astronomy Features
Power
Technology Assessment Reports
Sungrazing Comets

 

Best of NASA Science
NASA Science Highlights
Technology Features
Propulsion
Lectures & Discussions

Awards and Recognition   Solar System Exploration Roadmap   Contact Us   Site Map   Print This Page
NASA Official: Kristen Erickson
Advisory: Dr. James Green, Director of Planetary Science
Outreach Manager: Alice Wessen
Curator/Editor: Phil Davis
Science Writer: Autumn Burdick
Producer: Greg Baerg
Webmaster: David Martin
> NASA Science Mission Directorate
> Budgets, Strategic Plans and Accountability Reports
> Equal Employment Opportunity Data
   Posted Pursuant to the No Fear Act
> Information-Dissemination Policies and Inventories
> Freedom of Information Act
> Privacy Policy & Important Notices
> Inspector General Hotline
> Office of the Inspector General
> NASA Communications Policy
> USA.gov
> ExpectMore.gov
> NASA Advisory Council
> Open Government at NASA
Last Updated: 24 Jan 2011