Local Topographic Shielding and Radiation Shadows from Electron Irradiation on Europa
Abstract
A torus of magnetically trapped high-energy electrons and ions encompasses Jupitern and at Europa’s orbital radius the density of these particles is especially high, yielding a continuous bombardment of its surface by these energetic particles. The high surface irradiation dosage makes it unlikely that chemical signature of simple organisms remain detectable and also drives chemical and physical processing of the surface. Energetic electrons with energies less than 25 MeV preferentially impact Europa on its trailing hemisphere, while higher energy but lower intensity electrons irradiate the leading hemisphere. This effect arises from the prograde motion of the Jovian magnetic field relative to Europa and the opposite motion of electrons due to gradient-curvature drift. Therefore, the total energy flux is greater on the trailing hemisphere and this bombardment has been linked to albedo variations on Europa’s surface. In addition, energetic electrons gyrate about magnetic field lines. While energetic electrons are striking Europa’s surface, the gyro-motion and surface interaction causes the directional surface irradiation flux to be anisotropic, even if the source electron flux in Jupiter’s magnetosphere is isotropic Therefore, depending on the location, inclination, and orientation of the impact point, energetic electrons strike that point from limited local zenith and azimuth directions. We have computed the anisotropic flux of energetic electrons bombarding Europa’s surface as a function of longitude along the equator. We identify the most likely scenarios for how a local topographic features may produce a radiation shadow such when the irradiation flux is sufficiently oblique to the surface. We find that local topography can shadow certain surface points from electron irradiation, such that these points could preserve the chemical signatures of the original pristine ice surface or emergent inorganic and organic material from the putative subsurface ocean for longer times. That is, the radiation age of these points could be much longer than elsewhere with potentially detectable effects on surface chemistry.
- Publication:
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AAS/Division for Planetary Sciences Meeting Abstracts #45
- Pub Date:
- October 2013
- Bibcode:
- 2013DPS....4550105H