Europa's surface is an icy armor continuously bombarded by charged particles from Jupiter's magnetosphere. Electrons slam into the ice's crystal lattice, lose energy, and kick off chemical reactions. Until now, there was no tool to show how deep such bombardment pierces the ice. Now there is.

The authors took the Geant4-DNA program, which can simulate particle passage through matter, and taught it to work with two forms of ice: amorphous (disordered, like glass) and hexagonal (ordinary, like in earthly snowflakes). For each, they calculated how electrons scatter and lose energy. The result is a transport model ready for use in Monte Carlo simulations.

The model was applied to Europa. On the trailing hemisphere (the one facing backward along the orbit), electrons arrive slowly. They deposit their energy in a top layer less than a millimeter thick. On the leading hemisphere, electrons are fast and penetrate tens of centimeters. The difference is huge: like the difference between a raindrop hitting asphalt and a bullet burying itself in the ground.

This picture explains observations: near the equator of the trailing hemisphere, radiolysis products (substances born from radiation) gather in lenses. Slow electrons don't let them go deep, so chemistry concentrates at the surface. Now it's possible to model not only electrons but also ions and chemical transformations in ice with impurities — for example, with salts that make Europa's ice reddish.