Europa’s environment Chris Paranicas Johns Hopkins University Applied Physics Lab Member of the 2016 Europa Lander Science Definition Team September, 2016

Outline Question is where on Europa should we land to find material that is least radiation-processed? Lessons from Saturn images show persistent “plasma bull’s eye” centered on trailing apex and lens feature around leading apex due probably to energetic electrons What is the radiation environment and context of Europa? How do individual particles impact Europa (in a simple model without the plasma interaction)? What regions are likely to be most heavily impacted by particles that deposit a lot of energy into the surface? Is the leading or trailing more prefered? Where should we land so that the more pristine ice is most accessible?

Plasma “bull’s eye” centered at ~270oW on Saturn’s moon Dione, from Kirchoff and Schenk (2015) Some examples of surface modification

In this color map (IR/green/UV) from Cassini/ISS at Saturn, Schenk et al. (2011) show the difference between the plasma bull’s eye around 270oW and the faint energetic electron lens (near 90oW) on Tethys Environment

Schenk et al. (2011) presented IR/UV color ratio maps of 5 inner satellites of Saturn and found a dark “lens” feature on 2 of them (Mimas and Tethys) likely related to ~ MeV electrons. Maps are dominated by UV, 0.338 microns

Jupiter’s radiation belts from the models of Garrett et al Jupiter’s radiation belts from the models of Garrett et al. These models do reflect some structure in the belts, e.g., the notch in protons near Io’s orbit around 5.9 RJ (from Paranicas et al. 2009) JPL’s radiation model shows that Europa (at 9.4 RJ) is one of the moons of Jupiter in the most intense proton and electron radiation environments (from Paranicas et al. -- Europa book chapter, see plate)

Cassidy et al. (2013) illustrate that most cold to hot plasma will hit the trailing but more energetic particles can reach all points on the surface How does environment interact with surface?

Cartoon of electrons impacting a Saturnian moon showing preference for equatorial impact (from Paranicas et al. 2014)

On Europa, the electrons < 24 MeV form a lens (Paranicas et al On Europa, the electrons < 24 MeV form a lens (Paranicas et al. 2001; W. Patterson et al. 2012). Also expect a more latitudinally-extended lens on the leading hemisphere due electrons between about 24 and 100 MeV.

Due to precipitation, some regions have much lower fluxes of energetic electrons, Paranicas et al. (2007)

Predictions of electron bombardment patterns on Europa from the work of Truscott et al. (2011; IEEE)

From Cassidy et al. (2013) showing precipitation patterns on Europa of S++ as a function of energy (using data and modeling). All ions will likely have an energy-dependent evolution. More like the plasma bull’s eye At 10 MeV, more uniform

Leading/trailing comparisons from Paranicas et al Leading/trailing comparisons from Paranicas et al. (Europa book chapter), using S. Sterner’s transport calculation. These are applicable to the apex point. Dose versus depth at a point on the surface

Dose-depth on Europa based on trailing data (Paranicas et al. 2002) Dose-depth on Europa based on trailing data (Paranicas et al. 2002). Below 200 mm, the secondaries (bremsstrahlung photons) become important). The value 100 eV per 16 amu of material is a standard value to assess the creation/destruction of molecules. Uses J. M. Ratliff’s transport calculation.

EXTRAS

An ion summary from Bagenal et al. (2015)

Can see in the electron and ion densities, there is expected and unexpected variations in the cold plasma (from Bagenal et al. 2015)