Europa’s Tenuous Atmosphere

1. Europa’s Tenuous Atmosphere T. A. Cassidy, R. E. Johnson University of Virginia Thin “atmosphere” representative of surface composition Energetic ions and other radiation Surface Material Ejected Planetary Surface (e.g. rock or water ice)

2. Sodium Exospheres at Mercury and the Moon Na in Absorption (Taken during Mercury's transit of the Sun) Dawn Enhancement: Photodesorption North Dawn Sunset Mercury SUN Quarter Moon Eclipsed Moon Na in Emission Peak near subsolar point (due to photodesorption)

3. Near-surface atmosphere model schematic Atmosphere is marginally collisional Launch Molecules Stick or thermally desorb? Probability S Molecules that don’t stick are eventually lost to space Molecule lifetime is measured in minutes to days Of known or suspected atmospheric constituents:

4. 1-D Atmosphere Model Results Recent papers by Shematovich et al.; Smyth and Marconi O2 and H2 created by radiolysis Johnson et al., 1982 H2 quickly lost O2 thermalized near the surface O, H, OH: created by electron-impact dissociation of the atmosphere Example from Smyth and Marconi, 2006

5. Neutral Clouds / Torus The gravitationally unbound atmosphere E.g., top-down view of Europa’s Na torus E.g., side view of Europa’ O torus Burger and Johnson, 2004 Smyth and Marconi, 2006 Europa’s torus is dominated by H2 and O, detectable by its interaction with the plasma. The Na torus is much more tenuous, but detectable by ground based telescopes. There is ~10X more material in the neutral torus than the bound atmosphere

6. Brought to sea level on Earth, the neutral cloud would occupy a cube ~500 m per side. Io has a larger neutral ejection rate, but— Io’s torus slightly smaller (~1/3 the # of atoms and molecules) due to faster ionization. Burger and Johnson, 2004

7. Trace species And their spatial distribution

8. H2O Atmosphere 107 Thermal ions 106 Sputtering yields X Ion energy distribution 105 Sputtered H2O flux (cm-2s-1) Non-thermal ions 104 103 102 103 104 105 106 107 S ion energy (eV) • High-energy ions dominate sputtering. • Such ions precipitate uniformly • (because of their long bounce times) • Paranicas et al., 2002 • Thus: H2O ejection is roughly uniform over the surface. • (Thermal desorption contributes much less; negligible at this scale) H2O vertical column density

9. Trace species carried off with H2O Porco et al. (2003) Cassidy et al. (2008) Cassini ISS NAC Europa’s visible aurora Sum of 9 images of Europa in eclipse Europa’s surface from same perspective This is the only such observation available

10. Aurorae O2 Aurora HST UV, 1356Å (same perspective) Visible wavelength from previous slide: McGrath et al. (2004) Cassidy et al. (2007) Plasma or neutrals? Reminder: brightness = (neutral column density)X(rate coeff.)X(electron density)

11. Saur et al. (1998) McGrath et al. (2004) Saur et al. model: A plasma simulation that assumed a fairly uniform O2 atmosphere My approach: assume brightness variations are due (mostly) to neutral density variations

12. O2 UV Aurora Visible aurora Surface image (same perspective as obs) Simulations O2 simulation O2 reacts with dark terrain Na simulation Na ejected from dark terrain

13. But the neutral only approach is insufficient: It cannot explain temporal variability (Hansen et al., 2005) and ignores the complex plasma environment in Europa’s wake.

14. Plasma models Plasma properties: generally speaking, plasma gets much more dense and slightly colder near the surface Kabin et al., 1999/Liu et al., 2000 Saur et al., 1998 MHD model: Focused on large-scale plasma properties Assumed constant neutral density Focused on O2/electron interaction Assumed constant magnetic field Neutral models: Include more species and reactions, but neglect feedback to plasma

15. Different models have different assumptions • O2 scale height near the surface—what is the relationship between neutral density and plasma density? • Electron temperature near the surface: • 0.5 eV to 20 eV have been used in models Example: rate coefficient for electron-impact process (1356 Å emission from O2) Note: Rate (s-1) = (rate coefficient)x(electron density) Rate Coefficient (cm3s-1)

16. Io’s interaction HST ~1012 watts of mostly UV light Jupiter’s rotating magnetic field Conduction provided by plasma Jupiter Io Cassini From Russell et al., 2000 “There is no such thing as a unipolar inductor”

17. The time-variable Na torus/atmosphere Dawn on Leading: Na desorbs Night on leading: Na accumulates Orbit Plasma ejects Na from trailing hem. Leading hem. Sunlight

18. In-situ Detection of Trace Species Model ionosphere: Placed trace species in surface to see what would be detectable. Ion detection: A mass spectrometer in orbit could detect as few as 1x10-3 ions/cm3. Neutral detection: Na could be easily detected. Johnson et al., 1998