Investigation of the atmospheres of Europa, Ganymede, and Callisto with PEP/JUICE

1. Investigation of the atmospheres of Europa, Ganymede, and Callisto with PEP/JUICE Peter Wurz, Marek Tulej, Audrey Vorburger, and Nicolas Thomas Universität Bern, Physikalisches Institut, 3012 Bern, Switzerland (peter.wurz@space.unibe.ch, 41 31 631 44 05) Stas Barabash, Martin Wieser, Swedish Institute of Space Physics, S-981 28 Kiruna, Sweden Helmut Lammer Austrian Academy of Sciences, A-8042 Graz, Austria

2. Jupiter Icy Moons Explorer (JUICE) • The JUpiter ICy moons Explorer (JUICE) will perform detailed investigations of Jupiter and its system in all their inter-relations and complexity with particular emphasis on Ganymede as a planetary body and potential habitat. Investigations of Europa and Callisto would complete a comparative picture of the Galilean moons. • In early May 2012 ESA announced the selection of JUICE as L-Class mission, with launch in June 2022, and 7.6 years cruise to Jupiter. In February 2013 the scientific instruments for JUICE have been selected. • Most important, new light will be shed on the potential for the emergence of life in the galactic neighbourhood and beyond. The overarching theme for EJSM has been formulated as: • What are the conditions for planet formation and emergence of life? • How does the Solar System work? • To understand the Galilean satellites as a system, Europa and Ganymede are singled out for detailed investigation.

3. Particle Environment Package (PEP) • PEP consists of three units with elegant, modular design hosting sensors and electronics, and well-defined, minimal interfaces to the spacecraft • Zenith Unit (IRF, Sweden) • Nadir Unit (UBe, Switzerland) • JENI (APL / USA) • PEP combines remote global imaging with in-situ measurements, obtaining 3D plasma flows in less than 10s, and first-ever gas mass spectroscopy at the icy moons; • PEP uses mutual shielding, single to triple coincidence detection schemes in all sensors for operation in the harsh Jovian environment; • TRL≥6 building on direct flight & team heritage • Galileo, Cassini, Juno, Mars Express (ASPERA-3), Venus Express (ASPERA-4), Rosetta, SOHO, New Horizons, Chandrayaan-1, IMAGE, & RBSP. • PEP team includes world leaders on the outer planets and recognised providers of space hardware. JEI JoEE JDC Zenith Unit Nadir Unit NIM JENI JoEE JEI JDC JNA

4. Scientific Objectives of NIM The science goal of the Neutral gas and Ion Mass (NIM) spectrometer is the determination of the extended atmospheres of Europa,Ganymede and Callisto, in particular the neutral and the ionised component. The main scientific goals are: • The chemical composition of regular atmosphere produced by energetic particle and photon interaction with the surface • The ion composition of the ionosphere • Chemical analysis of geysers (if encountered) and their temporal evolution • Isotopic analysis when signal levels are sufficiently high. • Assist in the identification of the chemical nature various surface elements

5. NIM/PEP: A TOF-MS Instrument Time-of-flight technique All masses are measured simultaneously High dynamic range Mass spectra easy to interpret Robust, simple system Moderate need for resources D. Abplanalp, P. Wurz, et al., Adv. Space Res. 44 (2009) 870–878. 24. September 2014 5

6. Detection level ≈10–16 mbar Residual gas recorded at a total pressure of 5.0·10–10 mbar

7. Prototype Results: Mass Resolution PEP / NIM Krypton at 2·10–9 mbar P. Wurz, D. Abplanalp, M. Tulej, and H. Lammer, Planet. Planet. Sp. Science 74 (2012) 264–269.

8. Europa T.A. Cassidy, et al., Icarus 2009 This image shows the approximate natural colour appearance of Europa, predominantly ice-rich regions of the crust. Europa is about 3160 km in diameter, or about the size of Earth's moon. This image was taken on 7 September 1996, at a range of 677’000 km by the solid state imaging television camera onboard the Galileo spacecraft during its second orbit around Jupiter. Long, dark lines are fractures in the crust, some of which are more than 3’000 km long. The bright feature containing a central dark spot in the lower third of the image is a young impact crater 'Pwyll' some 50 km in diameter. Dark brown areas (ice-poor) represent rocky material derived from the interior, implanted by impact, or from a combination of interior and exterior sources.

9. Surface composition • Bright Areas (ice-rich regions) • H20, CO2, • SO2, Sx, • H2O2, ... • Dark Areas (ice-poor regions) • MgSO4 • xH20 • Na2SO4 • xH20 • Na2CO3 • xH20 • H2SO4 • xH20 • Possible extremophile bacteria • Cyanidium • Deinococcus radiodurans • Sulfolobus shibatae • Escherichia coli The surface composition of the dark areas is not well constrained by infra-red (IR) spectroscopy, even with high spectral resolution. Europa Atmosphere composition

10. Europa Atmosphere Model • NIM flyby operations • Full mass spectra at 5-sec cadence • Detection threshold 30 cm–3, in Europa’s radiation environment • dynamic range of > 105 • All species known in Europa‘s exosphere can be detected by NIM/PEP during the JUICE flybys • O, O2, H2, H2O, Na, SO2, SO, CO2, CO • Expected exospheric species from non-ice surface • Detection if surface concentration is >= 10–3 • Mg, MgO, NaO, Ca, CaO, Al, AlO, ... • Isotopes: • With a threshold of 30 cm–3 and a dynamic range of > 105 the D/H ratios can be resolved in the thermal component of H2 • 18O/16O from the O2 and H2O in the sputtered signal

11. Ganymede This Voyager 2 colour photo of Ganymede, the largest Galilean satellite, was taken on 7 July 1979, from a range of 1.2·106 km. The photo shows a large dark circular feature about 3200 km in diameter with narrow closely-spaced light bands traversing its surface. The bright spots dotting the surface are relatively recent impact craters, while lighter circular areas may be older impact areas. The light branching bands are ridged and grooved terrain first seen on Voyager 1 and are younger than the more heavily cratered dark regions. The nature of the bright region covering the northern part of the dark circular feature is uncertain, but it may be some type of condensate. Most of the features seen on the surface of Ganymede are probably both internal and external responses of the very thick icy layer which comprises the crust of this satellite. M.L. Marconi, Icarus, 2007

12. Ganymede Exosphere • Molecular oxygen (O2): Spencer et al. JGR 1996 • Leading / trailing side differences • Oxygen trapped in surface • Ozone detection: Noll et al. Science 1996 • Ozone gas trapped in ice • Oxygen atoms: Hall et al. ApJ, 1998 • Inferred vertical O2 column densities are in the range (1–10)·1014 cm2 • Localised emission regions near north and south pole • Oxygen atoms: Feldman et al. ApJ, 2000 • Correlation of oxygen emissions with magnetic field topology • Ionosphere: Eviatar et al. PSS, 2001 • Bound ionosphere, mostly molecular oxygen • Corona of hot oxygen atoms • Exosphere modelling: M.L. Marconi, Icarus, 2007 • H2O, O2, H2 • Surface densities up to 109 cm–2 (~ 10–7 mbar)

13. Callisto Liang, M.-C., B. F. Lane, R. T. Pappalardo, M. Allen, and Y. L. Yung (2005), Atmosphere of Callisto, J. Geophys. Res., 110, E02003, doi:10.1029/2004JE002322.

14. Radiation Environment • Coaxial closed cylinder shells • Three variable shielding layers • One fixed inner layer to represent the detector housing (1.5 mm of Titanium) • MCP as disc target

15. Radiation optimisation of NIM detector shield

16. NIM S/N Estimate at Europa • Assuming maximum high-energy flux of penetrating particles (electrons) of 3·10+8 e– / (cm2 s1). • No shielding of detector: • 500 background counts in each 0.5-ns bin of the TOF spectrum accumulated for 5 seconds. • This limits the dynamic range to about 3 decades • Reduces the life-time of the MCPs • Graded-Z shielding (Al/Ta) • Reduces the penetrating electron flux to 4·104 e–/(cm2 s) • In addition, secondary γ-radiation of 5·105γ/(cm2 s). • Shielding improves the dynamic range to >5 decades

17. Summary • The atmospheres of Europa, Ganymede and Callisto are largely unknown • Result of evaporation / sublimation and exogenic processes (sputtering) • Atmospheric species are directly related to the surface • With NIM / PEP we will characterise these atmospheres • Chemical composition • Contribution from non-ice material on the surface • Isotopic composition of major species