Electrons at Saturn’s moons: selected CAPS-ELS results A.J. Coates 1,2. G.H. Jones 1,2, C.S.Arridge 1,2, A. Wellbrock 1,2, G.R. Lewis 1,2, D.T. Young 3, F.J. Crary 3, J.H. Waite Jr. 3, R.E. Johnson 4, T.W. Hill 5 1. MSSL-UCL, UK 2. Centre for Planetary Sciences at UCL/Birkbeck, UK 3. SwRI, USA 4. University of Virginia, USA 5. Rice University, USA

CAPS instrument SensorMeasuresEnergy range (eV/q) Energy resolution (  E/E, %) Angle range (  ) Angle bin (  ) Ion mass spectrometer (IMS) Ion mass, energy and direction 1-50, x820x8 Ion beam spectrometer (IBS) Narrow ion beams; energy and direction 1-50, x1.41.5x1.4 Electron spectrometer (ELS) Electron energy and direction , x520x5 Three sensors + DPU, actuatorThree sensors + DPU, actuator Young et al., 2004Young et al., 2004 ELS: Coates et al., 1992, Linder et al., 1998, Lewis et al., 2008, 2010ELS: Coates et al., 1992, Linder et al., 1998, Lewis et al., 2008, 2010

Titan - Ionospheric plasma in the tail Ionospheric photoelectrons e i e T9 encounter Interval 1 – ionospheric photoelectrons at R T in tail: magnetic connection to sunlit ionosphere; plasma escape Interval 2 – ionospheric & magnetospheric plasma; light ions Role of ambipolar electric field in escape – similar to Earth’s polar wind – & lower mass from higher altitude Coates et al, GRL 2007a, PhTrRS A 2009, PSS in press 2011, Wellbrock et al in prep; Wei et al., GRL 2007 Seen on other encounters e.g. T15 (Wellbrock et al., paper in prep), T55-59 (Edberg et al., PSS 2011) Process seen at Mars, Venus; Frahm et al, 06,07, Coates et al, 2008, 2011 (PSS in press)

T75 – tail encounter (c.f. T9) e i Intermittent spectra showing photoelectron peak in tail regions 1 & 2 – plasma from dayside ionosphere MAG data consistent – Wei et al 2011

T63 – tail encounter e i Intermittent spectra showing photoelectron peak in tail regions 1 & 2 – plasma from dayside ionosphere 1 2 MAG data consistent – Wei et al 2011

6 Negative ions in Titan’s ionosphere - Unexpected! - Ram direction - Near closest approach Use ram velocity as a mass spectrometer m amu ~5.32E eV Confirmed in further low altitude encounters... T16 T17 T18 T19 Originally seen on TA in 2004… Coates et al, 2007b, GRL

7 Data from CAPS electron spectrometer (ELS) at Titan  E/E=16.7%, scanned through ram direction Always see negative ion population < 1400km Maximum mass 13,800 amu/q on this encounter (T16) Coates et al., Faraday Discussions, 2010

Heavy neutrals and positive ions: Waite et al, Science 2007, Crary et al. PSS 2009) Coates et al, GRL 2007b, PSS 2009, PhTrRSA 2009, Farad. Disc Sittler et al PSS 2009, Michael et al PSS 2011 Also Wellbrock et al talk 8 Waite et al., 2007 Aerosols (UVIS, solar occultation ) Liang et al, 2007 Titan’s ionosphere: hydrocarbon & nitrile rich Source of Titan’s haze - tholins Unexpected Negative ions: In ram direction at <1400km Heaviest (up to 13,800 amu/q) at lowest altitudes

9 First chemical model including negative ions (low mass), c.f. ELS data at 1015 km (T40) (Vuitton et al., PSS 2009) Hypothesized PAHs (Waite et al., 2007, Coates et al., 2007) & fullerenes (Sittler et al., 2010) Negative ions confirmed by RPWS-LP (Wahlund et al, 2010) Work continuing on composition (Vuitton et al), agglomeration (Lavvas et al), tholins in lab (Horst et al) – Titan meeting, St Jacut, mass analysis (Wellbrock et al) Production processes: Several considered and rates estimated Mainly dissociative electron attachment Loss processes: Several considered and rates estimated Mainly associative detachment Some photodetachment

10 E3 Negative ions in the plume Charged nanograins Jones et al., GRL, 2009 Coates et al., Icarus 2010, Faraday Discussions 2010 Unexpected discoveries from CAPS-ELS at Enceladus See also Hill et al talk, Kanani et al poster, Tokar et al 2009 GRL, Gurnett et al GRL 2011

Rhea’s O 2 and CO 2 atmosphere – from INMS and CAPS Teolis, B.D., et al., Science, 2010 In-situ neutral atmosphere measurements (INMS) Negative and positive ions picked up from atmosphere pinpoint near-surface source (CAPS) Also surface charging at Rhea (Jones et al. talk) and Hyperion (Nordheim et al MAPS meeting)

Enceladus’ auroral spot Pryor, Rymer et al., Nature 2011 Variability a monitor of emission Field aligned electrons a key observation

Ganymede and Europa: JUICE Weak, O 2 /H 2 O atmospheres Ganymede magnetic field Ionospheres present Need to measure: Upstream plasma conditions key for interaction – e.g. cold, hot electrons Photoelectrons to trace magnetic field to ionospheres Search for suspected negative ions at Europa, (e.g. Cl -, Vollwerk et al, 2001), also Ganymede 13 Johnson et al 2003 Khurana et al 1996

14 Atmosphere?Ionosphere?Magnetic fieldInteraction TitanDense, N 2 /CH 4 YesNoSubsonic EnceladusPlume, H 2 O products PlumeNoSubsonic Main ringsWeak, O 2 WeakNoSubsonic RheaWeak, O 2 and CO 2 WeakNoSubsonic GanymedeWeak, O 2, H 2 OWeakerYesSubsonic EuropaWeak, O 2, H 2 OWeakNoSubsonic

Conclusions Good electron measurements important at outer planet moons –Plasma environment –Ionisation effects –Photoelectrons – trace field connection –Surface charging –In-situ measurements of fleld aligned flux, Alfven wing structure – producing auroral spot Additional species in electron data –Negative ion measurements important also – composition, pickup ions –Charged nanograins 15