Searching for N 2 And Ammonia In Saturn's Inner Magnetosphere

1. Searching for N2 And Ammonia In Saturn's Inner Magnetosphere Polar Gateways Arctic Circle Sunrise 2008 29 January 2008 H. Todd Smith (Johns Hopkins University Applied Physics Lab) R. E. Johnson, E. C. Sittler, M. Shappirio, D. Reisenfeld, F. J. Crary, D. McComas, and D. Young

2. Research introduction • Research motivation: Presence/absence of nitrogen in Saturn’s magnetosphere provides valuable clues about satellite compositions and planetary/magnetopsheric processes • Locally formed N+ present in Saturn’s inner magnetosphere: Enceladus source? • Source characterization • Enceladus vs. Titan • Could N2 from Enceladus, seen by INMS, be the parent molecule? Is ammonia a possible source?

3. ~9.5 AU (~1.4 Billion km) from Sun 1 Saturn radius ~ 60,268 km BackgroundThe Saturnian system

4. BackgroundSaturn’s magnetosphere(based pre-Cassini results) • Neutral particle density > ion density (reverse of Jupiter)

5. BackgroundPrior knowledge about nitrogen • Before 2004, information about Saturn limited to Earth based, Pioneer 11 and Voyagers 1 & 2 observations • Voyagers 1 & 2 detected “heavy ions” (14-16 amu but nitrogen not confirmed) • Titan atmosphere mostly nitrogen

6. Predicted nitrogen source - Titan - Dense atmosphere (~95% Nitrogen) - Larger than Mercury - No intrinsic magnetic field Anticipated nitrogen source

7. Nitrogen predictions • Neutral densities too low for direct detection • Titan could produce N+ in inner magentosphere (6-10 Rs) • N2 shows same basic trend but with lower densities * Smith, H. T.; Johnson, R. E.; Shematovich, V. I.Titan's atomic and molecular nitrogen tori. Geophys. Res. Lett., Vol. 31, No. 16, L16804

8. Nitrogen Tori N N2 Prediction summary • Titan can generate nitrogen ions in inner magnetosphere (makes source identification more difficult) • Could nitrogen from Titan interact with icy satellites? • Could nitrogen exist on icy satellites (primordial or implanted)? N+/N2+

9. Detections • After July 1st, 2004 the fun began… We began searching for nitrogen using the Cassini Plasma Spectrometer (CAPS)

10. Cassini Plasma Science Instrument (CAPS) Electron Spectrometer Ion Mass Spectrometer Ion Beam Spectrometer Cassini Plasma Spectrometer

11. We Detected N+! • N+ detected on all orbits • Analyzed data from 22 orbits • July 2004 – October 2006 • Separated all data spatial and by energy distributions

12. Locations of N+ detections using Cassini Plasma Spectrometer (CAPS) Orbit Trajectories • Detected in inner magnetosphere (not near Titan) • Energy distribution indicates local ion source • Spatial distribution similar to OH torus • Highest N+ densities correspond to Enceladus orbital Shell • N+ enhanced in Enceladus’ orbit and during encounter

13. N+/W+ during Enceladus encounter (July 14, 2005) • N+ enhanced relative to W+ at both orbit crossings • Narrow Enceladus torus (Johnson et al. 2006)

14. Things are not as expected Dominant nitrogen source in vicinity of Enceladus orbit Credit: NASA/JPL/Space Science Institute - Mainly H2O ice - Geologically young surface - New images indicate source of E-ring

15. Enceladus observations concur Credit: NASA/JPL/Space Science Institute Credit: NASA/JPL/Space Science Institute • Enceladus “plumes” detected • Tiger stripes – south pole • Possible nitrogen source • Principal source of E-ring • Subsurface composition questions • Cassini Ion Neutral Mass Spectrometer (mass 28 detection ~4%) • Requires further investigation Credit: NASA/JPL/Space Science Institute

16. Testing N+ (CAPS) vs. Possible N2 Enceladus Source (INMS) (using computational models) CAPS N+ data • N2 Enceladus source (if present) could produce observed N+

17. The new nitrogen picture N Dominant nitrogen source N2 • Indirect evidence - possible Enceladus-generated neutral nitrogen torus Possible nitrogen source Nitrogen tori

18. Attempt to confirm nitrogen source(using CAPS data) • Search for N2+ • Could INMS mass 28 be N2 at Enceladus? • If present, no other nitrogen source species required to explain N+ detections • Is there any ammonia present? (implications for subsurface H2O melting temperature)

19. Search for N2+ in CAPS • N2+ difficult to detect in small amounts using CAPS • CAPS data supports mass 28 detection • Small amounts of N2+ possible • CO2+, CO+and C+ also possible CAPS ST Detector CAPS LEF Detector Possible N2+ detections

20. Search for N2+ in CAPS(continued) • Upper limit placed on N2+ • Note upper limit at 4.5-5.0 Rs similar to INMS (~4%)

21. Search for NHx+ in CAPS • Also very difficult to positively identify ammonia products in CAPS spectra • Developed technique of analyzing N+ spectral peak width • Atomic peaks narrower than molecular peaks CAPS calibration data

22. Search for NHx+ in CAPS (continued) • Trend toward atomic nitrogen as move further away from Saturn • Eliminated other species and instrument effects as the cause • Indicates Ammonia products (NHx+, x=1-4) with abundance diminishing with distance from Enceladus orbit • Examined CAPS data statistically by radial distance from Saturn to examine average N+ peak FWHM (W+ = water-group ions)

23. Search for NHx+ in CAPS (continued) • Upper limits of ammonia products in data closest to Enceladus’ orbit

24. Conclusions • Detected N+ in inner magnetosphere from Enceladus (Smith et al. 2005, 2007) • Modeled Enceladus vs. Titan nitrogen sources • N2 from Enceladus and N, N2 from Titan • No need for other parent nitrogen molecules • Searched for nitrogen source in CAPS data • Data indicates presence of ammonia products • Data best fit with N2+ (upper limits <4%) but identification less conclusive

25. It appears that David is beating Goliath (with nitrogen)