Searching for N 2 And Ammonia In Saturn's Inner Magnetosphere Polar Gateways Arctic Circle Sunrise 2008 Polar Gateways Arctic Circle Sunrise 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

H. Todd Smith Research introduction  Research motivation:  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 N 2 from Enceladus, seen by INMS, be the parent molecule? Is ammonia a possible source?

H. Todd Smith Background The Saturnian system ~9.5 AU (~1.4 Billion km) from Sun 1 Saturn radius ~ 60,268 km

H. Todd Smith Background Saturn’s magnetosphere (based pre-Cassini results)  Neutral particle density > ion density (reverse of Jupiter)

H. Todd Smith  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 Background Prior knowledge about nitrogen

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

H. Todd Smith  Neutral densities too low for direct detection  Titan could produce N + in inner magentosphere (6-10 Rs)  N 2 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 Nitrogen predictions

H. Todd SmithN N2N2N2N2 Nitrogen Tori Prediction summary N + /N 2 +  Titan can generate nitrogen ions in inner magnetosphere (makes source identification more difficult) (makes source identification more difficult)   Could nitrogen from Titan interact with icy satellites?   Could nitrogen exist on icy satellites (primordial or implanted)?

H. Todd Smith Detections  After July 1 st, 2004 the fun began… We began searching for nitrogen using the Cassini Plasma Spectrometer (CAPS)

H. Todd Smith Cassini Plasma Spectrometer Cassini Plasma Science Instrument (CAPS) Electron Spectrometer Ion Mass Spectrometer Ion Beam Spectrometer

H. Todd Smith  N + detected on all orbits  Analyzed data from 22 orbits  July 2004 – October 2006  Separated all data spatial and by energy distributions We Detected N + !

H. Todd Smith 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

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

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

H. Todd Smith Enceladus observations concur   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

H. Todd Smith Testing N + (CAPS) vs. Possible N 2 Enceladus Source (INMS) (using computational models)   N 2 Enceladus source (if present) could produce observed N + CAPS N+ data

H. Todd SmithN N2N2N2N2 Nitrogen tori The new nitrogen picture Possible nitrogen source Dominant nitrogen source   Indirect evidence - possible Enceladus-generated neutral nitrogen torus

H. Todd Smith Attempt to confirm nitrogen source (using CAPS data)   Search for N 2 +   Could INMS mass 28 be N 2 at Enceladus?   If present, no other nitrogen source species required to explain N + detections   Is there any ammonia present? (implications for subsurface H 2 O melting temperature)

H. Todd Smith   N 2 + difficult to detect in small amounts using CAPS   CAPS data supports mass 28 detection   Small amounts of N 2 + possible   CO 2 +, CO+ and C + also possible Search for N 2 + in CAPS Possible N 2 + detections CAPS ST Detector CAPS LEF Detector

H. Todd Smith   Upper limit placed on N 2 +   Note upper limit at Rs similar to INMS (~4%) Search for N 2 + in CAPS (continued)

H. Todd Smith   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 Search for NH x + in CAPS CAPS calibration data

H. Todd Smith   Examined CAPS data statistically by radial distance from Saturn to examine average N + peak FWHM Search for NH x + 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 (NH x +, x=1-4) with abundance diminishing with distance from Enceladus orbit (W + = water-group ions)

H. Todd Smith   Upper limits of ammonia products in data closest to Enceladus’ orbit Search for NH x + in CAPS (continued)

H. Todd Smith Conclusions   Detected N + in inner magnetosphere from Enceladus (Smith et al. 2005, 2007)   Modeled Enceladus vs. Titan nitrogen sources   N 2 from Enceladus and N, N 2 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 N 2 + (upper limits <4%) but identification less conclusive

H. Todd Smith It appears that David is beating Goliath (with nitrogen)