THE SEARCH FOR NITROGEN IN SATURN S MAGNETOSPHERE. Author: H. Todd Smith, University of Virginia Advisor: Robert E. Johnson University of Virginia

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1 THE SEARCH FOR NITROGEN IN SATURN S MAGNETOSPHERE Author: H. Todd Smith, University of Virginia Advisor: Robert E. Johnson University of Virginia Abstract We have discovered N + in Saturn s inner magnetosphere by using a combination of modeling and analysis of Cassini Plasma Science (CAPS) instrument data. The presence of N + in Saturn s magnetosphere has been a source of much debate since Voyager s detection of unresolved mass/charge amu ions in this region. Two principal nitrogen sources have been suggested: Titan s atmosphere and nitrogen compounds trapped in Saturn s icy satellite surfaces (Sittler et al 2004a, b). The latter may contain primordial nitrogen, likely as NH 3 in ice (Stevenson 1982; Squyers et al. 1983) or N + that has been implanted in the surface (Delitsky and Lane 2002). Here I present the initial nitrogen cloud modeling generated from Titan s atmosphere as well as our detection of N + in Saturn s magnetosphere in the range L~3.5 to ~9.5 using data collected by the CAPS during Saturn Orbit Insertion and the following orbit (Rev A). In addition to our nitrogen detection results, I present an initial examination of possible sources of these ions. Introduction Beyond Saturn s five inner icy satellites (Mimas, Enceladus, Tethys, Dione and Rhea) lies Titan, its largest satellite (Figure 1). A common feature of the giant planets in the outer solar system is the presence of toroidal clouds of neutrals and ions that reside within the planet s magnetosphere. This is material that is ejected from the planet s moons or ring particles and orbits within its magnetosphere. Once ionized, this ejected material is picked up and trapped in the planet s magnetic field where it resides until it is lost by a plasma process. Until recently, our analysis of this system relied on limited data gathered from terrestrial and Hubble Space Telescope observations and from three spacecraft (Pioneer 11 and Voyager 1 & 2) that passed through Saturn s magnetosphere. These data indicated both thermal and energetic plasmas composed of a light ion component (protons) and a heavier ion component. However, the earlier instruments were not able to determine if the heavy ions were oxygen and/or nitrogen. The arrival of the Cassini mission at Saturn on 1 July, 2004 is rapidly increasing our data on this region We initiated our research with modeling of Titan as a likely source of nitrogen in Saturn s magnetosphere. Nitrogen ions from Titan can diffuse inward become energized and can be implanted in and sputter these moons (Sittler et al. 2004), ultimately driving nitrogen chemistry (Delitsky and Lane 2002). After Cassini s arrival at Saturn, we then shifted our research focus to searching for indications of N + in the data. Here, we present the post-cassini arrival modeling results as well as the N + detections. Initial Neutral Cloud Modeling In preparation for Cassini s arrival at Saturn, we constructed a model to generate the topography of the neutral particle distributions and the source of nitrogen ions in Saturn s magnetosphere as shown in Fig 2a (Smith et al., 2004). A 3D Monte-Carlo Particle Tracking Model was used which accounted for electron & photo- ionization, electron & photo-dissociation charge exchange based on plasma parameters derived from Voyager data (Richardson and Sittler 1990). The initial Titan nitrogen source for this model is ~5 x per second (Michael et. al., Shematovich et. al.). Satellite gravitational effects (Titan, in particular) cause the nitrogen cloud to extend toward the inner magnetosphere. Fig.2b gives the source rate of fresh nitrogen ions for the cloud in Fig. 2a. Because nitrogen is ionized faster in the Smith 1

2 Figure 1. Saturnian system Figure 2. Titan generated Nitrogen Cloud Modeling Smith 2

3 inner magnetosphere the N + source rate peaks in this region despite the small neutral particle density. For the entire region, the N + source is 1.3 x per second which is several orders of magnitude smaller than the entire H 2 O source (~10 28 per second) estimated by Jurac et. al. (2004). Additionally, they estimates an icy satellite H 2 O source of ~0.9 x /s. If one assumes 3% of N for this source, then a potential satellite N source starts to compete with a Titan generated source. Nitrogen Detection Data collected by CAPS (IMS) just prior to Saturn Orbit Insertion (SOI) (30 June, 2004, 18:00 to 24:00 SCET) when Cassini was in Saturn s inner magnetosphere were analyzed. Figure 3 shows the LEF SOI ion counts vs. Time of Flight (TOF) channel (higher masses generally have longer TOFs) integrated over the entire 6 hour period. Each series represents a different energy band. The inset in figure 3 expands the region where nitrogen is expected with the red line showing the estimated N + peak (channel 258 based on computations & prototype calibration data). The initial data showed a peak in the spectrum where we expected to see N +. Figure 4 shows the data when only channel 258 is examined for energies below 1 KeV as a function of SOI time. This figure illustrates how the energy distribution of this shifts to lower energies as Cassini moves closer to Saturn consistent with a local pickup source. In figure 5 (Smith et al. 2005), we show this data for ions with energies around 333 ev because they produce the largest number of counts during the 6-hour period. This figure shows a reduced spectrum integrated over 6 hours in the vicinity of the peak in nitrogen flux at 333 ev. The dotted line shows a model fit to the spectrum with the N + peak on the left and the water group ion (referred to as W + representing the sum of O +, OH +, H 2 O + and H 3 O + ) peak on the right. We also examined all other species in the calibration data (at 375 ev) that could produce a peak in the vicinity of N +. Specifically, we considered N 2 +, CH 4 +, O 2 + and CO + however all of these species require a peak to the left of N + that is not present in our spectra. We also detected N + during the next orbit (Rev A) around Saturn when the spacecraft returned to the inner magnetosphere. We examined IMS data collected from 12:00 until 24:00 UTC on 28 October 2004 (DOY 302), covering the out-bound trajectory from ~6.2 to ~9.5 Rs from Saturn. We integrate counts at the peak in the energy spectrum (~333 ev) over the 12-hour period. Figure 6 (Smith et al. 2005) shows a model fit to the spectra. All other species in the calibration data at 375 ev in the vicinity of the N + peak again do not appear present. These results indicate the presence of N + on two passes through Saturn s inner magnetosphere. Smith 3

4 Figure 3. SOI six hour Spectrum Figure 4. SOI ion counts by energy and time Smith 4

5 Figure 5. SOI six hour reduced spectrum with ion fits Figure 6. Rev A twelve hour reduced spectrum with ion fits Nitrogen Detection Figure 7 (Smith et al. 2005) shows the average ion energies (10 minute integration intervals) for the estimated nitrogen peak for the SOI and Rev A passes. The average energies decrease as the spacecraft moves closer to Saturn and these energies appear lower that anticipated for an N+ source that is ionized near Titan. Figure 8 (Smith et al. 2005) shows the average ion count rates (10 minute integration intervals) for the estimated nitrogen peak for the SOI and Rev A passes. The lower portion of figure 8 shows the vertical height of Cassini relative to the ring plane with the icy satellite orbital shells identified as well. Notice these count rates counts increase as the spacecraft is closer to Saturn which may be indicative of icy satellite nitrogen sources vs. the originally postulated Titan source. Smith 5

6 Figure 7. SOI and Rev A average nitrogen ion energies Figure 8. SOI and Rev A average nitrogen ion count rates Smith 6

7 Summary & Conclusions The CAPS data clearly indicates the presence of nitrogen ions in the inner magnetosphere, and the low energies indicate that they are locally formed. Since the count rate increases near the icy satellite orbits, we have concluded that the inner icy satellites, and not Titan, are the nitrogen sources. The lack of identification to date of other nitrogen containing ions that must also be present (e.g., NH x +, NO +, etc.), means we can yet fully rule out that the nitrogen is from Titan and is locally ionized (Smith et al 2004). However, the latter source should appear strongly peaked between about 6-11 Rs, whereas the signal detected here appears to increase with decreasing distance from Saturn with the largest values close to the orbit of Enceladus, strongly suggesting an icy satellite source. Therefore, this is the first indication that a nitrogen containing species is present in the surfaces of the icy satellites. We will use the data from future passes to confirm this conclusion and to look for the related nitrogen species. In this way we hope to obtain a better understanding of the role of nitrogen in Saturn s magnetosphere and the possible consequences for satellite surface compositions. Acknowledgements I wish to recognize M. Shappirio, E.C. Sittler, D. Reisenfeld, R.E. Johnson, R.A. Baragiola, F. J. Crary, D.J. McComas, V. Shematovich, D. T. Young and the rest of the CAPS team for their contributions to this research. This work is supported by the Virginia Space Grant Consortium Graduate Research Fellowship, NASA Planetary Atmospheres, NASA Graduate Student Research, and CAPS Cassini instrument team programs. References Delitsky, M. L. and A. L. Lane (2002), Saturn s inner satellites: Ice chemistry and magnetospheric effects, J. Geophys. Res., 107(E11), Johnson, R. E (1990). Energetic Charged Particle Interaction with Atmospheres and Surfaces, Springer-Verlag, New York. Johnson, R. E., and Sittler, E. C. (1990), Sputter-produced Plasma as a Measure of Satellite Surface Composition: Cassini Mission, Geophys. Res. Letts. 17, Lanzerotti, L. J., Brown, W. L., Marcantonio, K. J., and Johnson, R. E. (1984), Production of Ammonia-Depleted Surface Layers on the Saturnian Satellites by Ion Sputtering, Nature, 139, p Jurac, S., J.D. Richardson (2004). A selfconsistent model of plasma and neutrals at Saturn: Neutral cloud morphology. Submitted for publication in J. Geophys. Res. Krimigis et al. (2005), Dynamics of Saturn's Magnetosphere from MIMI During Cassini's Orbital Insertion, Science, 307, Richardson, J.D., S. Jurac (2004). A selfconsistent model of plasma and neutrals at Saturn: The ion tori. Accepted for publication in Geophys. Res. Lett. Richardson, J.D., and E.C. Sittler, Jr. (1990), A Plasma Density Model for Saturn Based on Voyager Observations, J. Geophys. Res., 95, 12,019-12,031. Shematovich, V.I., R.E. Johnson, M. Micheal, and J.G. Luhmann (2003), Nitrogen loss from Titan, J. Geophys. Res., 108, No. E8, Sittler, et al., (2004a), Pickup ions at Dione and Enceladus: Cassini Plasma Spectrometer simulations, J. Geophys. Res. Vol. 109, A1. Sittler, E.C et al, Energetic Nitrogen Ions within the Inner Magnetosphere of Saturn, J. Geophys. Res. Submitted, 2004b. Sittler et al, Preliminary Results on Saturn s Inner Plasmasphere as Observed by Cassini: Comparison with Voyager, Geophys. Res. Lett. Submitted Smith, H.T., R.E. Johnson, and V.I. Shematovich (2004), Titan's Atomic Smith 7

8 and Molecular Nitrogen Tori, Geophys. Res. Lett. 31, 029GL Smith, H.T., et al., Discovery of Nitrogen in Saturn s Inner Magnetosphere, Geophys. Res. Lett Submitted Squyres, S., Reynolds, R., Cassen, P. (1983), The evolution of Enceladus. Icarus 53, Stevenson, D.J. (1982), Volcanism and igneous processes in small icy satellites, Nature, 298,142. Young, D.T. et al (2004), Cassini Plasma Spectrometer Investigation, Space Sci. Rev. 114, Young, D.T. et al (2005), Composition and Dynamics of Plasma in Saturn s Magnetosphere, Science, 307, Smith 8