1 Introduction

Saturn's equatorial magnetosphere is dominated at <20 Rs (1 Saturn radius, Rs = 60,286 km) by local origin charged and neutral atoms and molecules. W+, the water group ions (O+, OH+, H 2 O+, and H 3 O+), along with H+ and H 2 +, dominates Saturn's magnetospheric plasma sheet and ring current populations [Krimigis et al., 2005; Sergis et al., 2007; Thomsen et al., 2010]. W+ is currently understood to be mostly derived from one of Saturn's inner moons, Enceladus. Of four minor heavy ion species, C+, N+, O 2 +, and 28M+, the first two atomic and the latter two molecular, the origin of only one, O 2 +, is known. (Please note that we use the textual construct AM+a to denote a currently undetermined ion or set of ion species, M, which has a mass of A amu and a charge state of +a, where a is an integer, unless the ion is singly charged, in which case the charge state value 1 is typically suppressed.) In the case of ~28 amu, we do not know which of several molecular ion species dominates the 28M+ observations [see Christon et al., 2013, 2014a, referred to hereafter as papers 1 and 2, respectively]. In the case of ~56 amu, we are confident that 56M+ is Fe+, the atomic ion at that mass, but in the course of this paper will use 56M+ prior to and Fe+ after we present our identification of Fe+ as the species we observe. The dominant O 2 + population originates at thermal energies from photolysis of ring ice, has intensity that varies seasonally, spreads to the outer magnetosphere, and is subsequently accelerated to hundred keV energies [see Johnson et al., 2006 and paper 1, and references therein]. The minor ions are presumed to originate locally and have abundances relative to W+ of roughly ~10−2 [see DiFabio, 2012 and papers 1 and 2]. Saturn's suprathermal ion partial number densities (PND) typically peak radially at ~8–9 Rs, in the ring current region [Sergis et al., 2007; DiFabio, 2012; paper 2]. The O 2 + and 28M+ PNDs vary seasonally, having apparent maxima near solstices and observed minima around equinoxes (see papers 1 and 2). W+, C+, and N+ variations are described elsewhere [see Mauk et al., 2009; DiFabio, 2012]. 56M+ can then be thought of as an additional tool for investigating and understanding ion sources, transport, and acceleration in planetary magnetospheres. As well as at Saturn, we have now observed suprathermal 56M+ at Earth at radial distances from ~9 to ~30 Earth radii, RE, [see Christon et al., 2014b]. While high charge state solar wind Fe+8:+14 has long been known to be present in the near‐Earth magnetosphere [Christon et al., 1994], long‐term collections now also reveal 56M+ near Earth with no obvious charge exchange products between the solar wind Fe+8:+14 and 56M+. (Please note that we use the textual construct +a:+b to denote the range of charge states from +a to +b.) Below, we describe the relevant observations at Saturn, in particular, and of iron in planetary ionospheric and magnetospheric environs in general.

Interplanetary dust particles (IDPs, in which we include meteoroids, which are defined as particles larger than 5 µm in radius) [Rubin and Grossman, 2010], originating from comets and asteroids, impact all planets including Saturn and its rings. For example, ~16–60 tons of IDPs fall into Earth's atmosphere every day [Plane, 2012; Gardner et al., 2014]. For simplicity, we will refer to all nonaqueous, infalling interplanetary material collectively as cosmic material in order to distinguish it from “ice dust,” the micron‐ to nanometer‐sized water ice grains that are present throughout Saturn's magnetosphere and in high‐altitude noctilucent clouds at Earth. Because many types of cosmic material have a probability of containing some Fe, cosmic material is likely the original source of the Fe from which the Fe+ at Saturn derives. (Note also that higher‐energy charged particles, galactic cosmic rays or GCRs, also contain Fe and also impact Saturn's atmosphere and/or its rings producing secondary “splash” particles) [Chenette et al. 1980]. Higher‐energy charged particles such as these are addressed below near the end of this section. Cosmic material impacting Saturn and Titan likely forms thin metallic layers in their ionospheres similar to those found at Earth, Venus, and Mars [Withers, 2012] and expected to be present at planets and moons with atmospheres [Moses and Bass, 2000; Molina‐Cuberos et al., 2001, 2008]. We note here that our focus in this paper is on Saturn, although Titan is also a likely source of meteoric metal ions [Molina‐Cuberos et al., 2001]. However, we do not consider Titan as important Fe+ source as Saturn because of Saturn's much greater size (~550 times the surface area). At 80–90 km altitudes in Earth's ionosphere, near the mesosphere‐ionosphere boundary, thin layers of ionized and neutral meteoric metals, including Mg, Fe, Na, Ca, and K, are produced by meteoric ablation, evaporation from the molten meteoroid [Plane, 2003]. The basis for our discussion of the ionospheric Fe+ source herein is a model of the formation of a Fe/Fe+ layer at Saturn with which we address nominal thermal, or Jeans, escape of Fe+ from metal layers in Saturn's ionosphere. While the meteoric species are predominantly found in these layers, we are expressly interested here in their ionospheric escape after dispersal from these layers. Fe+ at Earth, for example, has been detected at altitudes far from the metal layers, up to altitudes of ~1000 km near the equator and at varying concentrations from low to high altitudes at different local times and latitudes [Grebowsky and Aiken, 2002].

Outward flowing ion conics at energies from ~30 keV to ~200 keV accompanied by ~20 keV to ~1 MeV electron beams are measured throughout Saturn's outer magnetosphere [Mitchell et al., 2009a]. They are observed at all local times on field lines that nominally map from well into the polar cap (dipole L > 50) to well into the closed field region (L < 10), strong evidence for ion and electron acceleration in Saturn's auroral zone. Saturn's south aurora sits ~1.5°–2° more equatorward and is wider than the north aurora as a result of the planetary magnetic dipole field's northward offset and has extremely variable boundaries. Two dedicated studies of the south auroral poleward boundary show consistent measures, where the south auroral colatitude (a) poleward (equatorward) edge ranges from 2° to 20° (6° to 23°) with median ~14° colatitude [Badman et al., 2006, 2014] and (b) poleward (equatorward) full width at half maximum ranges from ~7.5° to 16° (16° to 25°) with median ~16° ± 2° [Carbary, 2012, Figure 3]. This allows outflows at times from the southern aurora's equatorward edge at latitudes of ~74°(~65°) access to approximate dipole L locations into ~13.2 (~6.0), where L = cos−2(Λ) and Λ is the invariant latitude of the point where the L shell of a centered dipole intersects the planet (see Figure S14 in the supporting information and Søraas [1973]). The ion conics composition is typically dominated by light ions, with a few associated heavy ions, which flow exclusively outward. Associated electrons, measured by a different instrument, travel either outward, as do the ions, or bidirectionally. In one case, Badman et al. [2012] demonstrated upward bursts of 100–360 keV light ions and 250–600 keV heavy ions, as well as electrons with hundreds of keV energies, that mapped back to high‐latitude intense auroral arcs separated by dark regions poleward of Saturn's northern dayside auroral oval in Saturn's cusp region [Badman, 2012]. Note that in the imaging and neutral camera, INCA, the sensor which measured the outflowing suprathermal ions [Mitchell et al., 2009a; Badman et al., 2012], hydrogen, and oxygen is well separated, but INCA's capability for higher mass discrimination is limited [Mitchell et al., 2009a]; therefore, in discussions of the ion conics and beams, low mass species, H+, H 2 +, H 3 +, and/or (possibly) He+, are collectively referred to as “light” ions, and high mass species are referred to as “heavy” ions. The heavy ions are generically identified as “O+,” but they may include the carbon ions C+, CH 3 + (and other hydrocarbons, see, for example, the Saturn ionospheric chemistry of Moses and Bass [2000]), CO+, CO 2 +, OH+, H 2 O+, and/or Fe+ (and/or other meteoric ions such as Na+, Mg+, and/or Si+).

Outflowing suprathermal ions at Saturn may result from ionospheric processes similar to those observed at Earth, although the characteristic energies reported at Saturn are a factor of ~100 greater than those at Earth. These ion conic observations highlight that, in addition to the process of continual ion escape, there are likely two high‐latitude, planetary magnetic disturbance‐related dynamic escape locations known at Earth, the dayside cleft ion fountain and nightside auroral outflow, which probably have active analogs at Saturn. At Earth, gravitation disperses low‐velocity outflowing ions spatially, but separation of light and heavy escaping ions probably does not occur at Saturn because the escape velocity of Saturn's outflowing ions is about 3 times that at Earth. There are also powerful ionospheric escape events, reported at Earth and referred to as ionospheric mass ejections (IMEs) [Moore et al., 1999], that often contain appreciable amounts of the molecular ions N+2 and NO+ [Wilson and Craven, 1998], molecular ions that we observe in Earth's magnetosphere [Christon et al., 1994, 2014b]. These IMEs are known to be initiated by CMEs that significantly disrupt Earth's magnetosphere [Moore and Khazanov, 2010; Moore et al., 1999]. Below, we will refer to all these ionospheric escape processes collectively as ionospheric outflow because, even at Earth, details of processes in and/or contributions from ionospheric outflow regions are not necessarily well‐enough studied and/or correlated with common indicators of solar or geomagnetic activity. They are still not fully understood [see, e.g., Chandler et al., 1991; Horwitz and Moore, 1997; Moore et al., 2014].

Cosmic material has been observed impacting Saturn's main rings by Cassini [Tiscareno et al., 2013] and, as such, it likely contributes to and may even dominate the impurity composition of the main ring ices. Solstice and equinox temperatures of Saturn's main rings, at ~80–120 K and ~45–70 K, respectively [Flandes et al., 2010], are generally colder than those in Earth's atmosphere at altitudes of ~95 km in the E region where a layer of meteoric Fe+ has long been observed and modeled [Carter and Forbes, 1999; Grebowsky and Aikin, 2002; Feng et al., 2013]. Whether the source region at Saturn is the main rings or its ionosphere, a large fraction of cosmic material contains varying amounts of metallic Fe, iron oxides, and/or minerals containing the elements Fe, Mg, Na, K, C, Ca, and/or Si [Kopp, 1997; Anders and Grevesse, 1989; Jessberger et al., 2001; Plane, 2012]. Iron is a major element in most classes of meteorites [Mason, 1979]. Small IDPs also contain iron, as evidenced directly by Cassini's cosmic dust analyzer, CDA, observations during the cruise to Saturn in which two of the six small IDPs whose composition could be determined were found to be iron rich [Hillier et al., 2007a]. Olivine (which has the collective formula (Mg x ,Fe 1 − x ) 2 SiO 4 , 0 ≤ x ≤ 1), metallic Fe‐Ni, and various iron oxides and Fe‐inclusive minerals are present in cosmic material [Rubin, 1997; Bridges et al., 2010]. Likely sources of Fe and Fe m O n (wüstite, FeO, hematite, Fe 2 O 3 , and/or magnetite, Fe 3 O 4 ) are reduced or oxidized iron‐containing compounds in the cosmic materials in interplanetary space which are believed to be generated in large part by asteroid belt collisions [Grebowski and Aikin, 2002; Grün et al., 2001; Hutchison, 2007; Plane, 2012].

Local origin charged and neutral ice particles, gas clouds, and atomic and molecular heavy ions play active roles in Saturn's magnetosphere. Early in the Cassini mission it was noted that, while the ultraviolet imaging spectrograph measurement of the rings' reflectivity showed the main rings becoming brighter from the C ring to the A ring, the visual and infrared mapping spectrometer (VIMS) instrument data showed the outer A ring to be richer in the “mystery” iron‐based silicate dark material [Miner et al., 2007]. Jaumann et al. [2009] noted that the dark material appears throughout Saturn's system, most predominantly on Dione (~6.3 Rs), Rhea (~8.7 Rs), Hyperion (~24.5 Rs), Iapetus (~59 Rs), and Phoebe (~215 Rs). Additionally, Cassini discovered plumes of water vapor and icy material being ejected from large fissures at the south pole of the moon Enceladus. Enceladus' orbit is at ~3.95 Rs, and its ejecta contribute over time to form Saturn's E ring and OH torus. Long‐term CDA measurements have detected E ring particles far beyond the classical, optically determined 3–8 Rs equatorial E ring, that is, ~2–4 Rs away from the ring plane, outward to ~20 Rs, and inward almost to the outer edge of the A ring [see, e.g., Srama et al., 2011, Figure 10]. Icy matter between the F ring (~2.33 Rs) and the outer edge of the A ring (~2.27 Rs) extinguishes the radiation belts [Kollmann et al., 2011]. The Enceladus neutral cloud torus, fed by the moon's plumes and centered on its orbit, is the densest part of the E ring [Cassidy and Johnson, 2010] and is the second largest structure in the Saturn system next to the 40 Rs thick Phoebe ring at ~128–207 Rs [Verbischer et al., 2009].

Some matter from the E ring and the Phoebe ring appears to have common characteristics with ice contaminants on or in other objects, rings and moons, in the Saturn system. Micron‐sized and smaller E ring particles from Enceladus (also called grains and dust) are mostly pure water ice, but some, a few percent, have small amounts of contaminants such as silicates, CO 2 , NH 4 , N 2 , and hydrocarbons [Hillier et al., 2007b; Postberg et al., 2008; 2011]. CDA data show that ~98% of all resolved E ring icy particle mass spectra are dominated by water and water cluster ions, H–(H 2 O) n +, and another <1% of their observations represent meteoric metals [Postberg et al., 2008]. Postberg et al. [2008] note that with water clusters dominating most E ring spectra, the CDA cannot detect minor contributions of Fe+ in Saturn's environs as a result of a prominent water cluster, H‐(H 2 O) 3 +, at ~55 amu. CDA also observes various H+‐ and Na+‐water cluster peaks at ~59, ~63, ~77, and ~77 amu, some with amplitudes comparable to or greater than the peak at ~55 amu [Postberg et al., 2008]. Waite et al. [2006, 2009] identify neutral species count distributions they observe in the ~50–60 amu range as C 4 hydrocarbons. To our knowledge, there are no other reports of species in the ~50–60 amu range. If there is any Fe and/or Fe+ in the Enceladus plumes possibly hidden by these water clusters and/or hydrocarbons, it will likely experience the same fate as the other Enceladus materials we discuss below. From their model of Enceladus origin water‐based ions and neutrals in the inner magnetosphere, Jurac and Richardson [2007] estimated that ~17% of the dense neutral water products from Enceladus is deposited mostly onto the A ring, but the atomic and molecular water cloud particles probably reach all the main rings through ion‐neutral scattering and ring particle collisions. Clark et al. [2012] find that dark material from Phoebe's ring has spectral characteristics in common with the contaminants in Saturn's icy main rings and its moons, suggesting that some of Phoebe's dark material may reach the main rings, in particular, over time.

Saturn's main rings are primarily composed of constantly interacting water ice bodies of various sizes, from nanometer‐ to boulder‐sized objects and constantly changing clumps of the same. The main rings are visually an overall pale tan color with a reddish tint, especially the A and B rings [Estrada and Cuzzi, 1996; Cuzzi et al., 2009]. Ring ice appears to have a small (up to a few percent) contamination by UV‐absorbing nonaqueous material, which includes as a constituent, or is, Saturn's so‐called dark material [Cuzzi et al., 2009; Filacchione et al., 2012; Clark et al., 2012]. Cuzzi et al. [2009] reviewed various ring composition studies, noted that the ring contaminants have been ascribed to both organic and inorganic materials, discussed the pros and cons of the various source options, and demonstrated that a compositional interpretation for the UV absorber including nanohematite explains the observations better than an absorber having organics alone. Hematite is the iron ore that gives Mars and red earths their color [Morris et al., 1997]. Filacchione et al. [2012] devoted their study to investigating organic compounds as the prime contaminant sources and found that organics could explain the UV absorber, but they did not compare the pure organic fits to alternate inorganic(hematite)‐inclusive contaminant fits. Clark et al. [2012] found that the dark material appears to have significant components of nanophase metallic iron and nanophase hematite contributing to the observed UV absorption. By modeling both the ice and dark contaminant grain size distributions, Clark et al. [2012] demonstrated that multiple spectral features and the overall spectral shape of the dark material on Phoebe match those seen on the dark side of Iapetus, a few other moons, Saturn's rings Cassini Division, and the F ring, thus implying the material has a common composition throughout the Saturn system. After investigating many substances, Clark et al. [2012] concluded that the dark material could be a mixture of nanophase metallic iron (Fe), nanophase hematite (Fe 2 O 3 ), CO 2 and H 2 O, with possible traces of ammonia (NH 4 ), bound water, H 2 , or OH‐bearing minerals, trace organics, and other as yet unidentified materials. This mixture was found to be a simple and consistent explanation for the dark material in Saturn's environs. Taken together, these studies suggest that the contaminants in Saturn system ice with reddish tints likely have amounts of nanophase metallic iron and nanophase hematite.

Summarizing the current information on Saturn's rings, Cassini VIMS finds visual reddening in the A and B rings much stronger than that of the C and D rings or any of the other icy satellites, except possibly Rhea or Hyperion [Filacchione et al., 2012; Cuzzi et al., 2009]. Clark et al. [2011] find that nanophase hematite and nanophase metallic iron in a mixture with fine‐grained water ice and other materials are the most likely explanation for the spectral structure of the contaminant in the icy surfaces of the Saturn system. Notably, Clark et al. [2012] find that organic compounds, such as tholins, do not match the observed spectra as well as the tested mixture of nanophase iron dominated materials. Because of the abundance of water ice in the Saturn system, oxidation of metallic iron and wüstite, common components in meteorites, might be expected.

Unlike charged and neutral thermal energy atomic and molecular populations whose intensity maximum can often help identify a nearby source location, intensities of suprathermal ions are often determined by magnetospheric dynamics far from their origin. Ionospheric ions can propagate far away from their origin throughout the magnetotail, where they can be accelerated to keV energies in the crosstail current sheet followed, at times, by redistribution to distant locations in minutes or hours. Consequently, suprathermal ions from various sources are often thoroughly mixed by the time they are measured.

Several other possible Fe source candidates are briefly addressed here (an extended discussion and reference list is in section K of the supporting information). As noted above, MeV energy ions and electrons over the main rings are consistent with modeled splash albedo production resulting from bombardment of Saturn's atmosphere and/or its rings by high‐energy GCRs. Some portion of these secondary ions, or even the primary GCRs, might ultimately lose energy through successive collisions and charge exchanges to be observed at low‐charge state and hundred keV energies. A review of the literature (see section K of the supporting information) shows that 56Fe+Q ions are reported in both fast (Q Fe,HSSW = 7–12) and slow (Q Fe,LSSW = 8–14) solar wind, SW, from lower and higher in the Sun's corona, respectively; the corotating interaction regions, CIR, that form between fast and slow solar wind streams; solar energetic particle (SEP) events (Q Fe,SEP = 8–16) from solar flares; coronal mass ejections, CME (Q Fe,CME = 10–22), large‐scale outbursts of solar wind plasma and magnetic fields more massive than individual solar flares; galactic cosmic rays, GCR (Q Fe,GCR = 26), from the interstellar medium; and finally, the low‐charge state anomalous cosmic rays, ACR (Q Fe,ACR ≥ 1), and singly charged inner source pickup ions, ISPUI (Q Fe,ISPUI ≥ 1), from interstellar gas, dust, and grains. GCR Fe ions are likely fully stripped, as are probably all GCR ions. CME Fe ions have the highest charge states of solar origin Fe, 10 ≤ Q Fe,CME ≤ 22. For any of the galactic or solar origin ions, GCR, CME, SEP, CIR, and SW, to charge exchange down to Q Fe,SW = 1, charge exchange products should leave a trail of intermediate charge state Fe ions between their original Q values at 7 ≤ Q Fe ≤ 26 (at 8 to 2.15 amu/e, where we would observe them) to Q Fe = 1 (at 56 amu/e). Solar and/or galactic Fe might, through multiple charge exchange collisions, arrive at lower charge states. In section 3, we demonstrate that Fe at charge states intermediate between those of the solar wind (7 ≤ Q ≤ 14) and Fe+ are not observed, so that high charge state GCR, CME, SEP, CIR, and/or SW ions are not Fe+ sources. Upon discovering ~56 MeV Fe+ at low L values in Earth's trapped radiation belts, Mazur et al. [1999, 2000, 2008] attributed this population to the ISPUIs and not to ACRs. However, Fe+ has not been definitively identified in the ISPUI population, and the trapped Fe+ might have another explanation (originating from the ionosphere in a process similar to the one we suggest for Saturn, for example). We conclude that neither ACR Fe+ nor ISPUI Fe+ seems to be supported by a set of repeatable, persistent observations of the Fe+ component. They are only attributed by possible identifications which are not supported by subsequent studies or independent observations. The interested reader can read our extended discussion, and reference list supporting this conclusion is in section K of the supporting information. Therefore, we conclude that none of the higher‐energy solar and/or extrasolar candidates appear to be viable sources for the Fe+ at Saturn.

Below we investigate the singly charged heavy atomic ion species, 56M+, likely ionized and accelerated in Saturn's magnetosphere. Our study spans ~9.5 years of the Cassini mission. In the following sections, we describe the instrument and trajectory, the observations, and then compare and contrast our current understanding of possible suprathermal Fe+ sources.