Discriminating contamination from particle components in spectra of Cassini's dust detector CDA

ARTICLE IN PRESS Planetary and Space Science 57 (2009) 1359–1374

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Discriminating contamination from particle components in spectra of Cassini’s dust detector CDA F. Postberg a,b,, S. Kempf b,c, D. Rost d, T. Stephan e, R. Srama b, M. Trieloff a, A. Mocker b, M. Goerlich f a

Institut f¨ ur Geowissenschaften, Universit¨ at Heidelberg, Im Neuenheimer Feld 236, D–69120 Heidelberg, Germany Max-Planck-Institut f¨ ur Kernphysik, Saupfercheckweg 1, D–69117 Heidelberg, Germany c Institut f¨ ur Geophysik und extraterrestrische Physik, Universit¨ at Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany d School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Williamson Building, Oxford Road, M139PL, UK e Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA f Deutsches Zentrum f¨ ur Luft- und Raumfahrt e.V., Berlin-Adlershof, Rutherfordstraße 2, 12489 Berlin, Germany b

a r t i c l e in fo

abstract

Article history: Received 9 December 2008 Received in revised form 15 June 2009 Accepted 16 June 2009 Available online 15 August 2009

The Chemical Analyser subsystem of the Cosmic Dust Analyser (CDA) aboard the Cassini spacecraft performs in situ measurements of the chemical composition of dust in space. The instrument records time-of-flight mass spectra of cations, extracted from the impact cloud that is created by high-velocity particle impacts onto the detector target. Thus, the spectra not only show signals of particle components but also of ions from the target material and target contamination. The aim of this work is to determine which non-particle ions are to be expected in the spectra obtained in space operation at Saturn. We present an analysis of the contamination state of the instrument’s impact target. Beside investigations of the purity of the rhodium target surface, spectra from CDA calibration experiments at the dust accelerator facility are evaluated with regard to contamination signatures. Furthermore, contamination mass lines in spectra obtained by impacts of Jovian and Saturnian dust stream particles are analysed. Due to their small size and high speed, stream particle impacts predominantly produce ions from the target material and therefore the spectra are excellent probes of the contamination state of the target operating in space. With the exception of adsorbed hydrogen and carbon, the level of contamination is very low. Implications for CDA spectra of Saturnian E ring particle impacts are derived. The findings confirm the published interpretations. The low level of alkali metal contamination implies a significant sodium contribution in the composition of E ring ice particles. Additionally, ionisation thresholds for the occurrence of contamination mass lines can be utilised to set limits for the impact velocity. & 2009 Elsevier Ltd. All rights reserved.

Keywords: Cassini Saturn Planetary rings Interplanetary dust Enceladus Contamination

1. Introduction Due to the tenuous nature of interstellar and interplanetary dust, it is difficult to investigate by remote sensing. Therefore, in situ measurements carried out by spacecraft are the best way to explore this form of matter, and they deliver insights that are not accessible by other means. The Cosmic Dust Analyser (CDA) aboard the Cassini spacecraft is such an instrument, which provides information about speed, mass, direction, and chemical composition of impacting dust particles (Srama et al., 2004). The subsystem that delivers the chemical information is the so-called Chemical Analyser (CA). Its main data output are time-of-flight

 Corresponding author at: Max-Planck-Institut fur ¨ Kernphysik, Saupfercheckweg 1, D–69117 Heidelberg, Germany. Tel.: +49 6221 516543. E-mail address: [email protected] (F. Postberg).

0032-0633/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2009.06.027

mass spectra of positive ions (cations), extracted from a gas and plasma cloud created by high-velocity particle impacts onto a metal target. Thus, the spectra not only show signals of particle components but also of ions from the target material rhodium þ ðRh Þ and target contamination. These mass lines are, in principle, indistinguishable from those of particle ions. For a proper determination of particle composition, it is therefore crucial to have a good estimate of the identity and abundance of non-particle ions expected with respect to the varying given impact conditions. Figs. 1 and 2 are good examples of the contamination problem. They show representative spectra created by high velocity impacts of the tiny Jovian and Saturnian stream particles ðvi \100 km s1 ; rt20 nmÞ (Zook et al., 1996; Kempf et al. 2005a, b; Postberg et al., 2006). Under these conditions, the proportion of ions generated from the particle is very small and the spectra are dominated by target and contaminant ions. In both spectra the respective signatures represent more than 95% of all

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2. The Chemical Analyser

Fig. 1. Thirty co-added Jovian stream particle spectra recorded shortly before Cassini’s Jupiter flyby in October 2000. Of the main peaks, only Na, K, and probably O are due to particle constituents.

Fig. 2. Saturnian stream particle spectrum recorded in early 2004 before Cassini’s orbit insertion. Lower panel with logarithmic amplitude. These tiny particles þ ðr5 nmÞ almost exclusively produce target ðRh Þ and target contamination ions ðHþ ; Cþ Þ.

detected ions. A thorough statistical analysis of hundreds of spectra was ultimately required to identify particle constituents (Kempf et al., 2005b; Postberg et al., 2006). In this work, a thorough investigation of the contamination state of the CA impact target (CAT) is presented. Implications for the interpretation of CDA mass spectra are also derived, which in turn result in a better understanding of the composition of dust encountered by Cassini in the Saturnian system. We use different approaches to get a full picture of the target contamination. After a brief introduction to the instrument (Section 2), we will discuss the initial chemical purity of the target plate (Section 3.1), followed by a TOF-SIMS (time-of-flight secondary ion mass spectrometry) analysis of possible contamination of the target’s surface (Section 3.2). In Section 3.3, mass spectra of CDA calibration experiments obtained in a dust accelerator facility are evaluated. For the latter two sections, more detailed results are provided in respective appendices. CDA spectra obtained in space are analysed with respect to contamination signatures in Section 3.4. In the discussion chapter, we compare the findings from the previous sections and derive a plausible contamination state of the CDA target in Section 4.1. From that, a conclusive picture is drawn concerning the interpretation of E ring ice grain spectra obtained by CDA (Section 4.2).

The CDA consists of two independent instruments (Fig. 3): the Dust Analyser (DA) and the High Rate Detector (HRD). The HRD was designed to monitor high impact rates (up to 10 000 s1 ) in dust rich environments such as Saturn’s ring plane. It does not produce any compositional information and will not be discussed further here. The DA has three different subsystems: the entrance grid (QP detector), sensitive to the charge carried by the particle (Auer et al., 2002), the classical Impact Ionisation Detector (IID), ¨ et al., 1992a, b), and a similar to Galileo-type instruments (Grun time-of-flight (TOF) mass spectrometer, which is referred to as the Chemical Analyser (CA). Depending on the trajectory of the particle, it either hits the central Rh target (Chemical Analyser Target—CAT, diameter: 0.16 m), the surrounding Gold target (IIT, diameter: 0.41 m), the inner wall of the instrument, or one of the grids (Fig. 3). This work only deals with impacts on the CAT, because only these provide mass spectra. The other CDA components are described in greater detail by Srama et al. (2004). If a dust particle impacts onto the CAT with sufficient energy, the particle is partly vaporised and ionised, forming an impact cloud of ions and electrons together with neutral molecules and atoms. Besides particle components, the impact cloud also consists of contaminant material from the CAT’s surface and excavated target material. The instrument separates the charged species, from which the positive component is used to generate a time-of-flight (TOF) spectrum (see Fig. 4 for a more detailed explanation of the instrument operation). The spectra are logarithmically amplified, digitised at 8 bit resolution, and sampled at 100 MHz for a period of 6:4 ms after triggering. As the TOF is proportional to the square root of the mass, the multiplier signal ideally represents a mass spectrum for identical ion charges. In reality, this equation is influenced by the broad distribution of the initial ion velocities Dv0 , varying flight paths Ds, and plasma shielding effects (Hillier et al., 2006). The recording period of the high rate sampling mode enables the detection of ions with masses of up to approximately 190 atomic mass units (u), assuming that the instrument recording is triggered by the impact itself and the ions are singly charged. The spectrometer is sensitive to positive ions only. The mass resolution (m=Dm at full-width half-maximum), derived from laboratory experiments with the instrument, depends on the atomic masses of the ions. At 1 u, m=Dm is 10, increasing to 30 at 100 u and up to 50 at 190 u, although these average values vary ¨ with impact conditions (Stubig, 2002). In general, this does not allow a detailed analysis of complex chemical structures and only very limited differentiation of isotopes. Nevertheless, the mass spectra allow the classification of different projectile types such as silicates, water ice, organics, or Fe, Ni-particles.

3. Results 3.1. Initial state of the rhodium target plate The Rh plate used for the CAT was manufactured by the W.C. Heraeus GmbH and delivered in early 1992. Rhodium, a metal of the platinum group, is considered to be one of the most inert metals. The chemical analysis (Tack, 1992) of the material, which was carried out by Heraeus, certified a Rh purity above 99.5%. Iridium (0.15%) and silicon (0.12%) were identified as the main impurities. The manufacturing and conditioning of the CAT was done by the ¨ Luft- und Raumfahrt (DLR) in BerlinDeutsches Zentrum fur Adlershof. Micro-analysis of the material carried out by the DLR found that there were major Mn surface impurities over the whole plate. Furthermore, X-ray diffraction photography and scanning

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Fig. 3. Photograph and schematic of the Cosmic Dust Analyser (CDA). On the left picture, the reverse of the multiplier housing (MP) can be seen in the centre, behind which the inner (silvery) Rh target (CAT) and the outer Gold target (IIT) are visible. The two foil sensors of the High-Rate-Detector (HRD) are below the DA unit. The ‘‘IG’’ is the ion grid where the QI signal is sampled (Fig. 4). The entry grid (EG) is sensitive to the charge of the particle and forms the QP signal.

electron microscopy showed skin textures and a large number of cracks in which Ni, Fe, Si, Ca, and Al impurities could be found ¨ (Gorlich et al., 1999). This required heat treatment, subsequent polishing, and Ar plasma sputtering of the surface to produce the required structure and cleanliness of the surface. The sputtering of the CAT was carried out twice, each time removing the upper 50 nm of the entire surface. These elaborate processes finally produced a Rh plate in a satisfactory condition in 1997, a few months before Cassini’s launch. With the exception of the intrinsic Si impurities concentrated in ‘‘islands’’, a TOF-SIMS analysis of the surface at the DLR proved that all impurities had been reduced to a ¨ negligible level (Lura, 1997; Gorlich et al., 1999). After an outgassing incident during spacecraft assembly in the United States, the instrument was sent back to Germany, where it was subject to another cleaning procedure. The target and the accelerator grid were heated up to 200 3 C in an ultra-high-vacuum (UHV) chamber for several hours to restore its previous state of cleanliness. Despite precautions to prevent recontamination, it was inevitable that the instrument was exposed to a certain amount of contaminants before it reached interplanetary space. Though otherwise constantly purged by a flow of nitrogen ðN2 Þ, the target was exposed to potential contaminants for less than an hour while it was mounted inside the vacuum chamber of the dust accelerator facility in Heidelberg for a final function test where it stayed for three days. It should be noted that an absolutely pure metal surface—even that of a noble metal like Rh—becomes polluted when exposed to even a very thin atmosphere. Within a few seconds, at least a monolayer of surrounding atoms and molecules are adsorbed onto the metal’s surface. Thus, the clean state produced by the final heating procedure of the CAT at the DLR facilities could not be maintained. The N2 purging continued until a few hours before the rocket launch. The remaining N2 atmosphere within the closed instrument provided protection

against the outside gas environment until the spacecraft reached outer space.

3.2. TOF-SIMS analysis of the rhodium target plate In 2003, three samples of the Rh plate that had been used to manufacture the CAT (now operating in space) were analysed extensively by TOF-SIMS to examine possible surface contamination. In Tables 1 and 2, the main results of the analysis of Sample 3 are summarised. Details of the measurements and secondary ion images can be found in Appendix A. The samples have sizes of about 1 cm2 and had been stored for over 9 years in a sealed plastic bag. In contrast to Samples 1 and 2, Sample 3 experienced the same treatment as the CDA target now operating in space (see Section 3.1) and is thus considered to be the most reliable analogue. For Sample 3, positive secondary ions from six different randomly chosen areas (500  500 mm2 each) on an untreated strip were examined by TOF-SIMS. In addition, two of the areas were investigated for negative secondary ions. After sputter-cleaning of the Rh surface with 3 keV argon ions, two of the previously analysed areas were reinvestigated within a field of 100  100 mm2 for positive and negative secondary ions. The two other rhodium samples (Samples 1 and 2) were analysed at one location, first in their original states, and then again after sputter-cleaning, for both positive and negative ion emission. The secondary ion counts are normalised to the number of primary ion shots to provide comparable yields.1 1 As the absolute values for ion emission depend on a number of uncertain parameters (Stephan, 2001), values from different measurements—such as the results for the cleaned and the untreated surfaces—should be used for qualitative rather than quantitative interpretation.

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Table 2 Summary of the TOF-SIMS surface analysis of Sample 3, anions. Anions

F

H

C1;2 H1;2;3

CN

C2

Cl

O

C

48 u

Si

Untreated surface Y ð104 Þ 1962

1533

1473

963

644

393

303

186

72

2

Cleaned surface Y ð104 Þ 2212

8

38

286

30

32

18

o1

5

N.d.

‘‘Y’’ refers to the ‘‘yield’’—the number of detected secondary ions of the respective species per shot of primary ions ð104 Þ.

Fig. 4. A dust impact on the Chemical Analyser Target (CAT) creates a TOF spectrum. The CAT, which is composed of rhodium (Rh), is nominally held at a potential of about þ1000 V. An electrically grounded grid is mounted 3 mm in front of it. Following the projectile impact, a plasma cloud expands from the target surface. The strong electric field ð340 kV m1 Þ separates the plasma: while the electrons and negative ions are collected at the CAT (QC signal), the positive ions are accelerated towards the grounded grid. After passage through this accelerator grid, the ions drift over a distance of 0.1924 m through a weak electric field ð1:8 kV m1 Þ towards the multiplier mounted in the centre of the instrument. About 50% of the incoming ions are collected by concentric grids in front of the multiplier, where their integrated charge is registered by the QI channel. Behind these grids, the passing ions enter a strong field region ð100 kV m1 Þ for final acceleration towards the multiplier (þ2700 V). Ion arrival time is proportional to the square root of the mass-to-charge ðm=qÞ ratio of the ions. Their amplified signal (MP signal) is recorded as the positive ion TOF spectrum.

Table 1 Summary of the TOF-SIMS surface analysis of Sample 3, cations. Cations

Csa

Na

K

Si

Untreated surface Y ð104 Þ 1674 235 110 57

H

RhC C2 H2;3;4 Ca

28 21

RhH Ni

C RhOH Al

7 5

13

10

10

o1

o1

1

8

2

Cleaned surface Y ð104 Þ

3

4

3 o1

3 19

N.d. 1 9

o1

‘‘Y’’ refers to the ‘‘yield’’—the number of detected secondary ions of the respective species per shot of primary ions ð104 Þ. Notes: The values for the untreated sample surface are an average of all six Sample 3 measurements. For the evaluation of the cleaned surface, only one of the two measurements of Sample 3 could be used, since sputter-cleaning failed for the other procedure obviously did not work properly. a Caesium ðCsþ Þ is extremely abundant on Sample 3 only. In the analysis of Samples 1 and 2, Csþ emission only plays a subordinate role.

After being exposed to air for more than 9 years, the examined rhodium samples can be expected to be considerably more contaminated than the CDA target plate at Cassini’s launch. However, the Ar sputter-cleaning of the samples probably provided a surface similarly contaminated to the CAT (with Sample 3 providing the best analogue material). We conclude that contaminants that are not significant in the analysis of the treated or the untreated material are highly unlikely to play a role in the surface contamination of the CDA Rh target at the time of Cassini’s launch.

A major NaCl or KCl contamination during handling of the flight CAT (as seen for the untreated TOF-SIMS samples that have been touched with bare fingers) can be excluded.2 However, it is not impossible that the CAT experienced minor alkali metal contamination during its short exposure to air after the cleaning procedure (see Section 3.1). Due to the low ionisation energies of the alkali metals (Table 3), even traces—as seen in the cleaned TOF-SIMS samples—might be relevant in CDA spectra obtained in space. Of the other metals, only Ca, Ni, and Al in exceptional cases might play a minor role as contaminant signatures. According to the results of the TOF-SIMS analysis, abundant adsorption of H2 , N2 , and O2 onto the target surface occurred. For H2 and O2 this phenomenon is well known from impact ionisation experiments in the laboratory (see Section 3.3). Volatile hydrocarbons and C might also be adsorbed at the target surface. Regular heating of the Rh plate is applied during CDA space operation (see Section 3.4) to induce the desorption of volatile þ þ species (Lura, 1997). However, the resistance of RhC and RhOH , þ and to a lesser extent RhH , to the sputter-cleaning process is striking. Some H, O, and even C seem to have reacted with the Rh surface forming refractory compounds which are possibly not þ þ desorbable by heating. RhOH and RhH could have been formed after adsorption of residual gas within the UHV chamber shortly after or during sputter-cleaning. RhC at least partly stems from an equivalent reaction of Rh with hydrocarbons. This interpretation is supported by the detection of a species with mass 130.9 u which is only significant in the analysis of the sputter-cleaned surface. The respective cation likely is RhC2 Hþ 4 which must have formed from a reaction of Rh with hydrocarbon residuals. RhC might also stem from solid compounds within the target material, which reach at least some nm below the surface (the thickness of the layer removed by Ar sputter-cleaning). However, because of their low abundance, those persistent molecular ions would be expected to show up only as small signatures in CDA spectra in space operation. As CDA is only sensitive to cations, the abundant detection of F in the TOF-SIMS analysis is only of limited relevance. However, species with high ionisation energies like fluorine (Table 3) might also form a certain amount of cations at extreme impact velocities as provided by stream particles (see Section 3.4.2). Although Si is a common surface contaminant of intermediate abundance in the analysis of the untreated sample, a recontamination of the CDA target in the short period of air exposure after the sputter-cleaning is unlikely. In this regard, the state of the cleaned sample from the TOF-SIMS analysis should be applicable to spectra from impact instruments. Thus, Si contamination features above the level known from the intrinsic impurities of the Rh plate (see Section 3.1) are not expected. 2 The extremely abundant Csþ in the Sample 3 analysis is likely to be of only minor relevance and not indicative for the CAT. The Csþ emission from Samples 1 and 2 is much lower and the ratios with respect to Na and K are more plausible. On Earth Na and K are typically more than 1000 times more abundant than Cs.

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Table 3 Atomic masses, ionisation energies and electron affinitiesa of some relevant elements and molecules. Species

Main isotope mass (u)

E0 !Eþ (eV)

E0 !E (eV)

Hydrogen (H) Carbon (C) Nitrogen (N) Oxygen (O) Water ðH2 OÞ Fluorine (F) Sodium (Na) Magnesium (Mg) Aluminium (Al) HCN Nitrogen ðN2 Þ Carbon monoxide (CO) Silicon (Si) ðC2 Hx Þ ðC3 Hx Þ Sulphur (S) Chlorine (Cl) Potassium (K) Calcium (Ca) Carbon dioxide ðCO2 Þ Iron (Fe) Rhodium (Rh)

1 12 14 16 18 19 23 24 27 27 28 28 28 – – 32 35 39 40 44 56 103

13.60 11.25 14.53 13.62 12.62 17.42 5.14 7.65 5.99 13.61 15.58 14.01 8.16 8.90 8.05 10.36 12.97 4.34 6.11 13.78 7.91 7.46

0.76 1.60 40:00 1.46 40:00 3.40 0.55 40:00 0.43 o1:00 40:00 1.33 1.39 – – 2.07 3.62 0.50 0.02 40:00 0.16 1.14

a Electron affinity is the energy required to add an electron to the neutral. Thus, negative numbers mean that energy is liberated. Species with more negative electron affinities and high ionisation energies (e.g., O) tend to form negative ions. In the cases where energy is needed to form the negative species, the value is set to 40:00.

3.3. Contamination signatures in CDA laboratory spectra Several experiments and calibration campaigns with the CDA flight spare model have been carried out at the Heidelberg dust accelerator facility. The Van-de-Graaff accelerator is a high speed ð1270 km s1 Þ dust source for particles with sizes ranging from about 0:05 to 5 mm. The accelerator facility and the test setup are ¨ described in greater detail by, for example, Stubig (2002). Major calibration campaigns with the CDA Chemical Analyser subsystem were carried out between 1994 and 2002 by Posner (1995), ¨ Ratcliff et al. (1997), Srama (2000), Stubig (2002), Goldsworthy et al. (2003), and Kuhn (2002). A detailed phenomenology and interpretation of all contamination species relevant during CDA calibration and testing campaigns can be found in Appendix B. Here we summarise the results concerning the velocity thresholds of target and contamination signatures in the spectra obtained during those laboratory experiments. Faster impact speeds allow the ionisation of species with higher ionisation potentials (Table 3). For most species, velocity thresholds are found, above which a certain species regularly appears in CDA mass spectra. However, it has to be noted that those velocitythresholds obtained in the laboratory can only be used with caution for space measurements. Owing to the characteristics of the accelerator, particles with a certain impact speed have a comparably narrow size- and mass distribution.3 This is of course not the case in space, where particles with a given speed might have a much higher (lower) mass than in the laboratory, thus providing higher (lower) energy densities at the impact site. We assume that the crucial parameter to overcome the ionisation threshold of a certain species is the maximum energy density created by an impact. If the inflicted area is assumed to be proportional to the projected grain area, the energy density scales with m1=3 and v2 . This is in agreement with the observation of

3 Typical is a mass range of about two orders of magnitude for a given particle ¨ speed (Stubig, 2002).

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particle mass being a less important factor for an ionisation threshold. More complex models (Kissel and Krueger, 1987) also support our assumption that velocity is the key parameter for an impact’s maximum energy density and thus ion formation. 3.4. CDA contamination operating in space 3.4.1. Possible sources for contamination in space Even though an instrument situated on a spacecraft operates in ‘‘empty space’’, there are still several contamination sources. The most common are rocket engine exhaust, outgassing of spacecraft components, dust impacts, and the interplanetary medium (with the solar wind as the main contributor). The CDA target was subject to all those effects, after the instrument cover was removed on November 4th, 1997, about three weeks after Cassini’s launch. To reduce the impact of these sources, the CDA is equipped with a heating device, which raises the temperature of the Rh target up to 370 K within 2 h. The standard procedure is to maintain this temperature for another 8 h. These decontamination cycles have been carried out from September 2000 (DOY4 247) onwards, at intervals of a couple of months. The parameters of the heating cycles were defined in cooperation with the DLR in BerlinAdlershof on a basis of their extensive knowledge of similar bakeout procedures as a final cleaning method of metal surfaces. Tests performed by the DLR demonstrated that within the heating period, volatile substances like atmospheric gases, water or hydrocarbons were sufficiently desorbed from the target surface. Nevertheless, contamination in space is still an important issue:

 Contamination by combustion products of the rocket engine:





According to Guernsey (personal communication, 2006) the major combustion products of Cassini’s thrusters are: N2 : 42%, H2 O: 29%, CO: 17%, CO2 : 9%, H2 þ H: 2%. At the operating temperature of the target of about 210 K, only H2 O is a considerable thread since it can condense on the CA target. However, there is no evidence for contamination from this source. First, there is no significant increase of water signatures in stream particle spectra after the 98 min main engine burn during orbit insertion on July 1st, 2004. Second, 8 h heat-ups of the target on March 21st, 2004 and October 10th, 2004 to remove possible contamination did not alter the frequency of those mass lines in stream particle spectra either (Kempf et al., 2005b). Outgassing: Outgassing of volatile substances from spacecraft components is a well known but often poorly understood phenomenon. Due to the extremely low ambient pressure ðo1012 mbarÞ, and the aggressive radiation environment in interplanetary space, all forms of non-metal components (especially lubricants and different kinds of synthetic material) are in principal a potential source of contamination. The identification of outgassing products as a brownish layer on certain parts of the Genesis spacecraft (Burnett et al., 2005) demonstrated that the phenomenon is almost inevitable—even on spacecraft where the prevention of outgassing was of the highest priority. The layer occurred only on parts of Genesis exposed to the Sun, was about 5 nm thick and consisted primarily of photopolymerised hydrocarbons, siloxane, and a fluorine-bearing compound (Burnett et al., 2005; Calaway et al., 2007). Contamination by dust impacts: Each dust impact deposits a fraction of its material onto the target and thus in principle the 4

Day of year.

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instrument becomes increasingly contaminated over time. By far the most abundant CDA impactors are E ring grains. On crossings through the dense part of this ring the main instrument is in saturation ð41 impact=sÞ. From CDA’s High Rate Detector an impact rate in the order of 50 s1 for particles with r\0:9 mm can be inferred. Smaller particles can be neglected with respect to the total deposited mass (Kempf et al., 2009). The grains totally evaporate on impact forming an adiabatic expanding cloud. For the given conditions the proportion of the residue on the target can be expected to be less than 0.1% of the original impactor mass (A. Kearsley, personal communication, 2009). An upper deposition limit of 1% of the initial grain mass on the impact site yields a deposition rate of 5  1016 kg s1 during crossings of the dense part of the E ring (assuming an average ice grain size of 1:2 mm impacting with 50 s1 ). This would form a monolayer of particle constituents within several years. However, the detector is only exposed to the high impact rate found during E ring crossings for a few days per year in total. Moreover, only refractory compounds which cannot be desorbed by the decontamination heating cycles accumulate on the target over time. As the refractory compound of E ring grains is in the order of 1% or less (Postberg et al., 2008, 2009) it probably takes thousands of years until a refractory monolayer is accumulated. Currently no significant increase of such components in CDA spectra has been observed. However, in the course of the mission these effects have to be monitored, especially with respect to cations which the CDA is very sensitive to (like alkali metal ions). Impacts of stream particles and IDP’s can safely be neglected as a significant source of material deposition onto the Rh target. Interplanetary medium: With an average density of about 5 ions per cm3 at 1 AU solar distance and an average speed of  400 km s1 the solar wind dominates the particle flux in interplanetary space. It consists basically of ions of Hþ ð 95%Þ with a kinetic energy in the order of  1 keV and He2þ ( 4%, highly variable). Solar wind components are abundant enough to affect CDA measurements if accumulated on the Rh target over a long period. Cassini was exposed to fluxes of 2  108 cm2 s1 at Venus’ orbit to about 106 cm2 s1 at Saturn. However, during Cassini’s cruise phase, most of the time the CDA was not pointing towards the solar wind flux. The neutral interplanetary gas component, primarily H, is of minor relevance, as its flux is mostly far less than a tenth of the ionic solar wind (Galli et al., 2006).

þ

Fig. 5. Anti-correlation of Cþ and Rh ion abundance in spectra of Jovian stream particles. The percentage given represents the proportion of Cþ ions with respect to the total ion yield. The standard deviations of each data point are given as error bars.

Fig. 6. In Jovian stream particle spectra the proportion of C ions decreases with increasing total ion yield per impact (a good indicator for the impact energy). This trend is characteristic for ions released from a thin surface layer, e.g., of contamination.

Table 4 Velocity thresholds for specific cation species in CDA laboratory experiments with differing projectile materials. Projectile

Density (g cm3 )

Rh (km s1 )

Hþ a (km s1 )

Cþ (km s1 )

Oþ (km s1 )

Iron Aluminosilicate Aluminium Carbon Polystyrene

7.9 2.5 2.7 2.2 1.1

9 (3) 11 (5) 11 (5) 10 (6) 15c (9c)

15 13 12 15 14

15 (13) 15b (11b) 15 9b (4b) 13b (11b)

(22) 26 (26) (22) (25)d (25)d

þ

(6) (11) (8) (10) (12)

The first number refers to the speed above which the signature is generally present in spectra; the number in parentheses is the speed above which a signature is ¨ occasionally observed. Above the first value, the ion abundance of the species is increasing with increasing particle velocity. The numbers are inferred from Stubig (2002), Kuhn (2002), and Goldsworthy et al. (2003). Velocity threshold for alkali metals cations are not listed because they appear already at the lowest applied impact speeds ð 1 km s1 Þ. þ þ a Molecular hydrogen lines Hþ 2 and H3 occur as a less frequent companion of H . They are often detectable at intermediate impact speeds, whereas above  1 þ H2 disappear from CDA spectra (Goldsworthy et al., 2003). 20 km s1 Hþ 3 and above  35 km s b As the particle contains carbon, these values are not representative for the formation of contamination ions. þ c These values are not precise due to the overlap of a possible Rh mass line with signatures from hydrocarbon ions. d These values have to be taken with caution. For C and polystyrene projectiles, very few spectra of impacts above 25 km s1 could be recorded. Furthermore, the Oþ signature was of low significance in those spectra.

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3.4.2. Contamination features in CDA stream particle spectra Most of the potential contamination features identified in CDA calibration experiments are actually observed in space operation as well: alkali metals, H, C, O, and Si. However, it is important to evaluate to what extent the features are actually caused by contamination and identify those which might be a real signature of the particle’s composition. It is possible to investigate this by means of spectra due to the high velocity impacts of the tiny Jovian and Saturnian stream particles ðvi \100 km s1 ; rt20 nmÞ (Zook et al., 1996; Kempf et al., 2005a, b; Postberg et al., 2006). Under these conditions, the proportion of ions generated from the particle is expected to be very small and the spectra should be dominated by target and thus contaminant ions. Furthermore, the impacts provide energy densities that should be high enough to form positive ions of almost every species present, thus making them ‘‘visible’’ to the CDA detector. Note that stream particle sizes are far below, and the impact velocities far above, the parameters accessible during calibration experiments. Besides the target material Rh, H and C are the most abundant ion species observed in spectra of both stream particle types. In most cases, those three signatures represent more than 95% of all detected ions. Based on the derived small particle sizes, the amount of Hþ and Cþ in stream spectra exceeds by far the number of ions which can be of particle origin (Kempf et al., 2005b; Postberg et al., 2006). This observation implies that the majority of Hþ and Cþ result from target contamination. The clear anti-correlation þ between Cþ and the target ion Rh (Fig. 5) and decreasing Cþ ion proportion with increasing impact energy (Fig. 6) is in excellent agreement with Cþ coming from a layer on top of the Rh plate. The fact that the weakest impact of Saturnian stream particles only þ created a Cþ but no Rh signal is also difficult to explain without the invoking of an abundant carbonaceous surface contamination. In both Jovian and Saturnian stream particle spectra, Na and K features are observed (Kempf et al., 2005b; Postberg et al., 2006). In the Jovian case, alkali metal salts could be clearly identified as abundant particle constituents (Postberg et al., 2006). Therefore, Saturnian stream particle data are more appropriate for estimating the alkali metal contamination of the target. The analysis of stream particle spectra obtained prior to orbit insertion (Kempf et al., 2005b) showed mostly faint Na signatures in 45% of spectra, and a K signature in 15% of spectra. A recent investigation of a much bigger sample of over 2000 spectra obtained in Saturnian orbit delivered more reliable results. Here a Na signature shows up in 25–35%5 of spectra and a K signature in 15220%5 of spectra. This is consistent with the patchy distribution of alkali metal contamination observed in the TOF-SIMS analysis (Fig. 10). However, since it is unknown to what extent these features are due to particle constituents, the values can only be considered an upper limit for the contaminated target area. Therefore a CDA target free of alkali metal contamination is also consistent with the stream particle measurements. The first CDA spectra in space of two large ðr41 mmÞ Fe-rich interplanetary dust particles in 1999 (Hillier et al., 2007a) and many impact from the non-water dust population in the E ring (Postberg, 2007) show no traces of Naþ , which also implies alkali metals are not an abundant target contaminant. Oxygen ions were observed in Jovian as well as Saturnian stream particle spectra. Although the signatures might be partly due to particle ions, the relatively large proportion even in faint spectra (Postberg et al., 2006) hint at a significant contribution from target oxidation or contamination.

5 The uncertainty is due to the fact that many features are too faint to be counted as an unambiguous ion signal.

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A feature observed at 28 u, which in spectra from both stream particle types has been attributed to silicon (Kempf et al., 2005b; Postberg et al., 2006), is in principle also in agreement with þ þ fragment ions ðC2 Hþ 3 ; C2 H4 ; C2 H5 Þ from hydrocarbon surface contamination. However, this is unlikely for several reasons as described in the following section. It is inevitable that the intrinsic silicon impurity islands (see Section 3.1) of the Rh target occasionally contribute to CDA spectra. However, since those islands are rare (Table 1, Fig. 11), they cannot be responsible for all 28 u features observed in about two thirds of the stream particle spectra. The faint signatures at 19 u occasionally observed in both Jovian and Saturnian stream particle spectra—proposed as H3 Oþ in the original analyses (Kempf et al., 2005b; Postberg et al., 2006)—could also be due to Fþ contamination as indicated by the TOF-SIMS analysis (Table 2). Occasional weak signatures in spectra from Saturnian dust stream particles have been attributed to Nþ (Kempf et al., 2005b). However, it is unclear to what extent those features are of particle origin.

4. Discussion and conclusions 4.1. Contamination review of the CDA target In the following, a conclusive discussion considering the different aspects of the result section is presented. All potential contaminants are addressed, and substances not mentioned in this section are considered irrelevant for CDA contamination issues. The most cosmochemically relevant of these absent species are: B, Mg, Al, P, S, Ti, Cr, Mn, Fe, Zn, Pb, and all molecular species except hydrocarbons and the Rh-containing ions mentioned below. 4.1.1. Rhodium Although not a contaminant in a strict sense, the target bulk material is discussed here because it causes prominent nonparticle signatures in CDA mass spectra. Since Rh is not expected to be present in detectable amounts in solar system dust, the mass line at 103 u generally indicates that a significant amount of target þ material was excavated and ionised.6 Rh ions can be expected in spectra formed by impacts with energy densities above a certain threshold. Laboratory experiments have determined this threshold to be equivalent to impact speeds of 329 km s1 depending on þ the density of the impactor. Above 9211 km s1 Rh , ions are to be expected in every spectrum (Table 4). As an exception, very small particles like the Saturnian stream particles sometimes do not cause a Rh peak even though the energy densities are high enough. This is probably due to an substantial layer of C-rich material coating the target surface (see Section 4.1.9). 4.1.2. Alkali metals In the laboratory, Na and K contaminant ions are often detected simultaneously in similar amounts or with slightly more abundant Na ions. As CDA is extremely sensitive to alkali metals, even minor amounts leave a significant signal in the spectra, especially at very low impact velocities ðo5 km s1 Þ. However, after sputtering of the entire target, twice removing a layer of 50 nm deep, only traces of these metals remained at best. This was proven by TOF-SIMS analyses carried out at the DLR and the ¨ University of Munster (Lura, 1997). Only a minor recontamination of the target before or after Cassini’s launch is plausible, if any. The low Naþ and Kþ abundance observed on the cleaned Rh samples 6 An important exception is the NaðNaOHÞþ 2 ion in Na-rich E ring ice spectra (Type III) with a mass of 103 u.

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in Section 3.2 can be taken as worst case reference values. The spectra of Saturnian stream particle impacts confirmed the overall alkali metal contamination to be low. Upper limits of about 35% (Na) and 20% (K) of spectra show trace amounts of the respective alkali metal. Impacts of larger particles affect a larger target surface area than stream particles which are often smaller than 10 nm. Therefore, with larger projectiles, a greater proportion than in the case of stream particles may ionise enough alkali metals to reach the detection limit. However, spectra from impacts of Ferich particles larger than 1 mm recorded in interplanetary space and at Saturn (Hillier et al., 2007a; Postberg, 2007) often show no traces of Naþ in the spectra, which confirms that at least most of the CDA’s target surface is free of Na. Since alkali metals cannot be ruled out as part of Saturnian dust stream particles, the contribution from target contamination could be even lower or nonexistent. Although not apparent during the first years in Saturnian orbit, a slight contamination by salt deposition from Na-rich E ring particles and Enceladus plume grains on the target cannot be ruled out for the remaining mission. Li occasionally plays a role in laboratory spectra, but there is no indication for Li contamination of the CDA target in space. Cs was found to be abundant in the TOF-SIMS analysis of one of the samples. However, neither the analysis performed at the DLR nor stream particle spectra show clear evidence of Csþ . 4.1.3. Other metals Metal contaminations other than from alkali metals is not to be expected to play a significant role in CDA spectra. According to the TOF-SIMS analysis (see Section 3.2), rare and faint signals of Ca and Ni cannot be ruled out to be the result of contamination. The accelerator grid mounted 3 mm in front of the target (see Section 2) is composed of a copper-beryllium alloy. Since the grid is subject to bombardment by impactors, small amounts of these metals might be deposited on the target, increasing with the age of the instrument. However, no significant signs of these grid compounds have been observed in CDA mass spectra since Cassini’s launch. 4.1.4. Silicon The surface contamination from silicon bearing compounds was removed by the sputter-cleaning procedure. A recontamination during the short period of air exposure is very unlikely. The intrinsic silicon impurity of 0.12%, occurring as islands in the rhodium bulk material, potentially contributes to the ion yield observed in CDA spectra. However, the area distribution shown by the TOF-SIMS analysis (Fig. 11) implies that the frequency of such an event is clearly below 5%. 4.1.5. Nitrogen, oxygen and carbon dioxide Since the CDA interior was purged with N2 after the cleaning process of the Rh target, and was exposed to Earth’s atmosphere for a short period, these gases were adsorbed at the surface. Although the heating of the target should have induced a desorption of these substances, it has to be noted that the first of the decontamination cycles was carried out almost a year after the spacecraft launch. That may have allowed the formation of a thin oxidation layer (as indicated by the TOF-SIMS analysis). In contrast to Hþ and Cþ , Oþ does not appear in each stream particle spectrum and is far less abundant (Kempf et al., 2005b; Postberg et al., 2006). A detectable amount of Oþ can only be expected at high impact velocities. N cannot react with Rh and is removed by target heating. However, as a major combustion product of the rocket engines, N2 adsorption on the rhodium surface in between heating cycles has to be considered. The occasional and weak signatures in

agreement with Nþ after stream particle impacts are only consistent with a low level of N target contamination. COþ 2 has not been observed as a significant contaminant at any stage, neither in the TOF-SIMS analysis nor in CDA laboratory or space flight operation. 4.1.6. Halogens Chlorine (Cl) and fluorine (F) have been identified as surface contaminants in the TOF-SIMS analysis of the untreated Rh sample. The latter was still abundant after the sputter-cleaning procedure. Since both species favourably form anions rather than cations (F has the highest ionisation energies of all non-noble gases, see Table 3), they are unlikely to play a significant role in CDA E ring spectra and have never been observed in CDA laboratory experiments. However, the extreme energy density of stream particle impacts has never been simulated on Earth, and þ Cl was indeed identified in Jovian dust stream spectra (Postberg þ et al., 2006). Since there is no evidence for Cl in the Saturnian stream particles, the Cl in the Jovian counterpart was interpreted as a particle constituent. Based on the TOF-SIMS analysis, F can be expected to be far more abundant and, in contrast to Cl, even located below the Rh target surface. The infrequent occurrence of faint 19 u signatures in Jovian and Saturnian stream particle spectra can be interpreted as Fþ . Although not apparent during the first years in Saturn’s orbit, a slight contamination by NaCl deposition from Na-rich E ring particles and Enceladus plume grains on the target cannot be ruled out for the future mission. 4.1.7. Hydrocarbons Hydrocarbons have been entirely removed from the target during the cleaning process in 1997 (Lura, 1997). However, for a final function test, the instrument was exposed to air for less than an hour and then mounted inside the vacuum chamber of the dust accelerator in Heidelberg for a few days. Here, the instrument was inside a thin atmosphere with probable traces of oil from lubricant residues of the vacuum pumps. During the calibration campaigns, when the instrument was exposed to this atmosphere for much longer periods, signatures of C2 Hþ 3 (27 u) were observed in spectra. When operating in space, the regular heating of the target is expected to desorb all hydrocarbons. In both Jovian and Saturnian stream particle spectra, distinct spectra signatures at about 28 u were detected and attributed to Siþ . In principle, however, the signature is in some cases also in agreement with þ þ fragment ions of hydrocarbons ðC2 Hþ 3 ; C2 H4 ; C2 H5 Þ. Since the energy density at the impact location of stream particle impacts is too high for the survival of any chemical bonds, these molecular species may stem from a surface location further away from the impact crater where the energy density has dropped sufficiently (Neugebauer, 2001). Moreover, the abundant Hþ and Cþ could be explained as fragments of hydrocarbons after a high energy impact. Nevertheless, hydrocarbon signatures are considered to be insignificant for following reasons:

 The observation of hydrocarbon fragment ions as described by



Neugebauer (2001) also included C2 hydrocarbon fragment ions which were always detected concomitant with þ C 3 Hþ x 2C5 Hx species. This is inevitable due to the cracking of long chain hydrocarbons. Similar observations were made with impact ionisation experiments using organic impactors in ¨ Heidelberg on CDA flight spare target (Stubig, 2002; Goldsworthy et al., 2003) and a high resolution impact ionisation spectrometer (Srama et al., 2009). However, there is no indication for Cð2þnÞ species in stream particle spectra. Possible hydrocarbon contamination is expected to be homogeneously distributed over the entire surface (see Section 2). In

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contrast, peaks at 28 u only occur in about two thirds of the spectra, although the ionisation energies for the formation of C2 Hþ x were provided by each impact. In the case of the rare non-water spectra observed within Saturn’s E ring (Postberg, 2007), the impact conditions are much more likely to lead to hydrocarbon ion formation than for stream particles. However, in that case, only a few, mostly faint, signatures between 27 and 29 u have been found.

Nevertheless, target contamination by hydrocarbons cannot be ruled out entirely. Owing to the outgassing of volatile organic substances by the spacecraft, it is possible that these contaminants condense on the target surface in between CDA decontamination cycles. However, this process seems to be of minor importance so far.

4.1.8. Hydrogen The formation of Hþ is not expected at low impact velocities, while it has been frequently observed after high velocity impacts ¨ (Knabe, 1983; Krueger, 1996; Stubig, 2002). All metals can adsorb H on the surface or even incorporate H (such that it occupies interstitial sites or vacancies in the host metal’s lattice). A comprehensive introduction to this topic is given by Fukai (2005). The yield of Hþ in stream particle spectra obtained at Jupiter is in general many times higher than the number of atoms in the particles (Postberg et al., 2006). Therefore, it is clear that Hþ cannot stem from the particle alone. Furthermore, due to the negative enthalpy of solution, Rh cannot solve large amounts of H atoms in an equilibrium state at low ambient pressure to explain the observed proton abundance (Wipf, 2001). Although its presence as a contaminant is obvious, it is currently unclear where the H actually comes from. A possible explanation would be the entry of protons from the solar wind when the target is not shielded by a planetary magnetic field or the spacecraft. With impact speeds of several hundred km s1 , the protons will be implanted about 30–50 nm below the surface. Owing to the low target temperature ð 2102220 KÞ the diffusion coefficient of H in the metal lattice is very small. This might lead to an accumulation close to the surface where protons can be released through a high velocity impact. However, since Rh has a much lower ionisation energy, the ratio of Rh to H observed in spectra should be much higher if a considerable amount originates from the bulk material. More plausible is a mechanism in which H originates from a surface area which is many times larger than the area directly hit by the impactor. Potential sources of surface contamination would be H from the interplanetary medium and outgassing of spacecraft components. In addition to adsorbed elemental or molecular H, adsorbed hydrocarbons could be the reason for the abundant Hþ detection. However, those species should be desorbed during the decontamination heating cycles. Therefore, such a scenario would require the immediate re-adsorption of H-bearing compounds to explain the Hþ signals so soon (a few hours) after decontamination (e.g., on day 247 in 2000). We believe that the Hbearing compound cannot be removed sustainably by the decontamination heating.

In the spectra of Saturnian (Kempf et al., 2005b) and Jovian (Postberg et al., 2006) stream particles, Cþ is the dominant ion species, especially for impacts of the smallest grains, indicated by the lowest total ion yields. In these faint spectra, the amount of Cþ is often considerably higher than 50% of all detected ions, accounting for many times more atoms than the particles were likely to contain (Kempf et al., 2005b). No clear source for a potential C-bearing contamination can be identified. In laboratory spectra, the signature was usually explained as a fragment ion of contamination from lubricant oil originating from the vacuum pumps. Since this is not applicable in space, a more likely source is the outgassing of C-bearing compounds from the spacecraft. However, there are only weak indications of hydrocarbon fragment ions (see the respective section above). Furthermore, there is only few indications (if any) for Cþ ð2þnÞ ions such as those that appear in spectra with fast hydrocarbon projectiles in the laboratory (Srama et al., 2009; Goldsworthy et ¨ al., 2003; Stubig, 2002). Another potential source is solar wind implementation, although in much lower quantities than H. The most plausible explanation is a C-bearing contamination that for some reason appears primarily as Cþ and Hþ in the spectra. For example, an amorphous, C-rich contaminant which does not allow the abundant formation of Cx Hþ y ions. Such materials can be generated if hydrocarbons are exposed to UV radiation and degenerate as the C–C and C–H bonds are cracked. This results in a short-chain and C-rich amorphous layer, which ¨ cannot be removed by heating (Gohrlich, personal communication, 2007; Palumbo et al., 2004). However, the layer must have been established prior to September 2000 when the first stream particle spectra were encountered (Postberg et al., 2006) if not already at the recording of the interplanetary dust spectra in 1999 (Hillier et al., 2007a). As the TOF-SIMS analysis (see Section 3.2) indicates, C might þ also be present as target impurity. Thus, RhC which is detected not only by TOF-SIMS but also in spectra obtained in the laboratory (see Section 3.3) and space (Kempf et al., 2005b; Postberg et al., 2006, 2008; Hillier et al., 2007a; Postberg, 2007) probably stems at least in part from the target material itself. In contrast to Cþ , which cannot be expected at low impact velocities, þ RhC might show as a contamination spectral feature at lower speeds.

4.1.10. Summary In this section we summarise the general contamination potential of all discussed substances for CDA spectra. They are listed with decreasing relevance.

 Rh is the target’s bulk material. Cations can be expected



 4.1.9. Carbon The phenomenon of Cþ contaminant ions is known from high velocity impacts during CDA calibration experiments performed at the Heidelberg dust accelerator facility (see Section 3.3). However, the abundance of Cþ in stream particle spectra obtained near both Jupiter and Saturn is above all expectations, and Cþ is the least understood of all contaminant species.

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to be detectable in spectra at intermediate or high impact speeds. There are indications that the speed threshold varies significantly with the compactness and density of the particle (Table 4). C and H are the most abundant contaminant species. Their relevance increases with increasing impact speed. They form dominant signatures in all hypervelocity impacts at speeds above 50 km s1 . At low impact speeds (below approximately 10 km s1 ), they might not show up at all. O is a likely target contaminant. Since O is part of many possible impactors, too, the relevance as a contaminant is difficult to estimate. However, its cations only shows up at high impact speeds (above 20 km s1 ). Even then it is not always detectable and if present Oþ from contamination is always far less abundant than Cþ and Hþ . Traces of Na and K might be contaminants which are inhomogeneously distributed over the target surface. Only an

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upper limit could be inferred. Detectable amounts of Na compounds cover no more than 35% and K compounds less than 20% of the target area. If an alkali metal contamination exists, their relevance would increase with decreasing impact speed and increasing particle size. Na would probably be slightly more abundant than K. Even if present as contaminants, the relative abundance of both alkali metals in flight CDA spectra is small, however, in cases Naþ and Kþ (23 and 39 u) might be significant at very low impact speeds (below approximately 5 km s1 ). þ Molecular contaminant ions of RhC (115 u) and less abundant þ RhOH (120 u) are likely to cause occasional small signatures. Hydrocarbon contamination due to spacecraft outgassing is likely. Their relevance as molecular ions in CDA spectra however, appears to be negligible. There might be occasional minor signatures at 27, 28, and 29 u, particularly at low impact speeds. There are intrinsic silicon impurities within the target material which cause detectable signals at 28 u in 0.5–5% of spectra. Rare and weak contributions from F and Cl cannot be ruled out (only at the extremely high impact speeds of stream particles above 50 km s1 ), as well as from Ca, Ni, N, water, Cu, and Be (at lower impact speeds).

þ



4.2. Implications for the interpretation of E ring particle spectra So far, from the analysis of the data, recorded in 2004 and 2005, spectra of Saturnian E ring ice grains can be categorised into three main ‘‘families’’.

 Type I: Dominant water cluster mass lines H3 Oþ ðH2 OÞn , 



n ¼ 0–15; often traces of Na and K (Postberg et al., 2008) ( 70% of spectra). Type II: Water cluster H3 Oþ ðH2 OÞn, n ¼ 0–15 and a 29 u signature; mostly accompanied by other non-water mass lines; often traces of Na and K (Postberg et al., 2008) ( 25% of spectra). Type III: Na-rich water ice with dominant Naþ and Naþ ðNaOHÞn mass lines (Postberg et al., 2009). No or only faint pure water clusters H3 Oþ ðH2 OÞn ( 5% of spectra).

Typical sizes of E ring ice grains are 0.1–1 mm (Kempf et al., 2008). As demonstrated in Section 3.3, the energy density thresholds for the occurrence of contamination mass lines can be used to derive limits for the impact velocity (the particle mass only plays a subordinate role). This is of particular relevance since the standard method to derive impact velocities via signal rise times not very accurate for CAT impacts (Kempf et al., 2008). For geometrical reasons the possible impact speeds of E ring grains in prograde bound orbits onto Cassini can vary from about 4 to 25 km s1 , with 5210 km s1 being the most likely value in 2004 and 2005. However, the individual impact velocities vary for a given geometry of a Cassini ring plane crossing, reflecting a wide distribution of eccentricities and inclinations of E ring grains at any radial distance. The impact velocity regimes of potential contaminant mass lines from E ring particle impacts are part of the following sections. They are inferred from dust accelerator calibration experiments (see Table 4), which however, do not fully reflect the impact parameters encountered in space (see Section 3.3).

 Rhodium: Rhþ appears in about one third of water dominated 2005 E ring spectra of Types I and II evaluated by Postberg et þ al. (2008) with varying abundances. The Rh mass line appears often in combination with water cluster mass lines of the form





Rh ðH2 OÞn (Hillier et al., 2007b; Postberg et al., 2008). The abundance of neutral water molecules within the impact plasma and their strong electric dipole moments encourages the formation of such clusters. The occurrence and abundance þ of Rh can be used as an indicator for impact energy densities þ and thus impact speeds. The correlation of distinct Rh features with the formation of only short water clusters H3 Oþ ðH2 OÞn with no4, coupled with the fact that experiments by Timmerman (1989) proved a correlation of impact speed with cluster size, confirms this assumption. The density of E ring ice particles is below the density of projectiles used for CDA calibration experiments, whereas the masses are similar. Since formation of target ions is a function of particle density (see Table 4 and Section 3.3) the speed þ threshold for the presence of Rh is probably in the order of the highest threshold observed in the laboratory ð9 km s1 Þ. þ Therefore the observation of Rh in water ice spectra implies an impact speeds of over 829 km s1 for E ring ice particles. Due to their low density, water ice particles do not excavate þ much of the target material. Therefore, the intensity of Rh mass lines is always relatively small, even if the velocity threshold is substantially exceeded. Hydrogen: With water ice as the main component of the E ring impactors, H cannot be solely attributed to contamination. Hþ does appear in about 25% of E ring ice spectra recorded in 2005 (Postberg et al., 2008), mostly in low abundances. Elemental H cations are often accompanied by molecular species of the þ þ form Hþ 2 and H3 . The low H abundance and the occurrence of þ þ H2 and H3 both hint at energy densities just above the ionisation threshold (see Section 3.3). H2 O acts as an ‘‘Hreservoir’’ and the amount of hydrogen is effectively unlimited for each ice particle impact. Therefore, Hþ n ions appear in higher abundance than in the laboratory and the appearancethreshold for respective mass lines is expected to be slightly below the laboratory values for low density grains ð12 km s1 Þ. The formation of any Hþ n is indicative for impact speeds above 10211 km s1 . A simultaneous appearance of Hþ 2 sets an upper limit of about 35 km s1 and if Hþ 3 is also present, the impact speed was likely to be below 20 km s1. However, a nondetection of the molecular hydrogen ions has no definite implications regarding the impact speed. þ Although the velocity thresholds for the formation of Rh is þ lower, Hn appears in higher abundance if the threshold is substantially exceeded. Carbon: Due to its homogenous distribution as a target contaminant, the abundance of Cþ in spectra from ice particle impacts can in principle also be used as a speed indicator. In contrast to H, C is not an abundant particle component and the occurrence (8%) and peak intensities of Cþ during year 2005 ice particle impacts (Postberg et al., 2008) reflects the lower availability of C at the impact site. Cþ signatures probably only appear at the highest impact speeds possible for the prograde bound main population of E ring particles (above 13215 km s1 ). Alkali metals: Determining the influence of alkali metal contamination is of particular importance for E ring ice grains, as Naþ is a common signature which appears in most spectra of the icy main population (Postberg et al., 2008, 2009). The abundance of neutral water molecules within the impact cloud and their strong electric dipole moments encourage the formation of clusters with any type of ions present, and particularly Naþ. Thus, Na mass lines mostly appear in combination with respective mass lines of Na-water cluster ions of the form Naþ ðH2 OÞn (Fig. 7). Postberg et al. (2008) report distinct Na signatures in about 65% of Types I and II spectra. With faint signatures of Na or Naþ ðH2 OÞn included,

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Fig. 7. As well as the abundant water cluster mass lines of H3 Oþ ðH2 OÞn , this Type I spectrum shows peaks occurring due to Na ions clustering with water molecules ðNaþ ðH2 OÞn Þ. The spectrum has been slightly smoothed.

this number increases to 93% (Postberg et al., 2009). In about 6% of the spectra analysed by Postberg et al. (2009), Naþ is extremely abundant and the formation of Na-hydroxide clusters, Naþ ðNaOHÞn , is favoured over ordinary water clusters, Naþ ðH2 OÞn , which is a clear indicator for especially high Na concentrations (Steinbach and Buck, 2005). This E ring spectral type was named ‘‘Type III’’ (Fig. 8).

Neither the TOF-SIMS analysis, the laboratory experiments, nor other spectra recorded by CDA in space indicate amounts of Na as observed in Type III spectra. Furthermore, in impact experiments carried out in the laboratory with water ice (where contamination with trace amounts of Na could not be avoided) (Timmerman, 1989), only small Naþ and Naþ ðH2 OÞn contamination mass lines were observed; the Na contamination was never abundant enough for the formation of detectable amounts of Naþ ðNaOHÞn —the characterising signature of Type III spectra (Fig. 8). This clearly argues against Na from target contamination in Type III E ring spectra. In contrast to Na, distinct peaks of K, an element with even lower ionisation energy, are faint in Type III spectra (Fig. 8). This is not in agreement with Na from target contamination where Na and K are usually correlated. We conclude that Type III E ring spectra were caused by Na-rich ice particles (Postberg et al., 2009). Most Types I and II spectra only show traces of Naþ and Kþ and their respective Xþ ðH2 OÞn cluster ions. Although the low intensity of alkali mass lines is often in agreement with contamination, the frequency ð490%Þ and the irregularly observed high Na abundance is not. The much lower frequency and abundance of Naþ in Saturnian stream particle spectra (see Section 3.4.2) and other non-water impactors (see Section 4.1), imply that the Na contribution from particle compounds in Types I and II spectra dominates that from contamination. Moreover, CDA calibration ¨ experiments (Stubig, 2002; Goldsworthy et al., 2003) and the TOFSIMS analysis (see Section 3.2) show the amount of Naþ and Kþ contamination ions to be roughly similar, whereas in E ring ice grains Naþ is in most cases several times more abundant then Kþ . It should be noted that only an upper limit to the alkali metal target contamination could be inferred in this work. Therefore, it is also in agreement with our analysis that all the alkali metal ions observed in CDA E ring spectra stem from the grains.

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Fig. 8. This Type III spectrum clearly shows the distinct peak pattern of Naþ ðNaOHÞn following impacts of Na-rich ice particles. With a large enough proportion of Naþ , clusters with one or more NaOH molecules form (Na substitutes a H atom in the attached water molecule). Peaks of H3 Oþ ðH2 OÞn and Naþ ðH2 OÞx show very low intensities at best. The ion at mass 129 is Naþ ðNa2 CO3 Þ and indicates a sodium bearing salt species in the ice grain (Postberg et al., 2009). The spectrum has been slightly smoothed.

Although unlikely and not apparent during the first years in Saturn’s orbit, a slight contamination by salt deposition from Narich E ring particles and Enceladus plume grains on the target cannot be ruled out for the remaining mission (Section 3.4). Therefore, the possible accumulation of alkali metals on the CDA Rh target over years, due to the increasing number of spacecraft traversals of the E ring and Enceladus’ plumes, has to be monitored.

 Oxygen: With water ice as the main component of the E ring







impactors, O cannot be solely attributed to contamination. Some Type II spectra analysed by Postberg et al., 2008 contain a mass peak which can be interpreted as Oþ (16 u) shifted to a slightly lower mass, or CHþ 3 (15 u) shifted to a slightly higher mass. The occurrence of the signature is generally not correlated with Cþ and also occurs more frequently. Laboratory experiments and stream particle spectra show that O—which has a higher ionisation energy than C—only forms ions at impact energies high enough to form Cþ in great abundance (Section 3.3). Thus, the appearance of Oþ in Type II spectra is unlikely and the interpretation of this feature as CHþ 3 (15 u) more plausible. Even with ice grains as an abundant O-reservoir, Oþ mass lines, either from contamination or as particle component, can only be expected at impact speeds of about 20 km s1 of higher. RhC, RhOH: Some spectra of E ring particle impacts indicate the þ þ presence of RhC (115 u). Faint mass lines of RhOH (120 u) as a result of target contaminant might be formed, too. However, in many cases such signatures would be swamped by targetprojectile (Rh-water) clusters. Hydrocarbons: In Type II spectra, the mass lines of non-water ions are often in good agreement with hydrocarbon fragments. However, in most cases the respective signatures are much too large to be solely the result of target contamination ions. In a few spectra the 29 u signature—the characterising peak of Type II spectra (Postberg et al., 2008)—is small enough to be in agreement with the possible hydrocarbon target contamination. Silicon: In rare cases a silicon signature (28 u) from impurities embedded in the Rh target may be misinterpreted as a non water component in water ice particles.

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4.2.1. Summary In this section we summarise our findings with regarding the influence of CDA target contamination on spectra from E ring ice particles. H and C are the only contaminants of definite relevance for E ring spectra. They become visible as cations at the highest impact speeds possible for prograde E ring grains in all types of E ring spectra. There are no indicators for a distinct hydrocarbon contamination. However, an occasional contribution of hydrocarbon mass lines in Type II spectra cannot be ruled out. In these cases, a Type I particle might accidently be categorised as Type II. Contamination from other volatile compounds or ices is not significant or not relevant in the impact speed regime of E ring particles. It is not clear if the alkali metal species Na and K play a role as contaminant. It cannot be ruled out that the traces of Naþ and Kþ observed in Types I and II spectra are partially due to target contamination. However, possible alkali metal contamination is clearly below the Na content of Na-rich Type III E ring spectra. Other metals only play a subordinate role as contaminants, if any. In rare cases mass lines due to Si target impurities are possible. As a positive side effect, the contamination of the CDA rhodium target can be utilised as a tool to investigate impact dynamics. þ Besides Rh , the occurrence and abundance of contaminant mass þ þ þ lines from Hþ , Hþ 2 , H3 , C , and O can be used as an indicator for impact speeds of individual grains.

Acknowledgements ¨ Special thanks to Eberhard Grun, Jon Hillier, and Klaus Hornung for fruitful discussions. This project is supported by the DLR under Grant 500OH91019.

Appendix A. TOF-SIMS analysis of the rhodium target plate During the TOF-SIMS analysis, a 25 keV 69 Gaþ primary ion beam was used to sputter the uppermost monolayers of the rhodium sample. The primary ions transfer their kinetic energy to the target surface via a collision cascade with a typical dimension of approximately 10 nm (Fig. 9). Typically, less than 1% of the emitted atoms and molecules are emitted from the sample as secondary ions in a positively or negatively charged state. For a comprehensive introduction of the use of TOF-SIMS in cosmochemistry see Stephan (2001). This analytical method was chosen because the ionisation mechanism shares some basic principles with the impact ionisation method used to create CDA mass spectra. The amount of energy transferred to form the ions is roughly similar to that of a particle impact, especially for small particles sizes ðt1 mmÞ (Kissel and Krueger, 1987). It is thus expected that the abundance of emitted surface cations is to a certain degree comparable to impact ionisation instruments (Krueger, 1996). Potential volatile contaminants present in the air ðN2 ; O2 ; H2 OÞ and the Heidelberg accelerator’s vacuum chamber (H, H2 , H2 O, and hydrocarbons) are likely to be adsorbed within the short period of the CDA’s exposure, whereas refractory material like metals or silicates are not expected to have significantly contaminated the CDA target. Since the CDA is only sensitive to positive ions, the investigation of cation contaminants is generally of greater importance than anion contaminants. However, at high impact speeds, certain species with high ionisation energies (see Table 3), which preferably show up as negative ions in TOF-SIMS spectra (e.g., halogens or O), can also form positive ions and thus be detected by the CDA.

Fig. 9. The principle of secondary ion mass spectrometry: On the sample surface a primary ion beam generates secondary ions. The primary ions transfer energy in a collision cascade to the target atoms. Ion implantation (A), back-scattering (B), or forward scattering (C) may occur. Atoms from the target leave the sample after several collisions as secondary particles (a,b,c), with only a few of them being ionised. The secondary ions will then be mass separated and detected in a mass spectrometer. From Stephan et al. (2001).

Cations: The untreated sample surfaces were all heavily contaminated. Alkali metal ions from Cs, Na, and K are the predominantly detected element cations, followed by cations from Si, H, Ca, Ni, C, and Al at far lower abundances. Although TOF-SIMS is very sensitive to alkali metal ions, the intensity of Csþ on Sample 3 is unexpectedly high, and would be expected to be far below the Naþ and Kþ values. Thus, Sample 3 (in contrast to Samples 1 and 2) is likely to have experienced non-representative Cs contamination. There are several molecular species present in small amounts. The C2 Hþ 2;3;4 species are fragments from larger hydrocarbons. þ þ þ RhH , RhC , and RhOH are ions composed of the target material rhodium and a contaminant species. After Ar sputter-cleaning, the abundance of elemental contaminant ions decreased substantially. Only ions of alkali metals, calcium (Ca), hydrogen (H), and carbon (C) are still detectable þ þ (Figs. 10 and 11). Remarkably, RhC and RhOH (not shown) are detected in similar amounts as before cleaning, indicating a contamination layer with a thickness of at least several nanometers or an immediate reaction of Rh with residual gas after the sputter-cleaning. There are apparent differences in the homogeneity of the distribution of the contaminants on the surface. While Hþ , Cþ , and C2 Hþ x are almost homogeneously distributed (Fig. 11), the other contaminants listed in Table 1 have a more or less patchy appearance (Figs. 10 and 12). Naþ and Kþ show the most irregular distribution but both alkali metals are found at clearly correlated locations. Siþ and Niþ (not shown) are also detected at roughly identical locations. After sputtering, most of the cations are emitted much more uniformly from the þ surface, except for Al , Caþ , CaFþ , and Siþ (Fig. 12), which are concentrated on a few tiny islands on the Rh surface. For Si, this behaviour is only noticeable on Samples 1 and 2, but is in perfect agreement with the results obtained at the DLR (Lura, 1997) (see Section 3.1).

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Fig. 10. Sample 3 secondary ion images of 23Naþ and 39Kþ before (top row) and after (bottom row) Ar sputter-cleaning. The fields of view are 500  500 mm2 (5122 pixels, 80 primary ion shots/pixel) before sputtering and 100  100 mm2 (2562 pixels, 80 primary ion shots/pixel) after sputtering, respectively. All individual ion images shown in this work use the same field of view and the linear colour scale shown, where black corresponds to zero counts and red is used for the maximum intensity given below every image (e.g., 36 counts for Naþ before sputtering). The other number underneath each image is the integrated intensity of the entire field of view (e.g., 4:55  105 counts for Naþ ). The same labelling is used in subsequent figures containing secondary ion images. The untreated samples exhibits massive alkali metal contamination, which is highly concentrated in some places, whereas it is undetectable at other locations. On the sputtered sample, the abundance is clearly reduced. However, occasional traces of contamination remain.

Anions: F, H, Cl, C, and O are the dominant negatively charged ions. A few Si anions are also emitted. There is a range of C1;2 H 1;2;3 species, which are fragments from heavier hydrocarbons with  C2 H and C2 H 3 as the most abundant species. CN is also emitted in great abundance. Since N or N2 are not likely to form either anions nor cations, CN is a good proxy for N2 adsorption on the sample surface. After Ar-sputter-cleaning, the abundance of most of the contaminants is significantly reduced. However, the cleaning process is not as effective as for the positive species. F contamination, in particular, seems to be barely affected, and CN indicates adsorbed N2 being still present in significant amounts. Although experiments were carried out under ultrahigh-vacuum conditions, N2 traces are immediately adsorbed at the sputter-cleaned surface. Although there are slight variations, the distribution of anions on the surface is in general more uniform than the cation distribution. Cl and F have the patchiest appearance (Fig. 13).

The distribution is apparently linked to the surface structure which hints at a refractory material origin, most likely alkali metal  salts. E.g., Cl shows a patchy distribution similar to Naþ (Fig. 10) indicating NaCl to be the relevant surface contaminant. As in the case of cations, the anions are more evenly distributed after the sputtering process. The homogeneous distribution of volatile species shows that—in contrast to refractory materials—the surface adsorption of volatile materials (like H and hydrocarbons) does not depend that much on the structure of the metal surface.

Appendix B. Contamination in CDA laboratory calibration spectra In contrast to the target of the flight unit, which experienced a special cleaning treatment (see Section 3.1), the contamination state of the flight spare instruments used in the laboratory must

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Fig. 11. Secondary ion images before (top row) and after (bottom row) Ar sputter-cleaning from the same areas as in Fig. 10. In contrast to the metals, Hþ , Cþ , and þ hydrocarbons (e.g., C2 Hþ 3 ) show a less patchy distribution and the sputter-cleaning was not that effective. In contrast to all other cations shown, the RhC abundance þ slightly increases after sputtering and exhibits a homogeneous distribution. RhOH shows similar behaviour (not shown).

þ

Fig. 12. Secondary ion images after Ar sputter cleaning from the same areas as in Fig. 10. The example for Si is taken from the Sample 2 analysis. Though the total ion yield þ of Siþ has significantly decreased on the sputtered sample (Table 1), tiny islands of high Si abundance appear. The same applies to a lesser extent to Caþ , CaFþ , and Al .

Fig. 13. Sample 3 secondary ion images before (top row) and after (bottom row) Ar sputter-cleaning. As common in TOF-SIMS, the contamination produces more H anions  than H cations (Fig. 11). The H-distribution is almost homogeneous and most of it is removed by the argon sputtering. CN , Cl , and F show a patchy distribution similar to Naþ (Fig. 10). The sputter-cleaning is ineffective for F.

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be much worse. Thus, the abundance and frequency of contamination signatures observed in laboratory spectra can be treated as an upper limit. However, because of their rapid adsorption, the contamination of the surface with gas molecules as a consequence of air exposure should be comparable in both cases. At low impact speeds ð1210 km s1 Þ, the main contamination mass lines observed in all experiments were those of the alkali metals lithium (Liþ , 7 u), sodium ðNaþ , 23 u), and potassium ¨ 2002; Kuhn, 2002; (Kþ , 39 u) (Ratcliff et al., 1997; Stubig, Goldsworthy et al., 2003). Kþ and Naþ were observed in roughly similar abundance, Liþ only showed up as an occasional and much smaller mass line. For impacts with vi t5 km s1 , most spectra are ¨ dominated by Na and K signatures (Stubig, 2002; Goldsworthy et al., 2003). These elements—probably emitted from a thin inhomogeneous surface contamination layer (see Section 3.2)—have the lowest ionisation energies (see Table 3) and often produce the only significant spectral features at low impact speeds. There are also occasional smaller amounts of molecular ion species, which occur predominantly at low impact speeds (e.g., at 72–73 u, probably caused by silicone oil). With increasing impact energies, the proportion of alkali metal ions decreases. This is due to four reasons: (a) Species with higher ionisation energies are contributing. (b) Deeper layers of target and particle are affected. (c) Surface material is preferably ejected from the impact site at high angles from the surface normal (Hornung et al., 2000), reducing the chance of detection at the multiplier. (d) In accelerator experiments, fast particles are smaller than slow particles. The affected surface area becomes smaller.

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found indications that the mass line was not related to the target but to Al contamination of the Fe projectiles that have been used for the previous laboratory experiments as well (Posner, 1995). However, a chemical analysis of the Fe dust used for the accelerator shows that it contains only traces of Al, if any ðo0:5%Þ. However, the analysis of the same dust shot into aerogel (Stephan et al., 2006) revealed a high abundance of Al, probably on the surface of the particles. To further clarify the presence and origin of a possible Al contamination, we analysed a number of dust samples by secondary electron microscopy. Analysed dust samples comprised fresh (vacuum sealed) Fe dust, dust residues from the dust source chamber and from different sections of the Van-de-Graaff accelerator tube. While fresh dust samples and source chamber dust residues were essentially pure Fe dust, some Al contamination was found in dust residues extracted from sections within the accelerator tube (see Fig. 14). Consequently, the 27–28 u signature in most cases is not caused by target contamination. Our explanation is that the charged dust, which is focused by an aluminium baffle before it enters the Van-de-Graaff accelerator, abrades the Al, which then contaminates the surface of the dust grain or eventually becomes charged and accelerated as separate projectile. The Al-abrasion is believed to be much smaller than the Fe dust. This effect is probably responsible for the preferred appearance of 27 u signatures at high impact speeds (Posner, 1995): Due to the more or less constant energy provided by the accelerator, small Al fines only contribute at speeds above 10 km s1 whereas the larger Fe grains dominate the low speed regime. In high resolution Fe impact ionisation spectra of negative ions, a mass line at 25 u clearly indicates C2 H . This is consistent with a hydrocarbon contamination of the surface as observed during the

þ

At higher impact speeds, peaks due to the target material (Rh , þ þ 103 u) and the contaminants Hþ x , 1, 2, 3 u, C , 12 u, and O , 16 u occur. A pronounced signature at 27–28 u is also regularly recorded but is probably due to contamination of the projectile as is discussed further below. There is a strong velocity dependence for the occurrence and abundance of most of these þ mass lines (see Table 4). Rh appears at relatively low impact speeds and is roughly dependent on the density of the impactor. This is expected as model calculations indicate that the ion yield of target material critically depends on the particle/target density ratio (Krueger, 1985, 1996; Kissel and Krueger, 1987). Hþ and Cþ appear roughly above the same velocity threshold varying from  8 to 15 km s1 . The occurrence of the molecular hydrogen lines þ þ Hþ 2 and H3 as a companion of the H is in general less frequent. They are preferably detectable at impact speeds just above the Hþ 1 threshold, whereas above  20 km s1 Hþ 3 and above  35 km s Hþ disappear from CDA spectra (Goldsworthy et al., 2003). O 2 appears only at the highest impact speeds clearly above 20 km s1 . A low abundance mass line is occasionally observed at about 115 u. Although not always attributed as such by the authors, this þ is very likely to be the signature of RhC and it was observed with ¨ all projectile materials (Stubig, 2002; Goldsworthy et al., 2003). þ Such RhC ions are also observed in the TOF-SIMS analysis (Section 3.2). They could either be formed within the plasma cloud from a carbonaceous surface contamination and the metal target or alternatively be already present as preformed Rhcompounds on and below the surface of the target. The contaminant signature at 27–28 u represents a special case. Although observed in CDA as well as CIDA and PIA/PUMA calibration experiments, the origin of this signature was unclear. In early PIA/PUMA and CIDA experiments, the mass line was thought to be C2 Hþ 3 —a fragment ion of adsorbed volatile hydrocarbons, originating from oils used as a lubricant of the vacuum pumps. During the first CDA experiments in 1994, Posner

Fig. 14. Fe sphere (bright) with diffuse Al-rich abrasion attached to the right.

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TOF-SIMS analysis (see Section 3.2). However, respective cation peaks ðC2 Hþ 3;4;5 Þ are only weak for projectiles other than Fe (see above) and preferably appear at low impact speeds. References ¨ E., Srama, R., Kempf, S., Auer, R., 2002. The charge and velocity Auer, S., Grun, detector of the Cosmic Dust Analyser on Cassini. Planet. Space Sci. 50, 773–779. Burnett, D.S., McNamara, K.M., Jurewicz, A.J.G., Woolum, D.S., 2005. Molecular contamination on anodized aluminum components of the Genesis science canister. In: 36th Annual Lunar and Planetary Science Conference, Abstract no. 2405. Calaway, M.J., Burnett, D.S., Rodriguez, M.C., Sestak, S., Allton, J.H., Stansbery, E.K., 2007. Decontamination of Genesis array materials by UV ozone cleaning. In: 38th Annual Lunar and Planetary Science Conference, Abstract no. 1627. Fukai, Y., 2005. In: Fukai, Y. (Ed.), The Metal-Hydrogen System. Springer, Berlin, p. 2005 (second rev. and updated ed.). Galli, A., Wurz, P., Barabash, S., Grigoriev, A., Lundin, R., Futaana, Y., Gunell, H., ¨ Holmstrom, M., Roelof, E.C., Curtis, C.C., Hsieh, K.C., Fedorov, A., Winningham, D., Frahm, R.A., Cerulli-Irelli, R., Bochsler, P., Krupp, N., Woch, J., Fraenz, M., 2006. Direct measurements of energetic neutral hydrogen in the interplanetary medium APJ 644, 1317–1325. ¨ Gorlich, M., Lura, F., Stropahl, G., Siewert, F., 1999. Application of various analytical methods for the characterisation and failure analysis with the manufacture of a high purity rhodium-target for the CDA/ CASSINI (SM98-130/361). In: Spacecraft Structures, Materials and Mechanical Testing. ESA Special Publica¨ tion, vol. 428, Kaldeich-Schurmann, pp. 399–404. Goldsworthy, B.J., Burchell, M.J., Cole, M.J., Armes, S.P., Khan, M.A., Lascelles, S.F., Green, S.F., McDonnell, J.A.M., Srama, R., Bigger, S.W., 2003. Time of flight mass spectra of ions in plasmas produced by hypervelocity impacts of organic and mineralogical microparticles on a cosmic dust analyser. A&A 409, 1151–1167. ¨ Grun, E., Fechtig, H., Giese, R.H., Kissel, J., Linkert, D., Maas, D., McDonnell, J., Morfill, G., Schwehm, G., Zook, H., 1992a. The Ulysses dust experiment. A&A Suppl. Ser. 92, 411–423. ¨ E., Fechtig, H., Hanner, M., Kissel, J., Lindblad, B.-A., Linkert, D., Maas, D., Morfill, Grun, G., Zook, H., 1992b. The Galileo dust detector. Space Sci. Rev. 60, 317–340. Hillier, J.K., McBride, N., Green, S.F., Kempf, S., Srama, R., 2006. Modelling CDA mass spectra. Planet. Space Sci. 54, 1007–1013. Hillier, J.K., Green, S.F., McBride, N., Altobelli, N., Postberg, F., Kempf, S., ¨ E., 2007a. Interplanetary Schwanethal, J.P., Srama, R., McDonnell, J.A.M., Grun, dust detected by the Cassini CDA Chemical Analyser. Icarus 190, 643–654. Hillier, J.K., Green, S.F., McBride, N., Schwanethal, J.P., Postberg, F., Srama, R., Kempf, ¨ S., Moragas-Klostermeyer, G., McDonnell, J.A.M., Grun, E., 2007b. The composition of Saturn’s E ring. MNRAS 377, 1588–1596. Hornung, K., Malama, Y.G., Kestenboim, K.S., 2000. Impact vaporization and ionization of cosmic dust particles. Astrophys. Space Sci. 274, 355–363. Kempf, S., Srama, R., Hora nyi, M., Burton, M., Helfert, S., Moragas-Klostermeyer, G., ¨ E., 2005a. High-velocity streams of dust originating from Saturn. Roy, M., Grun, Nature 433, 289–291. Kempf, S., Srama, R., Postberg, F., Burton, M., Green, S.F., Helfert, S., Hillier, J.K., ¨ E., McBride, N., McDonnell, J.A.M., Moragas-Klostermeyer, G., Roy, M., Grun, 2005b. Composition of Saturnian stream particles. Science 307, 1274–1276. Kempf, S., Beckmann, U., Moragas-Klostermeyer, G., Postberg, F., Srama, R., ¨ E., 2008a. The E ring in the vicinity Economou, T., Schmidt, J., Spahn, F., Grun, of Enceladus I: spatial distribution and properties of the ring particles. Icarus 193, 438–454. Kempf, S., Beckmann, U., Schmidt, J., 2009. How the Enceladus dust plumes form Saturn’s E ring. Icarus, submitted for publication. Knabe, W., 1983. Masssenspektrometrische Untersuchung der Ionenbildung beim Einschlag schneller Staubteilchen. Ph.D. Thesis, Heidelberg University.

Kissel, J., Krueger, F.R., 1987. Ion formation by impact of fast dust particles and comparison with related techniques. Appl. Phys. A 42, 69–85. ¨ Krueger, F.R., 1985. Vergleichende Studie zur Ionenbildung aus Festkorpern bei verschiedenen Methoden schneller Energiedisposition zwecks semiquantitativer Determination von Massenspektren beim Einschlag kometaren anorga¨ nischen Staubes auf Metalloberflachen. Technical Report, Max-Planck-Institut, Auftrag Nr. 84/3716. Krueger, F.R., 1996. Ion formation by high- and medium-velocities dust impacts from laboratory measurements and Halley results. Adv. Space Res. 17, 71–75. Kuhn, S., 2002. Hochgeschwindigkeitsmessungen am Staubbeschleuniger mit der CDA—Flugersatzeinheit. Diploma Thesis, Heidelberg University. Lura, F., 1997. Cassini CDA/TDT Schlussbericht. Technical Report, DLR GmbH, Berlin-Adlershof. Neugebauer, R., 2001. Auf der Spur des Ursprungs und der Evolution von Ph.D. Thesis, Frankfurt a.M. ¨ ¨ ¨ Sekundarionen aus ION-Festkorper-St oßen. University. Palumbo, M.E., Ferini, G., Baratta, G.A., 2004. Infrared and Raman spectroscopies of refractory residues left over after ion irradiation of nitrogen-bearing icy mixtures. Adv. Space Res. 33, 49–56. Posner, A., 1995. Untersuchung von Flugzeit-Massenspektren im Rahmen der ¨ die Cassini-Mission. Kalibirierung des Cosmic Dust Analyzer Detektors fur Diploma Thesis, Heidelberg University. ¨ E., 2006. Postberg, F., Kempf, S., Srama, R., Green, S.F., Hillier, J.K., McBride, N., Grun, Composition of jovian dust stream particles. Icarus 183, 122–134. Postberg, F., 2007. A new view on the composition of dust in the solar system: results from the Cassini Dust Detector. Ph.D. Thesis, Heidelberg University. ¨ E., 2008. Postberg, F., Hillier, J.K., Kempf, S., Srama, R., Green, S.F., McBride, N., Grun, The E-ring in the vicinity of Enceladus II: probing the moon’s interior—the composition of E-ring particles. Icarus 193, 438–454. Postberg, F., Kempf, S., Schmidt, J., Brilliantov, N., Beinsen, A., Abel, B., Buck, U., Srama, R., 2009. Sodium salts in E ring ice grains from an ocean below the surface of Enceladus. Nature 459, 1098–1101. Ratcliff, P.R., Reber, M., Cole, M.J., Murphy, T.W., Tsembelis, K., 1997. Velocity thresholds for impact plasma production. Adv. Space Res. 20, 1471–1476. Srama, R., 2000. Vom Cosmic-Dust-Analyzer zur Modellbeschreibung wissenschaf¨ Munchen. ¨ tlicher Raumsonden. Ph.D. Thesis, Technische Universitat Srama, R., et al., 2004. The Cassini Cosmic Dust Analyser. Space Sci. Rev. 114, 465–518. Srama, R., Woiwode, W., Postberg, F., Armes, S.P., Fujii, S., Dupin, D., Omond-Prout, J., Sternovsky, Z., Kempf, S., Moragas-Klostermeyer, G., ¨ Mocker, A., Grun, E 2009. Mass spectrometry of hyper-velocity impacts of organic micrograins. Rapid Communications in Mass Spectrometry, submitted for publication. Steinbach, C., Buck, U., 2005. Reaction and solvation of sodium in hydrogen bonded solvent clusters. Phys. Chem. Chem. Phys. 7, 986–990. Stephan, T., 2001. TOF-SIMS in cosmochemistry. Planet. Space Sci. 49, 859–906. Stephan, T., Butterworth, A.L., Snead, C.J., Srama, R., Westphal, A.J., 2006. TOF-SIMS analysis of aerogel picokeystones—an analogue to Stardust’s interstellar dust collection. In 37th Annual Lunar and Planetary Science Conference, Abstract no. 1448. ¨ Stubig, M., 2002. New insights in impact ionization and in time-of-flight mass spectroscopy with micrometeoroid detectors by improved impact simulations in the laboratory. Ph.D. Thesis, Heidelberg University. ¨ Timmerman, R., 1989. Ladungsemmission bei Einschlagen schneller Mikroteilchen auf Wassereis. Ph.D. Thesis, Heidelberg University. Tack 1992. Test report 101 3 L001. Technical Report, Heraeus GmbH, Hanau. Wipf, H., 2001. Solubility and diffusion of hydrogen in pure metals and alloys. Phys. Scr. Vol. T 94, 43–51. ¨ Zook, H., Grun, E., Baguhl, M., Hamilton, D., Linkert, G., Liou, J.-C., Forsyth, R., Phillips, J., 1996. Solar wind magnetic field bending of Jovian dust trajectories. Science 274, 1501–1503.