THE COSMIC DUST ANALYSER ONBOARD CASSINI:

TEN YEARS OF DISCOVERIES

R. Srama (1,2), S. Kempf (3,13), G. Moragas-Klostermeyer (1), N. Altobelli (4), S. Auer (5), U. Beckmann (6), S. Bugiel (2,1), M. Burton (7), T. Economomou (8), H. Fechtig (1), K. Fiege (1,3), S. F. Green (9), M. Grande (10), O. Havnes (11), J. K. Hillier (9,19), S. Helfert (12), M. Horanyi (13), S. Hsu (1,13), E. Igenbergs (14), E. K. Jessberger (15), T. V. Johnson (7), E. Khalisi (1), H. Krüger (6), G. Matt (1), A. Mocker (1,2), P. Lamy (16), G. Linkert (1), F. Lura (17), D. Möhlmann (17), G. E. Morfill (18), K. Otto (13), F. Postberg (1,2,19), M. Roy (7), J. Schmidt (20), G. H. Schwehm (5), F. Spahn (20), V. Sterken (1,3), J. Svestka (21), V. Tschernjawski (17), E. Grün (1,13), H.-P. Röser (2)

Abstract the extended E ring (at least twice as large as predicted) allowed the definition of a dynamical The interplanetary space probe Cassini/ dust model of ‘s E ring describing the reached Saturn in July 2004 after seven years of observed properties. This paper summarizes the cruise phase. The German Cosmic Dust discoveries of a ten year story of success based Analyser (CDA) was developed under the on reliable measurements with the most leadership of the Max Planck Institute for advanced dust detector flown in space until Nuclear Physics in Heidelberg under the support today. This paper focuses on cruise results and of the DLR e.V.. This instrument measures the findings achieved at Saturn with a focus on flux interplanetary, interstellar and planetary dust in and measurements. CDA discoveries our since 1999 and provided related to the detailed dust stream dynamics, E unique discoveries. In 1999, CDA detected ring dynamics, its vertical profile and E ring interstellar dust in the inner solar system compositional analysis are published elsewhere followed by the detection of electrical charges of (Hsu et al. 2010, Kempf et al., 2008, 2010, interplanetary dust grains during the cruise Postberg et al, 2008, 2009). phase between and . The instrument determined the composition of interplanetary dust and the nanometre sized dust streams (1) Max Planck Institut für Kernphysik, Heidelberg originating from Jupiter‘s . During the (2) IRS, Universtität Stuttgart, D approach to Saturn in 2004, similar streams of (3) Universtität Braunschweig, D submicron grains with speeds in the order of 100 (4) ESAC, Madrid, ESP km/s were detected from Saturn‘s inner and (5) A&M Associates, Basye, USA (6) MPS, Kathlenburg-Lindau, D outer and are released to the (7) JPL, Pasadena, USA interplanetary magnetic field. Since 2004 CDA (8) Universtität Chicago, USA measured more than one million dust impacts (9) Open University, Milton Keynes, U.K. characterizing the dust environment of Saturn. (10) University of Wales Aberystwyth, U.K. The instrument is one of three experiments (11) Universtität Tromso, NOR (12) Helfert Informatik GmbH & Co KG, Mannheim, D which discovered the active ice located (13) LASP/Universtität Colorado, Boulder, USA at the south pole of Saturn‘s moon in (14) Universtität München, D 2005. Later, a detailed compositional analysis of (15) Universtität Münster, D the ice grains in Saturn‘s E ring system (16) LAS, Marseille, F lead to the discovery of large reservoirs of liquid (17) DLR Berlin, D (18) MPE Garching, D water () below the icy crust of Enceladus. (19) Universtität Heidelberg, Finally, the determination of the dust- (20) Universtität Potsdam, D magnetosphere interaction and the discovery of (21) Prag Observatorium, CR (23)

Srama et al., 2011 The Cosmic Dust Analyser onboard Cassini: Ten years of discoveries 1 enabling the science team today to derive invaluable compositional information of Saturn’s ring particles which cannot be achieved by other means and instruments (Postberg et al., 2009). A specially designed articulation mechanism allows the instrument to rotate about one axis and to independently track the dust RAM direction onboard the three axis stabilised Cassini spacecraft. A international team of scientists and engineers under the leadership of the MPIK in Heidelberg and, since 2011, at the University of Stuttgart, performs science planning, operations including commanding and instrument monitoring, data processing and Figure 1: The ISD flux measured by various science analysis. interplanetary spacecraft. The radiation pressure of the and interaction of the charged grains with The instrument consists of an Impact Ionisation the interplanetary magnetic field prevent small Detector (IID, Srama et al., 2004), a Chemical particles from penetrating the inner heliosphere. Analyser (CA) and a High Rate Detector (HRD). Various subsystems are based upon charge induction, impact ionisation, time-of- Keywords flight spectrometry and depolarisation of Cosmic Dust Analyser, Cassini, CDA, interplanetary PVDF foils (Simpson et al., 1985). This dust, interstellar dust, planetary ring, E ring, combination of subsystems provides reliable Enceladus, cosmochemistry, Saturn, water ice, information about individual dust impacts in , impact ionisation, PVDF Saturn‘s magnetosphere with its variable plasma and dust . Introduction Cruise Science The NASA-ESA space mission Cassini- Huygens reached the ring Saturn in July Interstellar dust in the inner solar system 2004 after seven years of interplanetary cruise Almost 20 years ago, interstellar dust was and is investigating Saturn with its identified inside our planetary system by the magnetosphere, its and ring system. The dust detector onboard the Ulysses spacecraft Cosmic Dust Analyser (CDA), a joint [Grün et al., 1993]. A flow of µm-sized development of the MPI Nuclear Physics in interstellar grains has been identified at a Heidelberg and the DLR Berlin, provides unique distance of up to about five AU from the Sun. insights into the dusty environment of the large The observed flux was 1.5x10−4 m−2 s−1 of planet. The CDA determines the speed (1 - 100 particles with a mean mass of 3x10−13 g giving a km s-1), mass (10-15 - 10-9 g), electric charge (1 mass flux of 5x10−17 g m−2 s−1. The results fC - 1 pC) and elemental composition (m/Δm = showed, that interstellar dust enters the Solar 20 - 50) of individual micrometeoroids and System with 26 km s−1 and its upstream derives dust densities between 10-9 m-3 and 10 direction of 259° longitude and +8° latitude was m-3. The instrument has a mass of 17 kg, found to be compatible with the direction of the consumes 12 W and its nominal data rates are interstellar gas [Landgraf, 2000]. This finding between 128 and 4192 bps (Srama 2000, 2004). lead to the integration of a special interstellar Many years ago, its time-of-flight mass dust (ISD) observation period around 1 AU solar spectrometer was manufactured under strong distance in the early cruise phase of Cassini in contamination guidelines and procedures 1999. This observation with very limited

Srama et al., 2011 The Cosmic Dust Analyser onboard Cassini: Ten years of discoveries 2 spacecraft resources provided the first unique particles in interplanetary space and a detailed discovery of CDA, the in-situ measurement of description can be find in Kempf et al., 2004. individual ISD grains in the inner solar system The detector geometry with two inclined charge [Altobelli et al., 2005b].1 An ISD flux of sensing grids allows an accurate determination approximately 2.5x10-5 m-2 s-1 was detected of the particle speed, electrostatic charge, mass, between 0.7 and 1.3 AU heliocentric distance of and directionality. The dust charges varied grains with compatible with values between 1.3 and 5.4 fC corresponding to determined by Ulysses of 3x10-16 kg (Fig. 1). particles masses between 1.3x10-13 and 9.5x10-12 The decrease of the ISD flux at close kg. Here, for the first time, a reliable grain mass heliocentric distances can be explained by the determination was derived3.2 under In-situ the measurements assumption of the dust potential radiation pressure filtering effect, preventing the of a grain surface potential of +5 V. The particle smaller grains to reach the innermost regions of the Solar System. On the other hand, the spatial % .333!%#"/%20."02" density of big ISD grains is enhanced by . gravitational focusing at close heliocentric " distances. The deep penetration of ISD into the !. !% solar system started a series of discoveries of the # * * * * * .333!%!3/.$01"02. most advanced dust detector ever flown on an $ interplanetary mission. % " !% # * * * * * Electrostatic charges of interplanetary .333!2.$/.#0%%01$ $ dust particles % Cosmic dust particles are embedded in the " !% $ * * * * * heliosphere and interact with UV light of the 2 .333!2.!/%.01$01# Sun and solar wind particles. They are affected % . by a variety of charging mechanisms leading " normally to an equilibrium potential at the grain !. !% 45*6789:)*+;<- 2 * * * * * surface of about +5V. UV photoelectron % .333!2$$/.$0%!011 emission dominates the charging process in . regions with low plasma densities like the " !. interplanetary space. Other contributing !% ! * * * * * charging processes are sticking and penetration # %"""!"../.$01$02. of plasma particles, and secondary electron $ emission due to the bombardment of highly % " energetic plasma particles. !% !!" !#" !$" !%" " The CDA instrument has the capability to &'()*+µ,- determine the electrical charge of incident dust grains with a sensitivity of approximately 1 fC. Figure 2: Measurement signals of the CDA entrance grid channel (QP) of IDPs carrying electrostatic Between 1 AU and 2.1 AU heliocentric distance Figure 3.1: QP signals whichcharges show between a clear 1.3 trapezoidaland 5.4 fC recorded feature causedduring the by charged grains travers- CDA registered six impacts showing a clear interplanetary cruise in 1999 and 2000 between ing through the entrance grid system. The reconstructed charge features using the parameters charge signal of interplanetary dust particles Earth and Jupiter. The reconstructed charge features given in Tab. E.4 are indicatedare indicated by blue by lines.thick lines The (Kempf broken et lines al., show2004). the signal baseline and the (IDP) (Fig. 2). This was the first interval. unambiguous From KempfThe et impact al. (2004 speeds). are 15 km s-1, 21 km s-1, detection of electrostatic charges± carried by dust 34 km s-1, 13 km s-1, 45 km s-1 and 34 km s-1.

Clearly, the signal reconstruction algorithm (appendix F.1) failed to fit the recorded signal. 1 The ISD observations were performed using extremely low data rates. Highly processed and compressed science data were transported Thisby the is instrument no surprise housekeeping since the data signal channel. model Furthermore, used (Eqs. no spacecraftF.1 - F.5 pointing) describes a signal recorded changes were allowed. linearly. Therefore for this QP-signal, t0 ...t3 and Qd were evaluated manually. It should be mentioned that although the non-linear signal recording does not affect the accuracy of the grid passing times, a significant degradation of the Qd accuracy cannot be avoided. Thus for the event detected at 1999-314T16:22:54, the given value for Qd is most likely underestimated. Srama et al., 2011 The Cosmic Dust Analyser onboard Cassini: Ten years of discoveries 3 The signals detected at 1999-289T14:50:31 and at 1999-344T14:28:55 clearly show a dip between t1 and t2 indicating that these grains were penetrating the QP detector far off the symmetry axis and close to the walls. Although the dip depth dependency upon the minimal wall distance was calibrated in laboratory experiments (Eq. 2.19), this result was not used to constrain the grain’s trajectory. This is mainly due to the fact that knowledge of the minimal wall distance alone does not allow one to draw quantitative conclusions about the impactor’s

31 properties are in agreement with the Laboratory calibration of the CDA flight spare interplanetary dust model predictions and the using the Van de Graaff dust accelerator in grain speeds are close to the predicted speed of Heidelberg indicates that the first particle had a circular Kepler velocities of micrometeoroids at mass of 9[+55,-18]x10-14 kg and struck CDA at the Cassini location during the time of detection. a speed of 18+/-10 km s-1. The second particle had a mass of 1.4[+1.9,-0.8]x10-12 kg and Composition of Interplanetary Dust impacted at a speed of 7.7±4.6 km s-1. The time of flight mass spectra for both impacts Particles are shown in Fig. 3. The mass resolution of the Early in Cassini's interplanetary cruise phase, spectra is reduced by special instrument settings, before the of the Jovian system, the CDA's due to bandwidth limitations. Calibration of the Chemical Analyser subsystem detected two spectrum for impact 1 was complicated by the interplanetary dust particles (Hillier et al., triplet of features between 4.5 and 5.2 µs. The 2007). During this time the instrument was not Rhodium peak (ions from the instrument target) in full science mode and serendipitous science is usually used for calibration onto a mass scale. data was returned with engineering and Comparison of the first impact spectrum with housekeeping data. The first IDP spectrum was the second allowed the Rhodium peak to be recorded on May 27th, 1999, when Cassini was reliably identified and mass scales for both 0.89 AU from the Sun. The second spectrum spectra to be derived. Numerical ion trajectory was recorded at a heliocentric distance of 1.87 modelling, comparison with laboratory AU, on the 10th November 1999. calibration spectra and solar system elemental

Apparent ion mass (amu) 1 5 10 20 30 40 50 60 80 100 120 140 160

Fe 100 s Impact 2

80 Rhs 60 H 40 Fe Rh 20 C f f RhC/Fe2 RhFe/Fe3 0 0

V) 3 Si µ * 6 K

Fe Fe f s Impact 1 Signal ( 100 Rhs 80 Rhf 60 H 40 C

RhFe/Fe3 20 RhC/Fe2 0 0 H O Si 2 * 2 K

0 1 2 3 4 5 6 Time µ(s) Figure 3: CDA Chemical Analyser time of flight mass spectra of two interplanetary dust particles (Hillier et al., 2007). The peaks labeled * are an instrument artefact. For clarity the spectra are shown inverted and magnified. The magnified view of the Impact 1 spectrum is magnified by a factor of 9, that of Impact 2 by a factor of 4.5. Carbon and are common contaminants of the Rhodium impact target but may also occur in the interplanetary dust particles.

Srama et al., 2011 The Cosmic Dust Analyser onboard Cassini: Ten years of discoveries 4 abundances indicate that the spectrum of impact 165 in year 2001 the CDA sensor recorded 7283 1 exhibits features of fast non-thermal (Fef and stream particle impacts including 836 time-of- 2 Rhf) and slow thermal (Fes, Rhs) ions . Both flight (TOF) mass spectra corresponding to a grains were found to be similar in composition, Jovian distance of 1.1 AU at the inbound predominantly Fe with surprisingly few traces of trajectory and about 2 AU on the outbound silicates. The apparent compositions of the trajectory. Similar to Ulysses and Galileo, particles, together with their detection locations Cassini CDA observed bursts of high dust and impact velocities indicate that they are in impact rates with durations between one and ten asteroidal-type orbits (Aten and Apollo) days. The time interval between the two strong although cometary-type orbital solutions are impact bursts at day of year (DOY) 251 and possible. DOY 266 in the year 2000 is approximately 15 days which agrees well with dust streams Composition of Jovian dust streams modulated by the IMF with a four sector structure (Kempf, 2007, Habil.). Postberg et al. Streams of nanometre sized dust grains (2006) analysed 287 spectra of these two major originating from Jupiter‘s moon Io were burst periods and he found nine distinct features discovered and investigated by the dust labeled with Fx (Fig. 4) in the mass spectrum instruments onboard the interplanetary space with positive ions. The dominant spectral missions Ulysses and Galileo (Grün et al., 1993, features are H+ (F1), C2+ (F2), C+ (F3), O+ Grün et al., 1996, Krüger et al., 2003, Krüger et (F4), Na+ (F5), Si+ (F6), S+/Cl+ (F7), K+ (F8) al., 2006, Graps et al., 2000). These particles are and Rh+ (F9). H+ and C+ are mainly attributed released by Io‘s volcanoes, become charged in to contaminations and Rh+ represents the target the plasma torus around Jupiter and are material. Therefore sodium, chlorine, sulfur and accelerated to speeds as high as 400 km s-1 potassium are the main particle components. leaving the Jovian system before coupling to the Due to a clear quantitative correlation NaCl is interplanetary magnetic field in interplanetary the parent molecule of the majority of the space. recorded Na+ and Cl+ ions. Silicates and sulfur The Cassini flyby of Jupiter in 2000 provided or sulfurous components might be another the unique opportunity to investigate the Jovian constituent of the stream particles, which have dust stream phenomenon by a sophisticated dust grain sizes of approximately 24 nm. From this detector. Between day 248 in year 2000 and day chemical fingerprint, recorded approximately 6 Dust streams

F1 Figure 4: Dust spectrometry of F3 F9 Io‘s volcanic ash recorded more than 80 Mkm away from Jupiter. F5 A sum spectrum of 30 TOF mass spectra of Jupiter‘s nanometre F4 F8 log amplitude F2 F6 F7 sized stream particles shows nine

major peak features indicating a 4574684984:((;!*+,-.<=4555=>?=%@!-A+.BCA2DE 56 NaCl rich composition. The lower spectrum shows a stretched y- '()*+,-./01/2(3(4553 scale (Kempf, 2007). $%& # 0 1 2 3 4 5 6 Time (µ!"

Figure 6.10: Sum spectrum of 30 typical TOF spectra. Nine major peak features can be identified (see Tab. 6.2). The lower spectrum shows a stretched y-scale. Note that the well defined shape of F7 only2 The shows peak up inshapes a sum reflect spectrum. the and angular distribution of the ion species in the impact plasma. The CDA instrument with its integrated linear TOF mass spectrometer does not compensate for different ion . The initial ion energy distribution is dependent on the particle impact energy and impact velocity. occurrence and strength of a spectral feature depends on a number of factors which have to be considered.

Postberg et al.Srama (2006 )et analysed al., 2011 287 The spectra Cosmic comprising Dust Analyser the first onboard two Jovian Cassini: dust Ten streams years of reg- discoveries 5 istered by CDA around the days 251 and 266 in 2000. The impact rates during this period is shown in Fig. 6.9; the data set is also discussed in Section 6.3.1. Later stream particle data were not considered in this analysis because later Jovian streams were recorded under different and constantly varying impact conditions and are of very limited use for a statistical analysis. Besides that, the first two streams provided the highest impact rates and showed the best peak definitions. The ion masses were assigned by means of the cross-correlation technique which was used successfully for calibrating the TOF spectra of Saturnian stream particles (Kempf et al., 2005a) and E ring particles (Postberg et al., 2007).

Tab. 6.2 lists properties of the 9 most distinct spectral features appearing in the analysed spectra. In general only the 4 most abundant (F1, F3, F5, F9) and F8 show a well defined shape. The weaker features are often irregular or barely separable from background noise in individual spectra. This can be improved by co-adding and creating sum spectra as shown in Fig. 6.10. In contrast to E ring particle impacts, the formation of molecular or cluster ions as is unlikely because of the high energy densities after a Jovian stream particle impact. Thus, all the distinct features shown in Tab. 6.2 are most likely of atomic origin.

Except for the broad features F2 and F7, the lines listed in Tab. 6.2 could be identified by the position of the peak maximum. The average maximum position of F2 cor- responds to a mass of 5.2 amu. Since no isotope with a mass of 5amu exists, F2 is likely to represent a doubly charged species. Because there is no clear evidence for B+ (isotopes with 10 and 11amu) in the spectra, a strongly shifted C2+ mass line is the best candidate. This assumption is supported by the fact that there is a clear anti-correlation between the abundance of C+ ions (F3) and F2.

Four main species which are familiar from calibration experiments can be found in almost

104 Figure 1

48.&$9%)'&6':$&;(9'*< / : +1,, +0,, ++,, +,,, =,, >,, 3,, ?,, 2,, -,, 1,, 0,, +,, 02' ' 32 ' +02 +2, ' 2

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0 '

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, ' 02 2, 32 +,, +02 +2, +32 0,, 002 02, 4$5.'67'0,,-

Figure 5: Discovery of stream particles by CDA originating from the Saturnian system during the approach phase to Saturn between January 10 and September 6 in 2004 (Kempf et al., 2005, ). CDA was powered off during the orbit insertion phase (SOI) between 20 June and 1 July indicated by the vertical stack of bars. The upper scale gives Cassini’s distance to Saturn in RS. In total, 1,409 impacts were detected: 546 before SOI and 863 impacts after SOI).

100 million km away from Jupiter, Io’s 20 than the magnetic field of Jupiter, the volcanoes are again identified as the source for escaping grains are either slower or smaller than the vast majority of stream particles detected. It in the Jovian case. In order to search for such is remarkable, that such an in-situ analysis far streams originating from Saturn, special away from its source allow us to learn about the observation campaigns were integrated in the thermodynamic and chemical properties of the approach phase to Saturn starting in January volcanoes and its plumes and even to look inside 2004. The planning efforts were rewarded a small planetary body. Based on these immediately by the detection of faint bursts of observations, Postberg et al. (2006) suggested a nanometre sized stream particles starting at condensation cascade inside the erupting distances as far as 1200 RS (70 Mkm) away volcanic gases around pryoclastic silicate cores. from Saturn (Fig. 5). The burst intensities and Sodium and potassium condenses prior to impact rates seemed to grow with decreasing sulfuric compounds (SO2) due to their high distance to Saturn implying an origin from the condensation , explaining the low Saturnian system. Before Cassini‘s orbit contents of sulfur in the mass spectra. insertion 546 impacts were detected. According to the instrument calibration, the impact speed Science at Saturn exceeded 70 km s-1 from a direction which excludes the so-called β-, which is in Discovery of Saturn Stream particles agreement with the model predictions of The potential mechanism to accelerate particles velocities above 100 km s-1, and which excludes within Jupiter and Saturn‘s magnetosphere12 to clearly bound interplanetary particles with their speeds exceeding the planet‘s low relative impact speeds (<20 km s-1). Besides was discussed by e.g. Horanyi et al. (2000) and the Jovian system, Saturn with its ring system is by Kempf (2007). In order to escape from now the second source of very tiny dust particles Saturn, positively charged grains have to be (masses below 10-21 kg) moving through our accelerated by the outward-pointing co- solar system with speeds above 100 km s-1. rotational electric field caused by Saturn’s Generally, it is even possible that particles from rotating magnetic field. Horanyi predicted the the Saturnian system enter the Jovian occurrence of tiny grains released by the magnetosphere and vice versa. Saturnian system along the ring plane. Since the What do we know about these particles besides magnetic field of Saturn is weaker by a factor of their masses and speeds? Is it possible to derive

Srama et al., 2011 The Cosmic Dust Analyser onboard Cassini: Ten years of discoveries 6 6.2 Saturnian dust streams

! ! &5(!6)7)'50* &;(!57<57) Figure 6: Co-added TOF mass spectra 9:- of the four composition types: silicates, alkali, ice and . The - , - CDA spectra of stream particles 8) - - 45 8), - indicate that the majority of the nano- " = grains consists of silicate material, about 30% of them have mantles of water ice and/or clathrate hydrates of (Kempf et al. 2005b). ! ! ! ! ! &'(!)'*! ! ! ! &.(!/)0123*/! ! !

723&5BE7)0F.*( ! 9:8)- - ,- 4 - + , - # 4+ $ ! >?% >?> >?% @?>

! ! " # $ % " # $ % A)B*!2C!C7)3:0!&D6(

Figurethe 6.6: compositionCo-added spectra of theof four nanometre compositiontypes sized produced byaccording selecting individual to the identified lines of each + + + + + + spectramicrometeoroids? with qe < 15fC and showing The answer Si , SiO 2 is, or yes, Fe (a although - silicate), Na individualor K (b - alkali), spectrum O (Fig. 6): silicate type for and H O+ (c - ice), and N+ and NH+ (d - nitrogen). In each case features of the other types by3 employing a special4 spectra processing spectra showing at least Si+ or SiO +, or Fe+ are still apparent. The inset shows the RhSi-cluster-ion line obtained by summing up all spectra 2 + withmethod a clear Rh usingline and anq e iterative< 15 fC relative method to the in Rh order peak position to (inpeaks,µs). From alkali Kempf type for spectra showing at least et al.magnify (2005b). the weak features. The most prevalent Na+ or K+ peaks, water ice type for spectra

mass lines, C+, O+, Na+, K+, Fe+, SiO2+ , and Rh+ showing both, O+ and H3O+ peaks, and + + ice, waswere found. identified There is no by indication, adding however, spectra of of water similar cluster ionsammonium(H2O)xH3 O type, with for spectra showing both, N x = 1,2,...strengthtypically and found with for similar impacts features. by water ice Then, grains each at low impactand energies NH3+ (Timmer- lines. 74% of the spectra are of mann and Grun,¨ 1991; see also Section 4.2.4). About 25% of the spectra also showed N+ and individual spectrum was+ scanned for these lines silicate type (32% of them simultaneously a line at 18amu which may be NH4 . Likely candidates for the frequent strong line at 28amu +and an +individual and improved+ mass scale+ was belong to the ice type), but only about 7% of the are CO and Si . There is evidence of SiO2 and Fe in the data which are building blocks of typicalallocated. silicate minerals A new such sum as spectrumolivine (Mg was,Fe) 2 calculatedSiO4. Laser-ionisation spectra experiments are of ice with type but show no silicate lines. silicatesusing such asthe olivine, improved serpentine, mass and scales. Murchison This meteoriteprocedure performed The to preponderance simulate impacts of spectra with silicate lines 1 1 + + at speedswas between repeated 23kms until and the 65kms resolution yielded of the Fe spectraland SiO 2 linessuggest (Ahrens that et al., the 2003 majority). of the detected grains There was also indication for a weak 28amu line in the laser experiments which was attributed + lines of the sum spectrum did not improve.1 consists+ of silicate material. If the water ice lines to Si . At stream particle impact speeds 100kms the number of Si ions will be larger Finally, a consistent data ⇥ set of calibrated are not+ due to target contaminations then about a than in the laser experiments. In addition, the 60amu line attributed to SiO2 correlates with the 28amuindividual line. Thus, mass assigning spectra the 28amu was obtained. line in the Saturnian Besides streamthird particle of mass the spectra silicate grains had ice mantles, + to Si seemsthe aforementionedto be the more natural ions, assumption. clear evidence for whereas solid ice grains are rare. H3O+, which is characteristic for water ice, was Saturn’s main ring is thought to be primarily A remarkable finding is the presence of lines at approximately 115amu, 119amu, and found. There is no indication, however, of water made of water ice, possibly containing clathrate cluster ions (H2O)xH3O+, with x = 1,2,... hydrates of ammonia or and a minor typically found for impacts by water ice grains amount of impurities97 most likely -bearing at low impact energies. About 25% of the silicate compounds. E ring particles dominantly spectra also showed N+ and a line at 18 amu consist of pure water ice. Ring particles (NH4+). Furthermore, there is evidence for Si+ originating from Enceladus plumes also contain (28 amu), SiO2+ and Fe+, which are the building impurities – probably tiny silicate inclusions or blocks of typical minerals like olivine organic compounds (Postberg et al., 2007). If the ((Mg,Fe)2SiO4). Carbon dominates all spectra plume particle inclusions are silicates, then the but is probably caused by target contaminations. Enceladus plumes are an important source of Four compositional types were defined Saturnian stream particles. In any case, in-situ

Srama et al., 2011 The Cosmic Dust Analyser onboard Cassini: Ten years of discoveries 7 6.1 Generation of dust streams

) ) ) ) ) ) studies of stream particles originating from the !"" 0 ! inner Saturnian system allowed the first and $" ) ;# only study of the composition of the ring system and of the moons. #" ) !"

%&'()*+%,&')-./0 !" ) ;

Dynamics and origin of stream particles <'=+7>)?>68=,(@)-A/' ) ) ) ) ) ) ! !" The small size of stream particles generally $

)-10 " leads to high charge-to-mass ratios such that $ electromagnetic forces significantly determine # 2 3 4 !" !# 5.,(,+6)78',(,8.)-9 0 their dynamics. To overcome Saturn’s , : the charge-to-mass ratio Q/m of the grain has to Figure 7 : Escape velocity of Saturnian stream Figure 6.1: Escape velocity (6.7) of Saturnian stream particles as a function of the initial po- be sufficiently large. This constraint setssition an versus theparticles dust radius as shown a function as a colour of scalethe initial (top panel). location The bottom panel shows the (distance from Saturn) versus the dust radius upper limit on the radius of a grain escapingequilibrium potential ⇤d used for calculating vex. The vertical dashed lines mark the outer edge of the A ring andshown the location as a of colour Saturn’s scale moons (top panel). and . The From bottom Kempf et al. (2005a). from the inner Saturnian system. There is also a panel shows the equilibrium potential Φ used for calculating the exit speed vex. The vertical dashed lower size limit, as Q/m has to be sufficientlyhappens approximately when lines mark the outer edge of the A ring and the small that grains circle along a magnetic field ⇧B location of Saturn’srL moons< Dione0.1 and Rhea. From (6.4) line and become tied to Saturn’s magnetic field. B Kempf et al. (2005a, Nature). (Alfven´ and Falthammar,¨ 1963), where Of the first order, only positive charged particles are accelerated outwards in Saturn‘s v md rL = ⇤ (6.5) ring icy grains. This impliesB Q that stream particles magnetosphere. The grain charge results from a | | d are of the same origin as E ring dust grains - competition between various charging processesis the particle’s Larmor radius and v the relative grain speed perpendicular to the B-field ⇤ like collection of ions or electrons, emissiondirection. of This constrainEnceladus. sets an lower limit on the grain size of stream particles Outside Saturn’s magnetosphere, the1 dynamics photo electrons and emission of secondary 2 of the grains⇥0 B are0 governedRp byµp their interaction s2 = 1 ⇤ . (6.6) electrons. There are two regions with a low min 2 3 d 10⌅dp r0 pr plasma density where the dust surface potential with the IMF, convected⇥ ⇤ by the0 solar wind. As the Cassini spacecraft crosses the compression is expected to be positive (photo emission)The magnetospheresand of with strong magnetic⌦ fields can↵ be envisioned as huge mass regions of the Co–rotation Interaction Regions which are considered as stream particlespectrometers: origin: only particles within a narrow size range bounded by Eqs. (6.6) and (6.3) can escape from the planetary(CIRs), gravitynot only field the and directionality form dust streams. of Numericalthe impacts simulations of the The outskirts of the A ring and theformation region of Jovian dust streams by Horanyi´ et al. (1993a) showed that the escaping particles changes with the field direction, but also the outside of approximately 7 Saturn radii form(Fig. a warped7, dust sheet – termed “ballerina dust skirt”. impact signal and rate vary with an increase of Kempf et al., 2005a). The dark region between 3 As an examplefield the model strength is applied (bursts to the properties occur). of Saturn’s At large magnetosphere. solar The lower and 6 Saturn radii correspond to negativeand upper limits for the stream particle size are distances the azimuthal component of the IMF surface potentials. It is a dynamical barrier that 1/2 1/2 3/2 3/2 dominates,⇤d RS and thusRS the out-of-ecliptic prevents nanodust particles located within smin or 10nm 2.5 1 ⇥ component1V r 0 of the dustr0 velocity should be inward this region to escape. Theoretically, the ⌅ ⇧ ⌅ ⇧ ⌃ ⌅ ⇧ ⌥ stochastic charging behaviour of nanodust affected most. As the inertial spacecraft velocity of about 10 km s-1 is small compared with the mitigates the negative potential barrier. dust speed, deviations of the impact direction 89 Nevertheless, in addition to the negative charging barrier, the sputtering of nanoparticles from Saturn’s location are attributed to a bending of the dust trajectories by the Lorentz in this dense plasma region limits the dust force due to the IMF. Monitoring the dust lifetime and indicates that only sputtering streams from Saturn’s magnetosphere is useful resistant material can survive and become for identifying their source, and for stream particles. This escape process has been understanding both the acceleration mechanism examined by Hsu et al. (2011). Combining CDA and the coupling between dust (including the composition analysis and the backward tracing simulation results, the authors suggested, that rings) and the magnetic field and the plasma environment in the magnetosphere (Hsu et al., Saturnian stream particles are dynamically old, 2011). silicateous relics released from plasma-eroded E

Srama et al., 2011 The Cosmic Dust Analyser onboard Cassini: Ten years of discoveries 8 G ring encounter 0.1 s-1. There is some evidence by laboratory experiments that the particle size was at least The optical properties of the G ring are 100 µm because a foil damage was not observed dominated by micron-sized dust grains, which in impact experiments with 60 µm iron particles (Kempf, 2005). counter radius impact # counter radius # of counts (µm) (µm) Assuming a particle moving in a gravitationally m1 1.2 356 M1 2.1 278 bound circular orbit, the impact speed during the m2 2.4 55 M2 4.2 46 crossing was approximately 8 km s−1 m3 4.9 3 M3 8.6 1 m4 6.7 0 M4 11.9 1 corresponding to grain radii exceeding 12 µm (Tab. HRD). Note, however, that this is only a TableTable 2: Mean 1 particle: Mean radius particle required radius to increment required an HRD to increment counter during the G ringan encounter HRD counter on 2005-248. during The the relative G ring speed encounter of the ring particles on to the lower limit for the size of the grain causing the 1 detector2005-248. was about 8The kms relative, the grains speed were of assumed the ring to be particles ice particles. to Note possible damage to the M sensor. The small that the radii given for the highest thresholds are the minimum grain size. the detector was about 8 km s−1 and the grains HRD sensor was not affected by the G ring were assumed to be ice particles. Note that the radii 1 about 8 kmsgiven corresponding for the highest with thresholds grain radius are exceeding the minimum 12 µm (see Tab. 1).crossing although the foil thickness of this Note, however,grain that size. this The is only instrument a lower limit has for a small the size detector of the grain causing thesensor is only 6 µm. damage tousing the M a sensor. 6 µm Thus, thick one foil can (counter only speculate mx), aboutand a the large size of this parti- cle. Since there was no damage to PVDF foils observed in laboratory experimentsThe HRD finding of extremely large ice detector (counter Mx) using a 28 µm thick foil. with grains smaller than 30 µm, the grain radius was probably significantly largerparticles at the outer G ring rim is consistent than this.Four The smallcounters HRD are sensor allocated was not to affected each sensor. by the G The ring crossing (Fig. 4) althoughcounters the foil thicknessrepresent of different this sensor signal is only amplitudes 6 µm (without and coating). Thiswith observations by the Cassini plasma camera seems to suggesttrigger thatthresholds. the particle which caused the damage was statistically notMIMI, which measures the depletion of relevant for the G ring population. electrons in the plasma environment. Large clumps of material obscure the motion of Distance to Saturn (R ) are expected to erode quicklyS by sputtering. 3 2.95 2.90 2.85 2.80 2.75 2.7 2.75 plasma particles gyrating around the field lines 1.4 ) m1

1 Because G ring dust is considered to be 1.2 m2 1.0 m3 of Saturn‘s magnetic field. 0.8hazardous for Cassini, the spacecraft trajectoryM1 0.6 M2 0.4 M3 HDR rate (s 0.2shall not cross the inner core of the G ring. 0.0 CAT

) 15

1 Traversals of the rims are allowed provided theIID WALL 10spacecraft uses its high gain antenna as a dustNOISE 5 DA rate (s shield. On day 248 of 2005 at 10:42 UTC 150Cassini passed the outskirts of Saturn’s G ring 100(Fig. 8). CDA’s big impact ionisation target with DUST RAM (°) 50 its0 field-of-view (FOV) of +/-45° was sensitive CDA 10:20 10:30 10:40 10:50 11:00 11:10 11:20 11:30 11:40 11:50 12:00 Hours 2005248...248 to ring particles only until about 10:40 h, while 12:00 0.4 Figurethe 3: Impact High rates Rate recorded Detector by the HRD (HRD, detector (upper Srama panel) et and al., by the 11:50 DA (middle panel) in the vicinity of the G ring. The gap between the two HRD 2004) with its PVDF foils and its significantly 11:40 ”impact spikes” is an artefact due to the incomplete transmission of the individ- 0.3 11:30

larger angular FOV could detect ring particles )

ual event times during this period. The masses corresponding with the 8 HRD S countersuntil are given about in Tab. 11:00 1. The shaded h. area Thus, marks periodsboth when detector the DA was 11:20 saturated. The lower panel shows the angle between impact direction of grains 0.2 in circularsubsystems prograde bound were orbits capable and the to detector detect boresight. ring particles. For angles larger 11:10

than about 80 the HRD starts to be insensitive. 11:00 During the encounter both HRD sensor foils 0.1

10:50 worked nominally. There were a significant Height above ring plane (R

10:40 number of impacts exceeding the higher 0.0 thresholds. At 2005-248T10:38:253 the M- 10:30 sensor3 registered an event exceeding its highest 10 2.75 2.80 2.85 2.90 2.95 3.00 Distance to Saturnian pole axis (R ) threshold M4. Unfortunately, this event probably S Figure 8: Spacecraft trajectory projected into a triggered a higher sensitivity of the M-sensor Figure 2: Spacecraft trajectory projected into a meridian plane of Saturn. The shadedmeridian area indicates plane the Gof ring Saturn. assuming The a vertical shaded height area of 960 km. leading to a background noise level during the indicates the G ring assuming a vertical height of -1 remaining tour with rates between 0.01 s and PVDF960 foil consists km (Kempf, of a thin film2005, of permanently Techn. Report). polarised material. A dust im- pact on the foil produces rapid local destruction of dipols causing a current pulse recorded by the electronics (see Fig. 1). The released current depends roughly on the grain size and on the impact speed. To make use of this dependence trigger the 3 28 µm thick PVDF foil with a sensitive area of 50 cm2impacts at least one of 4 counters with increasing thresholds. The detectors were calibrated with 6...65µm glass particles at the Munich plasma accelarator facility 1 1 with impact speeds between 2 kms and 12 kms . During these experiments no functional detector damage was observed.

Srama et al., 2011 The Cosmic Dust Analyser onboard2 G ringCassini: encounter Ten years of on discoveries day 248 9

On day 248 of 2005 at 10:42 UTC Cassini probably traversed through the out- skirts of Saturn’s G ring (Fig. 2). CDA’s Dust Analyser (DA) was sensitive to ring particles until about 10:40 while the HRD due to its significantly larger angu- lar FOV could detect ring particles until about 11:00 (see lower panel of Fig. 3). Thus, both detectors were capable to detect ring particles. During the encounter both HRD sensors worked nominally. There were a significant number of impacts exceeding the higher thresholds. At 2005-248T10:38:25 the M-sensor registered an event exceeding its highest threshold M4. Since then, the M1 counter of the 1 M-sensor shows a continuous low level noise (count rates between 0.01s and 1 0.1s ). Most probably this behavior is associated with the M4 event. Assuming a particle moving in a gravitationally bound circular orbit the impact speed was

2 Saturn‘s E ring Enceladus orbit, and a minimum of 8.000 km at the distance of Enceladus. The vertical thickness Saturn‘s outer E ring is one of the most exciting of the outer edge was determined to FWHM of astronomical objects in our solar system and it up to 40.000 km at 8 RS distance. Such a large allows to study the interaction of dust, plasma, thickness is caused by inclined submicron sized moons and the magnetosphere. The E ring is the particles, but this fact remained a major largest planetary ring in the solar system, challenge for the dynamical ring models. encompassing the icy satellites (r = M Dikarev (1999) included plasma drag effects to 3.07 R ), Enceladus (r = 3.95 R ), (r = S E S T the particle dynamics and he found a growth of 4.88 R ), Dione (r = 6.25 R ), and Rhea (r = S D S R the particles‘ semi-major axis. This allows the 8.73 R ). Optical remote sensing of the ring S grains moving in less eccentric orbits to cover shows a bluish color of the reflected light which the full radial range of the ring which in turn is typical for a narrow grain size distribution increases the dust lifetime. Horányi et al. (2008) between 0.3 and 3 µm. The Enceladus performed extensive numerical simulations of was proposed early as the dominant source of the long-term evolution of E ring particles ring particles since the edge-on brightness taking into account sputtering and dust particle profile peaks near the moon’s mean orbital erosion, explaining a ring which could even distance. For the same reason, Tethys was extend to distances as far as the orbit of . identified as a secondary E ring particle source Kernphysik keine klaren Hinweise auf das typischeIn-situ Ringteilchenmaterial investigations of Wassereisthe ring particles [2]. Al- by the by de Pater et al. (2004). Photometrical models lerdings wird schon seit langen aufgrund der leichtenCDA Verf detectorarbung¨ der helped Ringe to vermutet, unveil daß some die of the propose that for micron-sized grains the peak Ringkorper¨ winzige silikatische Verunreinigungensecrets enthalten. of Vermutlich the E ring. bestehen Srama die et Saturn- al. (2006) optical depth is τ 1.6x10−5 corresponding to Stromteilchen∼ aus den silikatischen Einschlussen¨ summarized im Wassereis der the Ringk firstorper¨ year des of . the in-situ aboutFreigesetzt 180 grains werden per die square Teilchen centimeter wahrscheinlich durchinvestigations Kollisionen of zwischen Saturn’s den dust einzelnen environment (Showalter,Eiskorpern¨ Cuzzi, in den & Saturn-Ringen. Larson, 1991). Anschließend The within werden the sie dann Titan aufgrund orbit by ihrer the winzigen Cassini dust Masse in den interplanetaren Raum geschleudert, anders als die mikrometergroßen Eispartikel vertical profile of the ring was investigated by detector CDA. In two publications a De Paterdes E-Rings, et al. (2004) die weit by schwerer Earth-bound sind infraredund die Anziehungskraft des Planeten nicht uberwinden¨ konnen.¨ comprehensive analysis of the CDA E ring data observations in 1995. The results showed a full– was given (Kempf et al., 2007; Postberg et al., width–half–maximum (FWHM) based on grain 2007). Kempf et al. (2007, 2010) investigated sizes dominating the optical cross section of Enceladus als Quelle des E-Rings the geometrical structure of the ring and about 9.000 km between the Mimas and the analysed the ring particle size distribution, while

FigureAbbildung 9: Discovery 3: Die of an linke active Abbildung dust source zeigt at diethe simuliertesouth pole Staubdichteof Enceladus in as der the Umgebung dominant source des Mondes of Saturn‘s E ring.Enceladus, Left: Simulated falls die dust Stauberzeugung density of impact aufgrund ejecta von in the Staubeinschl environmentagen¨ of der the dominierende moon Enceladus. Prozeß The w lineare;¨ representsdie linke the Abbildungtrajectory of stellt Cassini. die simulierte Right: Simulated Staubdichte dust um density den Mond of an aufgrundactive dust eines source Eisvulkanismus at the south pole in regionder of S Enceladus.udpolregion¨ Middle: dar. In beidenMeasured Fallen¨ dust markiert rate by the die CDA durchgehende subsystem Linie High die Rate Cassini-Trajektorie. Detector (HRD) during Die the flybymittlere and comparison Abbildung with vergleicht the two die model gemessenen predictions. Einschlagsraten The rate peaked mit den almost beiden one Modellrechnungen. minute before the closest approachQuelle: (C/A) M. Seiss,to the UniversitEnceladusat¨ surface Potsdam; and [3] is in agreement with an active source at the south pole (Spahn et al., 2006, Science). Untersuchungen der MPI-Physiker mit ihrem Staubdetektors an der vermuteten Quelle des E-Rings, des Eismondes Enceladus, legten die Grundlage fur¨ die Entdeckung von Eisvul- kanen auf dessen Oberflache¨ durch die Cassini-Kameras [3]. Am 14. Juli 2005 hatte sich SramaCassini et al., der 2011 Oberfl Theache¨ Cosmic des Enceladus Dust Analyser bis onboard auf 175 Cassini: Kilometer Ten genyearsahert.¨ of discoveries Das ermoglichte ¨ es 10 erstmals, die Staubverteilung tief innerhalb des Schwerkraftbereiches dieses Eismondes di- rekt zu messen und hierdurch die Produktion von frischen Ringteilchen untersuchen. Bisher hatte man angenommen, daß frische Staubteilchen durch ein Dauerbombardement der Ober- flache¨ des Enceladus mit Mikrometeoriten entweder interplanetaren oder E-Ring-Ursprungs erzeugt werden. Wahrend¨ ein Teil des freigesetzten Staubes eine annahernd¨ isotrope Wolke um den Mond selbst bilden wurde,¨ speisten die ubrigen¨ Staubteilchen den E-Ring des Plane- ten. Die Bildung von Staubhullen¨ um Monde aufgrund diesen Mechanismus wurde bereits mit

5 Postberg et al. (2007) studied the composition of chemical analysis of E ring grain compositions the ring particles. (see further below).

Enceladus ice geysers Extensive research is Another important source for ring particles are focused to the dust production mechanism of the secondary ejecta produced at the icy surface of ring’s main source, the icy moon Enceladus. the embedded moons. Ring particles, Producing fresh dust particles by impacts of fast interplanetary and interstellar dust are primary projectiles onto the moon’s surface (the so- impactors producing ejecta with broad size and called impactor-ejecta process) has been speed distributions. Ejecta particles with accepted as the most effective process. In 1997 velocities higher than the escape speed of the it was proposed that the E ring particles individual moons become ring particles (Spahn themselves constitute the main projectile source et al., 2006, PSS, Krivov et al., 2003). For this for replenishing the ring. However, energy process, they have to exceed speeds of 127 m s-1 considerations seem to favour the projectile flux (Mimas), 205 m s-1 (Enceladus), 338 m s-1 being dominated by particles of interplanetary (Tethys), 462 m s-1 (Dione) and 592 m s-1 and interstellar origin. Angular distributions of (Rhea). E-ring impactors are a relevant sources ejecta generated by various populations of for secondary ejecta for the inner moons and interplanetary dust particles (IDPs) were studied especially Enceladus and Tethys, where the by Colwell (1993). Based on this Spahn et al. ejecta mass production can be as high as 10 kg (1999) numerically studied the spatial ejecta s-1 or 3 kg s-1, respectively. However, only 0.014 distribution in the vicinity of Enceladus. Their kg s-1 are expected to escape from Rheas surface predictions were used to optimise the CDA due to E ring impactors, whereas the surface measurements during the close flybys of ejecta of interplanetary meteoroids are leading Enceladus in 2005. to a mass flux of 0.019 kg s-1 (Spahn et al., This flyby on 14 July 2005 at an altitude of 170 2006, PSS). Therefore the outer moon Rhea with km was supposed to become the most its encircled dust cloud is an excellent target in spectacular flyby of the Cassini mission. During order to investigate the infalling interplanetary this special observation CDA discovered a dust flux at Saturn. The low interplanetary dust collimated jet of micron-sized dust particles flux at the distance of Saturn of emerging from Enceladus’ south pole region Fimp = 1.8x10-16 kg m-2 s-1 is „amplified“ by the (Fig. 9). Spahn et al. (2006b) inferred from impactor ejecta process for icy surfaces comparison of numerical simulations with the according to M+ = FimpY S, with the cross CDA data that the particles emitted by the ice section S of the source satellite, the mass geysers at the south pole are the dominant production rate M+ and the yield Y = 2.64x10-5 source in the inner E ring. Recently, models for mimp0.23 vimp2.46 . mimp and vimp are the speed and grain condensation and growth in the geyser mass of the impactors (IDPs), respectively, and channels were developed based on the Cassini Y is the fraction of the ejected mass to that of data (Schmidt et al., 2008) which explain the the projectile and was determined by laboratory observed particle speed and size distributions experiments. Using a relative impact speed of within the dust plume. Scientists assume a the interplanetary grains with respect to the geyser venting area of approximately 200 m2 moon surface of 9500 m s-1 and the dominating with a total dust production rate of about 5 kg s-1 particle mass of 1x10-8 kg, the yield Y becomes of which ~10% escapes the satellites‘ gravity. as high as 18.000 for Mimas and 7.500 for Rhea, On the other side, the mass loss of the E ring making it possible to determine the was calculated to be approximately 1 kg s-1. interplanetary dust flux during one low altitude Significant insight into the processes of the ice flyby of the moon Rhea (~100 km altitude, geysers below the icy crust of Enceladus were within the moon‘s Hill sphere). The Cassini achieved by Postberg et al. (2009, 2011) by the flyby at Rhea on January 11th, 2011, occurred

Srama et al., 2011 The Cosmic Dust Analyser onboard Cassini: Ten years of discoveries 11 2004-200T00:00:00 - 2008-259T20:00:00 n m-3 1.0 4 0.20 0.040 0.0080 0.0016 ] S 0.00032 2 6.3E-05 1.3E-05 2.5E-06 5.0E-07 1.0E-07 0

-2 Altitude above ring plane [R

-4

0 5 10 15 Distance from Saturn [RS] Figure 10: Global apparent dust density measured by CDA in the saturnian system. The densities are color coded along the Cassini trajectory in the time range of 2004-200 to 2008-259 (Cassini prime tour). High impact rates and densities are observed outside the optically measured E ring, which was defined between 3 to 9 RS. Enhanced dust densities are found as far as 250.000 km away from the ring plane and extends to radial distances of 20 RS and beyond. with an altitude of only 75 km and the CDA necessary. The fragmented observation timeline instrument pointing towards the dust RAM of Cassini (pointing changes), the small amount direction made it possible to acquire valuable of pointing prime time for in-situ instruments data of highest quality for this purpose. The and the spacecraft flight rules (forbidden Cassini results of this ejecta cloud measurement are attitudes primarily caused by IR instrument currently analysed and they will be published radiator heating) made this task a major elsewhere. challenge. Generally ejecta cloud measurements around It was a big surprise to find dust particles at Saturn’s moons with Cassini-CDA are very altitudes as high as 100.000 km above the ring difficult due to the high background density of plane and at Saturn distances of 7 RS early in the the E ring particles. It was one of the major Cassini tour (e.g. 28 Oct 2004). The derived dust CDA goals to characterise the extension and densities of particles larger than 0.5 µm reach sharpness of the edges of the largest planetary values of 0.001 m-3. The process to calculate ring in our solar system. An in-situ instrument dust densities from measured impact rates like CDA is approximately 1000 times more require the knowledge of the relative impact sensitive than optical remote sensing speeds, the projected sensitive instrument target instruments to search and characterise faint area and of the instrument mass threshold, dusty rings. However, in order to achieve a which itself is impact speed dependent. global view of the outer ring system, long Furthermore, the relative impact speeds are integration times with adequate pointing are calculated under the assumption of circular

Srama et al., 2011 The Cosmic Dust Analyser onboard Cassini: Ten years of discoveries 12 Figure 11: Radial density 100

profile of Saturn’s E ring in N(r)=40/(r-1.5)^4.7 the ring plane derived from

CDA data between 2004 -1 N(r)=20/(r-2.8)^4.6 and 2008 with a power law 10 fit (blue line).

-2 ]

-3 10

-3 Density [m 10

10-4 fit_Ering.pro Srama 10-5 5 10 15 Distance [RS] prograde orbits, but it is known, that E ring look like with the eyes of a sensitive in-situ particles are also highly eccentric, leading to instrument? For this purpose, the dust fluxes different relative impact speeds. Finally, the measured by CDA during the Cassini prime tour measured dust rates have to be corrected by (DOY 200 in 2004 until DOY 260 in 2008) were further instrumental effects like the dead time, processed and corrected for instrument settings, checkout times or threshold and triggers spacecraft pointing and relative impact speeds settings. Impact rates varied by seven orders of (assuming circular prograde particles). The magnitude between 10.000 s-1 close to result of this extensive data compilation is Enceladus and 0.001 s-1 in the outer ring shown in Fig. 10. For this visualisation, the regions. Therefore any densities given in this apparent dust density was plotted along the paragraph are apparent densities which might Cassini trajectory in a Saturn centered differ from real dust densities by a factor of 10. coordinate system in an edge-on view to the ring Taking the (conservative) CDA lower mass plane. The achieved pattern looks like pieces of threshold MT of MT [kg] = 3.04x10-13 v-3.75 [km a gift ribbon wrapping Saturn. Hidden or black s-1] into account (Srama, 2010), the calculated areas along the track represent periods where dust densities are giving the minimum densities CDA was not able to record meaningful data. of particles in a given volume. Grain size This was primarily caused by long periods of distributions are following potential laws such inadequate pointing of Cassini (the dust RAM that tiny particles are more abundant than larger direction was not in the field-of-view of CDA). grains (Kempf, 2007). Assuming a typical Furthermore, only counter data were taken into relative impact speed of 8 km s−1, the lower account in order to avoid any influence of the mass threshold would be 1.2x10−16 kg. This science data rate. Very tiny dust impacts like mass corresponds to compact water ice particles nanometre sized stream particles were not with a diameter of 0.6 µm. Faster impact speeds considered in this global overview, since this decrease this mass threshold significantly: would totally change the picture: Stream particle Impact speeds of 20 km s−1 already produce data can cause CDA impact rates as high 1 s-1 at enough impact charge for water ice grains with distances of 30 RS or beyond and the derivation diameters above 200 nm to be detected. of a picture for the bound E ring particles would not be possible anymore. The highest dust E ring extension How does the E ring of Saturn densities in the inner Saturnian region (along

Srama et al., 2011 The Cosmic Dust Analyser onboard Cassini: Ten years of discoveries 13 Figure 12 : Three-dimensional apparent dust density profile (blue) along Cassini’s trajectory on DOY 247 in 2010 (orbit number 137). Two projections of the density profile are shown on the side panels (black and red curve). The vertical axis gives the dust density n in units of log(m−3). The density decreases from 1x10-3 m-3 at 15.5 RS down to 8x10-5 m−3 at 20 RS and an extension of the ring beyond Titan‘s orbit is visible. This discovery of an extended ring beyond Titan‘s orbit was confirmed in a later equatorial ring scan on DOY 290 in 2010 between 10 RS and 25 RS distance.

Enceladus orbit at 4 RS) are a few particles per special ring scan on DOY 247 in 2010 was cubic meter and they are consistent with former integrated in the Cassini observation timeline. modelling work. Surprising is certainly the high This observation with adequate dust RAM dust density inside Titan’s orbit at 20 RS with pointing covered a time period of approximately altitudes up to 4 RS (250.000 km) from the ring 16 hours between 15.5 RS and 21 RS and plane. In the outer region, fluxes below 1x10−6 provided new insights into the outer structure of m−3 are measured. the E ring as shown in Fig. 12. Between 16 RS The results shown in Fig. 10 were taken to and 20 RS the dust density decreases by one derive the radial profile of the E ring close to order of magnitude starting at approximately the ring plane. The overall dust density is 8x10-4 m-3 and ending at approximately 8x10-5 decreasing by three orders of magnitude m-3. In this scenario, the mass threshold of CDA between 6 RS and 16 RS radial distance (Fig. 11) was 4,7x10-16 kg (water ice grain with 1 µm and the density can be described by a power law diameter) for a relative impact speed of 5.6 km of the type n(r) = 20 (r - 2.8)-4.6. The applicable s-1 for prograde circular dust grains at 20 RS. range of this law is constrained to the radial Most particle are expected to move in eccentric region between 6 and 15 RS. At the inner orbits at this distance, leading to deviations in boundary, the CDA dead time of 1 s limits the the relative impact speed and the lower mass number of recorded particles (instrument threshold, respectively. The density along the saturation), and at the outer boundary the small trajectory seems to have some fine structure, but number of impacts within one time bin leads to we have to take into account that at 20 RS only a higher noise. Starting at a distance of r =6 RS up to seven dust particles were counted within a with a density of 0.095 m−3 the density falls time bin of 10 minutes leading to a high down to 2.0x10−4 m−3 at 15 RS. fluctuation of the calculated rate and density. The outer extension of the ring is not well defined, but recent results indicate an outer Nevertheless, this is the first in-situ boundary which lies even beyond Titan‘s orbit. measurement of Titan‘s dusty environment and Due to the fact that Cassini science is focused at we can take the calculated apparent density of 1 Titan in the region around 20 RS distance, a µm water ice particles to estimate the influx of

Srama et al., 2011 The Cosmic Dust Analyser onboard Cassini: Ten years of discoveries 14 water into Titan‘s . Titan moves charges between 1 and 5 fC on interplanetary with 5.6 km s-1 and the dust grains have a dust grains. variety of eccentricities and we take a • For two interplanetary particles detected at 0.9 conservative relative dust impact speed of only AU and 1.9 AU solar distance, the integrated 2 km s-1. If we take now a Titan cross section of time-of-flight mass spectrometer determined 2.1x1013 m2 we get a collection volume of the elemental composition. These grains were 4.2x1016 m3 s-1 meaning that 3.3x1012 ice predominantly iron with surprisingly few particles are raining down on Titan. This process traces of silicates. would deliver a total amount of 1.6 g s-1 of • CDA provided even the chemical composition water into Titan‘s atmosphere. However, the of Jovian dust stream particles: nanometre assumed relative impact speed of 2 km s-1 is sized particles originating from Jupiter‘s probably too low, and the influx of tiny stream moon Io escaping from the Jovian system particles was also not considered in this with speeds up to 400 km s-1. These grains estimation. primarily consist out of NaCl with possible contributions of sulfur and silicates (Postberg The overall amount of dust within Titan’s orbit et al., 2006). was very surprising. The questions that are • CDA discovered dust streams escaping from immediately raised are: Do these particles the Saturnian system. The first stream was belong to one big, faint and extended E ring, detected 70 Mkm away from Saturn. The which was defined between 3 and 9 Rs earlier in grains are composed out of silicate material the literature? Are there gaps or short scale and about 30% of them have mantles density gradients in the extended dust consisting of water ice and/or clathrate environment? Are there dust density hydrates of ammonia. Source regions for these enhancements or different dust mass grains, which are basically slower and smaller distributions along satellite orbits? The most than their Jovian counterparts, are the inner A likely answer is yes, there is one big E ring ring and the E ring outside of Dione‘s orbit. showing a very complex fine structure which is The streams are correlated with properties of not explained until today. the IMF. Furthermore, CIRs play a major role in the stream flux and directionality. Summary • The HRD subsystem of CDA detected the outer rim of Saturn‘s G ring in 2005. This paper is a first attempt to summarize Unfortunately, this event probably damaged selected scientific results of the Cosmic Dust one of the HRD foil sensors leading to a Analyser onboard Cassini. CDA is the most higher noise rate. Fortunately, this detector advanced dust detector ever flown in remained functional during the ongoing tour. interplanetary space. It combines subsystems to • CDA was one of the major instruments measure the dust particles properties like speed, discovering the dust plumes at the south pole mass, trajectory, composition and flux of Enceladus during the flyby in July 2005. simultaneously and with high reliability. The The active ice volcanoes eject submicron and scientific results reported in this paper are: micron-sized particles with a mass flux of • The scientific success of CDA started already approximately 5 kg s-1 of which 10% can during the early cruise phase in 1999 by the escape replenishing the faint E ring. The dust discovery of the interstellar dust flux at one plumes of Enceladus became the dominant AU distance, which provided the basis for dust source of the E ring. Ejecta particles future planetary missions to measure generated at the surfaces of the outer moons interstellar dust in the vicinity of the Earth. by interplanetary or E ring impactors are a • After the Earth flyby in August 1999, CDA second source for E ring particles. measured for the first time, electrostatic • During the global characterisation of the E

Srama et al., 2011 The Cosmic Dust Analyser onboard Cassini: Ten years of discoveries 15 ring CDA discovered an unexpected wide of Titan‘s orbit. vertical and radial extension. The vertical extension is by a factor of 10 higher than observed by optical instruments. The radial extension reaches the orbit of Titan and beyond. This lead to a new understanding of the dynamics of the outer ring.

The major discoveries of CDA which are not part of this paper are: • Measurements of the ice grain potential in Saturn‘s E ring which lead to a new plasma model of the magnetosphere (Kempf et al., 2006, PSS) • The in-situ characterisation of the E ring dust particles in the vicinity of Enceladus was published by Kempf et al. (2008, Icarus). For the first time, in-situ profiles of the vertical and radial ring profile as well as particle mass distributions were given. Based on these results, a dust injection model of the ice volcanoes at the surface of Enceladus was developed (Schmidt et al., 2008, Kempf et al., 2010). This dust injection model was the basis for a new dynamic model explaining how the Enceladus dust plume feeds Saturn‘s E ring (Kempf et al., 2010). The dust plume model predicted patterns of snow layers at the surface of Enceladus which are in agreement with recent observations. • Planetologists concluded from CDA data the existence of an of liquid water below the icy crust of Enceladus. This scientific highlight is based on the observation of sodium salts in the E-ring ice grains and was reported by Postberg et al. (2009, 2011).

The long list of discoveries still leaves room for future exploration. Today, CDA science observations focus on an in-depth analysis of ice grain compositions close to Enceladus, seasonal effects of Saturn‘s ring system, the determination of the interplanetary and Acknowledgements interstellar dust flux at Saturn, the search for The Cassini Cosmic Dust Analyser science planning, retrograde dust populations and possible operations and science analysis is supported by the asymmetries in the ring system, a global Deutsches Zentrum für Luft- und Raumfahrt e. V. understanding of the grain composition in (DLR) on behalf of the BMWi under the project grant Saturn‘s E ring and the dust populations outside 50 OH 1103.

Srama et al., 2011 The Cosmic Dust Analyser onboard Cassini: Ten years of discoveries 16 References Conference, 21-26 June 2009, Saint-Malo, France Hsu, H., S. Kempf, and C. M. Jackman (2010b), Altobelli, N., Kempf, S., Krüger, H., Landgraf, M., Observation of Saturnian stream particles in the Srama, R., and Grün, E.: 2005, in C. C. Popescu interplanetary space, Icarus, 206, 653–661, doi: and R. J. Tuffs (eds.), AIP Conf. Proc. 761: The 10.1016/j.icarus.2009.06.033. Spectral Energy Distributions of Gas-Rich Galaxies: Confronting Models with Data, pp 149 Hsu, H., S. Kempf, F. Postberg, M. Trieloff, M. E. Burton, M. Roy, G. Moragas- Klostermeyer, and R. Colwell, J. E.: A general formulation for the Srama (2011), Cassini dust stream particle distribution of impacts and ejecta from small measurements during the first three orbits at Saturn, planetary satellites, 1993, Icarus 106, 536 J. Geophys. Res., 116, in press. de Pater, I., Martin, S. C., and Showalter, M. R.: Hsu, H., Postberg, F., Kempf, S., Trieloff, M., Keck near-infrared observations of Saturn's E and G Burton, M., Roy, M., Moragas-Klostermeyer, G., rings during Earth's ring plane crossing in August Srama, R., Stream Particles as the Probe the Dust- 1995, 2004, Icarus 172, 446 Plasma-Magnetosphere Interaction at Saturn, Journ. Dikarev, V.: Dynamics of particles in Saturn's E ring: Geophys. Res., submitted (2011) effects of charge variations and the plasma drag Kempf, S.: 2005, What happened to the HRD, force, 1999, Astron. Astrophys. 346, 1011 Technical report, MPI Kernphysik Graps, A. L., Grün, E., Svedhem, H., Krüger, H., Kempf, S.: 2007, Saturnian Dust: Rings, Ice Horanyi, M., Heck, A., and Lammers, S.: Io as a Volcanoes, and Streams, Habilitation, Univ. source for the jovian dust streams, 2000, Nature Braunschweig 405, 48 Kempf, S., Beckmann, U., Moragas-Klostermeyer, Grün, E., Baguhl, M., Hamilton, D. P., Riemann, R., G., Postberg, F., Srama, R., Economou, T., Schmidt, Zook, H. A., Dermott, S., Fechtig, H., Gustafson, B. J., Spahn, F., and Grün, E.: The E ring in the A., Hanner, M. S., Hor ́anyi, M., Khurana, K. K., vicinity of Enceladus I: Spatial distribution and Kissel, J., Kivel- son, M., Lindblad, B. A., Linkert, properties of the ring particles, 2008, Icarus 193, D., Linkert, G., Mann, I., McDonnell, J. A. M., 420 Morfill, G. E., Polanskey, C., Schwehm, G., and Srama, R.: Constraints from Galileo Observations Kempf, S., Beckmann, U., and Schmidt, J.: How the on the Origin of Jovian Dust Streams,1996a, Nature Enceladus dust plume feeds Saturn‘s E-ring, 2010, 381, 395 Icarus 206(2), 446 Grün, E., Zook, H. A., Baguhl, M., Balogh, A., Kempf, S., Beckmann, U., Srama, R., Horanyi, M., Bame, S. J., Fechtig, H., Forsyth, R., Hanner, M. S., Auer, S., and Grün, E.: The electrostatic potential of Horanyi, M., Khurana, K. K., Kissel, J., Kivelson, E-ring particles, 2006, Planet. Space Sci. 54, 999 M., Lindblad, B. A., Linkert, D., Linkert, G., Mann, Kempf, S., Srama, R., Altobelli, N., Auer, S., I., McDonnell, J. A. M., Morfill, G. E., Phillips, J. Tschernjawski, V., Bradley, J., Burton, M., Helfert, L., Polanskey, C., Schwehm, G., Siddique, N., S., Johnson, T., Krüger, H., Moragas-Klostermeyer, Staubach, P., Svestka, J., and Taylor, A.: Discovery G., and Grün, E.: Cassini between Earth and of Jovian Dust Streams and Interstellar Grains by belt: First in-situ charge measurements of the Ulysses Spacecraft, 1993, Nature 362, 428 interplanetary grains, 2004, Icarus 171, 317 Hillier, J. K., Green, S. F., McBride, N., Altobelli, N., Kempf, S., Srama, R., Horanyi, M., Burton, M., Postberg, F., Kempf, S., Schwanethal, J. P., Srama, Helfert, S., Moragas-Klostermeyer, G., Roy, M., R., McDonnell, J. A. M., and Grün, E.: Interplanetary dust detected by the Cassini CDA and Gru ̈n, E.: 2005a, Nature 433, 289 Kempf, S., Chemical Analyser, 2007, Icarus 190(2), 643 Srama, R., Postberg, F., Burton, M., Green, S. F., Helfert, S., Hillier, J. K., McBride, N., McDonnell, Horanyi, M.: Dust streams from Jupiter and Saturn, J. A. M., Moragas-Klostermeyer, G., Roy, M., and 2000, Phys. of Plasmas 7(10), 3847 Grün,: Composition of Saturnian Stream Particles, Horanyi, M., Juhasz, A., and Morfill, G. E.: Large- 2005b, Science 307, 1274 scale structure of Saturn‘s E ring, 2008, Geophys. Krivov, A. V., Sremcevic, M., Spahn, F., Dikarev, V. Res. Lett. 35(4), 4203 V., and Kholshevnikov, K. V.: Impact-generated Hsu, H., Kempf, S., Postberg, F., Srama, R., dust clouds around planetary satellites: spherically Jackman, C., Moragas-Klostermeyer, G., Helfert, symmetric case, 2003, Planet. Space Sci. 51, 251 S., and Grün, E.: 2010, in AIP Conference Krüger, H., Geissler, P., Horanyi, M., Graps, A., Proceedings, Vol. 1216, pp 510–13, American Kempf, S., Srama, R., Moragas-Klostermeyer, G., Institute of Physics, USA, Interaction of the solar Moissl, R., Johnson, T., and Grün, E.: Impact- wind and stream particles, results from the Cassini generated dust clouds surrounding the Galilean dust detector, Twelfth International Solar wind moons, 2003, Geophys. Res. Let. 30, 3

Srama et al., 2011 The Cosmic Dust Analyser onboard Cassini: Ten years of discoveries 17 Krüger, H., Graps, A. L., Hamilton, D. P., Flandes, Srama, R., and Grün, E.: Dynamics of dust ejected A., Forsyth, R. J., Horanyi, M., and Grün, E.: from Enceladus: Application to the Cassini dust Ulysses jovian latitude scan of high-velocity dust detector, 1999, J. Geophys. Res. 104, 24111 streams originating from the jovian system, 2006, Planet. Space Sci. 54, 919 Srama, R.: Kombination von Messsystemen zur simultanen Messung der Eigenschaften von Landgraf, M., Baggaley, W., Grün, E., Krüger, H., kosmischen Staub, 2000, Ph.D. thesis, Technische and Linkert, G.: Modeling the motion and Univ. München distribution of interstellar dust inside the heliosphere, 2000, J. Geophys. Res. 105, 10343 Srama, R.: 2010, Cassini-Huygens and Beyond - Tools for Dust , Habilitation, Univ. of Postberg, F., Kempf, S., Srama, R., Green, S. F., Stuttgart Hillier, J. K., McBride, N., and Grün, E.: Composition of jovian dust stream particles, 2006, Srama, R., Ahrens, T., Altobelli, N., Auer, S., Icarus 183, 122 Bradley, J., Burton, M., Dikarev, V., Economou, T., Fechtig, H., Görlich, M., Grande, M., Graps, A., Postberg, F., Kempf, S., Hillier, J. K., Srama, R., Grün, E., Havnes, O., Helfert, S., Horanyi, M., Green, S. F., McBride, N., and Grün, E.: The E ring Igenbergs, E., Jeßberger, E., Johnson, T., Kempf, S., in the vicinity of Enceladus II: Signatures of Krivov, A., Krüger, H., Mocker-Ahlreep, A., Enceladus in the elemental composition of E-ring, Moragas-Klostermeyer, G., Lamy, P., Landgraf, M., 2008, Icarus 193(2), 438 Linkert, D., Linkert, G., Lura, F., McDonnel, J., Möhlmann, D., Morfill, G., Müller, M., Roy, M., Postberg, F., Kempf, S., Srama, R., Gruen, E., Hillier, Schäfer, G., Schlotzhauer, G., Schwehm, G., Spahn, J. K., Green, S. F., and McBride, N.: Composition F., Stübig, M., Svestka, J., Tschernjawski, V., of Saturnian E-ring Particles. Probing subsurface Tuzzolino, A., Wäsch, R., and Zook, H.: The Oceans of Enceladus? 2008, Intern. Jour. of Cassini Cosmic Dust Analyzer, 2004, Space Sci. 7(1), 68 Rev. 114, 465 Postberg, F., Kempf, S., Schmidt, J., Brillantov, N., Srama, R., Kempf, S., Moragas-Klostermeyer, G., Beinsen, A., Abel, B., Buck, U., and Srama, R.: Helfert, S., Ahrens, T. J., Altobelli, N., Auer, S., Sodium salts in E-ring ice grains from an ocean Beckmann, U., Bradley, J. G., Burton, M., Dikarev, below the surface of Enceladus, 2009, Nature 459, V. V., Economou, T., Fechtig, H., Green, S. F., 1098 Grande, M., Havnes, O., Hillier, J. K., Horanyi, M., Postberg, F., Schmidt, J., Hillier, J., Kempf, S., Igenbergs, E., Jessberger, E. K., Johnson, T. V., Kru Srama, R., A salt-water reservoir as the source of a ̈ger, H., Matt, G., McBride, N., Mocker, A., Lamy, compositionally stratified plume on Enceladus, P., Linkert, D., Linkert, G., Lura, F., McDonnell, J. Nature, doi:10.1038/nature10175, 2011 A. M., M ̈ohlmann, D., Morfill, G. E., Postberg, F., Schmidt, J., Brilliantov, N., Spahn, F., and Kempf, S.: Roy, M., Schwehm, G. H., Spahn, F., Svestka, J., Slow dust in Enceladus' plume from condensation Tschernjawski, V., Tuzzolino, A. J., Wäsch, R., and and wall collisions in tiger stripe fractures, 2008, Grün, E.: In situ dust measurements in the inner Nature 451, 685 Saturnian system, 2006, Planet. Space Sci. 54, 967 Simpson, J. A. and Tuzzolino, A. J.: Polarized Polymer Films as Electronic Pulse Detectors of Cosmic Dust Particles, 1985, Nucl. Instr. Meth. 187(A236) Spahn, F., Albers, N., Horning, M., Kempf, S., Krivov, A. V., Makuch, M., Schmidt, J., Seiß, M., and Sremcevi ́c, M.: E ring dust sources: Implications from Cassini's dust measurements, 2006a, Planet. Space Sci. 54, 1024 Spahn, F. and Schmidt, J.: Planetary science: Saturn's bared mini-moons, 2006, Nature 440, 614 Spahn, F., Schmidt, J., Albers, N., Horning, M., Makuch, M., Seiß, M., Kempf, S., Srama, R., Dikarev, V., Helfert, S., Moragas-Klostermeyer, G., Krivov, A. V., Sremcevi ́c, M., Tuzzolino, A. J., Economou, T., and Grün, E.: Cassini Dust Measurements at Enceladus and Implications for the Origin of the E Ring, 2006b, Science 311, 1416 Spahn, F., Thiessenhusen, K.-U., Colwell, J. E.,

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