White Paper for 2009 Planetary Decadal Study Mass Spectrometry of Atmosphereless Planetary Objects

Eberhard Grün1,2, Mihaly Horanyi2, Elmar Jessberger3, Sascha Kempf1,4, Harald Krüger5, Frank Postberg1, Ralf Srama1,6, Thomas Stephan7, Zoltan Sternovsky2

1Max-Planck-Institut für Kernphysik, Heidelberg, Germany 2LASP, University of Colorado, Boulder, CO, USA 3Institut fuer Planetologie, Westfälische Wilhelms-Universität, Münster, Germany 4IGEP, Universität Braunschweig, Braunschweig, Germany 5Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany 6Institut für Raumfahrtsysteme, Universität Stuttgart, Germany 7Department of the Geophysical Sciences, University of Chicago, Chicago, IL, USA

Contents Mass Spectrometry of Atmosphereless Planetary Objects...... 1 Executive Summary...... 2 Scientific Motivation...... 3 Interplanetary Meteoroids and the Zodiacal Dust Cloud...... 3 Stardust and Interstellar Processes ...... 4 Mercury...... 5 The Moon...... 5 Comets ...... 6 Mars System, Phobos and Deimos...... 7 Asteroids ...... 8 Jupiter's Satellites and Ring System...... 8 Saturn Icy Satellites and Ring system ...... 10 Uranus' Satellites and Ring system...... 11 Neptune's Satellites and Ring system ...... 12 The Pluto-Charon System...... 12 Kuiper Belt Objects ...... 12 Approach...... 13 Dust in Interplanetary Space...... 13 Reconnaissance of Atmosphereless Planetary Objects...... 13 Geology and Geochemistry of Planets, Satellites, and Small Bodies ...... 14 Ion Generation at Low Impact Speeds...... 15 Enabling technologies...... 17 Trajectory Sensor...... 17 Large-Area Mass Analyzer ...... 17 Dust Telescope ...... 18 COSIMA ...... 20 Active Dust Collector ...... 20 References...... 21

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Executive Summary Dust particles, like photons, carry information from remote sites in space and time. From knowledge of the dust particles’ birthplace and their composition, we can learn about the remote environment out of which the particles were formed. This approach is called “Dust Astronomy” which is carried out by means of a dust telescope in space.

Besides IR spectroscopy and gamma ray spectroscopy Dust Astronomy provides a novel method to directly analyze surface composition of atmosphereless planetary objects. In the latter case particulates ejected by meteoroid impacts from the surface of these objects are directly analyzed in an in-situ mass spectrometer. This method works down to fly-by or orbital speeds down to about 1 km/s. At lower speeds dust collection and in-situ analysis (like with the COSIMA instrument on Rosetta) are viable methods.

Targets for a dust telescope are interplanetary dust particles from nanometer to micron sizes which originate in comets and asteroids as well as interstellar grains which arrive from our galactic neighborhood and nearby star forming regions. All atmosphereless objects are enshrouded in clouds of dust particles that have been lifted by meteoroid impacts from the object’s surfaces. The particles move on ballistic trajectories, most of which re-collide with the object. In situ mass spectroscopic analysis of these particles provides spatially resolved mapping of the surface composition of the object. Even trace amounts of endogenic and exogenic minerals (e.g. salts), cyanogen-, sulfur-, and organic compounds which are embedded in ejected ice grains can be quantified with high accuracy. The achieved knowledge about surface-interior exchange processes could provide information about the internal composition of the satellites.

Dust from interstellar, interplanetary, or planetary sources is distinguished by accurately sensing their trajectories. Trajectory sensors use the electric charge signals that are induced when charged grains fly through the detector. Modern in-situ dust impact detectors are capable of providing mass, speed, physical and chemical information of individual dust grains in space.

Science questions: • Analyze the composition and origin of beta meteoroids and recently found nano particles in interplanetary space. • Identify the compositional differences between interplanetary micrometeoroids of cometary and asteroidal origin. • What is the composition of interstellar grains sweeping through the solar system? • What is the fraction of original stardust among interstellar grains? • Determine the composition of a comet by analyzing dust grains in its coma or tail. • Determine the composition of an asteroid by analyzing dust grains in its vicinity. • Analyze the composition of Mercury's and the Moon's surfaces by analyzing their ejecta clouds. What provinces of different composition can be identified? • Study the composition of Phobos and Deimos by analyzing dust grains in their environment and search for the putative dust rings of Mars. • Determine the compositions of the outer planets' dust rings and their satellite origin. • Measure the surface composition of the outer planets' satellites by their ejecta particles. What provinces of different composition can be identified? • Study the interior composition of satellites that display volcanism and geysers. • What is the composition of KBOs and how much variation is there?

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Scientific Motivation

Interplanetary Meteoroids and the Zodiacal Dust Cloud Zodiacal dust particles originate in a collisional cascade from bigger interplanetary meteoroids or by direct injection from comets (Grün et al., 1985). Interplanetary meteoroids itself are generated by collisions in the asteroid belt or by outgassing from comets (Ishimoto 2000, Dikarev et al., 2005). Ten micron sized and smaller grains are transported by the Pointing-Robertson effect towards the sun where they evaporate.

Streams of Nano-Dust The dust experiments on the solar orbiting Pioneer 8 and 9 spacecraft recorded a flux of sub- micron sized dust grains (~2 10-4 m-2 s-1) arriving from the solar direction (Berg and Grün, 1973; McDonnell et al. 1975). It was concluded that these particles move on hyperbolic orbits that leave the solar system. Zook and Berg (1975) called these particles beta-meteoroids and deduced that they were probably primarily generated as debris resulting from mutual collisions between larger meteoroids and were driven out of the solar system by solar radiation pressure (Zook, 1975). Measurements by the Helios spaceprobe (Grün et al., 1980) and the HITEN satellite (Svedham, et al. 1996, Grün et al., 2001) supported these observations in the ecliptic plane. Observations by Ulysses found this small particle stream also over the poles of the sun (Baguhl et al., 1995, Wehry and Mann, 1999, Wehry et al., 2004). These observations are interpreted to be due to beta-meteoroids affected by the solar wind magnetic field (Hamilton et al., 1996). Recently, the STEREO wave instrument has recorded a very large number of intense voltage pulses (Meyer-Vernet et al., 2009). It was suggest that these events are produced by impact ionisation of nano-particles striking the spacecraft at a velocity of the order of magnitude of the solar wind speed. Nanometer sized particles have such a high charge-to-mass ratio that the electric field induced by the solar wind magnetic field accelerates them very efficiently. The relation between the STEREO observations and the recordings of signals from putative nano-dust particles by the filmed Multi-Channelplate detectors on ISS (Carpenter et al., 2005, 2007) is unclear. The composition and the origin of these nano-particles are unknown.

Micrometeoroids Many spacecraft with in-situ dust detectors have explored interplanetary dust in radial direction from 0.3 AU (Helios, 80Gru) to 18 AU (Pioneer 10, Humes, 1980) from the Sun. The Ulysses spacecraft on its unique orbit nearly perpendicular to the ecliptic plane [Grün et al., 1992] measured the vertical density profile of the zodiacal dust cloud. However, little is known about the chemical composition from in-situ measurements. The Helios spacecraft carried two low mass resolution chemical dust analyzers. The impact spectra obtained between 1 and 0.3 AU from the Sun were classified into three categories: chondritic-type (45%), iron-type (35%), and unidentified (20%). Interstellar particles amounted to 10% of these particles and did not display significantly different compositions [Dalmann et al., 1977, Grün et al., 1981, Altobelli et al., 2006]. On the path to Saturn Cassini analyzed interplanetary particles. Two particles analyzed in interplanetary space at 0.9 and 1.8 AU showed iron to be a major component, being by far the most abundant metal in both particles [Hillier et al., 2007]. The question whether these particles were of cometary or asteroidal origin remains open since no accurate trajectory measurements were available.

Most of what we know about the composition of interplanetary dust comes from grains collected in the atmosphere. About half of the collected particles have elemental abundances that closely (within a factor 3) match the bulk abundances of CI or CM carbonaceous 4 chondrite meteorites (Table 1). However, the origin of these particles (cometary vs. asteroidal) is only guessed by very indirect methods.

Table 1. Composition of extraterrestrial stratospheric particles [Rietmeijer et al., 1998, Brownlee 1978 a, b, Flynn et al., 1978]

Chondritic particles Non-chondritic particles Aggregates Spheres FSN particles Silicate particles Abundance 55% 5% 30% 10% Chemical Chondritic with Chondritic but S/Fe = 1 to 0 Fe, Mg-silicates signature bulk carbon and no sulfur Fe/Ni = ∞ to 10 with sulfur sulfur both > 4 wt.% Mineralogical Pyroxene, Olivine, Magnetite, (Ni)- Olivine, properties Olivine, Pyroxene, Fe-sulfide Pyroxene, Pyrrhotite, Magnetite Enstatite, Fe- amorphous FE- sulfide Mg-Si-phase

• Analyze the composition and origin of beta meteoroids and recently found nano particles in interplanetary space. • Identify the compositional differences between interplanetary micrometeoroids of cometary and asteroidal origin.

Stardust and Interstellar Processes Dust grains condense in the expanding and cooling stellar winds from asymptotic giant branch (AGB), post-AGB stars, and also in supernova explosions. This so-called ‘stardust’ provides the seeds for ISD grains that grow further in cool interstellar clouds by accretion of atoms and molecules, and by agglomeration. On the other hand, interstellar shocks can efficiently destroy ISD grains by sputtering and high-speed grain-grain collisions (resulting in shattering or vaporization) behind shock fronts. In diffuse interstellar clouds, the grains lose their volatile constituents (e.g., H2O) due to UV irradiation (Greenberg et al., 1995). In denser regions, low velocity grain-grain collisions results in coagulation. Ultimately, ISD grains can either be destroyed in newly-forming stars, or become part of a planetary system. The matter in ISD grains is repeatedly recycled through the galactic evolution process (Dorschner and Henning, 1995).

The Solar system passes currently through a shell of material which is located at the edge of the local bubble (Frisch et al., 1999). It emerged from the interior of this bubble within the past 102 - 105 years. Since entering the cluster of clouds owing from the Scorpius-Centaurus direction, the Sun has encountered a new interstellar cloud at least once per 104 years. It is clear that sampling of dust from our LIC would greatly help to understand the nature and processing of dust in various galactic environments, and cast new light on the chemical composition and homogeneity of the ISM.

In 1992, after its Jupiter flyby, the Ulysses spacecraft positively identified interstellar dust penetrating deep into the Solar System. It detected impacts predominantly from a direction that was opposite to the expected impact direction of interplanetary dust grains. On average, the impact velocities exceeded the local Solar System escape velocity [Grün et al., 1994]. The motion of the interstellar grains through the Solar System was approximately parallel to the 5 flow of neutral interstellar hydrogen and helium gas, both traveling at a speed of 26 km/s [Baguhl et al 1995b]. The interstellar dust flow persisted at higher latitudes above the ecliptic plane, even over the poles of the Sun, whereas interplanetary dust was strongly depleted away from the ecliptic plane [Grün et al., 1997].

Ulysses measured the interstellar dust stream at high ecliptic latitudes between 2 and 5 AU. The masses of interstellar grains range from 10−18 kg to about 10−13 kg with a maximum at about 10−16 kg [Landgraf et al., 2000]. The in-situ dust detectors on board Cassini, Galileo and Helios, [Altobelli at al., 2003, 2005, 2006] detected interstellar dust in heliocentric distance range between 0.3 and 3 AU in the ecliptic plane.. The interstellar dust stream is strongly filtered by electromagnetic interactions with the solar wind magnetic field and by solar radiation pressure. Interstellar particles with optical properties of astronomical silicates or organic refractory materials are consistent with the observed radiation pressure effects [Landgraf et al., 1999].

En route to the comet Stardust collected and analyzed interstellar dust particles. The CIDA instrument provided the first high mass-resolution spectra of a few tens of presumably interstellar grains. The spectra indicate that the main constituents of interstellar grains are organic with a high oxygen and low nitrogen content. It was suggested that polymers of derivatives of the quinine type are consistent with the impact spectra recorded [Krueger et al., 2004].

• What is the composition of interstellar grains sweeping through the solar system? • What is the fraction of original stardust among interstellar grains?

Mercury Mercury is the smallest, the densest, and the planet with the oldest surface. It is like the moon heavily cratered with regions of smooth plains. However, unlike the moon, it has a large iron core, which generates a magnetic field about 1% as strong as that of the Earth. Mercury has a tenuous surface-bounded exosphere" [Domingue et al., 2009] containing hydrogen, helium, oxygen, sodium, calcium and potassium. Hydrogen and helium atoms probably come from the solar wind, water vapor is present, being released by a combination of processes such as: comets striking its surface, sodium and potassium are believed to result primarily from the vaporization of surface rock struck by micrometeorite impacts. The meteoroid flux is ten times higher than at Earth and hence the ejecta cloud is enhanced accordingly.

In 2011 the Messenger spacecraft will enter an orbit about the planet and will survey and map the entire surface with remote sensing instruments ranging from gamma rays to infrared wavelengths. No dust analyzer is included in the payload.

• Analyze the composition of Mercury's surface by analyzing its ejecta clouds. What provinces of different composition can be identified?

The Moon The Moon's surface is marked by impact craters caused by collisions with asteroids and comets. The largest impact basins (diameter up to 2000 km) called maria and are found almost exclusively on the near side of the moon. They are filled with dark solidified basaltic lava and 6 show relatively few impact craters. The far side of the moon is dominated by lighter-colored highlands, which display saturated cratering.

Blanketed atop the Moon's crust is a highly comminuted and "impact gardened" about 10 m thick surface layer called regolith. Beneath the finely comminuted regolith layer is what is generally referred to as the megaregolith. This layer is much thicker (on the order of tens of kilometres) and comprises highly fractured bedrock.

Geochemical maps were obtained from the Lunar Prospector gamma-ray spectrometer (GRS). GRS has identified several regions with high iron concentrations (Lawrence et al., 1998). The most important elements detectable by the GRS were uranium (U), thorium (Th), and potassium (K), radioactive elements which generate gamma rays spontaneously, and iron (Fe), titanium (Ti), oxygen (O), silicon (Si), aluminum (Al), magnesium (Mg), and calcium (Ca), elements which emit gamma rays when hit by cosmic rays or solar wind particles.

The lunar dust environment is expected to be dominated by submicron sized dust particles released from the Moon due to: a) the continuously ongoing bombardment by interplanetary meteoroids (Grün et al., 1985) and b) due to plasma effects induced large transient electric fields. To a good approximation, the impact produced ejecta is expected to form a spherically symmetric, continuously present cloud, while the electrically lofted population is expected to be concentrated over the terminators, and remain highly temporarily and spatially variable. Based on photometry, this second population has been suggested to have a characteristic radius of about 0.1 µm.

• Analyze the composition of the Moon's surface by analyzing its ejecta clouds. What provinces of different composition can be identified?

Comets Information about the composition of the non-icy component of comets comes from analyses of dust grains released from comets. High-resolution dust mass analyzers were employed by the Halley missions (Giotto, VeGa 1 and 2) in order to study the dust composition. Measurements by the impact spectrometers showed that in some spectra ions of the light elements H, C, N, O had the highest intensity peaks, while other spectra were dominated by ions of the rock-forming elements like Mg, Si, Ca, and Fe. The cometary dust particles represented by those spectra were called CHON and silicates, respectively. However, the term silicates appears to be too narrow since this category contains also Fe-sulfides, metal particles, etc. No grain consists purely of CHON or purely of silicates material and because even the smallest cometary particles appear to be fine scale mixtures of carbonaceous and inorganic phases, the proportion of the end member components varies from grain to grain [Kissel and Krueger, 1987]. About half the particles had an abundance ratio of carbon to any rockforming elements between 0.1 and 10. About one quarter of the particles had smaller ratios and one quarter bigger ratios [Sekanina et al., 2001, Fomenkova et al., 1994, Jessberger et al., 1988, 1991, Kissel at al., 1986a, 1986b, 1987, Langevin et al., 1987, Lawler et al., 1989, 1992].

Isotopic information was obtained only for a few among the most abundant elements in the mass spectra of the grains: C, Mg, Si, S, and Fe. The isotopic ratios — within large uncertainties — are generally normal. A clear indication of an isotopic anomaly has been found only for light carbon 12C/13C ratios that are much higher than the normal value of 89 7 and range up to 5000 [Solc et al., 1987]. The average bulk mineral component is in agreement with solar elemental abundance (CI chondrite) except for an excess of organic compounds H, C, N [Jessberger et al., 1988, Langevin et al., 1987]. The mineralogical composition of Halley dust resembles that of stony and iron meteorites [Schulze et al., 1997].

In January 2006 the Stardust sample capsule returned safely to Earth with thousands of particles from comet 81P/Wild 2 for laboratory study [Brownlee et al., 2006, Flynn et al., 2006, Hörz et al., 2006, Keller et al., 2006, McKeeken at al., 2006, Zolensky et al., 2006]. Impact tracks in aerogel were created by particles varying from dense mineral grains to loosely bound, polymineralic aggregates ranging from 0.01 to 100 µm in size. Residues in impact craters in aluminum foil targets were also analyzed.

The particles are chemically heterogeneous; however the mean elemental composition of comet Wild 2 particles is consistent with CI meteorite composition. The particles are weakly constructed mixtures of nanometer-scale grains with occasional much larger Fe-Mg silicates, Fe-Ni sulfides, Fe-Ni metal phases. A very wide range of olivine and low-Ca pyroxene compositions has been found. The organics are rich in oxygen and nitrogen compared with meteoritic organics. Aromatic compounds are present but less abundant than in meteorites and IDPs. The organics found in comet Wild 2 show a heterogeneous and unequilibrated distribution in abundance and composition. Hydrogen, carbon, nitrogen, and oxygen isotopic compositions are heterogeneous among particle fragments; however, extreme isotopic anomalies are rare, indicating that this comet is not a pristine aggregate of presolar materials. A single particle has been found with a ratio 17O/16O = 10−3 which is a factor 2.6 higher than the Solar System value and is similar to that of some presolar grains found in meteorites. The abundance of high-temperature minerals such as forsterite and enstatite appears to have formed in the hot inner regions of the solar nebula.

• Determine the composition of a comet by analyzing dust grains in its coma or tail.

Mars System, Phobos and Deimos First predictions of Phobos and Deimos dust rings date back 35 years ago (Soter, 1971). Dust released by meteoroid impacts onto the Martian moons should orbit Mars for some significant time giving rise to faint dust rings. Since that time numerous theoretical papers discussed the (supposedly) simple dynamics of such rings (for a review see Krivov and Hamilton, 1997). Krivov (1994) distinguishes 3 different populations of dust in the Martian environment: 1. the disk population of big grains (≥ 10 µm radius) oscillating around the orbits of Phobos and Deimos, 2. a subdisk population of micron-sized grains filling a larger toroidal volume down to the Martian atmosphere, and 3. the halo population of submicron-sized particles which are strongly affected by electromagnetic forces and rapidly leave the Martian environment. However, an observational test of these theoretical predictions is still missing.

Observations by the ill-fated Phobos mission provided indirect evidences for a gas/dust torus along the Phobos orbit (Dubinin et al., 1990, Baumgärtel et al., 1996). Astronomical observations first by the Viking spacecraft (Duxbury and Ocampo, 1988), HST in 2001 and 2006 (Showalter, et al., 2001, 2006) failed the detection of the dust rings, so far. However, in- situ methods are several orders of magnitude more sensitive to low spatial dust densities than remote sensing methods. Therefore, a clear signature of dust from Phobos and Deimos forming the Martian dust rings is expected in the data returned from an in-situ dust analyzer.

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• Study the composition of Phobos and Deimos by analyzing dust grains in their environment and search for the putative dust rings of Mars.

Asteroids Thermal emission from dust bands at low ecliptic latitudes has been detected, with the parallaxes pointing to the asteroid belt as the location of the source of emission [Low et al., 1984]. Collisions between asteroids were suggested as the origin of the dust bands [Dermott et al., 1984, Sykes, 1990] consisting of 10 to 100 micron-sized long-lived asteroidal fragments. Although dust instruments on various missions (Pioneer 10, 11, Ulysses, Galileo, and Cassini) have crossed the asteroid belt no spatial enhancements of micron sized asteroidal dust has been found. Nevertheless, bombardment by interplanetary and interstellar grains will increase the dust density in the immediate vicinity of an asteroid.

Several spacecraft had close fly-bys of various asteroids (Galileo, Deep Space 1, Stardust, Cassini, New Horizons, and Rosetta) or even went into orbit about an asteroid (NEAR Shoemaker and Hayabusa) and studied their composition by remote sensing methods (mostly infra-red spectroscopy). Hayabusa tried to pick-up from the surface some dust samples and is scheduled to return them to the Earth by 2010.

• Determine the composition of an asteroid by analyzing dust grains in its vicinity and how much variability is in the composition of different asteroid?

Jupiter's Satellites and Ring System Dust measurements by spacecraft contributed significantly to our current understanding of the Jovian system. The first observations of dust came from the Pioneer 10 and 11 spacecraft, which passed by the planet in 1973 and 1974, respectively. During their passage through the Jovian system, the in-situ dust detectors measured a flux which was, by three orders of magnitude, larger than the dust flux measured in interplanetary space (Humes et al., 1974). In 1979, the two Voyager spacecraft drastically changed our knowledge about Jovian system. Jupiter’s dusty rings were discovered on images from the Voyager cameras. Another discovery by Voyager was active volcanism on Io. At the time, there were conjectures that tiny dust grains entrained in Io’s plumes might be ejected into circumjovian space by electromagnetic forces (Johnson et al., 1980; Morfill et al., 1980).

Galileo discovered and studied impact-generated dust clouds surrounding all Galilean satellites. The impact rate of dust grains showed a sharp peak within about half an hour centered on closest approach to each satellite (Grün et al., 1998; Krüger et al., 1999a) indicating the existence of dust clouds surrounding all four Galilean moons (Krüger et al., 1999e). In the region between the Galilean satellites Galileo discovered a tenuous dust ring with a peak number density of 5×10-7 m-3 at Europa's orbit (Thiessenhusen et al., 2000; Krivov et al., 2002a). Furthermore the data indicates an increase in the number density between Europa and Io (Krivov et al., 2002b).

Metis, Adrastea, Amalthea and Thebe Jupiter's ring system was investigated with remote imaging from the Earth and from Voyager, Galileo, Cassini and New Horizons spacecraft, revealing significant structure in the ring: at least four components have been identified (Ockert-Bell et al., 1999; Burns et al., 1999; de Pater et al., 1999): the main ring, interior halo and two gossamer rings. The small moons Metis, Adrastea, Amalthea and Thebe are embedded in the ring system and act as sources of 9 ring dust via meteoroid impact erosion of their surfaces (Burns et al., 1999). The faint gossamer rings appear to extend primarily inward from the orbits of Amalthea and Thebe. During its last orbits about Jupiter Galileo made two passages through the gossamer ring system and led to a better understanding of its dust number density and size distribution (Krüger et al., 2009).

Detected particle sizes range from about 0.2 to 5 µm, extending the known size distribution by an order of magnitude towards smaller particles than previously derived from optical imaging [Showalter et al., 2008; de Pater et al., 2008]. The faint Thebe ring extension was detected out to at least 5RJ, indicating that grains attain higher eccentricities due to a shadow resonance as detailed by Hamilton and Krüger [2008].

Io With over 400 active volcanoes, Io is the most geologically active object in the Solar System (Lopez, 2006). This extreme geologic activity is the result of tidal heating from friction generated within Io's interior by Jupiter's varying pull. Several volcanoes produce plumes of sulfur and sulfur dioxide that reach heights of 500 km. Io's surface has more than 100 mountains that have been uplifted by extensive compression at the base of the moon's silicate crust. Io is primarily composed of silicate rock surrounding a molten iron or iron sulfide core. Io's volcanism is responsible for many of that satellite's unique features. Its volcanic plumes and lava flows produce large surface changes and paint the surface in various shades of red, yellow, white, black, and green, largely due to the sulfurous compounds. Numerous extensive lava flows mark the surface.

Io is the main emitter of gas, plasma, and dust into the Jovian magnetosphere shedding its material onto the neighboring satellites. Nanometer sized charged dust particles are accelerated by the giant Jovian magnetic field to high speeds and dispersed into interplanetary space. In 1992 the dust detector on board of Ulysses discovered intermittent collimated streams of dust particles from Jupiter (Grün et al., 1993; Baguhl et al., 1993). Modeling of the particles’ dynamics led to grain sizes of about 10 nm and impact speeds exceeding 200 km/s (Zook et al., 1996). The dust detector onboard the Galileo spacecraft investigated the distribution of the Io dust in the Jovian magnetosphere (Grün et al., 1998, Krüger et al., 2003a; 2003b; 2005). Frequency analysis of the observed impact rate confirmed that the tiny dust particles were released into the magnetosphere from the volcanic plumes on Io (Graps et al., 2000). In 2000 the Cassini spacecraft that flew by Jupiter and provided the first compositional analysis of the stream particles which were found to consist mainly of alkali salts, confirming their origin in Io’s volcanic plumes (Postberg et al., 2006).

Europa Europas' surface is composed of ice and is one of the smoothest in the Solar System. This young surface is striated by cracks and streaks, while craters are relatively infrequent. The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it. Heat energy from tidal flexing ensures that the ocean remains liquid and drives geological activity (Pappalardo et al. 1998; Greeley et al. 2004). All these surface features indicate that an exchange between the interior and the surface has occurred in the recent past on Europa and is maybe still ongoing.

On Europa, the presence of sulfate in endogenic surface areas (McCord et al., 1999; Carlson et al., 1999) may be an indication of oceanic constituents that were delivered to the surface through cracks or fractures in the ice shell (McCord et al., 1999; Greeley et al., 2004). Other non-ice colored materials and CO2 in multiple disrupted surface areas (lineae, bands, domes, 10 pits, and chaotic terrains) appear endogenic and secondary with respect to ice-dominated areas (Greeley et al., 2004), and could reflect interior processes and compositions (McCord et al., 1999; Fanale et al., 1999; Kargel et al., 2000; Zolotov and Shock, 2001; Hansen and McCord, 2008). Theoretical models of the oceanic composition (Kargel et al., 2000, Zolotov and Shock, 2001, Zolotov and Kargel, 2009) predict sulfate together with Na, K, Mg, Cl, and carbonate/bicarbonate solutes. Although some atmospheric alkalis (Leblanc et al., 2002; Brown, 2001) and surface CO2, (Hansen and McCord, 2008) may have oceanic origin, Cl and Mg remain to be detected at Europa to confirm the aqueous origin of impurities in disrupted surface areas.

It has been suggested that active venting may be present on Europa (e.g., Nimmo, Pappalardo, Cuzzy, 2007), which may be too tenuous to be observable by optical methods but some of the ejecta particles observed near the satellite by the Galileo dust detector may have been released in such vents.

Ganymede Ganymede's surface is heavily cratered but some bright terrain is being interpreted as caused by resurfacing by cryo- or liquid-water volcanism (Pappalardo, 2004; Showman et al., 2004). Impact cratering by km-sized objects also contributes significantly to exchange processes between the moon’s interior and the surface by excavating fresh underground material to the surface, redistributing material globally, and, in extreme cases, by penetrating to large depths (e.g. Turtle and Pierazzo 2001). On Ganymede in the last 109 years, most of the surface exchanges with the interior have likely been associated with impact cratering. The existence of a subsurface ocean is implied by Galileo magnetometer data (Khurana et al., 1998; Kivelson et al., 2000, 2004) and is also consistent with surface geological features (Pappalardo et al., 1999).

On Ganymede, the determination of particle composition in the vicinity of relatively young impact craters should give information about its subsurface composition, which is fundamental for understanding its evolution (Showman et al. 1997, Bland et al. 2009).

• Determine the compositions of the Jupiter's dust rings and their satellite origin. • Measure the surface composition of Jupiter's satellites by their ejecta particles. What provinces of different composition can be identified? • Study the interior composition of satellites that display volcanism and geysers.

Saturn Icy Satellites and Ring system The Saturnian system is governed by its giant ring system and the mercury-sized moon Titan that is covered by a thick atmosphere. Numerous icy satellites have been found inside the main rings and the extended dusty E ring. The enigmatic satellite Enceladus is located in the densest part of the E ring.

Also in Saturn's environment tiny dust stream particles were recorded by Cassini emanating from the ring system (Kempf et al., 2005). Compositional analyses of these particles directly provide the composition of their sources in the rings.

Enceladus During the flyby of Enceladus' South Pole in 2005 the Cassini Cosmic Dust Analyzer (CDA) detected a significant enhanced flux of dust particles about 1 min before the closest approach (Spahn et al., 2006). At about the same time the Ion and Neutral Mass Spectrometer detected 11 water gas, also at a peak rate (Waite et al., 2006). This gas plume was also seen by the Ultraviolet Imaging Spectrograph (Hansen et al., 2006) and, indirectly at an earlier flyby, by the magnetometer (Dougherty et al., 2006). These finding were compatible with a gas and dust source near the South Pole. Analyses of images taken by Cassini's camera showed plumes of ice grains emanating from the South Pole region (Porco et al., 2006). Some of these tiny ice grains escape the moon’s gravity, replenishing Saturn’s outermost r E-ring.

Analysis of the dust composition revealed that dust particles from Enceladus contain sodium salts (Fig. 1, Postberg et al., 2009). It was the combination of the sodium found in the dust 40 particles and the N2 and Ar in the gas phase that led to the unambiguous inference of a liquid water reservoir beneath Enceladus' surface that is in contact with the moon‘s rocky core (Matson et al., 2007; Glein et al., 2008; Postberg et al., 2008, 2009; Waite et al., 2009).

Figure 1. CDA Spectrum of an ice grain which hit the detector with v ~ 6 km s-1 near Enceladus. The spectrum shows pronounced signatures of sodium and potassium salts in a water matrix. NaCl and NaHCO3 could be identified to be embedded in ice particles from Enceladus on a percent level (Postberg et al. 2009).

Rhea The icy moon Rhea - the second largest moon of Saturn after Titan - is located at the outer edge of the visible part of the E ring about twice as far as form Saturn as Enceladus. Measurements by Cassini's electron detector MIMI/LEMMS showed an absorption feature near Rhea suggesting an enhancement of particulates close to that moon. This suggestion is supported by an enhanced count rate of dust particles in that region of space.

• Determine the compositions of the outer Saturn's dust rings and their satellite origin. • Measure the surface composition of Saturn's satellites by their ejecta particles. What provinces of different composition can be identified? • Study the interior composition of satellites that display volcanism and geysers.

Uranus' Satellites and Ring system Uranus has five main satellites (Miranda, Ariel, Umbriel, Titania and Oberon) and 22 small moons. The moons have relatively low albedos; ranging from 0.20 for Umbriel to 0.35 for Ariel. The moons are ice-rock conglomerates composed of roughly fifty percent ice and fifty percent rock. The ice may include ammonia and carbon dioxide

Inside the main satellites Uranus has a complicated ring system, consisting of thirteen distinct rings usually only a few kilometers wide. The rings are composed of extremely dark particles, which vary in size from micrometers to a fraction of a meter [Smith et al., 1986]. The plasma 12 wave experiments on Voyager 2 detected impacts of dust particles in the vicinity of Uranus (Gurnett et al., 1987).

Neptune's Satellites and Ring system The by far largest of 13 moons of Neptune is Triton. Triton has a retrograde orbit, indicating that it was captured from the Kuiper belt [Agnor and Hamilton, 2006]. Triton is one of the few moons in the Solar System known to be geologically active. Its crust is dotted with geysers believed to emit solid nitrogen particles. As a consequence, its surface is relatively young, with a complex geological history revealed in intricate and mysterious tectonic terrains [McKinnon and Kirk, 2007]. Triton has a tenuous nitrogen atmosphere

Neptune has a planetary ring system consisting of three main narrow rings and a broader, fainter innermost ring. These rings have a clumpy structure, the cause of which is not currently understood but which may be due to the gravitational interaction with small moons in orbit near them. The outermost ring contains five prominent arcs, which are believed to be corralled by the gravitational effects of Galatea, a moon just inward from the ring (Salo and Hänninen, 1998). Fly-by of Voyager 2 detected clouds of dust particles in the vicinity of Neptune (Gurnett et al., 1991).

The Pluto-Charon System Pluto and its companion Charon constitute binary dwarf planets. Two smaller moons orbit outside this binary system. It has been suggested that Pluto and Charon are immersed in a tenuous dust cloud. The cloud consists of ejecta from Pluto and Charon, released from their surfaces by impacts of micrometeoroids originating from Edgeworth–Kuiper belt objects (Thiessenhusen et al., 2002). The dust cloud may be dense enough to be detected by the New Horizons mission which is en-route to Pluto and the Kuiper Belt.

• Determine the compositions of the outer planets' dust rings and their satellite origin. • Measure the surface composition of the outer planets' satellites by their ejecta particles. What provinces of different composition can be identified? • Study the interior composition of satellites that display volcanism and geysers.

Kuiper Belt Objects More than 1000 Kuiper Belt Objects (KBOs) have been discovered outside about 30 AU since the first discovery in 1992. The outer Kuiper belt is believed to be the main repository of the short periodic comets (Levinson and Dones, 2007). Collisions among the KBOs (Stern, 1996) lead to fragments down to the size of dust grains. During the occasional tracking of the Voyager spacecraft beyond 6 AU the Plasma Wave instrument (Gurnett et al., 1997) reported significant dust impact rates out to about 30 and 50 AU. Measurements of the dust instruments on board the Pioneer 10 and 11 spacecraft found the signature of dust generated by Edgeworth-Kuiper Belt objects (Landgraf et al., 2000). While the dust concentration detected between Jupiter and Saturn were mainly due to the cometary components, the dust outside Saturn's orbit is dominated by grains originating from the Edgeworth- Kuiper Belt. Collisional fragmentation and erosion by impacts of interstellar dust grains onto the objects' surfaces release in total 5 107 g s-1 of dust from KBOs in order to account for the amount of dust found by Pioneer, which make the Kuiper disk the brightest extended feature of the Solar System when observed from afar.

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• What is the composition of KBOs and how much variability is there?

Approach Classical methods to analyze atmosphereless planetary objects are IR spectroscopy and gamma ray spectroscopy. Recently, a new method has been proposed to analyze dust particles as samples of planetary objects from which they were released. Dust particles, like photons, carry information from remote sites in space and time. From knowledge of the dust particles’ birthplace and their composition, we can learn about the remote environment out of which the particles were formed. This approach is called “Dust Astronomy” which is carried out by means of a dust telescope on a Dust Observatory in space (Grün et al., 2005).

In order to address the above scientific questions the methods employed have to - reliably distinguish dust particles from different sources, or - measure the trajectories accurately enough in order to determine the individual source object, or even - determine the launch position of the dust particle on the source object, and have to - measure the composition of the dust particle well enough to derive cosmo-chemical information.

Dust in Interplanetary Space Distinguishing interplanetary from interstellar dust requires the measurement of the impact speed onto a dust detector to about 10% accuracy. Besides interstellar dust flux is narrowly collimated. Both attributes can be determined on individual particles by a coarse trajectory measurement. In addition interstellar dust flux displays a strong seasonal variation and hence interstellar dust measurements can be optimized. At 1AU during the course of the year interstellar particles impact a dust instrument at speeds between 5 and 60 km/s from narrowly defined direction while interplanetary particles arrive at a detector from the spacecraft apex direction at a broad speed distribution around 20 km/s where fresh cometary dust maintains the high eccentricity of the original cometary orbit. Micron sized dust from the asteroid belt spirals on low eccentric orbits towards the sun. This orbital difference between young cometary and asteroidal dust provides a means to distinguish both types of dust in interplanetary space.

Beta-meteoroid or nano-particle flux are dominated by the repulsive solar radiation pressure force and their interaction with the interplanetary magnetic field that is carried by the solar wind away from the sun at high speed (400 to 800 km/s). These particles are too small for direct trajectory measurements but they are collectively characterized by high speeds and a flow away from the solar direction which will be modulated by solar wind parameters like speed and magnetic field direction.

Reconnaissance of Atmosphereless Planetary Objects Close fly-bys of a spacecraft carrying a dust telescope through the ejecta clouds of atmosphereless planets (Mercury and Pluto), of the Moon, asteroids, comets, of the Mars moons, of outer planetary satellites, or KBOs provides a means to characterize the geochemistry of these objects. Successful examples of this type are the fly-bys of the comets Halley and Wild 2, the passage of Jupiter by Cassini which analyzed the dust streams 14 emanating from Io, and the analyses of dust from the icy satellites of Saturn by the Cassini spacecraft while in orbit about Saturn.

Typical fly-by speeds range from 10 to 90 km/s for fly-bys in interplanetary space to 3 to 10 km/s for satellite fly-bys during a planetary orbit. Ejecta particles are concentrated in the Hill sphere of these objects (typically several to many radii away from the object) where low- velocity non-escaping particles are abundant. Because of the relatively small numbers of ejecta particles an appropriately sized dust analyzer (~0.1 m2 sensitive area) has to be used. Of course the dusty coma of a comet can extend to several 10,000 km form the nucleus and a much smaller instrument will yield statistically significant information.

Geology and Geochemistry of Planets, Satellites, and Small Bodies Orbital or rendezvous missions to planetary objects provide excellent means to obtain detailed geological and geochemical information of these objects by dust astronomical methods. Examples of such type missions are lunar missions, the Near-Shoemaker mission to 533 Eros, Messenger to Mercury, Rosetta to 67P/Churyumov-Gerasimenko, and the planned EJSM to Europa and Ganymede. So far only Rosetta carries a dust analyzer COSIMA (Kissel et al., 2007) capable of obtaining high mass resolution spectra of cometary particulate material.

The application of a modern dust trajectory sensor in combination with a chemical analyzer will yield additionally spatially resolved geological and geochemical information. It is carried out by measuring the trajectory and chemical composition of individual ejected dust particles in the vicinity of the satellite and its exosphere (Fig. 2). These measurements from low altitude orbit establishes a direct link to the grain’s origin on the surface and thereby provides information on the surface and subsurface composition. This data can provide key chemical constraints for revealing the satellite’s composition, history, and geological evolution.

Figure 2. Schematics of the proposed investigation. Ejecta particles lifted by meteoroid impacts from the satellites' surface are analyzed by a dust trajectory sensor in combination with a high-resolution dust mass spectrometer. By tracing back the trajectory to the surface compositional maps of the surface are generated.

In case active venting is present at the visited object (like at Io, Enceladus, Triton, and all active comets) then the analysis of ejected particles provides even information on the sub-surface composition as has been successfully demonstrated in the case of Enceladus (Postberg et al., 2009). It has been suggested that active venting may be present even on the Galilean moons Europa and perhaps Ganymede (Nimmo, Pappalardo, Cuzzy, 2007) where venting might be too tenuous to be observable by optical instruments and moreover may not be linked to obvious geographical features like the “Tiger Stripes” on Enceladus. In this case a dust telescope equipped with trajectory sensor might be the only instrument capable of precisely determining the source location of such a phenomenon at a much higher sensitivity than any optical observation can do. The analysis of ice grains which have been freshly ejected would not only be of geochemical and geophysical relevance, but also of high astrobiological potential. With a high resolution mass spectrometer 15 even the chemical investigation of life precursor molecules and biomarkers would be possible. By this means dust measurements could add significantly to the investigation of the habitability of planetary satellites like Europa and Ganymede.

The analysis of emitted solids (released either by meteoroid impact or active venting) by a dust telescope is complementary to studies by remote sensing methods (e.g. by infrared spectroscopy) and analysis of the gas phase (by an ion and neutral mass spectrometer). Interpretation of optical spectroscopy data is often not unique (e.g. Carlson et al 1999). This is of particular relevance for the hydrated salts exposed on Europa’s and Ganymede’s surfaces which probably come from the internal ocean (e.g., McCord et al. 1999, 2001). An impact like a dust telescope is very sensitive to different types of salts (e.g., NaCl or MgSO4) and carbonaceous material. Only direct sampling will provide unambiguous evidence for the origin of the surface materials and it is important that both solid and gas phases be measured.

Furthermore, complex molecules are protected when hidden and embedded in an ice matrix. After the ice grains are ejected from the moons' surfaces (either by micrometeoroid impact or active venting) detection with an in-situ dust instrument provides the best chance to detect intact large organic molecules even in Jupiter’s harsh radiation environment.

The speed of a spacecraft orbiting a planetary object with radius > 1000 km is > 1 km/s which sufficient to get chemical information from a highly resolved impact ionization mass spectrum, as it is the case from a dust telescope. Objects of this size include planets like Mercury and Pluto, the Moon, all Galilean satellites, and Neptune's moon Triton. At lower orbital speeds, i.e. at smaller planetary objects, dust collection and in-situ analysis by e.g. secondary-ion-mass-spectrometer, like the COSIMA instrument (Kissel et al., 2007) are the method of choice. Sample collection and return to Earth is possible, e.g. in aerogel (Tsou et al., 2003) at all speeds up to about 20 km/s. In this case an active dust collector, i.e. a combination of a dust trajectory sensor and the dust collector will not only facilitate finding the dust grains on the collector but also providing trajectory information with the collected particle.

Ion Generation at Low Impact Speeds. For direct impact ionization to occur the impact speed must be of the order of 10 km/s or more corresponding to an energy per projectile atom or molecule of > 10 eV. Accelerator experiments with the a dust telescope, demonstrated that dust impacts at 1-3 km/s yield high quality mass spectra (Figure 3). The presence of species which are easily charged (e.g., Na or Mg salts) drastically enhance the ion yield and the ‘visibility’ of such species (Figure 3).

Recent progress in the investigation of low energy ionization processes in a water matrix led to a greatly improved understanding of impact ionization spectra at impacts speed expected in an orbit around e.g. the Galilean moons (1 - 2.5 km/s). In this case the impact speed of ejecta onto the dust sensor during the satellite orbit phase can be as low as 1 km/s corresponding to less than 0.2 eV per water molecule. However, it has recently been shown that neutral molecular clusters and particles with kinetic energies much less than the ionization energy of atoms and molecules, fragment upon target or laser impact and display ion/charge separation in the fragments (Gebhardt et al., 1999; Wiederschein et al., 2009). This effect works particularly well in a water matrix and is due to preformed ions in the particles that are simply separated from their counterions on a fast timescale. These ions form cluster with any neutral water molecules but also with any impurity embedded in the ice matrix. 16

Figure 3: Spectrum recorded with the proposed detector type of an orthopyroxene dust grain (size ~0,6µm) with an impact speed of v ~ 2 km/s. Particle constituents, like the isotopes of Magnesium and Aluminum, can be quantified. The mass line of Na and at 29 amu are due to target contamination (no contamination counter measures were applied for these test shots).

For a particle impact a combination of two processes of ion formation occurs. While for large kinetic energy impacts (> 15 km/s) the plasma generation mechanism is dominating, the dispersion and charge separation mechanism (DCS-mechanism) is dominating at low kinetic energies (1-5 km/s) and exclusively operating in cases in which no plasma is formed anymore (Gebhardt et al., 1999). Both mechanisms can be distinguished by their correlated mass spectra. Mass spectra from impacts at lower energies following mostly the DCS-mechanism display a series of positively and negatively charged clusters and aggregates with minor intensities in ionized atomic and molecular species (Figure 4). In the other extreme large intensities at masses belonging to ionized atoms and molecules with low intensity in larger molecular aggregates are observed. The conditions in for the Cassini dust analyzer at Saturn (Postberg et al., 2009) belong to the low to intermediate energy case (3 – 15 km/s, Figure 4). In the case of an Europa or Ganymed orbiter only the low energy case is expected. The yield for charged species in the DCS-mechanism is slightly lower than for plasma ionization but still of the same order of magnitude (Abel et al., 2009; Charvat and Abel, 2007). Therefore, a dust telescope is well suited to determine the composition of particles with the impact velocities above 1 km/s.

Figure 4. Spectrum of a water ice particle obtained at ~ 4 km s-1 impact speed by the Cassini Cosmic Dust Analyzer in Saturn's E ring. Although mixing ratio of sodium in the grain is below 10 -6, a quantitative determination could be achieved.

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Enabling technologies

Trajectory Sensor The attainability of dust trajectory analysis was demonstrated by the measurement of dust charges with the Cassini dust instrument. Any meteoroid in interplanetary space is electrically charged. Because of the predominance of the photoelectric effect in interplanetary space over the charging by the solar wind plasma, meteoroids are usually charged positively to a potential of a few Volts (~+5 V). Cassini’s Cosmic Dust Analyser (CDA) reliably determined electrical charges on several interplanetary dust grains (Auer et al., 2002, Kempf et al., 2004). The dust charge was determined by measuring the electrical charge induced by the particle onto charge sensing grids in front of the dust impact detector (Srama et al., 2004).

The Dust Trajectory Sensor, DTS, makes use of the electric charge that all particles in space carry (Auer, 1975, Srama et al., 2006a, Auer et al., 2008). Dust particles' trajectories are determined by the measurement of the electric signals that are induced when a charged grain flies through a position sensitive electrode system (Fig. 5). Dust charges are measured in the range 10-16 to 10-13 C and dust speeds up to 100 km/s. A DTS has four sensor planes consisting of 16 wire electrodes each. Two adjacent planes have orthogonal wire directions. A charged dust grain flying through the set of wires generates an induced charge on the most adjacent wires. Each wire is connected to a Charge Sensitive Amplifier and the 16 wires of each plane are routed to a single multi-channel transient recorder (TR) digitizing the signals. The ASIC electronics were integrated in the laboratory model and induced signals of dust grains have been recorded at the Heidelberg dust accelerator facility (Fig. 6).

Figure 5. Dust Trajectory Sensor (laboratory model). Figure 6. Induced charge signals of a dust grain with primary charge of 4 fC passing through two planes of the Dust Trajectory Sensor.

Large-Area Mass Analyzer Previous dust analyzers had sensitive areas of 0.01 m2 or less. In order to cope with the low dust fluxes expected in interplanetary space and in the ejecta cloud of a planet or satellite a new high-resolution large-area mass analyzer of >0.1 m2 sensitive area has been developed. The schematic diagram of the cylindrically symmetric instrument is shown in the Fig. 7 (Srama et al., 2005, 2008; Sternovsky et al., 2007). The dust particle enters the instrument through the annular disk electrodes made out of highly transparent grid material. After 18 passing another (grounded) grid it impacts the target surface. The target has an opening in the center for the ion detector. Positive ions generated from the high velocity impact are accelerated away from the target by the applied +5 kV bias. By a positively biased grid the ions are reflected and directed towards the ion collector in the center. By the action of this reflectron a high mass resolution spectrum (M/ΔM > 100) of the ions is obtained.

Figure 7. Ion optics of the mass spectrometer. Figure 8. Laboratory model of a mass spectrometer with 20 cm in diameter.

Such a mass spectrum with a resolution five times better than the resolution of the Cosmic Dust Analyzer onboard Cassini is shown in Fig. 9. It displays the major elements of the Orthopyroxene projectile used in this experiment. The mass threshold is given by the charge threshold of approx. 10-15 C corresponding to a grain size of 0.4 µm (Europa, average rel. impact speed 1.4 km s-1) and 0.3 µm (Ganymede, average rel. impact speed 1.8 km s-1), respectively.

Figure 9. Mass spectrum of a 0.3 µm orthopyroxene grain impacting onto a silver target.

The analysis of beta or nano-particles in interplanetary space requires a modification of the mass analyzer since the instruments will be exposed to the solar UV flux which results in a high photo-electron flux.

Dust Telescope In-situ analysis of dust particles of speeds > 1 km/srequiers a Dust Telescope that is composed of two parts, the Trajectory Sensor and a Mass Spectrometer subsystem. This sensor is an in-situ instrument detecting and analyzing individual impacts of sub-micron and micron sized dust grains. First, the dust particles pass the Trajectory Sensor with its four 19 planes of wires measuring the induced electrical charge of the grains to derive their trajectory information. The grains impact on the plane target of the mass analyzer at the bottom of the instrument (Fig. 10) generating electrons and ions which are analyzed in a time-of-flight spectrometer (Table 2).

Figure 10. Dust Telescope (DUNE-SMEX version) with the four planes of wires of the Trajectory Sensor (top) and the time-of-flight mass-spectrometer (bottom). A particle impacting on the target generates electrons, ions, and charged cluster-molecules which are focused to the ion detector in the centre. Data acquisition is triggered by the electron signal collected at the target and by the ion signal.

Tab. 2: Dust Telescope measurement range

Dust grain property Measurement range [accuracy] Composition (TOF spectrum) YES, 1 to 500 amu Mass resolution M/ΔM ~200 (optional ~100,000 with Orbitrap) Size 0.01 - 60 µm (MS), 0.3 - 60 µm (TS) [<20%] Charge 10-16 C - 10-13 C [<5%] Speed 1 km s-1 - 20 km s-1 [<5%] Trajectory accuracy +/- 0.1° (@ 10 fC ... +/- 2° @ 0.1 fC)

Several versions of a Dust Telescope have been studied in the past (Tab. 3, Srama et al., 2005, 2008; Sternovsky et al., 2007). Owing to differing science and instrument requirements the total instrument masses do not scale directly with the respective sensitive areas. The DUNE SMEX version has the largest sensitive area and a Trajectory Sensor with four wire planes allowing a highly accurate trajectory determination. Leopard preserves the Trajectory Sensor properties but has smaller sensitive areas. The SODA version is the most compact version allowing accurate compositional measurements but with much reduced capabilities of trajectory sensing.

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Table 3: Dust Telescope instrument proposed for different missions. Mission (instrument) Sensitive area (m2) Total mass (kg) DuneXpress/SARIM 0.05 19 DUNE SMEX (Fig. 10) 0.06 15 LEO (Leopard) 0.024 6.5 Solar Orbiter (SODA) 0.023 3.5

COSIMA COSIMA, the COmetary Secondary Ion Mass Analyzer onboard the Rosetta mission to comet 67P/Churyumov-Gerasimenko is based on the analytic measurement method of secondary ion mass spectrometry (SIMS, Kissel et al., 2007). The chemical characterization will include the main organic components, as well as the mineralogical and petrographical classification of the inorganic phases. COSIMA covers a mass range from 1 to 3500 amu with a mass resolution M/ΔM ≈ 2000 at mass 100 amu. Cometary dust is collected on special, metal covered, targets, which are handled by a target manipulation unit. A pulsed primary indium ion beam (among other entities) releases secondary ions from the dust grains. These ions, either positive or negative, are selected and accelerated by electrical fields and travel through a drift tube and an ion reflector. A microsphere plate with dedicated amplifier is used to detect the ions. The arrival times of the ions are digitized, and the mass spectra of the secondary ions are calculated from these time-of-flight spectra. COSIMA will be the first instrument applying the SIMS technique in-situ to cometary grain analysis as Rosetta approaches the comet 67P/Churyumov-Gerasimenko in 2014.

Active Dust Collector Novel active dust collectors is a combination of a Dust Trajectory Sensor, DTS, with a dust collector. In this case, not only the trajectories of collected grains are determined and hence their origin is established, but also their impact positions into the collecting material can be determined to sub-millimeter precision.

Intact capture of hypervelocity projectiles in silica aerogel was successfully demonstrated (Tsou et al., 1990). For aerogels having densities about 10 kg/m3 or less, intact projectiles were lodged at the end of track. This concept has been extremely successful during Stardust’s fly-through of comet Wild2 coma at a speed of 6.1 km/s: many cometary particles have been intact captured in aerogel of 10 to 50 kg/m3 density (Brownlee et al., 2006).

In an active dust collector all particles impacting the collector must pass through the dust trajectory sensor. In order for DTS to function properly it requires a trigger signal which terminates the cyclic signal processing and starts the data read-out. In case of a metal collector this will be used as the target of a simple impact ionization detector with a grid in front of it. Even for impacts into aerogel it was shown that impact charges are collected (Auer, 1998) that can provide the necessary trigger signal for DTS. 21

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