EPSC Abstracts Vol. 9, EPSC2014-229, 2014 European Congress 2014 EEuropeaPn PlanetarSy Science CCongress c Author(s) 2014

SUDA: A Dust Mass Spectrometer for Compositional Surface Mapping for a Mission to

S. Kempf (1), N. Altobelli (2), C. Briois (3), E. Grün (1), M. Horanyi (1), F. Postberg (4), J. Schmidt (5), R. Srama (4), Z. Sternovsky (1), G. Tobie (6), and M. Zolotov (7) (1) LASP, University of Colorado at Boulder, Boulder, USA ([email protected]), (2) ESA, ESAC, Spain, (3) LPC2E, Orleans, France, (4) IRS, Stuttgart University, Stuttgart, Germany, (5) University of Oulu, Finland, (6) LPGN, Nantes, France, (7) Arizona State University, Tempe, USA

Abstract micrometeoroid impact on a Jovian moon produces a large number of ejecta particles whith a total mass on We developed a dust mass spectrometer to measure the the order of a few thousand times of that of the im- composition of ballistic dust particles populating the pactor [2]. These ejecta particles move on ballistic tra- thin exospheres that were detected around each of the jectories and most of them re-collide with the satellite Galilean moons. Since these grains are direct samples due to the lower initial speed. As a consequence, an from the moons’ icy surfaces, unique composition data almost isotropic dust exosphere is present around the will be obtained that will help to define and constrain moon [3, 7]. the geological activities on and below the moons? sur- In 1999, the dust instrument measured the face. The proposed instrument will make a vital con- density profiles of the tenuous dust exospheres around tribution to NASA’s planned mission the Galilean satellites Callisto, Ganymede, and Europa and provide key answers to its main scientific ques- [4]. The cloud density decreases asymptotically with 5/2 tions about the surface composition, habitability, the radial distance as r− , i.e. the cloud extent is only icy crust, and exchange processes with the deeper in- of a few moon radii. However, a spacecraft during a terior of the Jovian icy moon Europa. close flyby at Europa will detect a substantial number The SUrface Dust Aanalyser (SUDA) is a time-of- of ejecta particles. The initial speed of most ejecta par- flight, reflectron-type impact mass spectrometer, opti- ticles is smaller than the escape velocity, which in turn mised for a high mass resolution which only weakly is much smaller than the speed of an orbiting space- depends on the impact location. The small size (268 craft. The ejecta particles thus hit the dust detector × 250 171 mm3), low mass (< 4 kg) and large sensitive with the velocity of the spacecraft and arrive from the × area (220 cm2) makes the instrument well suited for apex direction. The dynamic properties of the parti- the challenging demands of the Europa Clipper mis- cles forming the ejecta cloud are unique and can be sion. A full-size prototype SUDA instrument was built clearly distinguished from any other kind of cosmic in order to demonstrate its performance through cali- dust likely to be detected in the vicinity of the satel- bration experiments at the dust accelerator at NASA’s lite. IMPACT institute at Boulder, CO with a variety of cos- mochemically relevant dust analogues. The effective 2. Instrument description mass resolution of m/∆m of 200-250 is achieved for mass range of interest m = 1-250. The SUrface Dust Analyser (SUDA) is a reflectron- type, time-of-flight impact mass spectrometer, which 1. Dust Exoclouds has heritage from the Cassini CDA and the Stardust CIDA instruments. The main challenge for the design The basic idea of compositional mapping [1, 6] is that of a dust mass spectro-meter is to achieve simultane- moons without an atmosphere are ensgulfed in clouds ously a high mass resolution, a sufficiently large sensi- of dust particles released from their surfaces by mete- tive area, and a compact design. The plasma ions pro- oroid bombardment. The ejecta cloud particles can be duced by the hypervelocity impact may have a broad detected and their composition analyzed from orbit or energy distributions of up to 100 eV, which limits the during a spacecraft flyby. The ejecta pro-duction pro- mass resolution of linear TOF dust spectrometer of 8 cess is very efficient: a typical interplanetary 10− kg reasonable size to about m/∆m = 50. The effect of Ejecta Particle K 39 H Pt-coated Pyroxene Na Mg 23 24 Al Si Ag Ag 27 28 109 107 Mg Mg 26 Entrance Dust Time of Flight 25

Grid Signal Si Si 2 C 29 30 O

Dust Charge 12 Ca 40 K 41 9

H 22 24 26 28 30 32 34 4 2 C H

Reflectron Grid log Amplitude Ag

Impact Charge Fe AgMg AgMg 109 Ions Acceleration 56 109 107 2 2 Li Ag 7 CH Fe 107

Ag) Ag) Reflecting Grid Signal 3 56 H 109 Fe 107 ( ( E field 56 27T18:28:46 − 06 -3000V / − K SpectrumGui * 2012

+3000V − MPI (c) 50 1002010−05−19T14:50:31 150 200 Ion Charge Mass (u) Acceleration Grid Ion Grid Signal H PPy-PMPV latex Na 23 Na 23

Target K C Accelerating 39 H 12 3 HCS? Ca 2 C 2

2010−139/15:07:04 40 3 E field 2 H 2 2 3 3 H H H 4 H 2 2 S/O 2 C H C C K 4 4 H C C C 32 K 3 5 2 C +2500V / 41 C 4 H 39 H C 4 2 S C C C -2500V Ion Sensor 34 2 H 3 H log Amplitude HCS?

TOF Signal C 25 30 35 40 45 50 30T20:42:44 − 08 H − Impact Time 5 K SpectrumGui * 2012 − 2 MPI H (c) C

2 4 6 8 10 12 5 log Amplitude 3 Time (µs) CH C C S/O H 32 4 2

Time C H 7 H 6 CH 5 C Au 4 C C

7 197 C 3 6 C H C 28T07:25:25 − 06 − K SpectrumGui * 2012 − MPI Figure 1: Function principle of the SUDA impact mass (c) 50 100 150 200 spectrometer. Mass (u) Na 23 Figure 2: Example spectra of a pyroxene particle im- the initial energy spread on the mass resolution is sig- pact on a silver target and of a latex particle on a gold nificantly reduced by employing a so-called reflectron target recorded with SUDA. acting as an electrostatic mirror [5]. The ion optics of large area reflectron mass spectrometers can be de- planetary, interplanetary, and interstellar origin. The signed using optimization methods to ensure simulta- vast majority of the dust impacts were slower than neously the good spatial and time focus-ing of ions. 4 km/s, which is similar to the typical ejecta impact The combination of a plane target, a set of ring elec- speeds onto a detector during the Europa Clipper fly- trodes and an hemispherical reflectron grid yields a bys at Europa. Fig. 2 shows two typical examples of good performance instruments (Fig. 1). The instru- SUDA impact spectra. ment size is 268 250 171 mm3 and the weight × × of 5 kg. Dust particles enter the aperture and fly through a set of shielding grids and reflectron grid be- References fore impacting on the planar, ring shaped target (Fig. 1). Even a relatively slow dust impact of typically 1.6 [1] S. Kempf et al. Dust Spectroscopy of Jovian Satellite km/s generates a sufficient amount amount of atomic Surface Composition. In European Planetary Science Congress 2009, pages 472–473, 2009. and molecular ions for the in-situ mass analysis of the grain?s material. A strong electric field generated by [2] D. Koschny and E. Grün. Impacts into Ice-Silicate Mix- the 2.5 kV bias potential on the target accelerates the tures: Ejecta Mass and Size Distributions. Icarus, 154, ions toward the ions detector, where they are detected 402–411, 2001. in a time-of-flight fashion focusing by the reflectron. [3] A. V. Krivov et al. Impact-generated dust clouds around The acquisiton of the mass spectra is triggered by the planetary satellites: spherically symmetric case. Planet. impact generated charge pulse detected by the charge Space Sci., 51, 251–269, 2003. sensitive electronics connected to the target. The re- tarding field of the reflectron was optimized to achieve [4] H. Krüger et al. Detection of an impact-generated dust cloud around ganymede. Nature, 399, 558–560, 1999. the best spatial and time focusing at the ion detector area in the center of the instrument. [5] B. A. Mamyrin et al. The mass-reflectron, a new non- magnetic time-of-flight mass spectrometer with high res- 3. Instrument Performance olution. JETP, 37, 45, 1973. [6] F. Postberg et al. Compositional mapping of plane- SUDA was tested using the IMPACT 3 MV dust ac- tary moons by of dust ejecta. Planet. celerator to simulate hyper-velocity impacts of cos- Space Sci., 2011. mic dust particles. We performed calibration experi- [7] M. Sremceviˇ c,´ A. V. Krivov, and F. Spahn. Impact- ments with powders of orthopyroxene and latex parti- generated dust clouds around planetary satellites: asym- cles. This choice of test particles covers a broad vari- metry effects. Planet. Space Sci., 51, 455–471, 2003. ety of materials representive for cosmic dust grains of