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Exogenous Dust at Saturn: Characterization and Consequences for the Saturn System

This study analyzes the exogenous dust populations at Saturn and explores the implications for ring erosion, moon surface composition, and the age of the rings. The data from three Cassini subsystems are used to reconstruct trajectories and determine impact velocities.

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Exogenous Dust at Saturn: Characterization and Consequences for the Saturn System

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  1. Exogenous Dust at Saturn N. Altobelli(1), S. Kempf(2), F. Postberg(3), A. Poppe(4), C. Fischer(3), T. Albin(5) and R. Srama(5) 1- ESA/ESAC, Madrid, Spain 2- LASP, Boulder, USA 3- University of Heidelberg, Germany 4- University of California, Berkeley, USA 5- University of Stuttgart, Germany CASSINI SCIENCE SYMPOSIUM, BOULDER, 2018

  2. MOTIVATIONS Saturn’s Ring erosion/’pollution’ Alteration of icy moons surface composition Deposition of oxygen in Saturn/Titan’s stratosphere I- CHARACTERIZING THE OUTER SOLAR SYSTEM DUST POPULATIONS … II- …CONSEQUENCES FOR THE SATURN’s SYSTEM AND.. III- …THE RING AGE DEBATE…

  3. The Challenge: exogenous-dust at Saturn: needle(s) in a haystack Analysis of all CDA-subsystems data 2004-2017

  4. The method(s): data from three CDA Subsystems Trying to get a consistent story !!! Entrance Charge Grid (‘QP’ data)  Trajectory reconstruction using impact velocity vector [Limitations: grains > 1 mu, saturation due to plasma environment] [Application: Ring Age paper, Kempf et al. 2018 ? ] Impact Ionization Detector (‘IID’ data)  Modeling of radial distribution of detections (example: Krivov et al. 2002 for Jupiter/Galileo dust instrument) [Limitations: impact speed only, error ~ 2, while √2 necessary to discriminate Saturn bound-from unbound] [Pros: good statistics, sensitive to sub-micron size range] Chemical Analyzer Target (‘CAT’ data)  Modeling of radial distribution/attempts of trajectory constraints (minimum) impact velocity determination from (non-water) TOF better than IID  lower statistics than IID  (much) larger error on impact speed vector determination than QP, but covers sub-micron regime, saturates in micron regime…

  5. CDA-entrance charge grid (ECG) subsystem: large grains regime (1-250 microns) 129 grains detected, at least 55 (at most 74) exogenous

  6. Injection speed at the Saturn’s Hill’s radius derived from trajectory back-propagation • Existence of a ‘micron-sized, slow IDP’ population, injection’s speeds peak at 4.5 km/s

  7. CDA-Chemical analyzer target CAT [ALL ~(1700) non-water grains analyzed sub-micron regime] Minimum impact velocities derived from TOFs by C. Fischer based on Fiege et al. 2014 Titan’s orbit CAT Aperture: 28 deg All possible velocity vectors Boresight • Existence of a ‘sub-micron sized, fast IDP’ population, injection’s speed peaks at 18.5 km/s

  8. JFCs/EKBs OCCs/HTCs MODEL (A. Poppe) COMPARISON MODELED INJECTIONS SPEED AT HILL’s RADIUS WITH MEASUREMENTS SLOW IDPS: - JFCs and EKBs FAST IDPs: - OCCs and HTCs 4.5 km/s peak 18.5 km/s peak DATA (ECD/CAT) [distrib normalized to max]

  9. RADIANT DIRECTION • MODEL vs DATA • MODELED GRAINS VELOCITY RELATIVE TO SATURN’s ORBIT VELOCITY AT HILL’s RADIUS • GRAINS ‘ABOVE’ THE WHITE LINE COME FROM SATURN’S ANTI-APEX • GRAINS ‘BELOW’ THE WHITE LINE COME FROM SATURN’S APEX JFCs EKBs A. Poppe’s model

  10. OCCs From A. Poppe’s model SLOW GRAINS (EKBs) HAVE BOTH APEX/ANTI-APEX COMPONENT (especially the large ones > 2 microns) SLOW GRAINS (JFCs) HAVE MAINLY APEX COMPONENT FAST GRAINS (OCCs) HAVE MAINLY APEX COMPONENT

  11. Entry points at Hill’s Radius from QP data with injection speeds 0-7 km/s (‘slow IDP’ domain) APEX APEX +- 90 deg Slow IDP grains appears to have both APEX/ANTI-APEX component (in agreement with modeled EKBs/JFCs)

  12. Entry points at Hill’s Radius from CAT data with injection speeds > 15 km/s (OCC domain) APEX +- 90 deg Fast IDP grains appears to have mainly Saturn’s APEX component (in agreement with modeled OCC dust)

  13. Analysis of heliocentric orbital elements of exogenous grains (ECG and CAT data) Large > 2 micron EKB grains Modeled distribution and data EKB grains can explain well (better than JFCs) our derived (a,e) distribution for the slow population…

  14. Kuiper-Belt grains can also explain well our derived (a,i) distribution for the slow population…

  15. Ecc I [deg] a [AU] a [AU] Fast particles, wide range of inclinations suggest an Oort Cloud or Halley type parent body… (injections speeds at R_H exclude JFCs/EKBO)

  16. INTERMEDIATE SUMMARY  We have established the existence of slow/fast Interplanetary Dust population at Saturn  Slow IDPS: EKBO, JFCs  Fast IDPs: OCCs/HTCs Can we also derive the relative abundance of each IDP type?

  17. Constraining the relative abundance of ‘slow/fast’ populations: Modeling the radial distribution of IID dust detections

  18. 3 populations fit: • SLOW IDPs (injection speed = 4.5 km/s), isotropically distributed at S/C location • FAST IDPs (injection speed = 18.5 km/s … ), from Saturn’s APEX +- 90deg, correctd for S/C motion • INTERSTELLAR DUST (‘standard direction and flux’) Radial distribution fitting method applied • DERIVE THE RELATIVE ABUNDANCES AT ‘INFINITY’ FOR SLOW AND FAST IDPs Gravitation focusing (From Colombo/ Spahn et al. 06)

  19. Derived: Abundance slow IDPs = 9e-9 /m3 Abundance fast IDPs = 2e-9 /m3Ratio slow/fast ~ 4 TOTAL TOTAL IDP Slow IDP population Fast IDP population ISD

  20. Landgraf et al. 2002 COMPARISON WITH PREVIOUS MISSIONS Pionner 10-11 data, grains size > 10 mu Poppe et al., 2010 • ‘IDP Dust Mix’ at 10 AU: • Kuiper Belt collision debris • Cometary grains (JFCs, Oorts Cloud/HTCs, active Centaurs) Han et al. 2012 Saturn ~ 1e-8/m3 total IDP Good agreement with CDA values

  21. ALTERNATIVE POSSIBLE POPULATION FOR ‘SLOW IDPs’  the active Centaurs hypothesis ‘Fresh grains’ released by Centaur cometary activity ‘Old’ grains from KB, drifting to Saturn by Poynting-Roberston drag Loneos (Epifani et al. 2011)

  22. WHAT DOES THIS HAVE TO DO WITH CONSTRAINING SATURN’S RING AGE ? The mass in-flux of micro-meteoroids is a crucial parameter to constrain the ‘Ring Exposure Age’ CUZZI and ESTRADA, 98 • CASSINI-CDA tells us: • 3.6e-16 kg/m2/s < Mass flux < 4.2e-15 kg/m2/s at Hill’s Radius distance • dominated by SLOW population [EKB/JFC dust] injection speeds < 5 km/s • flux can only have been higher in the past •  Tg < 1e8 years • - FAST IDP (and ISD) population too small, not abundant enough, to contribute significantly to the mass flux TYPICAL TOF OF IDP AT SATURN (silicate, magnesium rich)

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