Cassini Small Bodies Observations Ymir Atlas Tilmann Denk Freie Universität Berlin, Germany 10th Small Bodies Assessment Group Meeting (SBAG) Washington, DC – Thu 09 Jan 2014 2:20 p.m. EST (8:20 p.m. CET) T.D. gratefully acknowledges the support of this research by the Deutsches Zentrum für Luft- und Raumfahrt (DLR) in Bonn (Germany) (grant no.: 50 OH 1102). Cassini Orbiter Remote Sensing (ORS) • Imaging (ISS NAC, WAC) • Vis./near-IR spectrometer (VIMS) • IR spectrometer (CIRS) • UV spectrometer (UVIS) ORS FOVs: = 3.5° = 0.35° Denk -- Cassini Small Bodies Observations – page 2 Small Bodies or "Rocks" of Saturn "Alkyonides" 3 38 "Irregulars" "Ring rocks", 9 • Methone, Anthe, Pallene • Phoebe, Siarnaq, Albiorix, "Sheperds", Paaliaq, Ymir, Kiviuq, "Co-orbitals" Orbits between Mimas • Tarvos, Ijiraq, Erriapus, and Enceladus Skathi, Hyrrokkin, Suttungr, • Pan, Daphnis, Atlas Prometheus, Pandora Mundilfari, Thrymr, Narvi, • Discovered by Cassini etc. Janus, Epimetheus Bestla, Kari, Tarqeq, Aegaeon • Beyond 11 million km (> 180 R ) • Inside Mimas orbit; 4 "Lagrange Moons" S partly inside A ring or "Trojans" • 9 prograde, 29 retrograde • Cassini: disk-resolved • Telesto, Calypso • Discovered from Earth & spectra Helene, Polydeuces • Cassini: only disk-integrated • L4 & L5 moons of (except Phoebe) Tethys, Dione Denk -- Cassini Small Bodies Observations – page 3 "Ring rocks" Pan, Daphnis Denk -- Cassini Small Bodies Observations – page 4 "Ring rocks" Pan Daphnis Atlas 34 x 21 km 9 x 6 km 41 x 19 km Denk -- Cassini Small Bodies Observations – page 5 "F-ring Sheperds" Prometheus, Pandora 136 x 59 km 104 x 64 km Denk -- Cassini Small Bodies Observations – page 6 "F-ring Sheperds" Prometheus, Pandora 136 x 59 km 104 x 64 km Denk -- Cassini Small Bodies Observations – page 7 "Co-orbitals" "Problem": Δai < ΣRi Janus, Epimetheus [ The difference of objects' semi-major axes is smaller than Horse-shoe orbits the sum of the objects' radii ] aJA = 151,460 km aEP = 151,410 km rJA = 100 km rEP = 65 km => Exchange of orbits every 4 years Rotating coordinate system January Cartoon is not to scale 2014 Denk -- Cassini Small Bodies Observations – page 8 "Co-orbitals" Janus 203 x 153 km Epimetheus 130 x 106 km Denk -- Cassini Small Bodies Observations – page 9 Aegaeon Produces the G ring Semi-major axis: 167,500 km Orbit & rotation period: 19.4 h Object size: 1.5 x 0.5 km Denk -- Cassini Small Bodies Observations – page 10 "Alkyonides" Methone, Anthe, Pallene orbit between Mimas and Enceladus; P ~ 24...28 h øMethone ~ 3.2 km øPallene ~ 5.0 km Denk -- Cassini Small Bodies Observations – page 11 "Lagrange Moons" or "Trojans" Tethys or Dione Saturn Denk -- Cassini Small Bodies Observations – page 12 "Lagrange Moons" Polydeuces (Dione L5) Helene 3 x 2 km (Dione L4) 43 x 26 km Telesto Calypso (Tethys L4) (Tethys L5) 33 x 20 km 30 x 14 km Denk -- Cassini Small Bodies Observations – page 13 Irregular Moons 38 objects are known at Saturn 9 progrades, 29 retrogrades Range ~ 8...33 million km Sizes: 213 km (Phoebe); <40 km (all others) Phoebe Ymir Denk -- Cassini Small Bodies Observations – page 14 Irregular-Satellite Observations with Cassini: Motivation • Gain information on basic properties of these objects (like rotation period, size, shape, pole-axis orientation, color, ...; search for binaries) Denk -- Cassini Small Bodies Observations – page 15 Irregular-Satellite Observations with Cassini: Motivation • Gain information on basic properties of these objects (like rotation period, size, shape, pole-axis orientation, color, ...; search for binaries) • So far (2013): 17...20 periods, 2 shape models • Expected (2017): up to 32 periods, up to 13 shape models Paaliaq, 102° phase Ymir 0 days 1 day 2 days Denk -- Cassini Small Bodies Observations – page 16 Irregular-Satellite Observations with Cassini: Motivation • Gain information on basic properties of these objects (like rotation period, size, shape, pole-axis orientation, color, ...; search for binaries) • Periods, pole-axes orientations: Purely random, or shaped by a physical process? • Compare to each other and to other solar-system objects • Understand their role related to the albedo dichotomy of Iapetus and the reddish color of Hyperion • Understand the impact history of the Saturnian system (origin of the irregular moons) Denk -- Cassini Small Bodies Observations – page 17 Cassini Future Opportunities • Irregular moons until EOM • Small inner moons in Cassini Solstice Mission (example of rev 227) • Small inner moons in F-ring orbits Denk -- Cassini Small Bodies Observations – page 18 Irregular-moons accessible to the NAC 2014 to 2017 (EOM) • Synodic rotation - 01/14: Tarqeq • Pole/ shape/ - 03/14: Fornjot sidereal rotation - 09/14: Thrymr* - 12/14: Loge - 12/13: Siarnaq - 12/14: S/2004 S7 - 04/14: Kiviuq • Color - 07/14: Ijiraq - 04/15: Aegir - 08/14: Tarvos - 06/15: Bergelmir* - 12/13: Siarnaq - 05/15: Paaliaq - 06/15: Greip - 04/14: Kiviuq - 07/15: Hyrrokkin - 10/15: Narvi* - 07/14: Ijiraq - 10/15: Hati - 01/15: Albiorix - 01/16: Skoll* - 11/15: Bestla - 03/16: Surtur - 04/15: Ymir - 11/15: Narvi - 04/17: Paaliaq - 01/17: S/2004 S12 - 05/16: Albiorix - 08/17: Tarvos - 04/17: S/2006 S1 - 08/16: Erriapus - 06/17: S/2004 S13 - 12/16: Tarqeq - 09/17: S/2007 S2 Denk -- Cassini Small Bodies Observations – page 19 Cassini Future Opportunities: rev 227 Epimetheus 2,680 km / 16 m/pxl Atlas 20,640 km / ~130 m/pxl Prometheus 20,860 km / ~130 m/pxl View from north. Movements of objects: CCW. Sun direction: up. Denk -- Cassini Small Bodies Observations – page 20 Cassini F-ring and Proximal Orbits T125 T126 F-ring orbs.: 29 Nov 2016; rev 250 out Prox. orbs.: 22 Apr 2017; rev 270 out EOM: 15 Sep 2017; rev 293 peri Denk -- Cassini Small Bodies Observations – page 21 Cassini F-ring Orbits "Rocks" Opportunities <25,000 km Orbit Object Altitude Spatial. res.* Moon size* 253 Pandora 18,000 km 110 m/pxl 961x591 pxl 257 Daphnis 18,300 km 110 m/pxl 78x58 pxl Atlas 24,200 km 150 m/pxl 281x129 pxl 258 Pandora 24,100 km 150 m/pxl 720x443 pxl 259 Epimetheus 4,500 km 30 m/pxl 4760x3900 pxl 262 Epimetheus 9,600 km 60 m/pxl 2250x1840 km 264 Pan 23,500 km 140 m/pxl 244x147 pxl 269 Janus 7,900 km 50 m/pxl 4310x3240 pxl Atlas 11,100 km 70 m/pxl 612x282 pxl Aegaeon 22,800 km 140 m/pxl 10x4 pxl *...ISS NAC; phase angle ignored Denk -- Cassini Small Bodies Observations – page 22 Cassini F-ring Orbits Pan – rev 023, and rev 264 simulation Pan rev 264 244 x 147 pxl Denk -- Cassini Small Bodies Observations – page 23 Cassini F-ring Orbits Atlas – rev 009, 046, and rev 269 simulation Atlas rev 269 612 x 282 pxl Denk -- Cassini Small Bodies Observations – page 24 .