Approaching the internal structure of the nuclei of comets

Transcription

1 Approaching the internal structure of the nuclei of comets Anny-Chantal Levasseur-Regourd J. Lasue, E. Hadamcik Univ. Paris VI / Aéronomie IPSL-CNRS aclr@aerov.jussieu.fr Levasseur-Regourd Alicante, Approaching the internal structure of the nuclei of comets Outline 1. Motivation 2. Past understanding 3. Present results 4. Future projects Tiny hidden nucleus Comet McNaught, January 2007 Levasseur-Regourd Alicante,

2 1. Motivation Knowing the internal structure of comets nuclei mandatory to - understand their formation and evolution - prepare future missions In the absence of direct evidence, internal structure approached through - estimations of the density - determination of the properties of dust particles ejected from the nuclei - observations of fragmentation and catastrophic disruption events Rosetta artist view (ESA) Mini comae around fragments Note Icy comet nuclei small and very fragile LINEAR 1999 S4 disruption (HST) Levasseur-Regourd Alicante, Past understanding Densities 1P/Halley ( ) VeGa (IKI) and Giotto (ESA)! Low density dark nucleus - no detectable perturbations - in agreement with NGF models (0.2 to 0.8 g cm -3, Rickman et al. 1987)! Dust particles, from OPE+DID data - evolving with nucleus distance (time) - density! 0.1 g cm -3 and albedo! 4% Keller et al Halley/Giotto (MPI/ESA) Inner coma Inner coma Levasseur-Regourd Levasseur-Regourd Alicante, 2007 Log Nucleus distance (km) 4 et al. 1999

3 Fragmentation 26P/Grigg-Skjellerup (1992) from Giotto! Possible detection of a fragment by OPE! Fragility of cometary nuclei suspected 26P/G-S Main coma Secondary coma Remote observations - Shoemaker-Levy 9 discovery and evolution (tidal disruption) - Motion of icy fragments in C/1996 B2 Hyakutake! Sizes! m, densities g cm -3 (Desvoivres et al. 2000) - Also C/LINEAR 1999 S4 and 73P/S-W 3 disruptions McBride et al LINEAR, HST S-L 9, HST Levasseur-Regourd Alicante, 2007 Yakutake, HST 5 Hyakutake, 73P/S-W HST 3, Spitzer 3. Recent results Remote light scattering observations and simulations Observations of polarization of solar light scattered by dust, with laboratory simulations (reduced gravity) and numerical simulations (e.g. DDA)! Irregular particles, with size >!! Presence of fluffy -possibly fractal- aggregates (clue to the nucleus structure?)! At least bimodal population, silicates and more absorbing organics P red (") mixtures 50/50 in mass SiO 2(40±20)nm+C SiO 2+C (14±5)nm Data points and corresponding fits - red (near 670 nm) - green (near 523 nm) Polarization (%) SiO 2 SiO 2+C (95±20)nm Phase angle ( ) Levasseur-Regourd et al Alicante,

4 3. Recent results 81P/Wild 2 ( ) and Stardust! Rather rounded nucleus! Complex morphology with flat craters! Swarm detected at! 4000 km (Tuzzolino et al. 2004), possibly related to dust fragmentation! Estimated density g m -3 (Davidsson & Guttierez, 06) Brownlee et al Return capsule with impacts on Al foils and tracks in aerogel cells!direct evidence for low density dust aggregates Hölz et al Levasseur-Regourd Alicante, P/Tempel 1 (2005) and Deep Impact, Pre-Impact Worldwide observations! Aperiodic outbursts (nucleus crumbling ) High resolution images prior to impact! Flat layers, scarps, erosion features! Icy patches, leading to tiny jets at the limb! Impact craters Farnham et al Zone d impact Levasseur-Regourd Alicante, A Hearn et al. 2005

5 9P/Tempel 1 (2005) and Deep Impact, Post-impact Realistic impact experiment (m = 370 kg v = 10.2 km s -1 ) - crater hidden by dust - speed of fastest dust particles > 300 m s -1! Density! 0.35 ± 0.25 g m -3 (Estimated density! 0.45 g m -3 (Davidsson et al. 2006)! Super porous, porosity >! 80 %! Organics and water ice in subsurface Zone d impact A Hearn et al Levasseur-Regourd Alicante, 2007 Sunshine et al Future projects Rosetta mission Objectives Rendezvous (Rosetta orbiter) Landing on nucleus (Philae lander)! Characterization of its evolution from aphelion to perihelion! Detailed study of the nucleus: composition, mineralogy, structure Target 67P/Churyumov-Gerasimenko - a = UA, e = 0.632, i = nucleus! 3 km x 5 km - active areas! 7% at perihelion - density estimated! 0.1 to 0.37 g cm -3 Successful launch, 2 March 2004 Philae Estimated shape of 67P nucleus, assuming a Levasseur-Regourd Alicante, 2007 prograde (up) or retrograde (bottom) rotation Lamy et al

6 Rosetta milestones 2005, Earth swing-by, Tempel 1 impact observations (from.6 AU) 2007, swing-by in February and 2 nd Earth swing-by in November 2008, 2867 Steins (! 10 km, E-type) flyby on 5 September , 3 rd Earth swing-by in November 2010, 21 Lutetia (! 100 km, M-type) flyby on 10 July , Rendezvous, with Philae landing! 6 months later 9P/Tempel 1 Levasseur-Regourd Alicante, CONSERT experiment CONSERT, Cometary Nucleus Sounding by Radiowave Transmission with parts on both the orbiter and the lander (Kofman et al. 2007) Objectives: nucleus tomographic sounding from the determination of its dielectric properties (materials, voids, discontinuities, porosity) Instrumentation: radio-waves (90 MHz, 8 MHz band-pass) from the orbiter, with reception and re-emission by Philae! Information on celerity V and electric field E Levasseur-Regourd Alicante,

7 Preparation of Rosetta encounter Dust coma Optimisation of the mission from observations, numerical simulations and laboratory simulations CONSERT Optimisation of the science return from - measurements of the properties of ices-dust samples with relevant porosities, temperatures, concentrations - numerical simulations with 3D thermal models of the nucleus Other proposed missions ESA & Jaxa, Marco Polo, NEO sample return? ICAPS for accretion exp. on board ISS? Levasseur-Regourd Alicante, Philae artist view (ESA) Please do not forget comet nuclei in catastrophic disruption studies - Small - Fragile - Mostly hidden - Often suffering disruptions - Underdense - Possibly built of fluffy particles - Having suffered impacts More to be hopefully known in 2014 Thank you Levasseur-Regourd Alicante,