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Surface Composition of Icy Asteroids and the Special Case of Ceres

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Surface composition of icy asteroids and the special case of Ceres P. Vernazza (Laboratoire d’Astrophysique de Marseille) March 15, 2017 SOFIA Tele-Talk Examples of asteroids with Compositional links between asteroids and meteorites meteoritic analogues Falls Mass in the asteroid belt 6.2% 9.6% 80.0% 8.4% 1.9% 1.0% 1.1% 0.4% 2.5% 0.1% 1.5% 7.0% 4.6% 3.3% Total Total ~98% ~30% Vernazza & Beck 2016 ~2/3 of the mass of the asteroid belt seems absent from our meteorite collections 6 asteroid types, representing ~2/3 of the main belt mass (DeMeo & Carry 2013), are presently unconnected to meteorites. Asteroid spectral properties (DeMeo et al. 2009) Metamorphosed CI/CM chondrites as analogues of most B, C, Cb, Cg types? (1) Metamorphosed CI/CM chondrites as analogues of most B, C, Cb, Cg types? (2) This is very unlikely because: a) Metamorphosed CI/CM chondrites represent 0.2% of the falls whereas B, C, Cb and Cg type represent ~50% of the mass of the belt b) Metamorphosed CI/CM chondrites possess a significantly higher density (2.5-3 g/cm3) than those asteroid types (0.8-1.5 g/cm3) c) Metamorphosed CI/CM chondrites possess different spectral properties in the mid-infrared with respect to those asteroid types Why are most B, C, Cb, Cg, P and D types unsampled by our meteorite collections? The lack of samples for these objects within our collections may stem from the fact that they are volatile-rich as implied by their low density (0.8-2 g/cm3) and their comet-like activity in some cases. Main belt comets Jewitt 2012 Ceres (Kuppers et Water ice at the al. 2014) surface of Themis (Campins et al. 2010) The asteroid belt as a condensed version of the early solar system as the result of giant planet migrations? Grand Tack model Walsh+ 2011 Nice model Morbidelli+ 2005 Tsiganis+ 2005 Gomes+ 2005 DeMeo & Carry 2014 Because a large fraction of main belt asteroids appears unsampled by our meteorite collections, it seems logical, as a next step, to test a link between these asteroids and the other significant source of extraterrestrial materials, namely interplanetary dust particles (IDPs). Interplanetary dust particles (IDPs): the MAIN class of extraterrestrial materials Fluffy aggregates of: ! silicates (amorphous and crystalline) ! Sulfides ! Iron-nickel ! Carbonaceous matrix 3 classes: ! Pyroxene-rich ! Olivine-rich ! Phyllosilicate-rich IDPs differ from meteorites in being: ! Smaller (< 2 mm) ! More plentiful (~40,000 tons/year accreted by the Earth) ! Different in texture and composition Parent bodies of IDPs: asteroids? comets? IDPs as analogues of B, C, Cb, Cg, P and D types 1.2 BCG Asteroids Asteroids (P, D), Comets Pyroxene-rich IDPs Mixture of pyroxene-rich 1.0 and olivine-rich IDPs 1.6 Chondritic porous (CP) IDPs 762 1.4 P- and D-type asteroids 0.8 H-B BCG asteroids 526 Halley 1.2 0.6 45 1172 1.0 Emissivity 0.4 624 Norm. Reflectance 0.8 10 107 0.2 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 1 87 Wavelength (µm) 0.0 8 9 10 11 12 13 8 9 10 11 12 13 Wavelength (µm) Wavelength (µm) Vernazza et al. 2015 IDP-like surface composition for B, C, Cb, Cg, P and D types BCG Asteroids (B, C, Cb, Cg) Asteroids (P, D) 2.0 Model (CP IDPs’ building blocks) Model (CP IDPs’ building blocks) Red 3200 Trojans 154 1.5 Vernazza et al. 52 2015 45 Blue 1.0 31 Trojans 107 Norm. Reflectance 24 87 0.5 10 1 0.0 0.5 1.0 1.5 2.0 2.5 0.5 1.0 1.5 2.0 2.5 Wavelength (µm) Wavelength (µm) C-types: Surface composition dominated by crystalline pyroxene and amorphous silicates Vernazza et al. 2017 Jupiter Trojans (D-types): Surface composition dominated by crystalline olivine and amorphous silicates Vernazza et al. 2012 Summary figure Rocky / metallic Hydrated Icy 100 80 B,C,Cb,Cg,P,D Asteroids 60 (including Jupiter 40 Trojans) A,E,K,M,S,V Freq. (%) Freq. Ch, Cgh 20 0 5000 4000 Asteroid 3 3000 Densities kg/m 2000 1000 0 Vernazza et al. Asteroid Activity No No Yes 2015 "Dry" meteorites 100 80 Related 60 Meteorites 40 Aqueously altered Freq. (%) Freq. 20 chondrites (CM) 0 100 80 Hydrous? Related 60 IDPs 40 Freq. (%) Freq. Chondritic 20 Achondritic 0 Related Bodies - - Comets The case of Ceres 1200 (1) Ceres SOFIA 1000 800 ISO 600 400 Flux density (Jy) 200 0 1.15 1.10 1.05 1.00 Emissivity 0.95 0.90 5 10 15 20 25 30 35 Wavelength (microns) Comparison between Ceres and hydrated CC meteorites: Something is missing! 1.15 (1) Ceres 1.10 Nogoya (CM2) 1.05 1.00 Emissivity 0.95 0.90 Vernazza et al. 2017 1.15 (1) Ceres 1.10 Tagish Lake (C2) 1.05 1.00 Emissivity 0.95 0.90 1.15 (1) Ceres 1.10 Murray (CM2) 1.05 1.00 Emissivity 0.95 0.90 10 15 20 25 30 Wavelength (microns) Ceres: Surface composition dominated by crystalline pyroxene, hydrous silicates and carbonates 1.2 (1) Ceres Tagish Lake (60%) + Enstatite (40%) TL (31%) + Enst. (26%) + Am. Ol. (23%) + Magn. (12%) + Dol (8%) Murray (80%) + Enstatite (20%) 1.1 1.0 Emissivity 0.9 0.8 10 15 20 25 30 35 Wavelength (micron) Vernazza et al. 2017 Ceres and Hygiea: similarities and differences 1.4 (1) Ceres 1.2 (10) Hygiea 1.0 0.8 Norm. Reflectance Carbonates 1 2 3 4 Wavelength (microns) 1.15 (1) Ceres (10) Hygiea 1.10 1.05 Emissivity Local maximum 1.00 Local maximum 10 15 20 25 Wavelength (microns) Enstatite at the surface of Ceres: Endogenous or exogenous? Result of thermal evolution or impact contamination? Anhydrous pyroxene-rich dust Hydrous dust Carbonates Water ice Surface (as observed) Sub-surface (predicted) CI ? TL ? ? ? D ~ 200 km D ~ 400 km D ~ 940 km Enstatite as an endogenous component: Results of thermal evolution models McCord and Sotin 2005 Enstatite as an exogenous component: The case of the Beagle family The Beagle family belongs to the Themis family. Marsset et al. (2016) showed that the latter has a composition similar to pyroxene-rich IDPs. Beagle family Enstatite as an exogenous component: The case of the Beagle family 3 possible ages for the Beagle family Nesvorny et al. 2008 Other cases of contamination? The case of M-types Hardersen et al. 2005 Hardersen et al. 2005 Landsman et al. 2017 16 Psyche Takir et al. 2017 Conclusions / Open questions 1. The majority (~2/3 of the mass) of main belt asteroids appear unsampled by our meteorite collections. Instead, IDPs appear as plausible analogs for these objects. 2. Ceres’ surface = complex mixture of a) enstatite, b) ammoniated phyllosilicates, c) carbonates and d) water ice 3. What was the initial composition of Ceres? Is it connected to C-types or rather to P/D-types? Could Orcus and Ceres be twins? 4. Contamination of asteroid surfaces by IDPs: a reality? a common process? .
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