Asteroids, Trojans and Transneptunians Sonia Fornasier LESIA-Obs. de Paris/Univ Paris Diderot The solar system debris disks - Small bodies: Preserve evidence of conditions early in solar system history - TNO: cold and relatively unprocessed - They trace Solar System formation and evolution - Delivery of water and organics to the Earth 2 …and exo-systems disk debris ~400 long, highly elongated 1I/2017 U1 'Oumuamua • First interstellar asteroid discovered on 18 Oct. 2017 (William 2017) • Q=0.254 AU, e=1.197, i=122.6 °, v∞ = 25 km/s • very elongated shape (10:1 axis ratio and a mean radius of 102±4 m, assuming an albedo of 0.04, Meech et al., 2017) • No cometary activity, red spectrum similar to D-type (Jewitt et al., 2017) More than 700000 asteroids discovered Crust Wide diversity of composition, shape, structures mantle Pristine (D, C) silicaceous (S, A) Igneous (E,V) core S E C Rosetta (ESA) ρ=2.7 S: silicaceous asteroids, Steins: differentiated objet similar to the ordinary enstatite rich chondrite (space Vesta: differentiated Dawn (NASA) V weathering effects) object with internal structure (Density 3,5) Itokawa -parent body of HED achondrite Mathilde ρ=1.95 ρ=1.3 S C/D: carbonaceous and organic material, hydrated silicates ( liquid water in the past), small M-type: high density, densities, high porosity Rubble pile exposed nickel-iron Similarities with carbonaceus structure core of an early planet chondrites Shepard et al., 2017 Focus on primitive asteroids, TNOs The water problematic and evidence of its past and current presence in the main belt (aqueous alteration, ice) Families studies: the Themis/Beagle case Space weathering issues The Trojans population TNOs/Centaurs The water problematic in Asteroids ● Nebular snowline ● A lot of mass in H2O ● Big effect on accretion where condenses Dodson-Robinson et al. (2009) ● Significant impact on geochemical evolution ● Latent heat energy buffer ● Heat from serpentinization ● Resulting mineralogies ● Asteroids retain a record of the initial H2O distribution and evolutionary events ● They are a potential source of terrestrial volatiles Aqueous alteration low temperature (< 320 K) chemical alteration of materials by liquid water phyllosilicates, sulfates, oxides, carbonates, and hydroxides. Liquid 1) Icy planetesimals ( water ice condensed at ~160 –6 water K at P ~10 bar ( Cyr et al., 1998; Drake 2005) 2) heating mechanism allowing ice to melt but not to sublimate McSween et al., 2002) Radiactive decay of induction of materials 26Al or 60Fe with solar wind during the T-Tauri phase of the Sun Studies of meteorites indicates that this process took place 20 Mys after Solar System formation SONIA FORNASIER 7 (From Fornasier et al, 1999) JWST (From Rivkin et al, 2003) Asteroid Hydration Bands: correlation • Aqueous alteration of iron-bearing silicates results in hydrated minerals with both 0.7 and 3 micron absorption bands. • The 0.7 micron band can be “cooked’’ out at moderate temperatures, so the 3 micron band can appear alone. • The 0.7 micron band should never be seen without the 3 micron •. band, if it really only comes from hydrated minerals. • Extreme thermal processing can eliminate bothm band bands, or the μ asteroid may be anhydrous 0.7 Howell et al., 2011, Rivkin et al. 2015 Incidence of the aqueous alteration process on asteroids classes • The analysis of 625 C-complex visible spectra from the literature show that 45 ± 2 % have features in the VIS range (0.7 micron) attributed to hydrated silicates ! lower limit • Considering the 3 micron band studies , 70 ± 5 % of C-complex asteroid should be hydrated (Fornasier et al., 2014, Rivkin et al. 2015) Fornasier et al., 2014 Aqueous alteration region 2.6< Vilas 1994 < 3.6 AU -Very few NEO show Analysis of 625 C- signatures of hydrated complex asteroid asteroids: - 2099 Opik : 0.7 μm (Binzel et al. 2004) - 1996 FG3 : 3 μm band (Rivkin et al., 2013) - 1999 JU3 Ryugu: 0.7 μm (Vilas et al. 2008), not confirmed in further studies covering 65% of the surface (Perna et al., Fornasier et al., 2014 2017) 2.3 < Aq. Al zone <3.2 AU The 3 micron region - Signature of hydroxyl (2.7-2.8 micron) - Water ice : ~ 3.0, 3.1 and 3.2 micron, accurate position depend on ice state (crystalline or amorphous) , and temperature (Mastrapa et al., 2009) - Methane and organic materials: 3.3-3.4 micron - Ammonium (NH 4+): 3.1 micron (Ceres) - ….. All these diagnostic features can be fully investigated by JWST ! Ceres • Ceres is a special case – strange 3-µm band – Fe-rich phyllosilicates + brucite +carbonates – Water ice – Ammoniated clays + magnesium salt Milliken & Rivkin (2009) • Differentiated – Liquid H2O mantle? Dawn: evidence of a water ice layer under the surface at medium and high latitude • Albedo variations, geology • Herschel detected water vapor in 2012 - 6 km Lobate 2013 (r < 2.72AU, Kueppers et al. 2014) 18 km flows Ahuna moon: cryovulcan 92 km Credits: NASA/JPL- Occator crater: water ice Caltech/UCLA/MPS/DLR/ID A Themis, Cybele • Rounded band, centered ~3.1 µm – H2O frost/coating (Canpins et al., 2010, Rivkin and Emery , 2010) – Goethite (Beck et al., 2010) 24 Themis (a~3.13 AU) 65 Cybele (a~3.43 AU) Licandro et al. (2011) Rivkin & Emery (2010) • Detected on several more outer belt asteroids 3 micron region: 4 different types of bands sharp group Europa-like: (or Pallas band @3.15 types) µm 2.5 < a < 3.3 AU OH-stretching in hydrated minerals Ceres-like, Themis-like: band rounded 3-µm centered 3.05 band H2O μm brucite ice 2.5-3.3 AU 3.4 < a < 4.0 region AU region Asteroid Families -Most of the largest asteroids have families -most of the family members are homogeneous in composition in the 0.4-2.5 micron range (exception: Themis) Delbo’ et al., 2017 - A new family 4 billions year old recently discovered in the inner main belt (Delbo’ et al., 2017), and primordial asteroids identified, leftover of the original planetesimals - The study of family members of different age help understanding space weathering effects on primitive material The Themis/Beagle families • Themis is a statistically robust families in the asteroid belt (500-4000 members according to Zappalà et al. 1995-Nesvorny et al. 2012), dominated by primitive C- and B- type asteroids •formed ∼ 2.3 Gyr ago as a result of a large catastrophic disruption event • Themis is a source of main belt comets (133P, 176P, 238P, Jewitt et al., (2015) and P/2006 VW139) • Themis family is a reservoir of water ice in the outer main belt (Ice on Themis, hydrated silicates on several members) • Beagle : a young (< 10 Myr) sub-family within the Themis Themis family with 65 members up to 2 km of diameter (Nesvorny et al., 2012) Themis members : spectral heterogeneities • spectral variability of Themis members from visible and WISE data •no Beagle members red and dark (and this is not related to size bias) •bright Themis members have lower pIR/pv ratio, indicating a blue spectrum in the NIR region and/or the presence of absorption bands in the 3 μm region, potentially attributed to hydrated silicates, organics and eventually water ice. VIS slope for 119 members from Kaluna et Wise data for 211 Themis al., 2015; Fornasier et al. 2014 & 5 Beagle Themis Beagle Fornasier et al. 2014 Infrared (3.4-4.6μm) albedo vs vis albedo 26 Mai 2016, CIAS Origin of Themis/Beagle spectral variagation 3 possible scenarios: A) Themis parent body was heterogeneous in composition: differentiation of rock and ice where the core underwent mild temperatures and no high thermal metamorphism asteroids with different spectra come from different regions of the parent body (Campins et al., 2014 B) The projectile (~190 km) and the parent body (~380km) were different in composition C) Themis parent body was quite homogeneous spectral variety produced by SW young members are blue and bright, old ones become darker and redder SW acts as in silicate rich asteroids, as foreseen by Lazzarin et al. (2006) Space weathering effects Quite well understood for silicate rich surface: darkening and reddenig effect (proven from moon and Itokawa samples) Not yet well understood for primitive surface. Recent studies on irradiation of carbonaceous chondrites (CC) indicate different behaviour depending on carbon content and albedo of the surface (Lantz et al., 2017): Lantz et al., 2017 p > 8% : darkening and reddening, as in S-type asteroids p< 6% : brightening and blueing Trojan Asteroids • Widely thought to contain ice, but none detected; no sign of hydrated silicates • Featureless and red spectrum organics? (but none detected so far) Emery et al., 2011: amorphous and space weathered silicates (refractory mantle?) Emery & Brown (2004) • Emissivity spectra indicate fine-grained silicates and analogies to cometary dust (Emery et al.2006) • Low densities: Yang & Jewitt (2007) -Patroclus ~ 1.08±0.33 g cm-3 (Mueller et al., 2010, Buie 2014) - Hektor ~ 1.0 ±0.33 g cm-3 (Marchis et al., 2014), but 2.4 g cm-3 from lightcurve analysis (Descamps 2015) • Low thermal inertia (20 in SI in Patroclus, Marchis et al 2010)) indicate a fine regolith Trojans swarms Statistical analysis of about 150 Trojans (68 L5 and 74 L4) indicate that : • L5 swarm dominated D-type (73.5%), L4 more spectral variagation (but related to the C-type dominated Eurybates family) •Slope distribution of Trojans is similar to that of cometary nuclei and part of TNOs population, but narrower and peculiar compared to other populations •Any correlation slope-size Trojans Fornasier et al. 2004, 2007 Comets Scattered Centaurs Classical Plutinos HDR Fornasier Spectral slope The promise of JWST studies of asteroids Spatially resolved studies of the largest asteroids with NIRCAM: ~28 targets (most are primitive in composition) They are intact remnants of the pebble accretion in the early Solar System .