Asteroids, Trojans and Transneptunians. Sonia Fornasier LESIA-Obs. de Paris/Univ Paris Diderot

Transcription

1 Asteroids, Trojans and Transneptunians Sonia Fornasier LESIA-Obs. de Paris/Univ Paris Diderot

2 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

3 and exo-systems disk debris ~400 long, highly elongated 1I/2017 U1 'Oumuamua First interstellar asteroid discovered on 18 Oct (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)

4 More than asteroids discovered Wide diversity of composition, shape, structures Pristine (D, C) silicaceous (S, A) Igneous (E,V) Crust mantle core C S E ρ=2.7 Rosetta (ESA) ρ=1.3 Mathilde Itokawa S: silicaceous asteroids, similar to the ordinary chondrite (space weathering effects) ρ=1.95 S Steins: differentiated objet enstatite rich Dawn (NASA) V Vesta: differentiated object with internal structure (Density 3,5) -parent body of HED achondrite C/D: carbonaceous and organic material, hydrated silicates ( liquid water in the past), small densities, high porosity Similarities with carbonaceus chondrites Rubble pile structure M-type: high density, exposed nickel-iron core of an early planet Shepard et al., 2017

5 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

6 The water problematic in Asteroids Nebular snowline A lot of mass in H 2 O Big effect on accretion where condenses Significant impact on geochemical evolution Dodson-Robinson et al. (2009) Latent heat energy buffer Heat from serpentinization Resulting mineralogies Asteroids retain a record of the initial H 2 O distribution and evolutionary events They are a potential source of terrestrial volatiles

7 Aqueous alteration low temperature (< 320 K) chemical alteration of materials by liquid water phyllosilicates, sulfates, oxides, carbonates, and hydroxides. McSween et al., 2002) Liquid water 1) Icy planetesimals ( water ice condensed at ~160 K at P ~10 6 bar ( Cyr et al., 1998; Drake 2005) 2) heating mechanism allowing ice to melt but not to sublimate Radiactive decay of 26 Al or 60 Fe induction of materials 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

8 (From Fornasier et al, 1999) JWST (From Rivkin et al, 2003)

9 0.7 μm band 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 both bands, or the asteroid may be anhydrous Howell et al., 2011, Rivkin et al. 2015

10 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 Fornasier et al., 2014 Considering the 3 micron band studies, 70 ± 5 % of C-complex asteroid should be hydrated (Fornasier et al., 2014, Rivkin et al. 2015)

11 Analysis of 625 C- complex asteroid Fornasier et al., 2014 Aqueous alteration region 2.6< Vilas 1994 < 3.6 AU 2.3 < Aq. Al zone <3.2 AU -Very few NEO show signatures of hydrated asteroids: Opik : 0.7 μm (Binzel et al. 2004) FG3 : 3 μm band (Rivkin et al., 2013) JU3 Ryugu: 0.7 μm (Vilas et al. 2008), not confirmed in further studies covering 65% of the surface (Perna et al., 2017)

12 The 3 micron region - Signature of hydroxyl ( 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: micron - Ammonium (NH 4+ ): 3.1 micron (Ceres) -.. All these diagnostic features can be fully investigated by JWST!

13 Ceres Ceres is a special case strange 3-µm band Fe-rich phyllosilicates + brucite +carbonates Water ice Ammoniated clays + magnesium salt Differentiated Liquid H 2 O mantle? Dawn: evidence of a water ice layer under the surface at medium and high latitude Albedo variations, geology Herschel detected water vapor in (r < 2.72AU, Kueppers et al. 2014) 6 km Ahuna moon: cryovulcan 92 km 18 km Milliken & Rivkin (2009) Lobate flows Occator crater: water ice Credits: NASA/JPL- Caltech/UCLA/MPS/DLR/ID A

14 Themis, Cybele Rounded band, centered ~3.1 µm H 2 O 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

15 sharp group (or Pallas types) 2.5 < a < 3.3 AU 3 micron region: 4 different types of bands OH-stretching in hydrated minerals Europa-like: µm Ceres-like, band centered 3.05 μm brucite AU region Themis-like: rounded 3-µm band H2O ice 3.4 < a < 4.0 AU region

16 -Most of the largest asteroids have families Asteroid Families -most of the family members are homogeneous in composition in the micron range (exception: Themis) - 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 Delbo et al., 2017

17 The Themis/Beagle families Themis is a statistically robust families in the asteroid belt ( members according to Zappalà et al 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, and P/2006 VW139) Jewitt et al., (2015) 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 family with 65 members up to 2 km of diameter (Nesvorny et al., 2012) Themis

18 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. Wise data for 211 Themis & 5 Beagle VIS slope for 119 members from Kaluna et al., 2015; Fornasier et al Themis Beagle Infrared ( μm) albedo vs vis albedo Fornasier et al Mai 2016, CIAS

19 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)

20 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

21 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: -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) Yang & Jewitt (2007) Low thermal inertia (20 in SI in Patroclus, Marchis et al 2010)) indicate a fine regolith

22 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

23 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 PSF of NIRCam ~80km mid belt, ~45 km at inner belt -look for surface heterogeneities -Can spatially resolve binary systems! saturation may be an issue for brightest targets

24 The promise of JWST studies of asteroids, Trojans & TNOs Spectroscopy in NIR&MIR range: several features diagnostic of minerals and volatiles constraint on composition Devoted study of selected bodies, no big survey. Potential targets: - targets of space missions (Lucy, Psyche, New Horizon,..) - primitive asteroids / TNOs - small family members - binaries - small NEOs Composition, presence water (hydrated minerals, ice), presence of volatiles meteorites&asteroids connections, space weathering effects red colors and organic content masses and structure of primari and secondary in binary-system

25 JWST observations of asteroids 1000s NIRSPEC continuum sensitivity Pv=5% Rivkin et al., 2016 Trojan

26 activated asteroids, small NEO (warning: max nonsidereal tracking rate: 30 mas/s) size, albedo, thermal inertia MIRI observations Saturation limits asteroids are very bright in MIR: JWST will allow serendipitous asteroid detection (10s of asteroid /hr at ecliptic from estrapolations of Spitzer-IRAC) - physical properties of small asteroids/neos - Size frequency distribution of smaller popolation of asteroids/neos, that is currently inaccessible from ground based and space observatory Thomas et al., 2016

27 MIRI observations Spectroscopy: emissivity features (silicates, hydrated minerals, carbonates, very fine grained silicates) Emery et al., 2006 Barucci et al., 2008 Emissivity spectra of Trojans (strong plateau near 10 micron, broader emissivity at micron) indicate fine-grained silicates (Emery et al.2006) Barucci et al., 2008

28 Centaurs and Transneptunians

29 The Kuiper Belt: bulk properties ~2000 objects known including ~250 Centaurs 200,000 estimated with D > 100 km Cold ( about K) water ice expected Different dynamical populations Source of Centaurs, SPC Resonants Hot Classicals Cold Classicals Scattered and detacted objetcs 29

30 Color diversity Doressoundiram et al., 2008) Hot: different colors, larger Cold: small and red Large color diversity from «neutral» (i.e. solar)» to «very red» in all families No correlation between color and dynamical parameters (e.g. semi-major axis, perihelion distance ) Exception: «cold» classicals which ~all «very red» (primordial?) Centaurs have color bi-modality ( grey / very red) Sonia Fornasier

31 Surface composition Difficult, spectroscopy limited to less than 70 objets (but S/N good for half of them); 3 groups: -featureless -dominated by water ice -dominated by methane (largest bodies) Barucci et al., 2008 Barucci et al., 2008 Sonia Fornasier

32 Volatiles Schaller & Brown 2007, Brown et al Brown 2013 Sonia Fornasier

33 - WATER ICE Barucci et al Orcus Water ice No water ice Needs confirmation - All bodies > 700 km show water ice features. Methane only on the largest ones (Eris, Pluto, Makemake) - Ammonia on Charon, Orcus - Water ice often in crystalline form: T ~100/110 K while T(40AU) = K cryovolcanisme? Sonia Fornasier

34 Size and albedos: results from HERSCHEL M. Rengel sizes ranging from below 100 km to 2400 km (Pluto/Eris) albedos ranging from (a factor of 50!)! Very high albedos require resurfacing process to maintain such a high albedo over longer timescales Sonia Fornasier ~120 objects

35 DENSITIES: Densities & thermal properties Most objects < 400 km have density < 1 small rock-ice ratio + porosity Density increase with size in a bigger way than what expected due to compaction: Larger rockice ratios in large bodies? Formed by direct collapse from over dense regions of the solar disk? Thermal inertia (Lellouch et al., 2013): 2-5 MKS [J/ (m 2 s 0.5 K)], 2-3 orders of magnitude lower than compact ice, decreasing with r sun Emissivity (Fornasier et al., 2013) : Strong emissivity decrease at λ > 250 μm highly porous surface texture Sonia Fornasier

36 Lacerda, et al., 2014, ApJ, 793, L2 Albedo-colours distribution Brown et al Colours differences related to the heliocentric distance at the time of formation and to the presence of stable ices (Kuiper belt was originally at ~15-35 UA) Bright red objects formed beyond the methanol ice line. Dynamical families formed far away from the sun (detached, cold classical) have only this kind of objects 36

37 Classicals TNOs Mean = 8.5 % Hot Hot Classicals: hot (i> 5 ) vs cold (i<5 ) Vilenius et al Cold Mean = 14 % Cold Cold classical: -smaller -Red colors -Higher albedos ALBEDO DIAMETER 37

38 Centaurs albedo vs color GREY RED Grey vs red Centaurs Grey : ~ uniformly dark diversity of sizes Red : some albedo diversity, higher mean albedo - small sizes Somewhat situation to cold classicals (red = higher albedo, small) Duffard et al

39 Active Centaurs Colors Bimodality : Different composition/origin or cometary activity? 10% of the centaurs are active Sonia Fornasier

40 JWST High S/N ratio spectroscopy of TNOs in a wavelength range with a variety of diagnostic features Identification of irradiation products Activity on Centaurs Cryovolcanism Surface heterogeneities Binary discovery and characterisation Family members: are composition similar (like in Haumea family) or not? Sonia Fornasier

41 JWST Spectroscopy of TNOs Water Ice rich objects Ratio amorphous vs. crystalline water ice Surface temperature from bands profile Constraint presence of contaminants Volatile rich objects Detection of N2 (α- and β- phases ~ 4.2μm ) Detection on CO, CO2 patches (( μm) Photolysis products Featureless Bodies Separate silicate-rich objects from organic rich (> 3.2 μm) Detect and distinguish different irradiation products (tholins) Detect methanol CH3OH? Or minor water ice content Pinilla-Alonso et al., 2017

42 Concluding remarks JWST will allow to: understand the composition of different Solar System minor bodies in a wavelength range with several diagnostic bands constrain the water content and state detect and characterise organics and volatiles with important astrobiological implication Look for activity and cryovolcanism phenomena (activated asteroids, Centaurs, big TNOS) Thermal properties, size and albedo for small asteroids/neo Size frequency distribution for very small (few meters!) asteroids in serendipitous imaging (MIRI) 11-Apr-2014 Sonia Fornasier