Asteroids 2867 Steins and 21 Lutetia: results from groundbased observations and from the Rosetta fly-bys S. Fornasier 1,2, M.A. Barucci 1, M. Fulchignoni 1,2 1)LESIA-Observatoire de Paris, CNRS, UPMC Univ Paris 06, Univ. Paris-Diderot 2)Univ. Paris Diderot, Sorbonne Paris Cité 11 Sept SONIA FORNASIER

11 Sept. 2012SONIA FORNASIER 2867 Steins Barucci, Fulchignoni, Fornasier et al., 2005, A&A, 430, 313 …enstatite achondrite (aubrite) --- EL6 enstatite chondrite Atlanta Fornasier et al. 2007, Few information known when it was selected as target Spectroscopy  Steins belongs to the rare E class: high albedo ( ) asteroids, are thought to be mostly differentiated and to have experienced high heating episodes (T>1500K) Sharp band at 0.49  m due to sulfides like oldhamite Steins surface must be homogeneous (spectra at different rotational phases are similar (Dotto et al., 2009, A&A 494, 24)

11 Sept. 2012SONIA FORNASIER Steins: albedo Polarimetry with high albedo E-type asteroids ( Steins Polarimetric properties (slope, invertion angle and polarisation values) are consistent with high albedo E-type asteroids (Fornasier et al. 2006, A&A 449, 9) albedo of 0.45  0.1 Polarimetric slope gives an albedo of 0.45  0.1 Radiometry Obs. with SPITZER-IRS R_ effective = 2.46 ± 0.20 km P(R) = 0.40 ± 0.07 (Lamy et al. 2008, A&A 487, 1187) Steins emissivity spectrum is very similar to that of the aubrite meteorites and of the enstatite mineral ( Barucci et al., 2008, A&A 447, 665 ) Barucci et al., 2008, A&A 447, 665)

11 Sept. 2012SONIA FORNASIER4 21 Lutetia Big asteroid initially classified as M-type (metallic) because of its visible colours and moderate albedo values ( ) V+NIR spectrum not compatible with iron meteorites but with carbonaceous or enstatite chondrites Absorption bands at 0.43 μm (Belskaya et al, 2010, Lazzarin et al., 2004,2009) et 3 μm (Rivkin et al. 2000, Birlan et al. 2006,2010) associated to hydrated silicates (Barucci et al. 2005, A&A 430, 313

11 Sept. 2012SONIA FORNASIER5 P min =-1.30±0.07%  i nv =24.4±0.4 ° Large inversion angle is indicative of small particle size and/or high refractory material or inclusions Polarimetry Belskaya et al., 2010 Radiometry (SPITZER) I ≤ 30 JK −1 m −2 s −1/2  Lutetia is likely covered by a thick regolith layer (Lamy,et al. 2010) emissivity spectrum is different from that of the iron meteorites and similar to that of CV3/CO3 (Barucci et al., 2008) 21 Lutetia

SONIA FORNASIER ROSETTA MISSION 2 fly-by with 2867 Steins (5 Sept. 2008) and 21 Lutetia (10 July 2010) 02/03/ / / / P : from 05/ Sept OSIRIS

7 Closest approach took place at 18:38:20 UTC on 5 Sept at a minimum distance of 803 km (80 m/px on WAC ; 60 % of the surface resolved during the fly-by ROSETTA – 2867 Steins Fly-by diamond shape with a big crater (2.3 km) on the top Shape seems modelled by YORP effects (r_eq = 2.65 km) Rotation retrograde, close to perpendicular to the ecliptic No satellites found

8 Phase angle coverage: 0.27 < α< 141° Geometric albedo = 0.40 ± 0.01 Phase function is typical for high albedo E type asteroids

9 The 2km-sized crater has well defined rim: relatively young? Surface age: very young 150—400 Myr shape modelled by YORP effects Large crater would have disrupt a monolithic body + YORP effect Steins is probably a rubble pile asteroid

10 OSIRIS Spectrophotometry 10 Spectrophotomet ry  E-type Sharp 0.5 micron band due to sulfide (E[II]) The UV drop-off shows that the surface of Steins is made by iron- poor minerals. Keller et al. 2010, Science 327, Sept. 2012SONIA FORNASIER

11 Analysis of Steins’ surface composition G-mode statistical method applied to the OSIRIS disk resolved images reveal no significant variations with 95% of confidence level, confirming the great homogeneity of the surface. This homogeneity seems to be consistent with possible outcome of an impact that may have ejected the first layer of the regolith on the whole surface Leyrat et al. 2010)

21 Lutetia fly by : 10 July 2010, min distance 3160 km, best res. 60m/px asteroid’s dimensions are (126 ± 1) × (103 ± 1) × (95 ± 13) km 3 Irregular shape Several big craters of (up to 55 km) Body shaped by an extensive collision history 11 Sept SONIA FORNASIER ROSETTA – 21 Lutetia Fly-by

Matteo Massironi, UPD Surface Geomorphology Lutetia exposes fascinating morphology with many craters, grooves, ridges, graben, scarps etc. (Sierks et al. 2011)

Surface age: Old (3.6Gyr) and young regions (100 Myr) landslides 237 boulders identified of size > 100m Young" region shows craters completely buried under the regolith blanket. Blanket thickness: ~600 m (for d/D = 0.13) 11 Sept SONIA FORNASIER ROSETTA – 21 Lutetia Fly-by

11 Sept. 2012SONIA FORNASIER ROSETTA – 21 Lutetia Fly-by Albedo = nm Albedo variations found, up to 30% No absorption bands detected the observed surface has no Fe- rich pyroxene / olivine, nor hydrated minerals or organics 15 (Sierks et al. 2011) OSIRIS+RSI data: density = 3.4 ± 0.3 g/cm -3 LUTETIA may be a partially differentiated object (metallic core plus mantle composed of carbonaceous material (Weiss et al. 2012) (Weiss et al. 2012)

21 Lutetia: summary Asteroid fly-by was very successful Lutetia is a piece of rock, too dense for a rubble pile Old and young regions (10Ma-3.6Ga) Blanket in North Pole region suggests a thick layer of regolith/ejecta Lutetia exposes fascinating morphology with many craters, grooves, ridges, graben, etc. OSIRIS spectrophotometry data agree with the ground observations with extension to 250nm Different from any other asteroid visited so far No satellites found, size limit 160m from WAC images No exosphere found (Na)

11 Sept SONIA FORNASIER Thanks!

18 NAC best res. Image (100m/px) 5Sept UT: 18:28, dist:5200 km, phase=30° WAC best res. Image (80m/px) 5Sept UT: 18:38:15 dist:806km,phase=50 ° Copyright: ESA ©2008 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA Keller et al. 2010, Science 327, 190

Opposition Image NAC phase angle ~0.15°, km far away from Lutetia. 18 min before CA res. ~ 300m/px Albedo variation in landslide area Line features are visible Small impact feature

UNRESOLVED SPECTROPHOTOMETRY The Osiris data show a nice agreement with the groundbased spectrum, and reveal a flat behaviour, without strong drop-off, in the UV region.