1. Multi-color adaptive optics imaging of asteroid 1 Ceres Christophe Dumas, JPL - USA ESO Bill Merline, SwRI - USA Thierry Fusco, ONERA - France

2. Asteroid imaging - Spacecraft - HST - Large telescopes w/ Adaptive Optics Eros (Near) Gaspra (Galileo) Mathilde (Near) Vesta (HST) Ida (Galileo)

3. Science objectives Use adaptive optics to obtain high-angular resolution imaging of the surface of Ceres at several wavelengths in the near-infrared Although it is the largest minor body, we had very limited knowledge of what its surface looks like Measurement of its size, shape, direction of spin axis Location of main albedo/geological features, study of its impact history Search for close-in (~30 radii), sub-km satellites Ceres is a main target for the DAWN Discovery mission Piazzi (1801) Eugenia and Petit-Prince

4. DAWN mission Chris Russel PI (UCLA) Launch w/ Delta 7925H on June 2006 Vesta in Oct. 2011 Ceres in Aug. 2015 6-month study Extended mission to Pallas

5. Observations Instrument10m Keck-II + AO-NIRC2 Phase angle6 deg. Geoc. distance1.98 AU Angular diameter0.66” Rotation coverage100% - about 15 deg./filter NIRC2 is a 1024x1024 Aladdin-3 array which provides: –10/20/40 mas/pix platescales –JHKLM + NB filters –Low res (R~3000 for 3 pix slit) and high- res (R~5000 for 3 pix slit) grism spectroscopy (40mas/pix) –Coronagraphic masks. AO system at Keck permits to achieve 40- 80 km resolution on Ceres in the J-H-K spectral range, which is adequate for a first geological study of its surface.

6. Keck AO system Keck-AO: 349 elements deformable mirror. Wavefront sensor runs up to ~700Hz. Magnitude limit V~13-14, depending on seeing conditions. AO system located at left Nasmyth at f/15. Provides high-contrast diffraction-limited images of 40 milliarcsec at H band adequate for high-resolution mapping: more than 200 resolution elements covering the disk of Ceres. Real-time correction of atmospheric wavefront distorsion PSF with AO on & off

7. Previous results Although it is the largest main-belt asteroid, our knowledge of the surface of Ceres was limited due to the lack of contrasted albedo features (contrarily to Vesta). Low lightcurve amplitude < 0.04 mag. Pole direction and axial ratios were estimated from earlier AO images (Saint-Pé et al. 1993, Drummond et al. 1998) Detection of a low contrast feature in HST-FOC images (Parker et al. 2002). AO image of Ceres (Drummond et al. 1998) HST-FOC image of Ceres (Parker et al. 2002)

8. Data processing AO images need to be deconvolved to reach the optimal spatial resolution (correcting for the PSF halo). PSF were recorded continuously through the night in order to calibrate the instrument + seeing response. MISTRAL (Fusco et al. SPIE, 2003) is a deconvolution technique based on a maximum likelihood approach w/ additional constraints for knowledge of object contour and noise statistics. MISTRAL was used in myopic mode to take into account the PSF variations due to seeing. Log(Intensity) pixels halo PSF profile (2  m) Raw image Deconvolved image

9. 210 o 217 o 240 o 247 o 272 o 290 o 325 o 337 o 3o3o 10 o 32 o 42 o K-band deconvolved images of Ceres. Geometry is nearly equator-on. (Note: Longitude of central meridian is reported below each image). N

10. Feature #2: ~ 130km in diameter Feature #1: ~ 190km in diameter. Bright central region. - Brightness of the main dark geological units is 5-10% lower than average surface, both located in the Northern hemisphere. Additional, lower-contrast features also visible. - Main albedo marks are smaller than the 250km diameter “Piazzi” feature reported by Parker et al. (2002). - Contour extracted using edge enhancement techniques. No evidence for a tri- axial shape (longer axis would be 10 sigma off). Oblate spheroid (a=b) with: - polar diameter = 884 +/- 7km - equatorial diameter = 951 +/- 6km. Surface features and dimensions

11. K band Spectral dependence H bandJ band Albedo mark #1 reveals a spectral variation (composition?)

12. Summary Highest-resolution images available to date (~50km on the surface of Ceres). New HST and VLT images have been obtained since. Full coverage of Ceres surface at JHK reveal two main geological units (130-190km in diameter) located in the Northern hemisphere Low contrast features (~5-10% below average) Pole solution from Drummond et al. (1998) good within ~ 10 degrees Dimensions: 884km x 951km Still needs to be done: Precise determination of pole orientation using surface features as control points Photometric analysis to derive scattering properties and establish 3- color albedo maps of the surface of Ceres Investigate nature of dark regions, spectral dependence Search for sub-km close-in satellites by combining all images in our data-set