Multi-color adaptive optics imaging of asteroid 1 Ceres

Christophe Dumas, JPL - USA ESO

Bill Merline, SwRI - USA

Thierry Fusco, ONERA - France

Asteroid imaging

- Spacecraft

- HST

- Large telescopes w/ Adaptive Optics

Eros (Near)

Gaspra (Galileo)

Mathilde (Near)

Vesta (HST)

Ida (Galileo)

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

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

Observations

Instrument 10m Keck-II + AO-NIRC2

Phase angle 6 deg.

Geoc. distance 1.98 AU

Angular diameter 0.66”

Rotation coverage 100% - 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.

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 atmosphericwavefront distorsion

PSF with AO on & off

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)

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.

Lo

g(I

nte

nsi

ty)

pixels

halo

PSF profile (2m)

Raw image

Deconvolved image

210o 217o 240o 247o

272o 290o 325o 337o

3o 10o 32o 42o

K-band deconvolved images of Ceres. Geometry is nearly equator-on.(Note: Longitude of central meridian is reported below each image).

N

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

K band

Spectral dependence

H band J band

Albedo mark #1 reveals a spectral variation (composition?)

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