Abstract
With the adaptive optics (AO) system on the 10 m Keck-II telescope, we acquired a high quality set of 84 images at 14 epochs of asteroid (52) Europa on 2005 January 20, when it was near opposition. The epochs covered its 5.63 h rotation period and, by following its changing shape and orientation on the plane of sky, we obtained its triaxial ellipsoid dimensions and spin pole location.
An independent determination from images at three epochs obtained in 2007 is in good agreement with these results. By combining these two data sets, along with a single epoch data set obtained in 2003, we have derived a global fit for (52) Europa of diameters a x b x c = (379 x 330 x 249) +/- (16 x 8 x 10) km, yielding a volume-equivalent spherical-diameter of 3 root abc = 315 +/- 7 km, and a prograde rotational pole within 7 degrees of [RA; Dec] = [257 degrees;+12 degrees] in an Equatorial J2000 reference frame (Ecliptic: 255 degrees;+35 degrees).
Using the average of all mass determinations available for (52) Europa, we derive a density of 1.5 +/- 0.4 g cm(-3), typical of C-type asteroids. Comparing our images with the shape model of Michalowski et al. (2004, Astron.
Astrophys. 416, 353), derived from optical lightcurves, illustrates excellent agreement, although several edge features visible in the images are not rendered by the model. We therefore derived a complete 3-D description of (52) Europa's shape using the KOALA algorithm by combining our 18 AO imaging epochs with 4 stellar occultations and 49 lightcurves.
We use this 3-D shape model to assess these departures from ellipsoidal shape. Flat facets (possible giant craters) appear to be less distinct on (52) Europa than another C-types that have been imaged in detail, (253) Mathilde and (511) Davida.
We show that fewer giant craters, or smaller largest-sized craters, is consistent with its expected impact history.