New studies from ESA's Rosetta spacecraft fly-by of asteroid Lutetia in 2010 have just been published. Lutetia is a primordial planetesimal formed during the very early phases of the Solar System. Interestingly, it appears to have a differentiated structure: a metallic (iron) core overlaid by primitive (pre-solar) chondritic crust. This differentiated structure was unexpected for an asteroid of this size. The article's accompanying illustration shows Lutetia's interesting composite surface features, reflecting its complex history of collisions that accreted different types of material to its surface.
Some excerpts from the article at SpaceRef:
On its way to rendezvous with comet 67P/Churyumov-Gerasimenko, ESA's Rosetta spacecraft flew by the main-belt asteroid (21) Lutetia, reaching the closest approach, at a distance of about 3170 km, on 10 July 2010. From this unique vantage point, Rosetta gathered high-resolution images, spectra, and other data, providing scientists with a valuable dataset with which to probe this peculiar asteroid in great detail.
"The images collected by Rosetta during the flyby have uncovered, for the first time, the wide variety of craters and other geological features that scar the surface of Lutetia," notes Rita Schulz, Rosetta Project Scientist at ESA.
In addition to craters, other geological markers, such as lineaments and faults, represent an important window into the turbulent past of asteroids and other Solar System bodies. The remarkable images collected by OSIRIS during the flyby have revealed an intricate network of linear features covering long distances across Lutetia's surface, up to 80 km in some cases. Many of these features are the results of seismic phenomena that also caused deformations on pre-existing craters.
Prior to the flyby, one of the most puzzling aspects of Lutetia was its surface composition: different datasets have hinted at either a metallic or a chondritic composition, thus making the classification of this asteroid particularly problematic. Scientists have now addressed the issue by combining data gathered with four remote-sensing instruments on Rosetta - OSIRIS, VIRTIS, MIRO, and ALICE - which cover visible, infrared, microwave and ultraviolet wavelengths. The new data show that Lutetia has an unusual surface composition that does not fit into the schemes established before the flyby and may result from the complex collision history of the asteroid.
The peculiar composition of Lutetia, when considered along with its high density, raises the possibility that this asteroid might have a partially differentiated structure, with a metallic core overlain by a primitive chondritic crust. The only other differentiated asteroid that has been visited by a spacecraft is Vesta, one of the largest asteroids in the Solar System and significantly larger than Lutetia. Whereas it is reasonable to expect such an internal structure in asteroids as large as Vesta, it is still unclear whether this should be the case also for objects of Lutetia's size. Therefore the possible evidence of a differentiated structure suggested by the new data is particularly intriguing.