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Altwegg K.,University of Bern | Balsiger H.,University of Bern | Calmonte U.,University of Bern | Hassig M.,University of Bern | And 12 more authors.
Planetary and Space Science | Year: 2012

During the Rosetta flyby at asteroid Lutetia the ROSINA instrument tried to detect a thin exosphere of the asteroid. Although the instrument is sensitive enough to detect even very tenuous gases at a density level of 1 cm -3 the Lutetia exosphere could not be unambiguously detected due to spacecraft outgassing, which was not constant because of the changing solar aspect angle. An upper limit for a water exosphere density at the flyby distance of 3160 km of (3.5±1.0)×10 3 cm -3 was deduced from the measurements. © 2011 Elsevier Ltd.

Kuppers M.,European Space Agency | Moissl R.,European Space Agency | Vincent J.-B.,Max Planck Institute For Sonnen System Forschung | Besse S.,University of Maryland University College | And 6 more authors.
Planetary and Space Science | Year: 2012

More than 200 boulders are among the many prominent geological features seen on Lutetia by the OSIRIS cameras onboard Rosetta. Most are concentrated around the central crater in Baetica regio with a few more apparently associated with Patavium crater. The size range of boulders visible to OSIRIS is about 60-300 m. We model the trajectories of boulders ejected from the central crater and show that their distribution is consistent with most of them being created from that crater, similar to the situation on asteroid Eros where most of the boulders are believed to originate from Shoemaker crater. We evaluate various destruction mechanisms for ejecta blocks and conclude that, using current estimates of the number of small asteroids in the main belt, destruction by impacts of small (several meters diameter) projectiles limits the lifetime of the boulders (and the age of the central crater) to a maximum of 300 million years. Since several analyses of crater ages and size distributions also come up with surprisingly young ages, the size-frequency distribution of small main-belt asteroids (below the size currently reached by surveys) may warrant to be revisited. © 2011 Elsevier Ltd.

Bertini I.,University of Padua | Sabolo W.,Institute Astrofisica Of Andalucia | Gutierrez P.J.,Institute Astrofisica Of Andalucia | Marzari F.,University of Padua | And 10 more authors.
Planetary and Space Science | Year: 2012

On 2010 July 10 the ESA Rosetta mission flew by the large asteroid (21) Lutetia. One of the scientific goals of the onboard OSIRIS instrument was the search for satellites of the asteroid, with more than 20 images specifically dedicated to this topic. An observational campaign was devised with a selection of filters and exposure times tailored to maximize the possibility of detecting small companions and determining their bound orbits. Data were analyzed with suitable methods to remove cosmic ray hits and known background objects, in order to search for persistent detections of potential interesting flux sources. We found no unambiguous detections of a satellite larger than ∼160m inside the entire sphere of gravitational influence. Our search confirmed the absence of bound companions larger than ∼30m inside 20 primary radii. These limits are a factor of ∼30 smaller than the values reported so far from large ground-based telescopes using adaptive optics and from the Hubble Space Telescope. © 2011 Elsevier Ltd.

Massironi M.,University of Padua | Marchi S.,French National Center for Scientific Research | Pajola M.,University of Padua | Snodgrass C.,Max Planck Institute For Sonnen System Forschung | And 13 more authors.
Planetary and Space Science | Year: 2012

The OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) images acquired during the recent Rosetta fly-by of Lutetia (10th of July 2010), enabled us to unravel the long geological history of the asteroid. This is recorded on its highly varied surface which displays geological units of disparate ages. In particular, using images of the closest approach, five main regions (in turn subdivided into minor units) have been discriminated on the basis of crater density, overlapping and cross-cutting relationships, and presence of linear features (i.e., fractures, faults, grooves, troughs). Other regions, with still unclear stratigraphic position, were also recognized on images of lower resolution on the bases of geomorphological properties such as crater density, relationship with scarp and ridges, and sharp morphological boundaries. In this work the geological evolution of Lutetia surface is reconstructed through the description of its main units and related contacts. The oldest regions imaged during the closest approach (Achaia and Noricum) are pervasively affected by fractures and grooves and display surfaces so heavily cratered to be dated back to a period not far from the Late Heavy Bombardment (yielding Achaia a crater retention age of 3.6-3.7 Ga). A crater of 55 km diameter, named Massilia and corresponding to the Narbonensis region, cuts both Achaia and Noricum regions and probably represents the most prominent event of the Lutetia history. The considerable crater density on its floor and walls, the absence of discernable deposits related to the impact event, and the intense deformation of it floor - all attest to its relatively great age. The North Polar Cluster (Baetica region) is associated with smooth ejecta broadly mantling the surrounding units and displays few craters and no linear features, demonstrating its relatively young age (estimated at less than 300 Ma). The North Polar Crater Cluster is the product of superimposed impacts; the last one of 24 km of diameter excavated the pre-existing ejecta up to the bedrock which locally outcrops at the crater rim. The ejecta of this last impact were involved in several gravitational phenomena testified by the great variety of deposits made up of mega-boulders diamictons, fine materials, gravitational taluses and debris, and landslide accumulations. A part from the big cratering events generating Massilia and the North Polar Crater Cluster, the Lutetia geological history is also punctuated by minor events still recorded by its stratigraphic record well imaged by the closest approach data. © 2012 Elsevier Ltd.

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