Pebble accretion at the origin of water in Europa

Despite the fact that the observed gradient in water content among the Galilean satellites is globally consistent with a formation in a circum-Jovian disk on both sides of the snowline, the mechanisms that led to a low water mass fraction in Europa ($\sim$$8\%$) are not yet understood. Here, we present new modeling results of solids transport in the circum-Jovian disk accounting for aerodynamic drag, turbulent diffusion, surface temperature evolution and sublimation of water ice. We find that the water mass fraction of pebbles (e.g., solids with sizes of 10$^{-2}$ -- 1 m) as they drift inward is globally consistent with the current water content of the Galilean system. This opens the possibility that each satellite could have formed through pebble accretion within a delimited region whose boundaries were defined by the position of the snowline. This further implies that the migration of the forming satellites was tied to the evolution of the snowline so that Europa fully accreted from partially dehydrated material in the region just inside of the snowline.Comment: Accepted for publication in Ap

First disk-resolved spectroscopy of (4) Vesta

Vesta, the second largest Main Belt asteroid, will be the first to be explored in 2011 by NASA's Dawn mission. It is a dry, likely differentiated body with spectrum suggesting that is has been resurfaced by basaltic lava flows, not too different from the lunar maria. Here we present the first disk-resolved spectroscopic observations of an asteroid from the ground. We observed (4) Vesta with the ESO-VLT adaptive optics equipped integral-field near-infrared spectrograph SINFONI, as part of its science verification campaign. The highest spatial resolution of ~90 km on Vesta's surface was obtained during excellent seeing conditions (0.5") in October 2004. We observe spectral variations across Vesta's surface that can be interpreted as variations of either the pyroxene composition, or the effect of surface aging. We compare Vesta's 2 micron absorption band to that of howardite-eucrite-diogenite (HED) meteorites that are thought to originate from Vesta, and establish particular links between specific regions and HED subclasses. The overallcomposition is found to be mostly compatible with howardite meteorites, although a small area around 180 deg. East longitude could be attributed to a diogenite-rich spot. We finally focus our spectral analysis on the characteristics of Vesta's bright and dark regions as seen from Hubble Space Telescope's visible and Keck-II's near-infrared images.Comment: 13 pages, 11 figures, 3 table

Spectral properties of nine M-type asteroids

Reproduced with permission. Copyright ESO. Article published by EDP Sciences and available at http://www.aanda.org.International audienceAims. We present spectroscopic results for nine M-type asteroids (325 Heidelberga, 497 Iva, 558 Carmen, 687 Tinette, 766 Moguntia, 860 Ursina, 909 Ulla, 1280 Baillauda, and 1564 Srbija) in the 0.8−2.5 μm spectral region. One visible spectrum is also presented for the asteroid 497 Iva. These asteroids were observed during several runs between 2003 and 2007, and the main goal was to investigate the NIR spectral region of M-type asteroids. Methods. The data was obtained with SpeX/IRTF in Prism mode and Dolores/TNG in LR-B mode. Spectral analysis was performed by comparing the M-type spectra and the meteorite ones (χ2 approach) and the Modified Gaussian Model. Results. With one exception, the asteroids present positive slopes of the spectra, with no absorption features, in good agreement with the spectra of metallic meteorites. The analysis of the asteroid 766 Moguntia was done by means of χ2, MGM techniques, and the Shkuratov scattering law. We conclude that the mineralogy is dominated by olivine. Its NIR spectrum is similar to those of CO/CV meteorites

E-type asteroid (2867) Steins: flyby target for Rosetta

Reproduced with permission. Copyright ESO. Article published by EDP Sciences and available at http://www.aanda.org.International audienceAims. The mineralogy of the asteroid (2867) Steins was investigated in the framework of a ground-based science campaign dedicated to the future encounter with Rosetta spacecraft. Methods. Near-infrared (NIR) spectra of the asteroid in the 0.8−2.5 μm spectral range have been obtained with SpeX/IRTF in remoteobserving mode from Meudon, France, and Cambridge, MA, in December 2006 and in January and March 2007. A spectrum with a combined wavelength coverage from 0.4 to 2.5 μm was constructed using previously obtained visible data. To constrain the possible composition of the surface, we constructed a simple mixing model using a linear (areal) mix of three components obtained from the RELAB database. A space-weathering model was applied to the aubrite ALH-78113 spectrum. Results. The four new NIR spectra reveal no major absorption features. The best-fit model for the constructed visible-plus-NIR spectrum is represented by a mixture of 57% enstatite, 42% oldhamite, and 1% orthopyroxene. These results place Steins in a subdivision of the E-type class with objects like Angelina, Eger, and Nereus. This group is not sampled by the current collection of aubrite meteorites. Interestingly, the reddened aubrite spectrum also provides a good match to the Steins VNIR spectrum

Near infra-red spectroscopy of the asteroid 21 Lutetia. II. Rotationally resolved spectroscopy of the surface

Reproduced with permission. Copyright ESO. Article published by EDP Sciences and available at http://www.aanda.orgInternational audienceAims. In the framework of the ground-based science campaign dedicated to the encounter with the Rosetta spacecraft, the mineralogy of the asteroid (21) Lutetia was investigated. Methods. Near-infrared (NIR) spectra of the asteroid in the 0.8−2.5 μm spectral range were obtained with SpeX/IRTF in remote observing mode from Meudon, France in March and April 2006. We analysed these data together with previously acquired spectra - March 2003, August 2004. I-band relative photometric data obtained on 20 January 2006 using the 105 cm telescope from Pic du Midi, France has been used to build the ephemeris for physical observations. A χ2 test using meteorite spectra from the RELAB database was performed in order to find the best fit of complete visible + infrared (VNIR) spectra of Lutetia. Results. The new spectra reveal no absorption features. We find a clear spectral variation (slope), and a good correspondence between spectral variations and rotational phase. Two of the most different spectra correspond to two opposite sides of the asteroid (sub-Earth longitude difference around 180◦). For the neutral spectra a carbonaceous chondrite spectrum yields the best fit, while for those with a slightly positive slope the enstatitic chondrite spectra are the best analog. Based on the chosen subset of the meteorite samples, our analysis suggests a primitive, chondritic nature for (21) Lutetia. Differences in spectra are interpreted in terms of the coexistence of several lithologies on the surface where the aqueous alteration played an important role

Asteroid target selection for the new Rosetta mission baseline: 21 Lutetia and 2867 Steins

Reproduced with permission. Copyright ESO. Article published by EDP Sciences and available at http://www.aanda.org.International audienceThe new Rosetta mission baseline to the comet 67P/Churyumov-Gerasimenko includes two asteroid fly-bys. To help in target selection we studied all the candidates of all the possible scenarios. Observations have been carried out at ESO-NTT (La Silla, Chile), TNG (Canaries), and NASA-IRTF (Hawaii) telescopes, in order to determine the taxonomy of all the candidates. The asteroid targets were chosen after the spacecraft interplanetary orbit insertion manoeuvre, when the available total amount of ΔV was known. On the basis of our analysis and the available of ΔV, we recommended to the ESA ScienceWorking Group the asteroids 21 Lutetia and 2867 Steins as targets for the Rosetta mission. The nature of Lutetia is still controversial. Lutetia's spectral properties may be consistent with a composition similar to carbonaceous chondrite meteorites. The spectral properties of Steins suggest a more extensive thermal history. Steins may have a composition similar to relatively rare enstatite chondrite/achondrite meteorites

VLT/SPHERE observations and shape reconstruction of asteroid (6) Hebe

(6) Hebe is a large main-belt asteroid, accounting for about half a percent of the mass of the asteroid belt. Its spectral characteristics and close proximity to dynamical resonances within the main-belt (the 3:1 Kirkwood gap and the nu6 resonance) make it a probable parent body of the H-chondrites and IIE iron meteorites found on Earth.We present new AO images of Hebe obtained with the high-contrast imager SPHERE (Beuzit et al. 2008) as part of the science verification of the instrument. Hebe was observed close to its opposition date and throughout its rotation in order to derive its 3-D shape, and to allow a study of its surface craters. Our observations reveal impact zones that witness a severe collisional disruption for this asteroid. When combined to previous AO images and available lightcurves (both from the literature and from recent optical observations by our team), these new observations allow us to derive a reliable shape model using our KOALA algorithm (Carry et al. 2010). We further derive an estimate of Hebe's density based on its known astrometric mass

The remarkable surface homogeneity of the Dawn mission target (1) Ceres

Dwarf-planet (1) Ceres is one of the two targets, along with (4) Vesta, that will be studied by the NASA Dawn spacecraft via imaging, visible and near-infrared spectroscopy, and gamma-ray and neutron spectroscopy. While Ceres' visible and near-infrared disk-integrated spectra have been well characterized, little has been done about quantifying spectral variations over the surface. Any spectral variation would give us insights on the geographical variation of the composition and/or the surface age. The only work so far was that of Rivkin & Volquardsen (2010, Icarus 206, 327) who reported rotationally-resolved spectroscopic (disk-integrated) observations in the 2.2-4.0 {\mu}m range; their observations showed evidence for a relatively uniform surface. Here, we report disk-resolved observations of Ceres with SINFONI (ESO VLT) in the 1.17-1.32 {\mu}m and 1.45-2.35 {\mu}m wavelength ranges. The observations were made under excellent seeing conditions (0.6"), allowing us to reach a spatial resolution of ~75 km on Ceres' surface. We do not find any spectral variation above a 3% level, suggesting a homogeneous surface at our spatial resolution. Slight variations (about 2%) of the spectral slope are detected, geographically correlated with the albedo markings reported from the analysis of the HST and Keck disk-resolved images of Ceres (Li et al., 2006, Icarus 182, 143; Carry et al., 2008, A&A 478, 235). Given the lack of constraints on the surface composition of Ceres, however, we cannot assert the causes of these variations.Comment: 8 pages, 5 figures, 2 tables, accepted for publication in Icaru