Aqueous alteration on main belt primitive asteroids: results from visible spectroscopy

This work focuses on the study of the aqueous alteration process which acted in the main belt and produced hydrated minerals on the altered asteroids. The aqueous alteration is particularly important for unraveling the processes occurring during the earliest times of the Solar System history, as it can give information both on the asteroids thermal evolution and on the localization of water sources in the asteroid belt. We present new spectral observations in the visible region of 80 asteroids belonging to the primitive classes C, G, F, B and P. We combine the present observations with the visible spectra of asteroids available in the literature for a total of 600 primitive main belt asteroids. Our analysis shows that the aqueous alteration sequence starts from the P-type objects, practically unaltered, and increases through the F, B, C, and G asteroids. Around 50% of the observed C-type asteroids show absorption features in the vis. range due to hydrated silicates, implying that more than 70% of them will have a 3 $\mu$m absorption band and thus hydrated minerals on their surfaces. The process dominates in primitive asteroids located between 2.3 and 3.1 AU, that is at smaller heliocentric distances than previously suggested. The aqueous alteration process dominates in the 50--240 km sized primitive asteroids, while it is less effective for bodies smaller than 50 km. No correlation is found between the aqueous alteration process and the asteroids albedo or orbital elements. Comparing the $\sim$ 0.7 $\mu$m band parameters of hydrated silicates and CM2 carbonaceous chondrites, we see that the band center of meteorites is at longer wavelengths than that of asteroids. This difference on center positions may be attributed to different minerals abundances, and to the fact that CM2 available on Earth might not be representative of the whole aqueous altered asteroids population.Comment: Icarus, accepted for publication on 28 January 2014 Manuscript pages: 38; Figures: 13 ; Tables:

AVAST Survey 0.4-1.0 {\mu}m Spectroscopy of Igneous Asteroids in the Inner and Middle Main Belt

We present the spectra of 60 asteroids, including 47 V-types observed during the first phase of the Adler V-Type Asteroid (AVAST) Survey. SDSS photometry was used to select candidate V-type asteroids for follow up by nature of their very blue $i-z$ color. 47 of the 61 observed candidates were positively classified as V-type asteroids, while an additional six show indications of a 0.9 $\mu$m feature consistent with V-type spectra, but not sufficient for formal classification. Four asteroids were found to be S-type, all of which had $i-z$ values very near the adopted AVAST selection criteria of $i-z \le -0.2$, including one candidate observed well outside the cut (at a mean $i-z$ of -0.11). Three A-type asteroids were also identified. Six V-type asteroids were observed beyond the 3:1 mean motion resonance with Jupiter, including the identification of two new V-type asteroids (63085 and 105041) at this distance. Six V-type asteroids were observed with low ($<5\deg$) orbital inclination, outside of the normal dynamical range of classic Vestoids, and are suggestive of a non-Vesta origin for at least some of the population.Comment: 1 table, 3 figures, To appear to Icaru

Scattering V-type asteroids during the giant planets instability: A step for Jupiter, a leap for basalt

V-type asteroids are a taxonomic class whose surface is associated to a basaltic composition. The only known source of V-type asteroids in the Main Asteroid Belt is (4) Vesta, that is located in the inner part of the belt. However, many V-type asteroids cannot be dynamically linked to Vesta., in particular, those asteroids located in the middle and outer parts of the Belt. Previous works have failed to find mechanisms to transport V-type asteroids from the inner to the middle and outer belt. In this work we propose a dynamical mechanism that could have acted on primordial asteroid families. We consider a model of the giant planets migration known as the jumping Jupiter model with five planets. Our study is focused on the period of 10 Myr that encompasses the instability phase of the giant planets. We show that, for different hypothetical Vesta-like paleo-families in the inner belt, the perturbations caused by the ice giant that is scattered into the asteroid belt before being ejected from the solar system, are able to scatter V-type asteroids to the middle and outer belt. Based on the orbital distribution of V-type candidates identified from the Sloan Digital Sky Survey and the VISTA Survey colours, we show that this mechanism is efficient enough provided that the hypothetical paleo-family originated from a 100 to 500 km crater excavated on the surface of (4) Vesta. This mechanism is able to explain the currently observed V-type asteroids in the middle and outer belt, with the exception of (1459) Magnya.Comment: 10 pages, 4 figures. To appear in MNRA

Distribution of spin-axes longitudes and shape elongations of main-belt asteroids

Context: Large all-sky surveys provide us with a lot of photometric data that are sparse in time (typically few measurements per night) and can be potentially used for the determination of shapes and rotational states of asteroids. The method which is generally used to derive these parameters is the lightcurve inversion. However, for most asteroids their sparse data are not accurate enough to derive a unique model and the lightcurve inversion method is thus not very efficient. Aims: To fully utilize photometry sparse in time, we developed a new simplified model and applied it on the data from the Lowell photometric database. Our aim was to derive spin axis orientations and shape elongations of asteroids and to reconstruct distributions of these parameters for selected subpopulations to find if there are some differences. Methods: We model asteroids as geometrically scattering triaxial ellipsoids. Observed values of mean brightness and the dispersion of brightness are compared with computed values obtained from the parameters of the model -- ecliptical longitude $\lambda$ and latitude $\beta$ of the pole and the ratios $a/b$, $b/c$ of axes of the ellipsoid. These parameters are optimized to get the best agreement with the observation. Results: We found that the distribution of $\lambda$ for main-belt asteroids is not uniform (in agreement with findings of Bowell et al., 2014, M&PS, 49, 95) and is dependent on the inclination of orbit. Surprisingly, the non-uniformity of $\lambda$ distribution is larger for asteroids residing on low-inclination orbits. We also studied distributions of $a/b$ for several groups of asteroids and found that small asteroids ($D<25\,$km) are on average more elongated than large ones.Comment: 10 pages; Accepted for publication in A&

BVRI photometry of 53 unusual asteroids

We present the results of BVRI photometry and classification of 53 unusual asteroids, including 35 near-Earth asteroids (NEAs), 6 high eccentricity/inclination asteroids, and 12 recently-identified asteroid-pair candidates. Most of these asteroids were not reportedly classified prior to this work. For the few asteroids that have been previously studied, the results are generally in rough agreement. In addition, we merge the results from several photometric/spectroscopic surveys to create a largest-ever sample with 449 spectrally classified NEAs for statistical analysis. We identify a "transition point" of the relative number of C/X-like and S-like NEAs at H~18D~1km with confidence level at ~95% or higher. We find that the C/X-like:S-like ratio for 18<=H<22 is about two times higher than that of H<18 (0.33+/-0.04 versus 0.17+/-0.02), virtually supporting the hypothesis that smaller NEAs generally have less weathered surface (therefore, less reddish appearance) caused by younger collision ages.Comment: 18 pages, 9 table

Origin and Sustainability of The Population of Asteroids Captured in the Exterior Resonance 1:2 with Mars

At present, approximately 1500 asteroids are known to evolve inside or sticked to the exterior 1:2 resonance with Mars at a = 2.418 AU, being (142) Polana the largest member of this group. The effect of the forced secular modes superposed to the resonance gives rise to a complex dynamical evolution. Chaotic diffusion, collisions, close encounters with massive asteroids and mainly orbital migration due to the Yarkovsky effect generate continuous captures to and losses from the resonance, with a fraction of asteroids remaining captured over long time scales and generating a concentration in the semimajor axis distribution that exceeds by 20% the population of background asteroids. The Yarkovsky effect induces different dynamics according to the asteroid size, producing an excess of small asteroids inside the resonance. The evolution in the resonance generates a signature on the orbits, mainly in eccentricity, that depends on the time the asteroid remains captured inside the resonance and on the magnitude of the Yarkovsky effect. The greater the asteroids, the larger the time they remain captured in the resonance, allowing greater diffusion in eccentricity and inclination. The resonance generates a discontinuity and mixing in the space of proper elements producing misidentification of dynamical family members, mainly for Vesta and Nysa-Polana families. The half-life of resonant asteroids large enough for not being affected by the Yarkovsky effect is about 1 Gyr. From the point of view of taxonomic classes, the resonant population does not differ from the background population and the excess of small asteroids is confirmed.Comment: Accepted for publication in Icaru