Publication Year 2016 Acceptance in OA@INAF 2020-12-21T11:43:55Z Title Spectral characterization of V-type asteroids - I. Space weathering effects and implications for V-type NEAs Authors Fulvio, Daniele; PERNA, Davide; IEVA, Simone; Brunetto, Rosario; Kanuchova, Zuzana; et al. DOI 10.1093/mnras/stv2300 Handle http://hdl.handle.net/20.500.12386/29048 Journal MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY Number 455 MNRAS 455, 584–595 (2016) doi:10.1093/mnras/stv2300 Spectral characterization of V-type asteroids – I. Space weathering effects and implications for V-type NEAs Daniele Fulvio,1‹ Davide Perna,2 Simone Ieva,3 Rosario Brunetto,4 Zuzana Kanuchova,5 Carlo Blanco,6,7 Giovanni Strazzulla6 and Elisabetta Dotto3 1Departamento de F´ısica, Pontif´ıcia Universidade Catolica´ do Rio de Janeiro, Rua Marquesˆ de Sao˜ Vicente 225, 22451-900 Rio de Janeiro, RJ, Brazil 2LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universites,´ UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cite,´ 5 place Jules Janssen, F-92195 Meudon, France 3INAF – Osservatorio Astronomico di Roma, Via Frascati 33, I-00040 Monte Porzio Catone, Roma, Italy 4Institut d’Astrophysique Spatiale, CNRS, UMR-8617, Universite´ Paris-Sud, batimentˆ 121, F-91405 Orsay Cedex, France Downloaded from https://academic.oup.com/mnras/article/455/1/584/984768 by guest on 06 November 2020 5Astronomical Institute of the Slovak Academy of Sciences, 059 60 Tatranska´ Lomnica, Slovakia 6INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, I-95123 Catania, Italy 7Dipartimento di Fisica e Astronomia, Universita` di Catania, Via S. Sofia 78, I-95123 Catania, Italy Accepted 2015 October 1. Received 2015 October 1; in original form 2015 August 19 ABSTRACT Among main belt asteroids, V-types belonging to Vesta’s dynamical family are known as ‘Vestoids’ while those lying outside Vesta’s family as ‘non-Vestoids’. V-types have also been found within the population of Near Earth Asteroids (NEAs). Several questions on Vesta, the V-types, and the Howardite–Eucrite–Diogenite meteorites are still unsolved, such as the genesis of each class/subclass, their evolution and mutual relationship, and the existence of other basaltic parent bodies. We present new NIR (0.8–2.4 µm) spectroscopic observations of seven non-Vestoids, carried out at the Telescopio Nazionale Galileo (TNG – INAF). We derived a number of spectral parameters (BI and BII centres, band separations, and BI slopes) and compared them with available spectra of V-types belonging to different subclasses (102 V- types in total), to highlight possible spectral differences useful to shed light on the questions mentioned above. We also considered the data from ion irradiation experiments performed on different samples of eucrites, simulating space weathering effects. Net discrepancies are seen for the BI slope distributions: NEAs show a distribution strongly different from all other V-type subclasses. Ion irradiation experiments induce strong effects on BI slope values and, as irradiation proceeds, the BI slope of eucrites quickly increases, changing the overall aspect of their VIS–NIR spectra (0.4–2.5 µm). Space weathering may explain the whole range of spectral slopes observed for all V-type subclasses. An exception is represented by NEAs, where moderate space weathering effects are evidenced. We propose that this is due to tidal perturbations exposing ‘fresh’ unweathered surface grains during close encounters with the Earth, as previously found for Q-type NEAs. Key words: methods: laboratory: atomic – techniques: spectroscopic – meteorites, meteors, meteoroids – minor planets, asteroids: individual: Vesta– infrared: general. Sanctis et al. 2012). The Dawn spacecraft data show that the 1 and 1 INTRODUCTION 2 µm pyroxene signatures (hereafter BI and BII, respectively) are Asteroid (4) Vesta is the largest differentiated main belt object present ubiquitously across Vesta surface and confirm that Vesta showing a basaltic crust. The basaltic composition of its surface surface composition is similar to that of the Howardite–Eucrite– has been discovered by McCord, Adams & Johnson (1970), later Diogenite meteorites (HEDs). Nevertheless, HEDs lithologies are on confirmed by many observations (e.g. McFadden, McCord & present on Vesta with spectral variations at both large and small Pieters 1977; Binzel et al. 1997; Gaffey 1997; Vernazza et al. scale across its surface and, overall, Vesta mineralogy indicates a 2005) and strengthen by the extensive and detailed data provided by complex geological and collisional history (De Sanctis et al. 2012; the Dawn spacecraft’s visible and infrared spectrometer VIR (De Jaumann et al. 2012; Russell et al. 2012). In recent years, several tens of asteroids have been found to show a surface composition similar to that of Vesta and have been clas- E-mail: [email protected]; [email protected] sified as ‘V-types’. Many of these objects belong to the Vesta’s C 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society Space weathering on V-type NEAs 585 Table 1. Orbital parameters, absolute magnitude, diameter, and dynamical subclass of the observed asteroids. Object a (au) ei(◦) H Diameter (km) Dynamical subclass (1914) Hartbeespoortdam 2.406 0.148 5.678 12.0 8.4 Low-i (1946) Walraven 2.293 0.236 8.166 12.0 8.4 Fugitives (2486) Metsahovi¨ 2.269 0.080 8.410 12.4 7.0 Fugitives (2566) Kirghizia 2.450 0.079 5.081 12.3 7.3 Low-i (2912) Lapalma 2.289 0.071 7.276 12.4 7.0 Fugitives (3153) Lincoln 2.423 0.130 7.707 12.7 6.1 IOs (3869) Norton 2.453 0.126 4.362 13.0 5.3 Low-i dynamical family (‘Vestoids’) and are believed to derive from the oid belonging to the Vestadynamical family, as defined by Nesvorny´ 1 huge collisional event responsible for the large craters near the south (2012) using the Hierarchical Clustering Method (HCM). V-types Downloaded from https://academic.oup.com/mnras/article/455/1/584/984768 by guest on 06 November 2020 pole of Vesta (Thomas et al. 1997; Marchi et al. 2012). The dis- outside the limit of the Vesta dynamical family are collectively covery of V-types outside the limits of Vesta’s dynamical family known as non-Vestoid. Among them, some objects could actually (Zappala et al. 1995; Migliorini et al. 1997; Carruba et al. 2005) be fugitive members that escaped the family via the combined influ- called this picture into question and, although, dynamical simula- ence of mean motion and secular resonances and Yarkovsky effect tions showed that some ‘non-Vestoids’ could be fugitives from the (Nesvornyetal.´ 2008). Following the definition of Nesvornyetal.´ Vesta family (Nesvornyetal.´ 2008;Roigetal.2008), many other (2008), we consider fugitive any basaltic asteroid with semimajor non-Vestoids do not show any clear dynamical link to Vesta, sug- axis <2.3 au and comparable e and i with the Vesta family. Among gesting, thus, that they could be fragments of distinct differentiated the remaining non-Vestoids, we consider two subclasses which de- parent bodies (Lazzaro et al. 2000; Duffard et al. 2004; Alvarez- serve a special attention: (i) the low-i subclass, inner main belt Candaletal.2006). The existence of other basaltic parent bodies V-types with low inclination (i < 6◦ and 2.3 < a < 2.5 au) that seems to be also supported by recent studies on the HEDs. As men- cannot be reconducted to a dynamical origin from Nesvornyetal.´ tioned before, reflectance spectra of Vesta and V-type asteroids are (2008) and (ii) the MOVs subclass: middle and outer main belt V- similar to those of the HEDs. This unique agreement was consid- types which should have crossed the 3:1 resonance with Jupiter to ered the main evidence that Vesta is the parent body of the HEDs, have an origin compatible with Vesta. This last hypothesis is highly although Vesta itself was not believed to be their direct source. The unlikely (Roig et al. 2008). Non-Vestoids in the inner main belt not more realistic scenario foresaw a link between Vesta and the HEDs belonging to any of the previously defined subclasses (fugitives, via basaltic fragments that reached the Earth through complex dy- low-i, and MOVs) are here defined as Inner Others (IOs). Finally, namical processes (see Binzel et al. 2004 and references therein). V-type NEA is any V-type asteroid with perihelion q < 1.3 au. In this However, recent measurements on oxygen isotopic composition of work, we consider a total of 102 V-types: 33 vestoids, 17 fugitives, HEDs have indicated that, although most of them come from the 18 low-i, 11 IOs, 2 MOVs, and 21 NEAs. same basaltic parent body, few others do not match this view (see for instance Scott et al. 2009). 3 OBSERVATIONS AND DATA REDUCTION In this study, we present new NIR (0.8–2.4 µm) spectra for seven non-Vestoids obtained at the Telescopio Nazionale Galileo (TNG- We observed seven non-Vestoids with missing NIR spectra (at the INAF, La Palma, Spain). We analysed and compared the NIR spec- time of the observations; in the meanwhile an NIR spectrum of tral properties of these asteroids and other available V-types belong- (2912) Lapalma has been published in De Sanctis et al. 2011a). ing to different classes and subclasses, to highlight possible spectral Orbital parameters and absolute magnitude H of the targets are given differences that can help to shed light on the origin and evolution in Table 1 (values are taken from the Minor Planet Center website2), of V-types. Previous works (Duffard et al. 2004; Moskovitz et al. together with their estimated diameters assuming an albedo of 0.4 2010; De Sanctis et al. 2011a,b) did not find any strong difference (a characteristic value for basaltic asteroids) and the non-Vestoids between Vesta, Vestoids, and non-Vestoids.