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Space Weathering of Asteroidal Surfaces* A&A 517, A60 (2010) Astronomy DOI: 10.1051/0004-6361/201014061 & c ESO 2010 Astrophysics Space weathering of asteroidal surfaces Influence on the UV-Vis spectra Z. Kanuchovᡠ1,2,G.A.Baratta1, M. Garozzo1, and G. Strazzulla1 1 INAF-Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy e-mail: [email protected] 2 Astronomical Institute of Slovak Academy of Sciences, 059 60 Tatranska Lomnica, Slovakia Received 14 January 2010 / Accepted 27 April 2010 ABSTRACT Context. The surfaces of airless bodies in the Solar System are continuously altered by the bombardment of micrometeoroids and irradiation by solar wind, flares, and cosmic particles. Major effects of this process – space weathering – are darkening and “reddening” of the spectra of surface materials, as well as a “degrading” of absorption features. Aims. We studied the changes induced by energetic ion irradiation in the ultraviolet-visual-near-infrared (UV-Vis-NIR) (0.2–0.98 μm) reflectance spectra of targets selected to mimic the surfaces of airless bodies in the inner Solar System. Our chosen targets are olivine pellets, pure or covered by an organic polymer (polystyrene), which is transparent before irradiation. Polystyrene is used as a template for organic matter of low volatility that can be present on asteroidal surfaces. Moreover we measured the changes induced by ion irradiation in the absorption coefficient of the polymer. The purpose was to have a tool to better compare laboratory with observed spectra and distinguish between planetary objects with pure silicate surfaces and those whose surface is covered by organic matter exposed to cosmic ion bombardment. Methods. The samples were irradiated in vacuum, at room temperature, with 200 keV protons or 200–400 keV argon ions. Before, during, and after irradiation diffuse reflectance spectra were acquired. Polystyrene films were also deposited on quartz substrates and irradiated while transmittance spectra were recorded. Results. We measured the variations of the absorption coefficient of polystyrene as a function of ion fluence. We showed that after ion irradiation the diffuse reflectance spectra of the samples covered by organics exhibit a much more significant variation than those of pure silicates. The spectra of targets made of olivine plus polystyrene can be fitted by using the measured absorption coefficient of polystyrene. Conclusions. The results obtained for pure olivine extend to the UV the spectral range of previous experiments. The data concerning the absorption coefficient of polystyrene are available on our web site (http://web.ct.astro.it/weblab/dbindex.html)and can be used to compare laboratory with astronomical spectra. This will possibly allow us to obtain information about organic matter on the surface of a given object, as well as on the relevance of the exposure to cosmic ions (space weathering). Key words. minor planets, asteroids: general – methods: laboratory – techniques: spectroscopic 1. Introduction objects (TNOs) and comets) (Moroz et al. 2004), and materi- als typical for main-belt asteroids such as different kinds of sili- It is well known that the surfaces of airless bodies in the Solar cates (Brunetto & Strazzulla 2005; Brunetto et al. 2006b; Sasaki System are continuously altered by the bombardment of mi- et al. 2001; Strazzulla et al. 2005; Loeffler et al. 2009, 2008). crometeoroids and irradiation by solar wind, flares, and cosmic ff ff Up to now, the e ects of space weathering have not yet been particles. Major e ects of this process – space weathering – are studied on organic materials of low volatility. Such materials – darkening and “reddening” (spectral reflectance increases with macromolecules or molecular solids – could be very abundant on wavelength) of the spectra of surface materials, as well as “de- small objects in the outer Solar System, where they could be ac- grading” of absorption features (Chapman 1996; Pieters et al. creted directly from the nebular material at the beginning of the 1993). Up to now most of the laboratory simulations of space Solar System or they could be produced by energetic processing weathering, in particular ion irradiation, have been conducted by of simple ices (e.g. Palumbo et al. 2008). The same molecules looking at changes in the spectra of relevant materials at visible μ or their space weathered daughters could be present on closer and near-IR (NIR) wavelengths (0.5–2.5 m). objects such as asteroids or even Mars satellites (Phobos and Investigated materials include carbon-based species such as Deimos, e.g. Shkuratov et al. 1987). frozen hydrocarbons, present on the outer Solar System ob- jects and interstellar dust grains (Brunetto et al. 2006a), red According to the work of Hendrix & Vilas (2006), the space ff bitumens (asphaltite and kerite, materials which are the most weathering e ects could be more evident in the UV range than similar to the less evolved organic matter of trans-neptunian in the visible and NIR ones. It was found a correlation between the visible-near-IR (VNIR, 550–1640nm) and near-UV (NUV, Data table is only available in electronic form at the CDS via 300–400 nm) spectral slopes of S-class asteroids and ordinary anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via chondrites, which suggests that surfaces that are spectrally http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/517/A60 redder in the VNIR are also spectrally bluer in the NUV Article published by EDP Sciences Page 1 of 10 A&A 517, A60 (2010) (Hendrix & Vilas 2006). In the spectra of most lunar-like and asteroid-like minerals an absorption feature in NUV appears as a result of a transition between volume scattering and surface scattering. The strength of this feature depends on the presence of opaque/nonopaque materials (e.g. pyroxenes, feldspars, and olivines all exhibit a UV drop-off in the 150–450nm range that is not seen in iron-bearing minerals as ilmenite and magnetite). The relative blue spectral slope in the NUV can be explained by a decrease in the strength of the UV absorption feature due to weathering-derived nanophase iron. This suggests that surfaces of different materials will manifest weathering in different ways. The UV wavelength region is suggested to be a more sensitive indicator of weathering (and thus age), because the changes in this spectral region should be more evident than those at longer wavelengths for the same exposure to weather- ing processes. In some cases, the effects of space weathering in UV-visible wavelengths can include also a spectral reversal besides the bluing of spectral reflectance which means that rel- atively dark surfaces in the visible range are relatively bright at shorter wavelengths (Hendrix & Vilas 2006). In our Laboratory for Experimental Astrophysics (LASp) in Catania (Italy) we started with a new series of experiments aimed at studying the effects of ion irradiation in solid materials in a spectral range that includes the NUV (0.2–0.98μm) region. Here we present the results obtained so far that concern ion irradiation of olivine samples and olivine samples that were pre- pared with a coat of polystyrene films. Olivine was used as an example of the asteroidal surface and polystyrene was chosen Fig. 1. Scheme of the vacuum chamber: solid samples are held on a as a template for organic materials of low volatility, possibly finger. The chamber is interfaced to an ion implanter. Bi-directional re- present on the surfaces of some small Solar System objects. flectance spectra are acquired with a halogen/deuterium lamp source Moreover, we measured the changes induced by ion irradiation and the NUV-Vis-VNIR spectrometer. in the absorption coefficient of the polymer. The purpose was to have a tool to better compare laboratory with observed spectra and distinguish between planetary objects with pure silicate sur- faces from those whose surface is contaminated by organic mat- ter exposed to cosmic ion bombardment. Polystyrene is meant are below 1 μm to avoid macroscopic heating of the sample. The to mimic a kind of organic material (i.e. a spectrally neutral ions impinge the surface of the sample at an angle of 45◦. one) freshly exposed at the surface of minor objects in the Solar / System. During experiments the bi-directional reflectance transmit- tance spectra are achieved using NUV-VIS-NIR spectrometer Polystyrene (Poly(1-phenylethane-1,2-diyl), (C H ) )isan 8 8 n with two sources (halogen and deuterium lamp), which cover aromatic vinyl polymer made from the aromatic monomer the spectral range 200–980nm. However, the background signal styrene (C H CH=CH ). Structurally, it is a long hydrocarbon 6 5 2 of the deuterium lamp decreases rapidly below 250 nm. For this chain, with a phenyl group attached to every other carbon atom. reason we considered as relevant only the data at longer wave- Polystyrene is a transparent thermoplastic substance existing lengths. The lamps, with a partially overlapping spectral region in solid state at room temperature. The strong absorption of around 450 nm, were used separately, and the two parts of the polystyrene in the UV spectral range (with the onset at 280 nm) achieved spectra were subsequently joined. For the reflectance is well known (e.g. Partridge 1967). measurements configuration, the scattered light is collected by the spectrometer at an angle of 45◦ with respect to the incident radiation that is normal to the sample surface. Because the ion 2. Experimental apparatus beam forms an angle of 45◦ with the target plane and of 90◦ with / Iradiation experiments were carried out at room temperature in- the collected NUV VNIR radiation, spectra can be obtained be- side a stainless steel vacuum chamber (P < 10−7 mbar), faced fore, during and after the irradiation without tilting the sample.
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