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We define a new approach to model asteroidal space weathering. We started from recent results of ion irradiation experiments (60-400 keV) of meteorites and silicates to give an accurate description of space weathering, and we included its effects in the Shkuratov model. We found that the reddening and darkening process (in the range 0.3-2.5 [micro]m) does not significantly affect the position or relative intensities of the mafic silicate absorption features and it mainly affects the continuum of reflectance spectra. This continuum is parameterized by a [C.sub.S] coefficient, which is strongly related with the number of displacements per unit area (damage parameter); we consequently obtained an exposure time curve, and corresponding astrophysical timescales. We applied this new description of space weathering to model observed spectra of Asteroid 832 Karin, in the 0.4-2.4 [micro]m spectral region. The obtained exposure time is slightly lower than the age of the impact and collisional breakup which originated the Karin asteroidal family, i.e., about 5.75 x [10.sup.6] years. Keywords: Asteroid, surfaces; Spectroscopy; Experimental techniques; Meteorites

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.07.001 Byline: Rosario Brunetto (a)(b), Giovanni Strazzulla (b) Keywords: Spectroscopy; Experimental techniques; Radiation chemistry; Meteorites; Asteroid surfaces Abstract: Ion irradiation experiments have been performed on silicates (bulk samples) rich of olivine, pyroxene, and serpentine to simulate the effects of space weathering induced on asteroids by solar wind ions. We have used different ions (H.sup.+, He.sup.+, Ar.sup.+, Ar.sup.2+) having different energies (from 60 to 400 keV) to weather the samples, probed by Raman spectroscopy and UV-vis-NIR reflectance spectroscopy. All the irradiated materials have shown reddening and darkening of reflectance spectra in the 0.25-2.7 [mu]m spectral range. We have found that the increase of the spectral slope of the continuum across the 1-[mu]m band is strongly related with the number of displacements caused by colliding ions because of elastic collisions with the target nuclei. The spectral slopes have been compared, at increasing ion fluence, with those from irradiated Epinal meteorite. We show that formation of nuclear displacements by solar wind ion irradiation is a physical mechanism that reddens the asteroidal surfaces on a time-scale lower than 10.sup.6 years. Author Affiliation: (a) Dipartimento di Fisica, Universita di Lecce, via Arnesano, I-73100 Lecce, Italy (b) INAF-Osservatorio Astrofisico di Catania, via S. Sofia 78, I-95123 Catania, Italy Article History: Received 23 March 2005; Revised 15 June 2005

Most ion irradiation experiments relevant to primitive outer Solar System objects have been performed on ice and silicate targets. Here we present the first ion irradiation experiments performed on natural complex hydrocarbons (asphaltite and kerite). These materials are very dark in the visible and have red-sloped spectra in the visible and near-infrared. They may be comparable in composition and structure to refractory organic solids on the surfaces of primitive outer Solar System objects. We irradiated the samples with 15-400 keV H+, N+, Ar++, and He+ ions and measured their reflectance spectra in the range of 0.3-2.5 [micro]m before ion implantation and after each irradiation step. The results show that irradiation-induced carbonization gradually neutralizes the spectral slopes of these red organic solids. This implies a similar space weathering trend for the surfaces of airless bodies optically dominated by spectrally red organic components. The reduction of spectral slope was observed in all experiments. Irradiation with 30 keV protons, which transfers energy to the target mostly via electronic (inelastic) collisions, showed lower efficiency than the heavier ions. We found that spectral alteration in our experiments increased with increasing contribution of nuclear versus electronic energy loss. This implies that nuclear (elastic) energy deposition plays an important role in changing the optical properties of irradiated refractory complex hydrocarbon materials. Finally, our results indicated that temperature variations from 40 K to room temperature did not influence the spectral properties of these complex hydrocarbon solids. Keywords: Radiation chemistry; Organic chemistry; Spectroscopy