Space Weathering: from Laboratory to Observations

Mem. S.A.It. Suppl. Vol. 11, 159 Memorie della c SAIt 2007 Supplementi

Space weathering: from laboratory to observations

R. Brunetto1,2, V.Oroﬁno1, and G. Strazzulla2

1 Dipartimento di Fisica, Universita` degli Studi di Lecce, Via Arnesano, I-73100 Lecce, Italy 2 Istituto Nazionale di Astrofisica – Osservatorio Astrofisico di Catania, Via Santa Sofia 78, I-95123 Catania, Italy e-mail: [email protected]

Abstract. An ongoing research program in our laboratories is focusing on the eﬀects of laser ablation and ion irradiation on silicates, meteorites, and ices, as a simulation of space weathering on Solar System minor bodies (asteroids, Trans-Neptunian Objects, etc.). Spectroscopic results show a general reddening and darkening of the various materials in the 0.3-2.7 µm range. Laboratory data are then compared with observations, through spectral characterization and scattering models, indicating that space weathering is a very eﬃcient process both in the inner and outer Solar System. In particular, we demonstrated that the majority of TNOs and Centaurs can develop an organic crust mantle produced after irradiation of simple C-bearing molecules. Another relevant result is that the exposure to surface space weathering of asteroid 832 Karin, as calculated from our experiments and models, is in agreement with a dynamical time-scale, i.e. the age of the corresponding Karin family.

Key words. Solar System: Minor planets, asteroids – Solar System: Kuiper Belt - Comets: general – Methods: laboratory – Molecular processes

1. Introduction Also minor bodies in the outer Solar System show the effects of space weather- Irradiation by cosmic and solar wind ions, ing: a great variety of spectral colors is ob- and bombardment by interplanetary dust par- served for Trans-Neptunian Objects (TNOs) ticles (micro-meteorites), induces relevant sur- and Centaurs (Doressoundiram et al. 2002), face modifications on airless bodies of the and this can be only partially explained by a Solar System. different composition. Such alteration processes, known as space In our laboratories in Lecce and in Catania, weathering, affect the spectral properties we perform laser ablation and ion irradiation of silicate-rich objects, inducing progressive experiments on silicates, meteorites, and ices, darkening and reddening of the asteroids re- as a simulation of space weathering on Solar flectance spectra (Hapke 2001; Marchi et al. System minor bodies (asteroids, TNOs, etc.). 2005). Weathering effects are studied through reflectance and transmittance spectroscopy, usu- Send offprint requests to: R. Brunetto ally in the 0.3-2.7 µm range. Details of the 160 Brunetto et al.: Space weathering experiments experimental setup can be found elsewhere In Fig. 1 we show a typical weathering ex- (Brunetto & Strazzulla 2005; Brunetto et al. periment on Fe-poor San Carlos olivine. The 2006a). Laboratory data are then compared effects of space weathering can be easily de- with observations, through spectral and scatter- scribed by an exponential continuum. Indeed, ing models. if we compute the ratio between spectra of ir- Here we present a few results of our ongo- radiated and unirradiated samples, the contri- ing research program. bution of silicate bands almost disappear, and a continuum curve is left, with decreasing in- tensity moving from the NIR to the UV spec- 2. Inner Solar System tral range. This space weathering process does not significantly affect the position or relative Many asteroids in the inner Solar System show intensities (areas) of the mafic silicate absorp- high silicate content (Marchi et al. 2005). We tion features (lower panel in Fig. 1). We model conducted micro-meteorite bombardment sim- the weathering continuum by fitting with an ulations using a nanosecond pulsed UV ab- exponential curve given by Ratio = W(λ) = lating laser on silicate targets (Brunetto et al. Kexp(CS /λ), where λ is the wavelength, K is 2006a). Such simulations show spectral red- a scale factor that change according to the nor- dening and darkening (UV-vis-NIR range), malization of the spectra, and the parameter CS and indicate that the weathering timescale rules the strength of the exponential curve, i.e. for micro-meteorite bombardment in the near- it is a measure of the effects of space weather- Earth space is about 108 years. ing. The quantity W(λ) is what we call weath- We also simulated solar wind heavy-ion ir- ering function. radiation on asteroids: we irradiated ordinary The CS parameter is strongly related with chondrite Epinal (H5), which is a good ana- the number of displacements per unit area logue for silicate-rich asteroids, with Ar++ (60 (damage parameter), and it becomes more neg- keV), producing strong reddening and dark- ative as the space weathering effect increases ening. Comparing spectra of irradiated sam- (Brunetto et al. 2006b). In Fig. 2 we show ples of Epinal with those of some silicate-rich the experimental damage curve and its applica- NEOs, the timescales for this process were es- tion to asteroid 832 Karin. Using our weather- timated to be in the order of 104 − 106 years at ing function in conjunction with the Shkuratov 1 AU from the Sun (Strazzulla et al. 2005). scattering model (Shkuratov et al. 1999), we Performing irradiation experiments with fitted the observed reflectance spectra of as- different ion species (H, He, N, Ar, etc.) and teroid 832 Karin, and estimated that Karin’s energies (30-400 keV), we investigated the surface accumulated about 8 × 1018 displace- physical mechanism producing weathering ef- ments per unit area (Brunetto et al. 2006b). fects. We obtained that the increasing of the This would correspond to an exposure of about spectral slope of the continuum above the 1- 2 × 106 years at 2.9 AU from the Sun (about micron silicate band is strongly related with Karin’s semi-major axis). the number of displacements caused by col- This result for the exposure to surface liding ions inside the sample, i.e., the elastic space weathering of asteroid 832 Karin is in collisions with the target nuclei (Brunetto & good agreement with a dynamical time-scale, Strazzulla 2005). i.e. the age of the corresponding Karin fam- This is very similar to what has been ob- ily. Indeed, a disruptive impact rejuvenated served in amorphization of other materials Karin’s surface 5.8 × 106 years ago, originat- relevant for Astrophysics (namely forsterite, ing the Karin family (Nesvorny´ et al. 2002). graphite, and diamond) after ion irradiation ex- This scenario is coherent with the model of a periments (Brunetto et al. 2005a). This sug- very efficient space weathering process caused gests that asteroidal weathering is more con- by solar wind ions; it has also been confirmed nected with a physical damage process, rather by recent discovery of an increasing trend for than chemical. the visible spectral slope of Main Belt aster- Brunetto et al.: Space weathering experiments 161

0.8 Olivine powder

0.7

0.6 e c n a

t 0.5 c e l f e 0.4 R after 7x1015 Ar+/cm2, 200 keV 0.3

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0.4 0.8 1.2 1.6 2.0 2.4 Wavelength (µm)

1.0

0.9 e

c 0.8 n a t c e l

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e r Dash: irradiated olivine powder d

e 0.6 µ o Dot: model (C = -0.29 m) i S t a

R 0.5

0.4

0.4 0.8 1.2 1.6 2.0 2.4 Wavelength (µm)

Fig. 1. Upper panel: reﬂectance spectra of powdered San Carlos olivine, before (solid) and after (dashed) ion irradiation. Lower panel: ratio plot of the spectra shown in the upper panel; the dotted curve represents the model obtained using the weathering function W(λ). 162 Brunetto et al.: Space weathering experiments

0.5 1.0 1.5 2.0 2.5

0.00 Circles: (32532) Thereus 2.0 103 eV/16amu

-0.05 rejuvenating processes 1.8

-0.10 e

c 1.6 n a

-0.15 t c ) e 832 Karin l f m

µ e ( r

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-0.20 d C e l 89 eV/16amu

Experimental a c

damage curve S 1.2 -0.25 (dot: error bars)

space weathering -0.30 1.0

Methanol virgin

-0.35 0.8

0.0 0.4 0.8 1.2 1.6 2.0 d (x1019 displacements/cm2) 0.5 1.0 1.5 2.0 2.5 Wavelength (µm)

Fig. 2. The CS parameter as a function of a dam- Fig. 3. Scaled (0.8 µm) reflectance spectra of age parameter (displacements per unit area), as cal- methanol ice (T = 77 K), before and after ion irradi- ibrated from ion irradiation experiments. The cor- ation (doses indicated), qualitatively compared with responding value for asteroid 832 Karin is also in- the spectrum of Centaur (32532) Thereus. cluded, as derived from model of the continuum using the weathering function. oids as a function of exposure to solar wind an organic residue is synthesized that may pro- ions (Lazzarin et al. 2006). duce a crust of refractory material, as in the case of comets in the Oort cloud (Strazzulla et al. 1991). 3. Outer Solar System Using 200–400 keV ions, we performed Methane and methanol are simple molecules irradiation experiments of frozen (16–80 K) relevant for the composition and chemistry of methanol, methane, and benzene (as a template TNOs and Centaurs. For instance, methane for aromatic compounds), monitored by vis– NIR (0.65-2.7 µm) reflectance spectroscopy. (CH4) ice is present on Pluto (Owen et al. The aim was to compare the induced color 1993), and on TNO 2005 FY9 (Licandro et al. variations with the observed spectra of some 2006); methanol (CH3OH) ice has been possi- bly identified on Centaur Pholus (Cruikshank Centaurs and TNOs. et al. 1998). In Fig. 3 we show scaled reflectance spec- As simulated by laboratory experiments, tra of methanol ice (T = 77 K) before and after simple frozen molecules are destroyed by ener- ion irradiation, compared with the spectrum of getic particles and by UV photons, with conse- Centaur (32532) Thereus. The spectral slope of quent modification of their band intensities and Thereus is compatible with the presence of an formation of other molecules (Brunetto et al. irradiation mantle on its surface (Brunetto et al. 2005b). Furthermore, when carbon is present, 2006c). Brunetto et al.: Space weathering experiments 163

Similar results are obtained for a relevant Series, 6, 120 number of TNOs and Centaurs, showing that Brunetto, R., Baratta, G. A., Domingo, M., & about 70% of the observed objects have spec- Strazzulla, G. 2005b, Icarus, 175, 226 tral slope well reproduced by irradiation of Brunetto, R., & Strazzulla, G. 2005, Icarus, methanol, methane, and benzene (Brunetto et 179, 265 al. 2006c). So, the surface colors of these ob- Brunetto, R., Romano, F., Blanco, A., Fonti, jects are compatible with the presence of a S., Martino, M., Orofino, V., & Verrienti, C. refractory organic crust developed after pro- 2006a, Icarus, 180, 546 longed irradiation by cosmic ions. Brunetto, R., Vernazza, P., Marchi, S., Birlan, M., Fulchignoni, M., Orofino, V., & 4. Conclusions Strazzulla, G. 2006b, Icarus, in press. Brunetto, R., Barucci, M. A., Dotto, E., & Our comparison between laboratory experi- Strazzulla, G. 2006c, ApJ, 644, 646 ments and astronomical observations indicates Cruikshank, D. P., et al. 1998, Icarus, 135, 389 that space weathering is a very efficient pro- Doressoundiram, A., Peixinho, N., de Bergh, cess. In the outer Solar System, in particular, C., Fornasier, S., Thebault,´ P., Barucci, the majority of TNOs and Centaurs can de- M. A., & Veillet, C. 2002, AJ, 124, 2279 velop an organic crust mantle produced after Hapke, B. 2001, J. Geophys. Res., 106, 10039 ion irradiation of simple C-bearing molecules. Lazzarin, M., Marchi, S., Moroz, L. V., Regarding the inner Solar System, the Brunetto, R., Magrin, S., Paolicchi, P., & spectral slope of asteroids increase as a func- Strazzulla, G. 2006, ApJ, 647, L179 tion of exposure to solar wind ions: a relevant Licandro, J., Pinilla-Alonso, N., Pedani, M., case is given by asteroid 832 Karin, whose ex- Oliva, E., Tozzi, G. P., & Grundy, W. M. posure to surface space weathering, as calcu- 2006, A&A, 445, L35 lated from our experiments and models, is in Marchi, S., Brunetto, R., Magrin, S., Lazzarin, agreement with a dynamical time-scale, i.e. the M., & Gandolfi, D. 2005, A&A, 443, 769 age of the corresponding Karin family. Nesvorny,´ D., Bottke, W. F., Jr., Dones, L., & Levison, H. F. 2002, Nature, 417, 720 Acknowledgements. We thank the groups of Owen, T. C., et al. 1993, Science, 261, 745 Experimental Astrophysics in Lecce and Catania, Shkuratov, Y., Starukhina, L., Hoffmann, H., & for precious support in performing the experiments, Arnold, G. 1999, Icarus, 137, 235 and useful scientific comments. Strazzulla, G., Baratta, G. A., Johnson, R. E., & Donn, B. 1991, Icarus, 91, 101 References Strazzulla, G., Dotto, E., Binzel, R., Brunetto, Brunetto, R., Baratta, G. A., & Strazzulla, R., Barucci, M. A., Blanco, A., & Orofino, G. 2005a, Journal of Physics Conference V. 2005, Icarus, 174, 31