Effects of Space Weathering on Reflectance Spectra of Ureilites: First Studies C.A. Goodrich 1 , J. Gillis-Davis 2 , E. Cloutis 3 , D. Applin 3 , D. Takir 4 , C. Hibbitts 5 , R. Christoffersen 6 , M. Fries 7 , R. Klima 5 and S. Decker 8 1 Lunar and Planetary Institute, USRA, Houston, TX 77058 USA (goodrich@lpi.usra.edu); 2 HIGP, Univ. HI Manoa, Honolulu HI USA (gillis@higp.hawaii.edu); 3 Univ. Winnipeg, Winnipeg, MB Canada; 4 SETI, Mountain View CA USA; 5 JHU/APL, Laurel, MD USA; 6 Jacobs, NASA JSC, Houston TX USA; 7 ARES, NASA JSC, Houston TX USA; 8 Oberwesel Germany.
Space Weathering of Ureilite Meteorites: Motivation OSIRIS-REx • Dark asteroids are of interest as primitive, volatile-rich, Hayabusa 2 based on spectral resemblence to carbonaceous chondrites. Lucy • However, not all dark asteroids are primitive! • Ureilites are differentiated meteorites with very high carbon contents: avg. 4.5 wt% C Ureilites Ureilites graphite Tagish Lake CI CM pyroxene CR CV CO CK CB Ungrouped Chondrites olivine OC RC EC 1 mm 0 1 2 3 4 5 6 7 8 9 Optical photomicrograph carbon content (wt.%)
Space Weathering of Ureilite Meteorites: Motivation VNIR reflectance spectra of ureilites (some similarities to CC): C-complex asteroids (DeMeo et al. 2009) F asteroids (Tholen 1984) 0.25 Main group ureilites: exhumed mantle rocks • Mg-rich olivine and pyroxene (various ratios) 0.20 • various amounts metal, sulfide, graphite • various degrees of shock CO reflectance 0.15 • spectral similarities to CO, CV CV Almahata Sitta (2008 TC 3 ): ureilitic regolith* 0.10 2008 TC 3 • AhS ureilites = clasts in regolith • CM highly shocked, fine-grained, porous 0.05 • some have spectral similarities to CM • spectral similarities to C-complex asteroids 0.00 0.4 0.8 1.2 1.6 2.0 2.4 • 2008 TC 3 = F (Tholen) wavelength ( m) How to distinguish differentiated dark Data Sources: asteroids from primitive dark asteroids? Main group ureilites: Cloutis et al. (2010) AhS: Hiroi et al. (2010); Goodrich et al. (2018) CC: Cloutis et al. (2011, 2012a,b) 2008 TC 3 : Jenniskens et al. (2009) *Herrin et al. (2010); Goodrich et al. (2015)
Space Weathering of Ureilite Meteorites: First Studies • Space weathering of ureilites has not previously been studied. • Based on space weathering studies of CC* significant effects expected . 3 ureilitic samples from Almahata Sitta (2008 fall): interior mantle regolith graphite oliv oliv oliv MS-MU-038 MS-MU-022 MS-MU-025 olivine-rich, olivine + orthopyroxene very fine-grained low-shock, medium-shock, highly-shocked, abundant graphite low abundance graphite, metal dispersed graphite, metal, sulfide *Hiroi & Pieters (1991); Gillis-Davis et al. (2013, 2015); Keller et al. (2015)
Laser Heating Experiments: (HIGP, U. HI) • Samples powdered and sieved to <75 m grain size • Micrometeorite bombardment simulated with pulsed laser irradiation • Reflectance spectra measured at 6000 12000, 24000, 36000, 48000 laser shots Laser pulse heating, with mass spectrometer ▪ 1064 nm, 20-Hz, 6 ns pulse, 30 mJ, 0.25 mm spot ▪ Simulate micrometeorite impacts ~ 20 mm in size ▪ Vacuum 10e-7 torr, residual gas analyzer 100 amu References: Gillis-Davis et al. (2015, 2017) Yamada et al. (1999)
Space Weathering of Ureilite Meteorites: Results MS-MU-038 fresh 0.12 MS-MU-038, 48000 laser shots fresh 0.10 reflectance 0.08 48,000 shots MS-MU-038 fresh 0.14 0.06 MS-MU-022 fresh MS-MU-025 fresh 0.04 0.12 interior mantle 0.02 0.10 MS-MU-022 fresh 0.12 MS-MU-022, 48000 laser shots reflectance 0.08 0.10 regolith fresh reflectance 0.08 0.06 0.06 0.04 48,000 shots 0.04 MS-MU-038, 48000 laser shots 0.02 MS-MU-022, 48000 laser shots 0.02 MS-MU-025, 48000 laser shots MS-MU-025 fresh 0.12 0.00 MS-MU-025, 48000 laser shots 0.5 1.0 1.5 2.0 2.5 wavelength ( m) 0.10 fresh reflectance 0.08 increasing irradiation 0.06 48,000 shots 0.04 darker, redder, decreased spectral contrast 0.02 0.5 1.0 1.5 2.0 2.5 wavelength ( m)
Space Weathering of Ureilite Meteorites: Results Raman MS-MU-022 100 nm D G MS-MU-025 • Raman spectrum of fresh -038 shows olivine and highly crystalline graphite. • After irradiation the carbon peaks have broadened and the D band is as intense as the G band, both 100 nm indicators of disordered carbon. SEI
Comparisons with Asteroid Classes AhS ureilite MS-MU-038 AhS ureilite MS-MU-038 AhS ureilite MS-MU-022 AhS ureilite MS-MU-022 1.30 AhS ureilite MS-MU-025 AhS ureilite MS-MU-025 0.24 Other AhS ureilites AhS ureilites (Hiroi et al. 2010; Goodrich et al. 2018) S Asteroid 2008 TC 3 A 1.25 Q L R 0.20 1.20 D L S Xe 750/550 nm ratio T 1.15 K 0.16 K albedo 1.10 Xe Xk 0.12 fresh Xc X 1.05 Cg Cg T Xc B Cb D 1.00 0.08 fresh Ch Cgh Xk B Cg Cgh C Cb 0.95 X Ch 0.04 0.90 0.80 0.84 0.88 0.92 0.96 1.00 1.04 0.9 1.0 1.1 1.2 1.3 750/550 nm ratio 450/550 nm ratio Increasing irradiation move away from C-complex, toward D, T, some X. Asteroid 2008 TC 3 may not have been significantly space weathered.* *Consistent with Hiroi et al. (2010) SMASS II means: DeMeo et al. (2009) and absence of solar gases in AhS (Ott et al. 2010; Albedo data: Mainzer et al. (2011) Downes et al. 2015; Riebe et al., this meeting).
Comparisons with Asteroid Classes 2.0 2.0 interior mantle regolith D D 1.8 1.8 550 nm-normalized reflectance 550 nm-normalized reflectance T T 1.6 1.6 X X 1.4 1.4 X k X k K X C 1.2 1.2 X C C gh C gh 1.0 1.0 0.8 0.8 MS-MU-025 fresh MS-MU-038 fresh MS-MU-025, 48000 laster shots MS-MU-038, 48000 laser shots 0.6 0.6 0.4 0.8 1.2 1.6 2.0 2.4 0.4 0.8 1.2 1.6 2.0 2.4 wavelength ( m) wavelength ( m) 2.0 D No exact spectral matches with 1.8 550 nm-normalized reflectance asteroid class means T 1.6 X for fresh or laser-weathered ureilites 1.4 X k X C 1.2 But, C gh 1.0 none of these samples individually represents bulk properties of ureilitic 0.8 MS-MU-022 fresh MS-MU-022, 48000 laster shots regolith. 0.6 0.4 0.8 1.2 1.6 2.0 2.4 wavelength ( m)
Conclusions (So Far) • Space weathering results in significant changes in the reflectance spectra of ureiltes. • Results suggest that non-space weathered ureilitic regolith may resemble C-complex asteroids, whereas space weathered ureilitic material may resemble D, T, or some X type asteroids. • Space weathering experiments on a series of ureilite samples with controlled variations in petrologic properties are essential to understand the range of spectral signatures of ureilitic asteroids. • Especially need to study fine-grained, highly-shocked AhS ureilites most likely to represent the bulk of asteroid 2008 TC 3 (ureilitic regolith).
Future Work • Space weathering experiments on a series of ureilite samples with controlled variations in petrologic properties. • Space weathering experiments on fine-grained, highly-shocked Almahata Sitta ureilites most likely to represent the bulk of asteroid 2008 TC 3 (ureilitic regolith). • Include UV (to 140 nm) and Raman, to examine changes in carbon phases. • TEM of space weathered powdered to understand microstructural changes that affect optical properties.
Extra Slide (see Goodrich et al. this meeting) • Almahata Sitta sample UOK 91A is a friable breccia of hydrated CC material with clasts of ureilitic minerals and other chondrites (OC, EC). • Suggested to represent significant fraction of the mass of 2008 TC 3 . Spectra of 91A show 2.7 m band! • AhS 91A chips and powders AhS ureilites [27] 0.12 Asteroid 2008 TC 3 [2], albedo estimated C-complex asteroid types [28,29] CM - Nogoya and Murchison (RELAB) 0.10 2.7 m band 0.08 reflectance Cgh 2.7 mm band C water of hydration F Ch on a ureilitic asteroid! 0.06 CM 2008 TC 3 0.04 CM 0.02 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 wavelength ( m)
Development of Diverse Recent Breakup. Regolith on Ureilitic Asteroid Fragments drift into Earth-crossing orbits EL EH Main Group Ureilites R EL Ang H many (shallow 2008 TC 3 impacts Typical regolith) Polymict CC LL Ureilites CC L (deep regolith) Goodrich et al. (2015)
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