EFFECTS OF LASER SPACE WEATHERING ON DERIVED IRON OXIDE CONTENT IN SAN CARLOS OLIVINE, PYROXENE, AND ANORTHOSITE Logan K. Magad-Weiss Department of Geology and Geophysics University of Hawai`i at Mānoa Honolulu, HI 96822 ABSTRACT Space weathering is the alteration of a planetary surface due to impacts from micrometeorites and irradiation due to the solar wind. The effects of space weathering lead to darkening, reddening, and affects mineral absorption band depth. The magnitude of these changes may be related to the amount of iron present in the minerals. Mineral spectra darken and redden with increased space weathering. These changes in reflectance are related to maturity. On a plot comparing the ratio of near infrared (NIR) to visible (VIS) versus VIS reflectance of lunar samples, it was shown that a correlation existed between iron oxide content and maturity Lucey et al. [1995]. On the basis of this correlation Lucey et al. [1995, 2000] developed an algorithm that mitigates the spectral effects of space weathering and yields estimates of iron oxide content. It was later improved by Shkuratov et al. [1999], who created a method to determine iron content, as well as the degree of maturity for a set of Earth based telescopic images. We used a Nd:YAG 1064 nm pulsed laser to simulate the micrometeorite component of space weathering on olivine, pyroxene, and anorthosite samples. Spectra of these samples in incremental steps from fresh to fully space weathered were measured from 0.35 to 2.5 µm. Finally, we used the spectral-based calculation of Lucey et al. [2000] to estimate the iron content of these laser space weathered samples and compare that to the known composition of these samples. The goal of this work is to show the reliability and accuracy of the Lucey equation to mitigate space weathering and estimate iron composition. These results will help us to determine if the equation needs to be reevaluated in order to account for mineral dependent space weathering rates. INTRODUCTION The work of Lucey et al. [1995] and Lucey et al. [2000] determined that there is a trend between the effects of iron oxide content and maturity on the absorption of light. This work was further improved in Shkuratov et al. [1999] who determined a method of mapping iron composition as well as the degree of maturity for a set of Earth-based telescopic images. Dr. Paul Lucey of the Hawai`i Institute of Geophysics and Planetology (HIGP) derived a formula that linearizes the relationship between iron content and maturity. An iron parameter θfe, and negates the effects of space weathering. θfe= -arctan{[(r950/r750)-y0fe]/r750-x0fe]} (1) x0fe and y0fe are the 750nm reflectance and 950nm/750nm ratio values of the optimized origin R950 and R750 are the reflectance values of a remotely observed location at the wavelength indicated by the subscript (Lucey et al. [2000]). 47
Iron content and maturity form two orthogonal trends when input on a graph of 950nm/750nm reflectance ratio on the y-axis and 750nm reflectance on the x-axis. Iron maturity increases in the up and left direction, while iron oxide content increases in the down and left direction, radial to the optimized origin. The angle created by the trend is the iron parameter θfe. Increasing values of θfe correspond to increasing iron oxide content (Figure 1a). The maturing trend line may not converge at the optimized origin for all minerals, and as a result the best way to determine the existence of the proposed optimized origin is to perform this experiment on samples of San Carlos olivine, pyroxene, and anorthosite in order to determine if the trend lines are different, as well as if the Lucey equation can affectively negate the effects of space weathering. Figure 1: a.) The NIR/VIS versus VIS plot for sample return sites and stations observed by Clementine. The location of the optimized origin defines the iron parameter θfe. b.) Plot of FeO content measured for returned lunar soils versus the spectral iron parameter θfe from remote measurements by Clementine (Lucey et al. [2000]). It was not until Sasaki et al. [2001] that the effects of space weathering were accurately recreated in the laboratory. They used a 6-8ns pulsed 1064nm wavelength laser to simulate 48
micrometeorite impacts and create submicroscopic iron particles (SMFe). These SMFe particles affect optical mineral spectra by causing darkening, reddening, and loss of spectral absorptions. This was tested on olivine samples, as well as pyroxene samples, and it was found that olivine s absorption was more affected by space weathering than pyroxene. Figure 2: Absolute spectra of olivine from Sasaki et al. [2001] By firing laser pulses at San Carlos olivine, pyroxene, and anorthosite samples, it effectively melted and vaporized the samples while they were held under high vacuum (1-2 x 10-6 torr). Utilizing a turbopump attached to the vacuum removed any gases that would chemically react with or oxidize the SMFe. METHODS Dr. Jeff Gillis-Davis of the HIGP had olivine, pyroxene, and anorthosite samples of known iron composition. The wt% FeO for olivine, pyroxene, and anorthosite were 9.8 wt%, 18.8wt%, and <1wt% respectively. Mineral compositions were measured using the electron microprobe in the Department of Geology and Geophysics (G&G) at University of Hawai`i at Mānoa and were provided for the experiment. The iron compositional information was used to plot the different iron compositions and degrees of maturity on a NIR/VIS versus VIS plot. These samples are similar to olivine or pyroxene that can be found on the moon, which enabled accurate recreation of a space weathering effects. Melting and vaporization of the sample in a vacuum prevented iron in the melt and vapor from being oxidized. In order to determine the effects of space weathering, a laser technique developed by Sasaki et al. [2001] was adopted, and along with an improved version implemented by Dr. Jeff Gillis- Davis. Sasaki et al. [2001] used a compressed powder while simulating the effects of space weathering. Dr. Jeff Gillis-Davis technique however involved using a loose powder. On the lunar surface, much of the regolith is loose and has not been compressed. Measuring the changes in absolute reflectance for the loose powder therefore will more accurately simulate a lunar surface, and how space weathering alters it. In order to simulate space weathering and take reflectance measurements, a Nd:YAG 1064 nm pulsed laser, vacuum, and Analytical Spectral Devices Inc. (ASD) FieldSpec FR spectrometer at HIGP were used. These tools were of utmost importance 49
in developing, accurately measuring, and precisely mapping the effects of iron composition on absorption to determine if the effects of space weathering on the Lucey equation are negligible. Once the samples are were safely placed in the vacuum and the vacuum was pumped down to 1x10-6 torr, laser pulses were shot at the sample with a voltage of 0.9kV at a frequency of 20Hz producing approximately 0.6W. The goal of this is to vaporize the sample so that any impurities in the sample are removed, and the iron composition can be accurately measured. Once the sample was removed from the vacuum, the powder was poured into a disk, the powder was flattened, and then rotated on the disk over one minute rotations. Flattening of the powder removed local topography that might cause shadowing effects, and affect the absolute reflectance. The samples absolute reflectance was taken between 350-2500nm, and plotted to display the optical changes. Once the data was collected, the Lucey equation was applied in order to examine the relationship between iron composition and maturity on a NIR/VIS versus VIS plot. The data was then plotted using computer software, and determined whether the effects of space weathering were negligible in the Lucey equation, or if the equation needed to be reevaluated. In addition, the results showed whether or not the trendlines for NIR/VIS versus VIS plot converged back to the theoretical optimized origin for San Carlos olivine, pyroxene, and anothosite, or if trendlines did not connect back to the optimized origin. RESULTS Forty minutes of space weathering on San Carlos olivine reproduced the changes in the optical spectra that were found in Sasaki et al. [2001]. Increased space weathering produced the darkening and reddening affect that was found in Sasaki et al. [2001], and is shown in the decrease in absolute reflectance with increased space weathering. The curve for San Carlos olivine went from absolute reflectance peaks of approximately 0.69 and 0.79 at about 700nm and 2200nm to approximately 0.29 and 0.59 at the same wavelengths after 40 minutes of space weathering. The decrease in absolute reflectance can be attributed to nanophase iron produced by the laser space weathering of the San Carlos olivine. Absolute reflectance curves were also obtained for the anorthosite and pyroxene samples. Figure 3: Reflectance spectra of San Carlos Olivine from Fresh to 40 minutes of space weathering 50
rmalized Reflectance at 750nm When reflectance data for all three of the minerals are plotted, the effects space weathering has on the minerals becomes apparent (See Figure 3). The anorthosite was least affected by the space weathering, and saw little change in its optical spectra. Most of the change seen occurred in the visible light region of the spectra, and there was little change in near-infrared region (NIR). This is due to the composition of anorthosite, which is very low in iron (<1 wt%). It is instead very rich silicic mineral anorthite. In this case, the composition of the anorthosite was approximately An75, but lunar samples can have even higher anorthite content at around An90-95. Pyroxene had a similar trend to San Carlos Olivine in that with increased space weathering, there was a decrease in the absolute reflectance. There was also a reddening effect that was seen in both the San Carlos olivine and the pyroxene. The reddening seen in olivine (See Figure 3b) was more pronounced in the visible end of the spectrum than the near-infrared. The same trend was see in the pyroxene, however it was not as large as the olivine. As shown in Sasaki et al. [2001], the pyroxene was less affected by the space weathering than the olivine. The reflectance of pyroxene was also initially lower than olivine. The fresh powdered pyroxene was a dark gray color while the fresh powdered olivine was light green. a.) San Carlos Olivine Normalized Reflectance A1 b.) 1.3 0.3 200 1200 2200 Wavelength (nm) SC-Olivine Fresh SC-Olivine 5min SC-Olivine 10min SC-Olivine 20min SC-Olivine 30min Figure 4: a.) Absolute spectra of fresh, 20 minutes, and 40 minutes of space weathering for San Carlos olivine, pyroxene, and anorthosite. b.) Spectra normalized to 750nm for fresh, 5 minutes, 10 minutes, 20 minutes, 30 minutes, and 40 minutes of space weathering. 51
950nm/750nm Application of the Lucey equation did successfully linearize the relationship between iron composition and maturity on the NIR/VIS versus VIS plot. It is not a perfect line, but there is a strong linear correlation for all of the three minerals. The trends of the San Carlos olivine and anorthosite both go towards a hypermature end member with increased space weathering. Pyroxene however, does not appear to go towards a hypermature end member. When pyroxene is subjected to increased space weathering, there appears to be an increase in the value of θfe, rather than a constant value. If this were the case, it would suggest that pyroxene has an increase in the FeO content with increased space weathering. It is unlikely that the amount of FeO in the mineral would increase due to space weathering, therefore, pyroxene may not follow the relationship proposed by Lucey et al. [2000]. Based on the trends of the lines, pyroxene would also not connect back with San Carlos olivine and anorthosite at a theoretical optimized origin. This brings into question whether the Lucey equation can be applied to all minerals present on the moon, or if there are other factors that have not been taken into account that may not accurately simulate lunar processes. Further testing on all three of the minerals will help determine if the Lucey equation needs to be modified, or if the data obtained was an exception. 1 0.95 0.9 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.5 40 Minutes 40 Minutes Space Weathering Fresh NIR/VIS vs VIS Plot 30 Minutes 20 Minutes 10 Minutes 40 Minutes Space Weathering Fresh Fresh 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 750nm Anorthosite SC-Olivine Pyroxene Linear (Anorthosite) Linear (SC-Olivine) Linear (Pyroxene) Figure 5: Absolute reflectance data of San Carlos olivine, pyroxene, and anorthosite on an NIR/VIS vs. VIS plot. CONCLUSIONS AND FUTURE WORKS The statements regarding nanophase iron causing darkening and reddening effects on absolute spectra with increased space weathering hold true as displayed by the data obtained. San Carlos olivine was affected more than pyroxene as found in Sasaki et al. [2001], and absolute reflectance decreased for each of the minerals with increased space weathering. The anorthosite experienced minor changes in its optical spectra with increased weathering due to the lack of iron in its composition. Though darkening and reddening were observed in the spectra, the change was not as pronounced as the iron bearing minerals with increased space weathering. Reddening occurred more intensely for the San Carlos olivine and pyroxene in the visible part of the spectrum than the near-infrared. Anorthosite, like San Carlos olivine and pyroxene had more reddening in 52
the visible part of the spectrum than the near-infrared. The minor changes in the optical spectra of the anorthosite compared to the San Carlos olivine and pyroxene further supports the claim that nanophase iron is responsible for darkening and reddening effects with increased space weathering. The Lucey method appears to effectively negate the effects of space weathering. It does appear however that the method may need to be re-evaluated to determine the accuracy it has in determining the FeO content and maturity in pyroxene. The trend of the pyroxene did not appear to go towards a hypermature end member however trend was seen in San Carlos olivine and anorthosite. In the future, at least two more 40-minute space weathering sessions will take place for all of the minerals in order to determine the reproducibility of the experiment. In addition, there is a second equation derived in Lucey et al. [2000] that calculates the weight percent FeO. wt% FeO= (17.427 x θfe) -7.565 (2) This equation was calibrated for samples from the Clementine Mission, and thus must be calibrated for the samples that we are working with in order to determine if they accurately calculate the FeO content of the samples. ACKNOWLEDGEMENTS A special thanks to Hawai`i NASA Space Grant for their generous funding and support. Also a special thanks to Dr. Jeff Gillis-Davis for all of his help in conducting this research, and Dr. Paul Lucey for providing the equation to test. REFERENCES Hapke, B. (2001). Space weathering from Mercury to the asteroid belt. Journal of Geophysical Research, 106(5), 10039-10073. Lucey, P. G., Taylor, G. J., & Malaret, E. (1995). Abundance and Distribution of Iron on the Moon. Science, 268(5214), 1150-1153. http://dx.doi.org/10.1126/science.268.5214.1150 Lucey, P. G., & Blewett, D. T. (2000). Lunar iron and titanium abundance algorithms based on final processing of Clementine ultraviolet-visible images. Journal of Geophysical Research, 105(8), 20297-20305. http://dx.doi.org/10.1029/1999je001117 Sasaki, S., Nakamura, K., Hamabe, Y., Kurahashi, E., & Hiroi, T. (2001). Production of iron nanoparticles by laser irradiation in a simulation of lunar-like space weathering. Nature, 410(6828), 555-557. http://dx.doi.org/10.1038/35069013 Shkuratov, Y. G., Kaydash, V. G., & Opanasenko, N. V. (1999). Iron and Titanium Abundance and Maturity Degree Distribution on the Lunar Nearside. Icarus, 137(2), 222-234. http://dx.doi.org/10.1006/icar.1999.6046 Yamada, M., Sasaki, S., Nagahara, H., Fujiwara, A., Hasegawa, S., Yano, H.,... Otake, H. (1999). Simulation of space weathering of planet-forming materials: Nanosecond pulse laser irradiation and proton implantation on olivine and pyroxene samples. Earth, Planets, and Space, 51(11), 1255-1265. http://dx.doi.org/10.1186/bf03351599 53