EFFECTS OF SPACE WEATHERING ON THE TROJAN ASTEROIDS by April A. Deet Bachelor of Science in Physics June 2002 The author hereby grants to MIT permission to reproduce and to distribute publicly paper and electronic copies of this thesis document in whole or in part. Copyright @ 2002 Massachusetts Institute of Technology. All rights reserved. MASSACHUSETTS INSTITUTE OF TECHNOLOGY SEP 28 2017 LIBRARIES ARCHIVES Signature redacted Signature of Author_ Department of Physics June 7, 2002 Certified by Professor Richard P. Binzel Thesis Supervisor Accepted by__Signature redacted Professor Timothy L. Grove Chairman, Undergraduate Thesis Committee 77 Massachusetts Avenue Cambridge, MA 02139 MIT~uraries http://Iibraries.mit.edu/ask DISCLAIMER NOTICE * The pagination in this thesis reflects how it was delivered to the Institute Archives and Special Collections. * Thesis was submitted to the Institute Archives without all the required signatures. U EFFECTS OF SPACE WEATHERING ON THE TROJAN ASTEROIDS by April A. Deet Abstract Trojan asteroids orbit at Jupiter's L4 and L5 points. They are included in the D-class of asteroids because of their steep spectral slope. According to spectra of other asteroid classes, the larger the diameter is of a D-class asteroid, the redder the asteroid should be in the visible spectrum. We examined a total of fifteen asteroids, five (from the SMASS 1 data set) were small, and ten (newly collected data) were large. The actual results did not match our expected results, most likely due to the large error bars and the small data set. Space weathering may affect Trojans in the same way as it does other asteroid classes. To know with certainty, further investigation is needed. 3 Dedication This thesis is dedicated to Shannon for also caring about asteroids, fostering my interest in space exploration, and giving me hope, to all of my EAPS friends, and to Mom and Dad. 4 Table of Contents Abstract 2 Dedication 3 Table of Contents 4 List of Tables and Figures 5 1. Introduction 6 2. Goals 9 3. Data Reduction 10 4. Discussion 12 5. Additional Comments 36 6. Conclusion 37 References 39 5 List of Tables and Figures Figure 4-1: Wavelength vs. reflectance for 2594 Acamas. 13 Figure 4-2: Wavelength vs. reflectance for 5233 1988 RL10. 14 Figure 4-3: Wavelength vs. reflectance for 5257 1988 RS10. 15 Figure 4-4: Wavelength vs. reflectance for 9694 Lycomedes. 16 Figure 4-5: Wavelength vs. reflectance for 9713 Oceax. 17 Figure 4-6: Wavelength vs. reflectance for 11273 1988 RN11. 18 Figure 4-7: Wavelength vs. reflectance for 11869 1989 TS2. 19 Figure 4-8: Wavelength vs. reflectance for 23463 1989 TX11 20 Figure 4-9: Wavelength vs. reflectance for 24454 2000 QF198. 21 Figure 4-10: Wavelength vs. reflectance for 25895 2000 XN9. 22 Figure 4-11: Wavelength vs. reflectance for 1143 Odysseus. 23 Figure 4-12: Wavelength vs. reflectance for 1749 Telamon. 24 Figure 4-13: Wavelength vs. reflectance for 2920 Automedon. 25 Figure 4-14: Wavelength vs. reflectance for 3317 Paris. 26 Figure 4-15: Wavelength vs. reflectance for 3451 Mentor. 27 Table 4-1: Table of the asteroids and their slopes. 28 Figure 4-16: Plot of perihelion distance 'q' vs. slope 'm' for all asteroids. 29 Figure 4-17: Plot of inclination 'i'vs. slope 'i' for all asteroids. 30 Figure 4-18: Plot of eccentricity 'e' vs. slope 'm' for all asteroids. 31 Figure 4-19: Plot of H magnitude vs. slope 'm' for all asteroids. 32 Table 4-2: Table of orbital components and correlation coefficients. 33 Figure 4-20: Plot of asteroids in numerical order vs. slope. 34 II 6 Chapter 1 Introduction Asteroids are believed to be planetesimals left over from the formation of our solar system. A myriad inhabit our solar system, provoking many observations and much scientific research. Most asteroids are located in the Main Belt, which is the group of asteroids found between Mars and Jupiter. One other grouping of asteroids, which will be discussed further, are the Trojan asteroids. Trojan asteroids orbit at 5.2 AU synchronously with Jupiter. Max Wolf, the founding director of the Heidelberg Observatory, discovered the first one in October 1906. More than one century earlier, J.L. LaGrange proved that in the restricted three-body problem, regions of stability exist at +600 and -60* from a planet. This means that objects, such as asteroids, can exist at those points (Shoemaker et al., 1989). These places later came to be known as the LaGrange points and this is where the Trojan asteroids are found. The low albedo (reflectivity of a body) and distance of the Trojans made detection difficult. Despite this difficulty, several hundreds of these objects are known today, according to the Minor Planet Center (List of Jupiter Trojans, n.d.). About twice as many had been found in the L4 than in the L5 group as of mid- 1988, but that was only due to the L5 point being located near the Milky Way, making detections there difficult (Shoemaker et al., 1989). Further study of the Trojans will offer insight as to their much-debated origin. There are 7 approximately 300 Trojans asteroids that we know of, and assuming the same size distribution for Trojans and main belt asteroids, there are about half as many Trojans as main belt asteroids (Shoemaker et al., 1989). The Trojan asteroids have an uncertain origin, though it was most likely in an area rich with frozen volatiles, such as water ice and CH4, which would correspond to a greater solar distance than their present one. Trojans tend to be redder with increasing distance from the Sun (Gradie and Veverka 1980). Trojans and cometary nuclei have similar colors. Most dark asteroids do not show spectroscopic evidence of organic materials. Despite this, the University of Hawaii used their 2.2-meter telescopes to study 18 dark objects with longer wavelengths in the near infrared in order to gain better resolution in the search for spectral signatures of organic materials. These primitive, dark surfaced objects studied were found to be redder than the sun. 944 Hidalgo, 2101 Adonis, and 2212 Hephaistos are good candidates for future observations, for they have not yet been observed in the near infrared (Dumas et al. 1998). Asteroids can be classified according to their spectra. In the commonly used Tholen classification scheme, the Trojan asteroids are included in the D class, which has steep spectral slopes (Tholen and Barucci, 1989). The D class has members throughout the outer asteroid belt as well as in the Trojan clusters. The larger the diameter of a D-type asteroid, the redder the asteroid is in the visible spectrum (roughly 0.4 to 0.7 micrometers). These characteristics may be due to progressive chemical and temperature effects among hydrocarbons (Dumas et al. 1998). 8 Spectroscopy Spectroscopy is the study of the reflectance spectrum, or light reflected from, a body. By looking for absorption lines in the spectrum, we can determine the chemical composition of that body. Through spectroscopy of asteroids, we learn about their composition, and in turn we learn about the composition of the primordial solar nebula. Eventually, asteroid spectroscopy will give us a better model of the formation of our solar system. 9 Chapter 2 Goals Determining a feasible project required extensive reading about the observations of other astronomers. It was also necessary to note the types of instruments used and the quality of information gained. Though it appears that these characteristics are absolute, the surfaces of asteroids change over time, and sometimes very dramatically. This is often due to events, such as impacts and reactions with the solar wind, occurring at the surfaces. These events can change the spectra of asteroids, influencing their interpretation. Building upon this, we come to the question of whether or not a correlation of asteroid size and composition exists. Observations of S-class asteroids (the most common type) show a correlation of size and spectra of both main belt asteroids and NEAs, but the outer belt asteroids have not yet been examined. In order to detect a possible interdependence of these properties, it is necessary to collect spectra of differently sized asteroids, and then to inspect the resulting spectral slopes. 10 Chapter 3 Data Reduction Much of the data for this project were collected at the Magellan telescopes in Chile. A visible spectrograph with a wavelength range of about 0.4-0.9 micrometers and a CCD were the instruments used. A total of ten asteroid spectra were available from this source. Five asteroid spectra from the Small Main Belt Asteroid Spectroscopic Survey (SMASS), taken at the MDM Observatory in Arizona were also used (Xu 1994, and Xu et al., 1995). This resulted in a total of 15 asteroids with a range of estimated diameters from 24km to 150km (Conversion of Absolute Magnitude to Diameter, n.d.). In order to find if there is a correlation between size and composition, it was necessary to reduce the data in such a way that it could be examined and interpreted. I did this by using a software package known as Image Reduction and Analysis Facility (IRAF). There were seven main steps to the data reduction procedure. The first part of the process involved use of the function known as apall. Two- dimensional images were input into apall, and one-dimensional spectra were output. The next two steps consisted of applying the functions refspec and dispcor to the data, respectively. The former associated the files we were reducing with a wavelength reference file by putting this information into the file header. The latter converted pixels to wavelength, with 25 angstroms/pixel.