Signature redacted EFFECTS OF SPACE WEATHERING ON THE TROJAN ASTEROIDS SEP LIBRARIES

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1 EFFECTS OF SPACE WEATHERING ON THE TROJAN ASTEROIDS by April A. Deet Bachelor of Science in Physics June 22 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. 22 Massachusetts Institute of Technology. All rights reserved. MASSACHUSETTS INSTITUTE OF TECHNOLOGY SEP LIBRARIES ARCHIVES Signature of Author_ Signature redacted Department of Physics June 7, 22 Certified by Professor Richard P. Binzel Thesis Supervisor Accepted by Signature redacted Professor Timothy L. Grove Chairman, Undergraduate Thesis Committee

2 * MIT~uraries 77 Massachusetts Avenue Cambridge, MA 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.

3 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.

4 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.

5 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 1 4. Discussion Additional Comments Conclusion 37 References 39

6 5 List of Tables and Figures Figure 4-1: Figure 4-2: Figure 4-3: Figure 4-4: Figure 4-5: Figure 4-6: Figure 4-7: Figure 4-8: Figure 4-9: Figure 4-1: Figure 4-11: Figure 4-12: Figure 4-13: Figure 4-14: Figure 4-15: Table 4-1: Figure 4-16: Figure 4-17: Figure 4-18: Figure 4-19: Table 4-2: Wavelength Wavelength Wavelength Wavelength Wavelength Wavelength Wavelength Wavelength Wavelength Wavelength Wavelength Wavelength Wavelength Wavelength Wavelength vs. vs. vs. vs. vs. vs. vs. vs. vs. vs. vs. vs. vs. vs. vs. reflectance reflectance reflectance reflectance reflectance reflectance reflectance reflectance reflectance reflectance reflectance reflectance for for for Acamas RL RS1. for 9694 Lycomedes. for 9713 Oceax. for RN11. for TS2. for TX11 for QF198. for XN9. for 1143 Odysseus. for 1749 Telamon. reflectance for 292 Automedon. reflectance for 3317 Paris. reflectance for 3451 Mentor. Table of the asteroids and their slopes. Plot of perihelion distance 'q' vs. slope 'm' for all asteroids. Plot of inclination 'i' vs. slope 'i' for all asteroids. Plot of eccentricity 'e' vs. slope 'm' for all asteroids. Plot of H magnitude vs. slope 'm' for all asteroids. Table of orbital components and correlation coefficients. Figure 4-2: Plot of asteroids in numerical order vs. slope

7 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 196. More than one century earlier, J.L. LaGrange proved that in the restricted three-body problem, regions of stability exist at +6 and -6* 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

8 7 approximately 3 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 CH 4, 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 198). 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, 211 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.4 to.7 micrometers). These characteristics may be due to progressive chemical and temperature effects among hydrocarbons (Dumas et al. 1998).

9 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.

10 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.

11 1 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.4-.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 15km (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. Twodimensional 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. The wavelength calibration came from a lamp of known source with known

12 11 wavelengths. The fourth step of this operation was to use the calibrate function, which took care of the extinction correction, meaning that we have compensated for the effects of the absorption and scattering of light. This was crucial, because extinction depends on wavelength. The next step was to divide each asteroid spectrum by an appropriate standard star spectrum. The standard star used was L93-11 on UT October 1, 21. By doing this, and then normalizing the resulting spectra to.55 im, we produced spectra that could be directly compared with one another. The sixth step was to write an ASCII text file using the function wspectext. This file held the wavelength versus counts information for every asteroid and star that there was data on. Finally, I plotted graphs of wavelength versus reflection for all of the asteroids, which allowed me to determine the slope 'm' through the equation: m =- R(X1) / R(Xo) (eq. 1) where 'R(X)' is the data point value for reflectance at some value X, X 1 is.8pm, and Xo is.55ptm.

13 - MMM 12 Chapter 4 Discussion Upon embarking this project, we formed two distinct hypotheses. The first was that space weathering was responsible for slope changes on the target asteroid population. We thought this was a good possibility because other groups of asteroids have this trend. The second hypothesis was that space weathering does not cause spectral changes on the Trojan asteroids. To test both hypotheses, I plotted the values for reflectance versus wavelength for the ten small asteroids and then for the five larger (SMASS) asteroids.

14 Acamas 3-4 2] C # wavwength (ntcrometers).8.9 I Figure 4-1: Plot of wavelength vs. reflectance for 2594 Acamas.

15 *, RLIO 3 2 C, *.* 4~I A.* 1~ /~*4.4.4 U I wavelength (micrometer) Figure 4-2: Plot of wavelength vs. reflectance for RLIO.

16 *. I MM RS1 9 S U S U S * 4'. * *4 4, 44gI%~~ % 4, 4 * #' V ~-- S * 4, * * S 4 4 -I wavelength (micrometers) 4 - -~.8.9 I Figure 4-3: Plot of wavelength vs. reflectance for RSIO.

17 * Lycomedes I- U * * 4 *b *44 * * , 4 n4~ n I vavelength (Micrometers) I Figure 4-4: Plot of wavelength vs. reflectance for 9694 Lycomedes.

18 Oceax wavelength (mkcromdtmr) Figure 4-5: Plot of wavelength vs. reflectance for 9713 Oceax.

19 RNI1 3, Wavwengt (nicrorntws) Figure 4-6: Plot of wavelength vs. reflectance for RN11.

20 TS *-.3 4 Q5 Q6.7 Q8 G9 amaength(mnckmmtens) Figure 4-7: Plot of wavelength vs. reflectance for TS2.

21 * ITXI 3- U C I- 1 Q3 Q4.5 Q6.7 YuWSeingdh (nriair wrs).8 Q9 1 Figure 4-8: Plot of wavelength vs. reflectance for TX11.

22 CF WNVO M (Wncrrmeters) Figure 4-9: Plot of wavelength vs. reflectance for QF198.

23 * S S 6 * * I *-W toa 4 W 4e Wav- ntomms) Figure 4-1: Plot of wavelength vs. reflectance for XN9.

24 Odysseus C wavelength (m-dcrometers) Figure 4-11: Plot of wavelength vs. reflectance for 1143 Odysseus (SMASS).

25 3, I Telamon wavelength (micrometers) Figure 4-12: Plot of wavelength vs. reflectance for 1749 Telamon (SMASS).

26 Automedon 2 Ai,9W~' ' wavelength (rnicrwmters) Figure 4-13: Plot of wavelength vs. reflectance for 292 Automedon (SMASS).

27 Paris wmvelength (nicromtter) Figure 4-14: Plot of wavelength vs. reflectance for 3317 Paris (SMASS).

28 Mentor U S *I vavelength (nirometers) 1 Figure 4-15: Plot of wavelength vs. reflectance for 3451 Mentor (SMASS).

29 28 Following this, I calculated the slope 'm' (eq. 1) for each object, and then plotted those values against the respective eccentricities, inclinations, semi-major axes, and H magnitudes. ASTEROID SLOPE H MAGNITUDE SOURCE 2594 Acamas 1, New RL New RS1. 1, New 9694 Lvcomedes New 9713 Oceax 1, New 12318RN11, New TS2 1, Ne-w T111, New QF198 1, New XN9 1, New 1143 Odysseus 1, SMASS 1749 Telamon SMASS 292- Automedon SM~ASS 3317 Padis 1,1735 8,3 SMASS 3451 Mentor 1, SMASS Table 4-1: Slopes and H magnitudes of asteroids, and their source.

30 Wthesis data I*snass data - perihelion distance vs. slope 1.6 K- K2 z t _-._ _I_ q 5.1 Figure 4-16: Plot of perihelion distance 'q' vs. slope 'i' for all asteroids.

31 3 W thesis data 17- U smass data inclination vs. slope I I& to 1.3- I Figure 4-17: Plot of inclination 'i' vs slope 'i' for all asteroids.

32 *thesis data m smass data eccentricity vs. slope I T -_- 1.4 T IV I I 1.3 A L I- ~ Figure 4-18: Plot of eccentricity 'e' vs. slope 'i' for all asteroids.

33 * thesis data * siass data H magnitude vs. slope a U' 1.3 I I T- _ I ii H mag Figure 4-19: Plot of H magnitude 'H mag' vs. slope 'im' for all asteroids.

34 33 In other asteroid families, space weathering events, such as micrometeorite impacts, cause spectral changes. This makes the slopes of the asteroids redder (corresponding to a higher slope value). Larger asteroids are older, and therefore also redder (because there is more time for space weathering to act upon them). Because of this, we expected negative correlation coefficients for H magnitude as well as perihelion distance, and positive correlation coefficients for eccentricity and inclination. The following is a table of correlation coefficients for each of the orbital components of concern. ORBITAL COMPONENT CORRELATION COEFFICIENT H Magnitude Perihelion Distance Inclination Eccentricity Table 4-2: Correlation coefficients for slopes and orbital components. Equations for determining correlation coefficients assume small to no error, with error responsible for lowering the value of a correlation coefficient. This can make borderline cases fall below a value for which they would otherwise be highly correlated. Taking the degrees of freedom into account, perihelion distance (q) is not correlated at the 95% level, while eccentricity is almost correlated at the 95% level, and H magnitude and inclination are significantly correlated at the 95% level.

35 34 In order to see whether or not good correlation coefficients can happen by chance or not, I constructed a graph that should show no correlation at all. This is of the asteroids in numerical order verses slope, as is shown below. Numencal Order vs. Slope 1.61 A A IA A o 1.3 A A A A. A A 1.1 A astroids In numerical order Figure 4-2: Plot of asteroids in numerical order vs. slope.

36 35 The correlation coefficient for this graph is very low, only This means that the numerical order of the asteroids and their slopes have nothing to do with one another. This also tells us that if the orbital elements we graphed were not related to the slopes, we should expect similar correlation coefficients. Since our correlation coefficients are significantly higher, it is reasonable to assume that the slopes are somewhat dependent upon the H magnitude, eccentricity, inclination, and perihelion distance of the asteroids. There are two ways to explain the positive correlation coefficients that we thought would be negative. First, the error bars are large enough that the correlation coefficient does not tell us what's true or not. Second, space weathering might not be creating the slopes. Instead, it may be composition causing it. While the physical processes going on with these bodies should be essentially the same as with the inner belt asteroids, the way the surface compositions and their spectra are affected may be different. So it is possible that the slope correlation with size is due to a real compositional difference, or maybe space weathering acts differently upon the Trojan asteroids. In addition, when organic surfaces (such as the surfaces of the Trojans) age, they first become more red, and later they start to become darker (Binzel, personal communication). When they darken, they become grayer, which means that they are less red. This means that very young surfaces and very old surfaces could both be much less red than the potentially very red middle aged surfaces.

37 36 Even though we got the positive correlation values that we expected for inclination and eccentricity, there is still no obvious trendline. Though this could be due to a lack of space weathering, it is more likely due to the large error bars and the small data set.

38 37 Chapter 5 Additional Comments Upon inspection of the graphs of wavelength versus reflectance, one may have noticed the apparent absorption feature in 1988 RS1 (Figure 4-3) between.8 and.9 micrometers. There are a variety of explanations for the feature. It is possible that the feature is not real. The airmass match between the standard star and asteroid were good though, so it's most likely not due to atmospheric effects. Also, a preliminary second reduction yielded the same results, so there was no error in the reduction process. Finally, something may have occurred at the telescope, but there is no record of anything abnormal in the log to back this up. Therefore, it may in fact be a real absorption feature that would be due to the asteroid's composition. However, this would require further study and is beyond the scope of this paper.

39 38 Chapter 6 Conclusion Trojan asteroids are D-type asteroids that orbit at Jupiter's L4 and L5 LaGrange points. D-type asteroids are red-sloped, so in order to see a correlation of size and spectra, it was necessary to collect spectra of very differently sized asteroids. To determine whether or not spectral slopes and sizes are related for Trojan asteroids, I plotted slope versus H magnitude, slope versus inclination, slope versus eccentricity, and slope versus perihelion distance, and then examined the respective correlation coefficients. We expected negative correlation coefficients for H magnitude and perihelion distance since higher H magnitude values and larger perihelion distances correspond to smaller asteroids, and therefore less steep slopes. Positive correlation coefficients were expected for inclination and eccentricity for similar reasons. The actual correlation coefficients did not match what we expected. All four orbital components had positive values, and only H magnitude and inclination were correlated at the 95% level. Out of the four graphs produced, the only one of which met our expectations was slope versus inclination. This may have happened for one of two reasons. First, the correlation coefficients do not take the large error bars into account, preventing us from getting very accurate numbers for the correlation coefficients. Because of this, the correlation coefficients do not accurately depict what is happening with the asteroids. Also,

40 39 if our data set were larger, then we would have a more obvious trendline. Second, the spectral slopes might not be affected by space weathering events. Though the same physical processes happen on Trojans as any other asteroids, surface compositions vary by asteroid type, so the way that the surface compositions are affected by space weathering might also be different. To make any definitive conclusions, the error bars would need to be smaller or there would have to be a correlation coefficient equation that takes the error bars into account. With this, we might find that space weathering does indeed affect Trojans the same way as it affects S-type asteroids.

41 4 References Barucci et al., Compositional Surface Variety Among the Centaurs. Astronomical Journal 117, Binzel, R.P., Sauter, L.M Trojan, Hilda, and Cybele Asteroids: New Lightcurve Observations and Analysis. Icarus 95, Conversion of Absolute Magnitude to Diameter. (n.d.) Retrieved April 12, 22, at Dumas et al Near Infrared Spectroscopy of Low-Albedo Surfaces of the Solar System: Search for the Spectral Signature of Dark Material. Icarus 133, Fink et al., The Steep Red Spectrum of 1992 AD: An Asteroid Covered With Organic Material? Icarus 97, Gradie, J., Veverka, J The Composition of Trojan Asteroids. Nature 283, Hartman, W., Tholen, D Comet Nuclei and Trojan Asteroids: A New Link and a Possible Mechanism for Comet Splittings. Icarus 86, Jewitt, D Kuiper Belt Objects. Annual Review of Earth and Planetary Sciences 27, Jewitt et al. 2. The Colors of KBOs from Keck and Subaru. American Astronomical Society DPS meeting #32, #2.7. Lazzarin, M., Barucci, M.A Spectroscopic Investigation of the Centaurs. American Astronomical Society DPS meeting #3, #51.P12. List of Centaurs and Scattered-Disk Objects. (n.d.). Retrieved November 15, 21, at List of Jupiter Trojans. (n.d.). Retrieved November 15, 21, at Shoemaker et al., Trojan Asteroids: Populations, Dynamical Structure and Origin of the L4 and L5 Swarms. Asteroids i, Spaceflight Now. Students Find Centaurs and Unique Asteroids. (2). Retrieved November 15, 21 at 12centaurs/ Tholen, D.J., Barucci, M.A Asteroid Taxonomy. Asteroids 11, Xu, S., CCD Photometry and Spectroscopy of Small Main Belt Asteroids. Ph. D. Thesis, Massachusetts Institute of Technology. Xu et al., Small Main Belt Asteroid Spectroscopic Survey: Initial Results. Icarus 115, 1-35.