THE DIRTY DOZEN: NIR SPECTRAL and MINERALOGICAL INTEPRETATIONS for 12 Vp-TYPE ASTEROIDS AS CANDIDATE VESTOIDS

46th Lunar and Planetary Science Conference (2015) 1775.pdf

THE DIRTY DOZEN: NIR SPECTRAL AND MINERALOGICAL INTEPRETATIONS FOR 12 Vp-TYPE ASTEROIDS AS CANDIDATE VESTOIDS. P. S. Hardersen1, V. Reddy2, and R. Roberts1. 1University of North Dakota, Department of Space Studies, 4149 University Avenue, Stop 9008, Grand Forks, ND 58202-9008, Harder- [email protected]; 2Planetary Science Institute, Tucson, AZ 85719.

Introduction: Asteroid taxonomic classifications (5235) Jean-Loup, (5560) Amytis, (6331) 1992 FZ1, are usually one of the first efforts to broadly constrain (6976) Kanatsu, (17469) 1991 BT, (29796) 1999 the characteristics of individual asteroids. Various tax- CW77, and (30872) 1992 EM17. The combination of onomic schemes have been devised and they can usu- existing evidence suggests that all eight of these aster- ally be applied to a relatively large number of asteroids oids are likely Vestoids and have analogs associated [1,2]. While useful for broadly grouping asteroids with with the howardite or eucrite meteorites. This initial similar optical properties, they are not designed to pro- work showed that these eight asteroids are likely de- vide rigorous compositional or mineralogic determina- rived from the surface of (4) Vesta. tions [1], nor are they able to associate an asteroid with Observations and Methodology: All observations a specific meteorite type. for the 12 asteroids reported here were taken at the This work is a continuing effort to better constrain NASA Infrared Telescope Facility (IRTF) on the the abundance of basaltic asteroids in the main asteroid nights of August 25 and 26, and September 3 and 4, belt. At least two discrete populations of basaltic aster- 2014 (UT). Near-infrared (NIR) spectra were obtained oids probably exist: 1) those asteroids that originated using the upgraded SpeX spectrograph utilizing the from the surface of (4) Vesta that were ejected via col- prism mode (~0.7-2.5 microns), the 0.8” wide slit, and lisions early in solar system history (i.e., Vestoids), with the dichroic open. Weather conditions were clear and 2) those basaltic asteroids originating from unique, with low relative humidity for all four nights. non-Vesta basaltic parent bodies. The Vestoid group The Vp-type asteroids in this work include (2011) has been primarily characterized via taxonomic classi- Veteraniya, (3648) Raffinetti, (5875) Kuga, (9147) fication (i.e., V-type), family identification, and spatial Kourakuen, (9553) Colas, (8149) Ruff, (11699) 1998 proximity to (4) Vesta [3,4]. A much smaller subset of FL105, (12073) Larimer, (15237) 1988 RL6, (31414) Vestoids has been mineralogically characterized and Rotaryusa, (32940) 1995 UW4, and (40733) 1999 associated with the HED meteorites [5,6,7]. SM17. See Table 1 for details of the observing circum- In contrast, knowledge of the abundance of outer stances. See Table 2 for orbital element information for main belt basaltic asteroids is very poorly constrained, each asteroid. but suggested to be no greater than the mass of the Data reduction took place utilizing a new version Vestoid population [8]. Few candidate outer belt basal- of Spextool to support the upgraded SpeX spectro- tic asteroids have been identified and include (1459) graph and array [15]. Spextool allows spectral back- Magnya, (7472) Kumakiri, and (10537) 1991 RY16 ground sky subtraction, sub-channel pixel shifting to [9,10]. Analysis of the distribution of V-type asteroids align spectra, telluric corrections, and spectral averag- in the main belt suggests the existing basaltic asteroid ing and display. Telluric corrections of the water vapor population should dominate near (4) Vesta in the as- bands at ~1.4- and ~1.9-microns are the most critical to teroid belt Sunward of the 3:1 mean-motion resonance deriving a high-quality average asteroid NIR reflec- at 2.5 AU [11]. tance spectrum. Each asteroid is typically paired with a Project Details: The original data set for this re- G-type main sequence star that is within ~5 degrees of search includes 650 asteroids that are members of the the asteroid on the sky to empirically model the atmos- Vp taxonomic class and have effective diameter (Deff) phere. A solar analog star is also observed to correct and albedo estimates derived from Wide Field Infrared the overall asteroid NIR spectral slope when non-G2V Survey (WISE) data [12,13]. This project will attempt stars are used for telluric corrections. The result is the to more rigorously characterize ~125 Vp-type asteroids creation of an average asteroid NIR spectrum that near (4) Vesta and ~14 Vp-type asteroids that are locat- mimics an asteroid’s actual reflectance of sunlight. The ed in the outer main belt beyond 2.5 AU. Our primary data reduction process is summarized as follows: As- efforts are to better constrain the basaltic asteroid pop- teroid/Sun = (Asteroid/Extinction Star) / (Solar Ana- ulation and determine the accuracy of the Vp taxonomy log/Extinction Star). in identifying likely basaltic asteroids. Two analysis methods were utilized to isolate ab- Past Work: [14] reported NIR spectra, pyroxene sorption features, derive continuum-removed features mineral chemistries, and potential HED meteorite ana- through the use of linear continua, to determine ab- logs for eight Vp-type asteroids: (3867) Shiretoko, sorption band centers and band areas, and calculate 46th Lunar and Planetary Science Conference (2015) 1775.pdf

Band Area Ratios (BAR) [16,17,18]. These tools are most applicable to asteroids exhibiting absorption fea- Asteroid a (AU) e i tures caused by the presence of surficial pyroxene (2011) Veteraniya 2.38696 0.14896 6.18976° (3648) Raffinetti 2.41586 0.10598 7.89430° and/or olivine group minerals. Basalt is spectrally (5875) Kuga 2.37928 0.05028 6.46969° dominated by pyroxene and mineralogically includes (8149) Ruff 2.32341 0.14183 6.58176° pyroxene and plagioclase feldspar as the only major (9147) Kourakuen 2.19155 0.10546 5.81666° minerals. Temperature corrections to continuum- (9553) Colas 2.19854 0.11659 1.91970° removed absorption band centers were applied using (11699) 1998 FL105 2.40446 0.08206 5.08340° the techniques from [19,20]. Laboratory-based mineral (12073) Larimer 2.41650 0.08300 6.23803° (15237) 1988 RL 2.39221 0.14859 7.32786° and meteorite calibrations from [19,21] were used to 6 (31414) Rotaryusa 2.26053 0.15169 5.37720° constrain the average surface pyroxene chemistries for (32940) 1995 UW 4 2.18896 0.13507 8.11072° each asteroid, which were then compared to pyroxene (40733) 1999 SM17 2.41738 0.14789 7.50672° chemistries for the HED meteorites to determine the (4) Vesta 2.36154 0.09002 7.13394° mostly likely meteorite analog for each asteroid. Table 2. Orbital elements for 12 Vp-type asteroids and Anticipated Results: We will present the average (4) Vesta. Orbital elements from JPL Horizons ephem- NIR reflectance spectra for the 12 asteroids, along with eris service. spectral mineral identifications, and associated band centers, areas, and BARs. Average pyroxene chemis- References: [1] Tholen D. J. (1984) Ph.D. thesis, tries will also be determined for those asteroids exhib- U. Arizona. [2] Bus S. J. and Binzel R. P. (2002) Ica- iting pyroxene-dominated features, which will then be rus 158, 146-177. [3] Zappala V. et al. (1995) Icarus, compared to pyroxene chemistries of the HED meteor- 116, 291-314. [4] Binzel R. P. and Xu S. (1993) Sci- ites. ence, 260, 186-191. [5] Burbine T. H. et al. (2001) These results will: 1) likely determine an additional Meteoritics & Planet. Sci., 36, 761-781. [6] Kelley M. number of Vp-type asteroids that can be labeled as like- K. et al. (2003) Icarus, 165, 215-218. [7] Moskovitz N. ly Vestoids derived from (4) Vesta, 2) continue testing A. (2010) Icarus, 208, 773-788. [8] Moskovitz N. A. the accuracy of the Vp taxonomy and its ability to iden- (2008) Icarus, 198, 77-90. [9] Hardersen P. S. et al. tify basaltic asteroids, and 3) possibly identify any (2004) Icarus, 167, 170-177. [10] Duffard R. and Roig asteroids that are not-basaltic. F. (2009) Planet. Space Sci., 57, 229-234. [11] DeMeo Acknowledgements: This work is funded by F. E. and Carry B. (2014) Nature, 505, 629-634. [12] NASA Planetary Astronomy Program grant Carvano J. et al. (2010) Astron. Astrophys., 510, A43. #NNX14AJ37G. We thank the NASA Infrared Tele- [13] Mainzer A. et al. (2012) Astrophys. J., 745, 1-9. scope Facility (IRTF) for their assistance in obtaining [14] Hardersen P. S. et al. (2014) Icarus, 242, 269-282. the data necessary for this research. The authors thank [15] Cushing M. et al. (2014) Astrophys. Source Code Amy Mainzer and the WISE team at JPL for helping Library, record ascl:1404.0017. [16] Reddy V. et al. us define our original sample size for this project and (2011a) Icarus, 216, 184-197. [17] Reddy V. et al. their continuing assistance and support. (2011b) Icarus, 212, 175-179. [18] Lindsay S. S. (2013) DPS, 45, #112.04. [19] Burbine T. H. et al. Asteroid Date (UT) # of spectra V mag (2009) Meteoritics & Planet. Sci., 44, 1331-1341. [20] (2011) Veteraniya 8/26/14 10 15.12 Reddy V. et al. (2012) Icarus, 217, 153-168. [21] (3648) Raffinetti 9/3/14 10 16.23 Gaffey M. J. et al. (2002) Asteroids III, 183-204. (5875) Kuga 9/3/14 10 15.37 (8149) Ruff 9/3/14 10 16.51 (9147) Kourakuen 8/26/14 10 15.59 (9553) Colas 9/3/14 10 16.85

(11699) 1998 FL105 9/4/14 10 16.40 (12073) Larimer 9/3/14 10 16.92

(15237) 1988 RL6 9/3/4/2014 20 16.85 (31414) Rotaryusa 9/3/14 10 16.40

(32940) 1995 UW 4 9/4/14 14 16.53 (40733) 1999 SM17 9/4/14 14 16.80 Table 1. Observational data for the 12 asteroids observed at the NASA IRTF as part of this project.