P31A-1878: Toward a taxonomy of asteroid spectra in the 3- µm region

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Authors: Andrew Rivkin1, Ellen S Howell2, Joshua P Emery3, Eric L Volquardsen4, Francesca E DeMeo5

Author Institutions: 1. JHU/APL, Laurel, MD, USA; 2. Arecibo Observatory, Arecibo, Puerto Rico; 3. U. Tennessee, Knoxville, TN, USA; 4. NASA IRTF, Hilo, HI, USA; 5. MIT, Cambridge, MA, USA

Introduction and observations: The 3- µm spectral region is critical for measuring the presence and distribution of volatiles. On asteroids, absorptions due to OH in minerals and H2O as ice have been reported in this spectral range by several worker since the 1970s (for instance, [1-5]) With the installation of the SpeX instrument at the NASA IRTF [6], observations of asteroids in the 2-4 µm region (“LXD mode”ù) are now commonplace. As a result of the ease of observations, a de facto SpeX LXD-mode asteroid survey has been ongoing for the past decade. At this writing, 97 low-albedo asteroids (C complex, D and T asteroids, and low-albedo members of the X complex), which are the focus of this work, have been observed 142 times. Band shapes and classes: A range of 3- µm band shapes is seen in the asteroid population. Several “Type examples”ù of the 3- µm classes have been identified: “Pallas types”ù, with roughly linear band shapes and band minima in the 2.5-2.85 µm region (where atmospheric water vapor precludes ground-based observations), “Ceres types”ù with a narrow band centered near 3.05 µm, and “Themis types”ù with a broader, rounded absorption near 3.1 µm. Both Ceres and Themis have had compositional fits made to their spectra [4,12], but while we expect objects with similar spectra to share similar compositions, formal fits to other Ceres- and Themis-type objects remain future work. Takir and Emery find similar band shapes in their sample, though they use different names for their groupings [5]. We are moving toward developing more quantifiable criteria for classifications. One possible approach involves the band depths at 3.2 and 2.9 µm. When these are plotted against each other, an array of points with similar band shapes but differing band depths is found in both the asteroid data and mineralogical and meteoritical spectra [13,14], all appearing as Pallas types by inspection. In addition to that array, a second cluster of points is found where the 3.2 µm band depth ‰¥ 2.9 µm band depth: the non-Pallas types. There is an excellent correlation between the Ch asteroids in the Bus-DeMeo taxonomy and the Pallas types: 29 of 31 Ch objects in the sample are clearly Pallas-types, with the remaining two ambiguous but likely Pallas-types. Future Work:The use of band depth plots shows promise of separating these spectral types from one another, but more work is required to identify the optimal wavelengths. In addition, we have begun exploring the use of principal component analysis (PCA), as has been done for shorter-wavelength data. We also hope to use data from other asteroids and perhaps icy satellites to help guide our analysis. References [1] Lebofsky, L. A. (1978), MNRAS 182. [2] Jones, T. D. et al. (1990), Icarus 88. [3]Rivkin, A. S. et al. (2002), in Asteroids III, U. Arizona Press, Tucson. [4] Rivkin, A. S. and Emery, J. P. (2010), Nature 464. [5] Takir, D. and Emery, J. P. (2012), Icarus 219. [6] Rayner, J. T. et al. (1998), Proc. SPIE 468-479. [7] Tholen, D. J. (1984), PhD Thesis, University of Arizona, 1984. [8] Bus, S. J. and Binzel, R. P. (2002), Icarus 158. [9] DeMeo, F. E. et al. (2009), Icarus 202. [10] Chapman, C. R. (1976), GCA 40. [11] Rivkin, A. S et al. (2011), Icarus 216. [12] Milliken, R. E. and Rivkin, A. S. (2009), Nature Geos. 2. [13] Salisbury, J. W. et al. (1991), Icarus 92. [14] Hiroi, T. et al. (1996), Met. Plan. Sci, 31.

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