Reflectance Spectra of Members of Very Young Asteroid

Reflectance Spectra of Members of Very Young Asteroid

Reflectance Spectra of Members of Very Young Asteroid Families

40th Lunar and Planetary Science Conference, The Woodlands TX, USA.

Poster #2258. Poster Session I, Tues., 24 March 2009.

C.R. Chapman, B. Enke, W.J. Merline, D. Nesvorn, P.M. Tamblyn, & E.F. Young (SwRI, Boulder CO)
Introduction: Several asteroid families or clusters have very
young dynamical ages [1]. The Veritas family of C-type
asteroids, Karin cluster within the S-type Koronis family, and the
Iannini cluster (S-type) formed about 8.3, 5.8, and <5 My ago, respectively. A family associated with Datura is extremely young, 0.45 My [2]. If space weathering modifies asteroid spectra on timescales comparable to or longer than several My, then we may expect different spectra for members of these recently formed families than for older families. We use the SpeX instrument on the IRTF (in low-resolution prism mode) to study infrared reflectance spectra of members of these young families. We observe selected young asteroids, controls (e.g. members of the Themis family and non-Karin members of the Koronis family), and G-type comparison stars. Observations: Spectra were obtained on 11 nights between June 2005 and July 2006. Although the sky quality was generally good, we had some difficulty characterizing extinction resulting in slight uncertainties in the overall slopes of spectra. Also there is jitter for faint asteroids beyond 2m and near the m and near the 1.4 and 1.9m water bands. An average spectrum is calculated m and near the m water bands. An average spectrum is calculated for each asteroid each night. Nightly averages are then averaged to yield spectra shown in the bottom half of this poster for asteroids observed on multiple nights. Averages for each asteroid family are shown in white-on-black immediately below. Discussion: The timescale for space weathering processes that modify reflectance spectra of S-type asteroids is an important issue [3]. It can be seen that spectra for the young Iannini and Karin family asteroids are slightly less red (and/or have shallower bands) than typical Koronis members, which may be a subtle reflection of their comparative youth. However, if space weathering modifies ordinary chondrite-like spectra to mature S-type spectra (like the Koronis family), then the Iannini and Karin family asteroids are near the end of their spectral evolution after only a few million years. The very young asteroid Datura is a Q-type with a deep 1m and near the m band and has only begun to evolve toward S-type maturity. Assuming that the Veritas and Themis families are of the same inherent composition, then the different average spectra for these families may reflect space weathering trends. Whereas Themis spectra show a U-like shape in this wavelength range, the young Veritas members tend to show a slightly bluish slope with a hint of an inverted U-like shape, as has been previously noted [4,5]. It is possible that the effects of space weathering are more prominent at these longer wavelengths for C-types than for S-types. PROJECT GOALS: The goals of our project are to study, with various telescopic techniques, the physical properties of members of very young dynamical families. These families, identified by D. Nesvorny and his colleagues, were formed at discrete and well-determined times less than 10 million years ago. Most asteroid families are believed to be hundreds of millions or even billions of years old. Our expectation is that young families, as compared with typical families, may provide clues to processes that presumably take place on relatively short timescales. Processes in which we are interested include space weathering, temporary preservation of near-surface volatiles, satellite formation and evolution, the Yarkovsky Effect (which we can help to calibrate), and understanding the initial/early configurations of asteroid families as a check on hydrocode simulations and other theoretical insights concerning the formation of families. PROJECT APPROACH: Our approach is to observe known members of the Karin cluster (a=2.87 AU, e=0.044, i=2.1), the Veritas family (a=3.17 AU, e=0.065, i=9.3), the Iannini cluster (a=2.64 AU, e=0.267, i=12.2), and younger families like Datura, plus controls (e.g. non-Karin members of the Koronis family, also small members of the C-type Themis family), using a variety of telescopes, instruments, and techniques. Our approach is to make coordinated space-based and groundbased observations of relevant family members, obtaining low- and mediumresolution spectrophotometry, radiometry, adaptive optics imaging, and lightcurve photometry. In this poster we present our final reductions for IR reflectance spectra obtained with SpeX on the IRTF during June 2005 through July 2006. Some members of our team, observing with the IRTF SpeX remotely from the Boulder, Colorado, offices of SwRI, from back-left to front-right: Bill Merline, Clark Chapman, Eliot Young, and Peter Tamblyn. References: [1] Nesvorny D. et al. (2003) Ap.J. 591, 486-49m water bands. An average spectrum is calculated 7. [2] Nesvorny D. et al. (2006) Science 312, 149m water bands. An average spectrum is calculated 0. [3] Chapman C.R. (2004) Ann. Rev. EPS 32, 539m water bands. An average spectrum is calculated -567. [4] Chapman C.R. (2008) ACM #839m water bands. An average spectrum is calculated 1. [5] Ziffer J. (2008) AAS/DPS#40, #60.06. Average spectra for asteroids, grouped by family. All spectra are from 0.8 to 2.45m and near the m, normalized to 1.0 reflectance at 1.5m and near the m (but vertically displaced by 0.2). Within each part of the figure, asteroid numbers from top to bottom are: Datura: 1270. Iannini: 1547, 151032, 4652, 81550, 87239m water bands. An average spectrum is calculated . Karin: 10783, 832. Koronis: 1289m water bands. An average spectrum is calculated , 1423, 167, 208, 2574, 3032, 311, 4863, 5338, 534. Veritas (Part 1): 1079m water bands. An average spectrum is calculated 3, 1086, 15066, 19m water bands. An average spectrum is calculated 845, 2147, 28022, 28546, 29m water bands. An average spectrum is calculated 89m water bands. An average spectrum is calculated 1. Veritas (Part 2): 31743, 49m water bands. An average spectrum is calculated 0, 49m water bands. An average spectrum is calculated 622, 559m water bands. An average spectrum is calculated 2, 559m water bands. An average spectrum is calculated 4, 6343, 7612. Themis: 1027, 2114, 229m water bands. An average spectrum is calculated 7, 359m water bands. An average spectrum is calculated 1, 4470, 468, 49m water bands. An average spectrum is calculated 2, 656, 859m water bands. An average spectrum is calculated 1, 9m water bands. An average spectrum is calculated 0. Veritas Karin Photo by Y Chapman. This work is supported primarily by the NASA Planetary Astronomy Program. Themis Comparison of S-like families: Koronis exhibits trends for slightly more weathering than Iannini (which has a slightly deeper 1m and near the m band, measured between 0.9m water bands. An average spectrum is calculated m and near the m and 1.4m and near the m) or than Karin (less reddened overall). But the differences from Koronis are subtle and not the same for Karin and Iannini. Datura has a deep 1m and near the m band and resembles a mostly unweathered, olivine-rich ordinary chondrite. Koronis Iannini Concluding Thoughts The extremely young Datura family is represented by a fresh, olivine-rich spectrum with a deep 1 micron band, suggesting little space weathering. The small, dynamically young S-type asteroid families (Karin and Iannini) have evolved rather far toward mature S-type reflectance characteristics in the 0.8 2.5 micron region in just a few million years. There may be less rapid spectral evolution exhibited by Veritas family members, provided that their inherent carbonaceous mineralogy is similar to that of Themis family members. For these C-type families, the differences in spectral slope are opposite in our wavelength range from the differences observed in the visible.

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