Michael S. Kelley^a,1,2, Michael J. Gaffey^b,1, Vishnu Reddy^c,1, Juan A. Sanchez^d,1

^aDepartment of Geology and Geography, Georgia Southern University, Statesboro, Georgia 30460
^bDepartment of Space Studies, University of North Dakota, Grand Forks, North Dakota 58202
^cPlanetary Science Institute, Tucson, Arizona 85719
^dMax Planck Institute for Solar System Research, Katlenburg-Lindau, Germany
¹Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under Cooperative Agreement No. NNX08AE38A with the National Aeronautics and Space Administration, Science Mission Directorate, Planetary Astronomy Program.
²Currently at Science Mission Directorate, NASA Headquarters, Washington DC

Near-Earth asteroids (NEAs) are interesting as both a threat to the Earth and as the immediate sources for most meteorites. We observed NEA (4953) 1990 MU using the NASA Infrared Telescope Facility (IRTF) and University of Hawaii (UH) telescopes on Mauna Kea to constrain its surface composition and origin. The surface composition of 1990 MU is similar to ordinary chondrites (H chondrites). The calculated olivine and pyroxene chemistry of 1990 MU (Fa_13.5±1.3 and Fs_12.7±1.4) are consistent with the olivine and pyroxene chemistry ranges for H chondrites (Fa_15-21 and Fs_13-19) (Dunn et al. 2010), although the estimated Fa value is at the lower end of H chondrite range. The olivine abundance ratio of 1990 MU (0.57±0.03) is slightly higher but not inconsistent with H chondrites (0.47-0.55±0.03). The radar circular polarization ratio (same circular polarization state or SC/opposite circular polarization state or OC) (Benner et al. 2008) of 1990 MU is 0.36±0.03, which is higher than the mean SC/OC ratio for S-type NEAs (0.270±0.079). 1990 MU SC/OC value is also higher than those of (25143) Itokawa (0.27±0.04), (4179) Toutatis (0.29±0.01) and (433) Eros (0.28±0.06) suggesting a rougher surface at decimeter scale (Benner et al. 2008). We constrained the diameter of 1990 MU (4.4 km) using the average albedo at 0.55 μm of H chondrites (0.21) and absolute magnitude (H) of 14.1 (Flower and Chillemi, 1992). This diameter is higher than the 2.8 km value from Harris et al. (2011) using an albedo of 0.52 for 1990 MU. This albedo value is unusually high for H chondrites, which have an albedo range of 0.12-0.30. We compared olivine and pyroxene chemistries of 1990 MU with main belt asteroid (6) Hebe, probable parent body of H chondrite meteorites and IIE irons (Gaffey and Gilbert, 1998), and found that 1990 MU has more high-calcium pyroxene than Hebe. Fayalite and ferrosilite values of the two asteroids are consistent with H chondrites but don’t overlap each other. The differences could be due to compositional variations observed on Hebe by Gaffey and Gilbert (1998), although the observed rotational variation in spectral parameters does not match well with those of 1998 MU.

Reference
Kelley MS, Gaffey MJ, Reddy V and Sanchez JA (2014) Surface Composition of Near-Earth Asteroid (4953) 1990 MU: Possible Fragment of (6) Hebe. Icarus
[doi:10.1016/j.icarus.2014.01.015]
Copyright Elsevier

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