Pyroxene Mineralogies of Near-earth Vestoids

Introduction: For any near-Earth asteroid (NEA) on a possible collision course with Earth, a number of factors must be determined to evaluate the object’s potential hazard. One of these factors is the object’s mineralogy, which can be used to estimate the density and, therefore, the mass of the object. This work discusses how to determine the mineralogies of NEAs that have reflectance spectra similar to howardites, eucrites, and diogenites (HEDs). Among near-Earth asteroids, we have identified seven HED-like, pyroxene-dominated objects (3908 Nyx, 4055 Magellan, (5604) 1992 FE, (6611) 1993 VW, (52750) 1998 KK17, (88188) 2000 XH44, 2005 WX) using spectra acquired during the MIT-UH-IRTF Joint Campaign for NEO Spectral Reconnaissance. The average pyroxene mineralogies of these seven asteroids are estimated through the application of two techniques that derive pyroxene mineralogies from an object’s Band I and II centers. All band centers are corrected for the effects of the low surface temperatures of asteroids. We show in detail the steps needed to calculate the pyroxene mineralogies of asteroids and the uncertainties in the calculations. We hope to identify possible meteoritic analogs for near-Earth V-type asteroids, which we speculatively call “Vestoids” because of their possible relationship to Vesta. Data: All objects (Figure 1) were observed using SpeX, a medium-resolution near-infrared spectrograph [1] on the NASA Infrared Telescope Facility (IRTF) located on Mauna Kea. Appropriate solar analog standard stars were used to produce the final reflectance spectra, normalized to unity at 0.55 μm. Asteroid (88188) 2000 XH44 was observed on two different dates with the second set of observations having higher signal-to-noise. Only the 2000 XH44 spectrum with the higher signal-to-noise is plotted in Figure 1. Visible data are available for four of the objects. All objects (Figure 1) have the characteristic pyroxene absorption features typical of HEDs. All objects have strong pyroxene bands centered near ~0.94 μm and ~1.95 μm. The objects with visible spectra all have very strong UV (ultraviolet) features. Pyroxene Mineralogies: To calculate pyroxene mineralogies, band centers must be determined. Band I centers for the asteroid spectra with visible data were calculated using the method of Storm et al. [2]. Only SpeX data were fit. A linear slope that was derived from a straight line tangent to the two reflectance peaks on each side of Band I was divided out. For asteroids without visible data, no slope was divided out and only the Band I minima were calculated for those objects. No linear slopes were divided out to calculate Band II centers since none of the objects appeared to be significantly sloped over this wavelength region and it is unclear if the bands extend far past 2.5 μm. We believe our calculated Band II minima are equivalent to the Band II centers.