Composition of Near-Earth Asteroid (4179) Toutatis

Surface composition of near-Earth asteroid (4179) Toutatis is consistent with an undifferentiated L-chondrite composition. This is inconsistent with early observations that suggested high pyroxene iron content and a differentiated body.Comment: 15 pages, 2 figures 1 table. Accepted for publication in Icaru

Phase Angle Effects on 3-micron Absorption Band on Ceres: Implications for Dawn Mission

Phase angle-induced spectral effects are important to characterize since they affect spectral band parameters such as band depth and band center, and therefore skew mineralogical interpretations of planetary bodies via reflectance spectroscopy. Dwarf planet (1) Ceres is the next target of NASA's Dawn mission, which is expected to arrive in March 2015. The visible and near-infrared mapping spectrometer (VIR) onboard Dawn has the spatial and spectral range to characterize the surface between 0.25-5.0 microns. Ceres has an absorption feature at 3.0 microns due to hydroxyl- and/or water-bearing minerals (e.g. Lebofsky et al. 1981, Rivkin et al. 2003). We analyzed phase angle-induced spectral effects on the 3-micron absorption band on Ceres using spectra measured with the long-wavelength cross-dispersed (LXD: 1.9-4.2 microns) mode of the SpeX spectrograph/imager at the NASA Infrared Telescope Facility (IRTF). Ceres LXD spectra were measured at different phase angles ranging from 0.7o to 22o. We found that the band center slightly increases from 3.06 microns at lower phase angles (0.7o and 6o) to 3.07 microns at higher phase angles (11 o and 22o), the band depth decreases by ~20% from lower phase angles to higher phase angles, and the band area decreases by ~25% from lower phase angles to higher phase angles. Our results will have implications for constraining the abundance of OH on the surface of Ceres from VIR spectral data, which will be acquired by Dawn starting spring 2015.Comment: 12 pages, 1 figure, 2 table

Exploring Exogenic Sources for the Olivine on Asteroid (4) Vesta

The detection of olivine on Vesta is interesting because it may provide critical insights into planetary differentiation early in our Solar System's history. Ground-based and Hubble Space Telescope (HST) observations of asteroid (4) Vesta have suggested the presence of olivine on the surface. These observations were reinforced by the discovery of olivine-rich HED meteorites from Vesta in recent years. However, analysis of data from NASA's Dawn spacecraft has shown that this olivine-bearing unit is actually impact melt in the ejecta of Oppia crater. The lack of widespread mantle olivine, exposed during the formation of the 19 km deep Rheasilvia basin on Vesta's South Pole, further complicated this picture. Ammannito et al., (2013a) reported the discovery of local scale olivine-rich units in the form of excavated material from the mantle using the Visible and InfraRed spectrometer (VIR) on Dawn. Here we explore alternative sources for the olivine in the northern hemisphere of Vesta by reanalyzing the data from the VIR instrument using laboratory spectral measurements of meteorites. We suggest that these olivine exposures could be explained by the delivery of olivine-rich exogenic material. Based on our spectral band parameters analysis, the lack of correlation between the location of these olivine-rich terrains and possible mantle-excavating events, and supported by observations of HED meteorites, we propose that a probable source for olivine seen in the northern hemisphere are remnants of impactors made of olivine-rich meteorites. Best match suggests these units are HED material mixed with either ordinary chondrites, or with some olivine-dominated meteorites such as R-chondrites.Comment: 62 pages, 12 figures, 4 tables; Icarus, Available online 30 January 2015, ISSN 0019-1035, http://dx.doi.org/10.1016/j.icarus.2015.01.01

Ground-based Characterization of Hayabusa2 Mission Target Asteroid 162173 Ryugu: Constraining Mineralogical Composition in Preparation for Spacecraft Operations

Asteroids that are targets of spacecraft missions are interesting because they present us with an opportunity to validate ground-based spectral observations. One such object is near-Earth asteroid (NEA) (162173) Ryugu, which is the target of the Japanese Space Agency's (JAXA) Hayabusa2 sample return mission. We observed Ryugu using the 3-m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii, on July 13, 2016 to constrain the object's surface composition, meteorite analogs, and link to other asteroids in the main belt and NEA populations. We also modeled its photometric properties using archival data. Using the Lommel-Seeliger model we computed the predicted flux for Ryugu at a wide range of viewing geometries as well as albedo quantities such as geometric albedo, phase integral, and spherical Bond albedo. Our computed albedo quantities are consistent with results from Ishiguro et al. (2014). Our spectral analysis has found a near-perfect match between our spectrum of Ryugu and those of NEA (85275) 1994 LY and Mars-crossing asteroid (316720) 1998 BE7, suggesting that their surface regoliths have similar composition. We compared Ryugu's spectrum with that of main belt asteroid (302) Clarissa, the largest asteroid in the Clarissa asteroid family, suggested as a possible source of Ryugu by Campins et al. (2013). We found that the spectrum of Clarissa shows significant differences with our spectrum of Ryugu, but it is similar to the spectrum obtained by Moskovitz et al. (2013). The best possible meteorite analogs for our spectrum of Ryugu are two CM2 carbonaceous chondrites, Mighei and ALH83100.Comment: 23 pages, 7 figures, 4 tables, accepted in Monthly Notices of the Royal Astronomical Society Main Journa

Lithologic Mapping of HED Terrains on Vesta using Dawn Framing Camera Color Data

The surface composition of Vesta, the most massive intact basaltic object in the asteroid belt, is interesting because it provides us with an insight into magmatic differentiation of planetesimals that eventually coalesced to form the terrestrial planets. The distribution of lithologic and compositional units on the surface of Vesta provides important constraints on its petrologic evolution, impact history and its relationship with Vestoids and howardite-eucrite-diogenite (HED) meteorites. Using color parameters (band tilt and band curvature) originally developed for analyzing lunar data, we have identified and mapped HED terrains on Vesta in Dawn Framing Camera (FC) color data. The average color spectrum of Vesta is identical to that of howardite regions, suggesting an extensive mixing of surface regolith due to impact gardening over the course of solar system history. Our results confirm the hemispherical dichotomy (east-west and north-south) in albedo/color/composition that has been observed by earlier studies. The presence of diogenite-rich material in the southern hemisphere suggests that it was excavated during the formation of the Rheasilvia and Veneneia basins. Our lithologic mapping of HED regions provides direct evidence for magmatic evolution of Vesta with diogenite units in Rheasilvia forming the lower crust of a differentiated object.Comment: Accepted for Meteoritics and Planetary Science special issue for Composition of Vesta/Dawn Missio

Characterizing Volatiles and Organics on Asteroid (162173) Ryugu

Outline: Overview of Hayabusa2 mission and its target asteroid Ryugu; Spectroscopic laboratory measurements under asteroid-like conditions of carbonaceous chondrites; Recent ground-based observations of carbonaceous asteroids relevant to Ryugu; Implications of meteorite spectroscopic measurements & asteroid observations for Hayabusa2s observations of Ryugu

Chelyabinsk meteorite explains unusual spectral properties of Baptistina Asteroid Family

We investigated the spectral and compositional properties of Chelyabinsk meteorite to identify its possible parent body in the main asteroid belt. Our analysis shows that the meteorite contains two spectrally distinct but compositionally indistinguishable components of LL5 chondrite and shock blackened/impact melt material. Our X-ray diffraction analysis confirms that the two lithologies of the Chelyabinsk meteorite are extremely similar in modal mineralogy. The meteorite is compositionally similar to LL chondrite and its most probable parent asteroid in the main belt is a member of the Flora family. Intimate mixture of LL5 chondrite and shock blackened/impact melt material from Chelyabinsk provides a spectral match with (8) Flora, the largest asteroid in the Flora family. The Baptistina family and Flora family overlap each other in dynamical space. Mineralogical analysis of (298) Baptistina and 9 small family members shows that their surface compositions are similar to LL chondrites, although their absorption bands are subdued and albedos lower when compared to typical S-type asteroids. A range of intimate mixtures of LL5 chondrite and shock blackened/impact melt material from Chelyabinsk provides spectral matches for all these BAF members. We suggest that the presence of a significant shock/impact melt component in the surface regolith of BAF members could be the cause of lower albedo and subdued absorption bands. The parent asteroid of BAF was either a member of the Flora family or had the same basic composition as the Floras (LL Chondrite). The shock pressures produced during the impact event generated enough impact melt or shock blackening to alter the spectral properties of BAF, but keep the BAF composition largely unchanged.Comment: 48 pages, 11 figures, 7 table