62 research outputs found
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
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