753 research outputs found
Thermal evolution and sintering of chondritic planetesimals III. Modelling the heat conductivity of porous chondrite material
The construction of models for the internal constitution and the temporal
evolution of large planetesimals, the parent bodies of chondrites, requires
information on the heat conductivity of the complex mixture of minerals and
iron metal found in chondrites. It is attempted to evaluate the heat
conductivity of a multi-component mineral mixture and granular medium from the
heat conductivities of its mixture components. Random mixtures of solids with
chondritic composition and packings of spheres are numerically generated. The
heat conduction equation is solved in high spatial resolution for a test cube
filled with such matter. From the heat flux through the cube the heat
conductivity of the mixture is derived. The model results for porous material
are consistent with data for compacted sandstone, but are at odds with
measurements for H and L chondrites. The discrepancy is traced back to shock
modification of the currently available meteoritic material by impacts on the
parent body over the last 4.5 Ga. This causes numerous micro-cracks that act as
additional barriers for heat transfer. The void structure in meteorites is
different from that which probably existed in the pristine material of the
parent bodies. The results obtained for the heat conductivity of the pristine
material are used for calculating models for the evolution of the H chondrite
parent body which are fitted to the cooling data of a number of H chondrites.
The fit to the data good.Comment: 19 pages, 8 figures, accepted by Astronomy & Astrophysic
Composition of Jupiter irregular satellites sheds light on their origin
Irregular satellites of Jupiter with their highly eccentric, inclined and
distant orbits suggest that their capture took place just before the giant
planet migration. We aim to improve our understanding of the surface
composition of irregular satellites of Jupiter to gain insight into a narrow
time window when our Solar System was forming. We observed three Jovian
irregular satellites, Himalia, Elara, and Carme, using a medium-resolution
0.8-5.5 micro m spectrograph on the National Aeronautics and Space
Administration (NASA) Infrared Telescope Facility (IRTF). Using a linear
spectral unmixing model we have constrained the major mineral phases on the
surface of these three bodies. Our results confirm that the surface of Himalia,
Elara, and Carme are dominated by opaque materials such as those seen in
carbonaceous chondrite meteorites. Our spectral modeling of NIR spectra of
Himalia and Elara confirm that their surface composition is the same and
magnetite is the dominant mineral. A comparison of the spectral shape of
Himalia with the two large main C-type asteroids, Themis (D 176 km) and Europa
(D 352 km), suggests surface composition similar to Europa. The NIR spectrum of
Carme exhibits blue slope up to 1.5 microm and is spectrally distinct from
those of Himalia and Elara. Our model suggests that it is compositionally
similar to amorphous carbon. Himalia and Elara are compositionally similar but
differ significantly from Carme. These results support the hypotheses that the
Jupiter irregular satellites are captured bodies that were subject to further
breakup events and clustered as families based on their similar physical and
surface compositions
Spectral properties and geology of bright and dark material on dwarf planet Ceres
Variations and spatial distributions of bright and dark material on dwarf
planet Ceres play a key role in understanding the processes that have led to
its present surface composition. We define limits for bright and dark material
in order to distinguish them consistently, based on the reflectance of the
average surface using Dawn Framing Camera data. A systematic classification of
four types of bright material is presented based on their spectral properties,
composition, spatial distribution, and association with specific
geomorphological features. We found obvious correlations of reflectance with
spectral shape (slopes) and age; however, this is not unique throughout the
bright spots. Although impact features show generally more extreme reflectance
variations, several areas can only be understood in terms of inhomogeneous
distribution of composition as inferred from Dawn Visible and Infrared
Spectrometer data. Additional material with anomalous composition and spectral
properties are rare. The identification of the composition and origin of the
dark, particularly the darkest material, remains to be explored. The spectral
properties and the morphology of the dark sites suggest an endogenic origin,
but it is not clear whether they are more or less primitive surficial exposures
or excavated subsurface but localized material. The reflectance, spectral
properties, inferred composition, and geologic context collectively suggest
that the bright and dark material tends to gradually change toward the average
surface over time. This could be because of multiple processes, i.e., impact
gardening/space weathering, and lateral mixing, including thermal and aqueous
alteration, accompanied by changes in composition and physical properties such
as grain size, surface temperature, and porosity (compaction).Comment: Meteoritics and Planetary Science; Dawn at Ceres special issu
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
Detection of Rotational Spectral Variation on the M-type asteroid (16) Psyche
The asteroid (16) Psyche is of scientific interest because it contains ~ 1%
of the total mass of the asteroid belt and is thought to be the remnant
metallic core of a protoplanet. Radar observations have indicated the
significant presence of metal on the surface with a small percentage of
silicates. Prior ground-based observations showed rotational variations in the
near-infrared (NIR) spectra and radar albedo of this asteroid. However, no
comprehensive study that combines multi-wavelength data has been conducted so
far. Here we present rotationally resolved NIR spectra (0.7-2.5 microns) of
(16) Psyche obtained with the NASA Infrared Telescope Facility. These data have
been combined with shape models of the asteroid for each rotation phase.
Spectral band parameters extracted from the NIR spectra show that the pyroxene
band center varies from ~ 0.92 to 0.94 microns. Band center values were used to
calculate the pyroxene chemistry of the asteroid, whose average value was found
to be Fs30En65Wo5. Variations in the band depth were also observed, with values
ranging from 1.0 to 1.5%. Using a new laboratory spectral calibration we
estimated an average orthopyroxene content of 6+/-1%. The mass-deficit region
of Psyche, which exhibits the highest radar albedo, also shows the highest
value for spectral slope and the minimum band depth. The spectral
characteristics of Psyche suggest that its parent body did not have the typical
structure expected for a differentiated body or that the sequence of events
that led to its current state was more complex than previously thought.Comment: 21 pages, 8 figures, 2 tables, published in The Astronomical Journa
Evaluation and Reconciliation Education from a Social Innovation Lens: A Case Study of the Haida Gwaii Instituteās Reconciliation Studies Semester
In 2015, the Truth and Reconciliation Commission of Canada released a report highlighting the impacts of residential schools on
Indigenous people, and presented Calls to Action to redress this legacy and move forward on a path of reconciliation. Two years later, in 2017, the Haida Gwaii Institute (HGI) launched the Haida Gwaii Semester in Reconciliation Studies. Since 2010, the HGI has been offering educational programming on Haida Gwaii, British Columbia, the ancestral and unceded territory of the Haida Nation. This research is the result of a three year partnership with the HGI as they piloted and evaluated the Reconciliation Studies Semester (RSS). This work has been guided by the tenets of community-based participatory research (CBPR), which prioritizes relationships as the basis for meaningful research between communities and researchers (Leeuw, Cameron, & Greenwood, 2012). My relationship with the HGI has been fundamental in exploring the RSS and evaluating its strengths and challenges. In this work, the HGI expressed a desire to evaluate the RSS based on the challenges encountered during the program pilot that were not predicted when the program was developed in 2015. I sought to explore the application of social innovation tools for the purposes of program evaluation through document reviews, discussions, participant observation, and five separate visits to Haida Gwaii. I first developed a conceptual framework of best practice, which can theoretically be applied to any organization undertaking transformative education and program evaluation in cross-cultural, complex environments. This framework was developed by exploring four main bodies of literature: systems change and social innovation, transformative learning, critical Indigenous literature, and program evaluation. This framework was applied to the RSS initially without any context, to strictly compare the program to these best practice criteria. Then, I used a multi-level perspective framework to explore niche, regime, and landscape activities which took place between 2015 and 2018 and may have impacted the program (McGowan, Westley, & Tjornbo, 2017). These activities were then overlaid onto the best practice criteria to contextualize the strengths and challenges faced by the HGI when developing and piloting the program. Through this analysis, I determined that the RSS as a program sought to include a variety of perspectives from the Haida and Haida Gwaii communities, and utilized the concepts of Two-Eyed Seeing in the program. Two-Eyed Seeing allows Indigenous and Western epistemologies and pedagogies to be integrated and taught in the classroom without being juxtaposed or compared to one another (Iwama, Marshall, Marshall, & Bartlett, 2009). The most significant challenge faced by the RSS was developing a program evaluation, which they were unable to create simultaneously with curriculum due to capacity and expertise constraints. Finally, the national-level discussions on reconciliation fluctuated widely during this time period, which may have influenced student perception and experience (Laucius, 2017; Liberal Party of Canada, 2015). The findings emphasize the importance of building flexibility into program design, co-developing evaluation and program content, and including local perspectives to contextualize and ground the program in place. As the HGI moves forward, the lessons learned from the RSS will improve programming for the organization and can be used as a template for reconciliation-based education in the future
Asteroid (354) Eleonora: Plucking an odd duck
During a survey of the S-type asteroids, Gaffey et al. (Gaffey, M.J., Bell, J.F., Brown, R.H., Burbine, T.H., Piatek, J., Reed, K.L., Chaky, D.A. [1993]. Icarus 106, 573ā602) identified Asteroid (354) Eleonora as anomalous with a 1 Ī¼m absorption feature ā¼2.5 times stronger than any S-asteroid of comparable size. Subsequent investigation revealed significant differences in the 1 Ī¼m absorption feature between the visible & very near-infrared CCD spectra (Ī» \u3c ā¼1.0 Ī¼m) and other spectral data sets for this asteroid. There were also significant spectral differences among the several CCD survey spectra (SMASS-I, SMASS-II & S3OS2) of Eleonora. These differences could potentially arise from spectral variations across the asteroid surface, from observational phase angle differences, from surface temperature differences, from viewing geometry for a nonspherical body, or from the use of standard stars with deviated to different degrees from a true solar standard. In June 2011 Asteroid (354) Eleonora was observed over two nights using the NASA Infrared Telescope Facility (IRTF) at Mauna Kea Observatory in order to test these possible scenarios and to better understand the nature and history of Eleonora and its relationships to other asteroids and to the meteorites. Analysis of this data set has eliminated the following options as the cause of the differences in the 1 Ī¼m absorption feature within the CCD data sets and between the CCD data sets and the other spectral data: (1) rotational spectral variations; (2) variation in surface composition with latitude; (3) observation phase; (4) surface temperature variations with differing heliocentric distance in the asteroidās elliptical orbit; (5) spectral effects of viewing geometry for a nonspherical body; and (6) differences in spectral standard stars. We conclude that none of the CCD spectra of (354) Eleonora are reliable, and that within the limits of their spectral coverage, analyses of the three CCD spectra would produce significantly different ā and generally unreliable ā indications of surface mineralogy. An effort needs to be made to determine whether ābadā CCD spectra are rare with the case of (354) Eleonora being an uncommon occurrence or whether there is a broader problem with the CCD asteroid survey data sets, and if so, how to identify the ābadā spectra. While CCD survey spectra show apparently irreconcilable differences, the near-infrared spectra of (354) Eleonora from various observers show only minor differences, primarily in the overall spectral slope, most of which can be attributed to slight differences in the standard stars used to calibrate the data. In June 2011, 226 near-infrared (ā¼0.76ā2.5 Ī¼m) spectra of (354) Eleonora were obtained using the SpeX instrument on the NASA Infrared Telescope Facility at Mauna Kea Observatory. These spectra were consistent with the six sets of NIR spectra obtained for Eleonora by previous observers. The primary variation observed in this new data set was an approximately 10% variation in spectral slope between ā¼0.8 Ī¼m and ā¼1.6 Ī¼m during the rotation period of the asteroid. Mineralogically diagnostic spectral parameters extracted from this new data are most consistent with a surface assemblage of fine-grained intimately mixed olivine (ā¼60ā70%, ā¼Fo61ā71) and low nickel (\u3cā¼7ā8% Ni) NiFe metal. The Fo estimate is consistent with previous estimates (Fo66Ā±5) by Sanchez et al. (Sanchez, J.A., Reddy, V., Kelley, M.S., Cloutis, E.A., Bottke, W.F., NesvornĆ½, D., Lucas, M.P., Hardersen, P.S., Gaffey, M.J., Abell, P.A., Le Corre, L. [2014]. Icarus 228, 288ā300), but not with the estimate (ā¼Fo90) of Sunshine et al. (Sunshine, J.M., Bus, S.J., Corrigan, C.M., McCoy, T.J., Burbine, T.H. [2007]. Meteorit. Planet. Sci. 42, 155ā170). The surface assemblage appears to contain a small component (ā¼8ā10%) of igneous pyroxene (weakly constrained at ā¼Fs50Wo10). The parent lithology of the surface regolith may be similar to a pallasite assemblage, although none of the three known types of pallasites are good mineralogical matches
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
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