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Victoria E. Hamilton

Publications and source records attributed to Victoria E. Hamilton.

11 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2023

Phyllosilicate Decomposition on Bennu Due to Prolonged Surface Exposure

The Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission to carbonaceous asteroid (101955) Bennu performed detailed mapping with a suite of instruments to characterize the composition and geology of its surface. Here we use data from the OSIRIS-REx Thermal Emission Spectrometer (OTES) instrument to investigate the relationship of OTES-derived spectral indices to other derived data products from OTES, the OSIRIS-REx Camera Suite (OCAMS), and the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) at global and local scales. We quantitatively confirm that high values of the OTES silicate stretching slope (from ∼10 to ∼12 μm) in midday spectra that are indicative of thin and/or patchy dust cover are strongly associated with low thermal inertia (high porosity), low albedo boulders on Bennu. These high porosity boulders have brecciated textures with embedded clasts that likely originated on Bennu's parent body or during its disruption. The high porosity of these boulders is a key factor in the local production of the dust or its entrapment, as some large, brecciated boulders with a lower porosity have little evidence of dust. A second OTES spectral parameter, the silicate bending band depth near 22.7 μm applied to early evening spectra, is not correlated to thermal inertia, but is weakly to strongly correlated to albedo, OVIRS-derived 1.05 μm and 2.74 μm band depths, OVIRS-derived hydrogen abundance, and modeled nanophase troilite abundance. In several regions on Bennu there is a strong spatial relationship between these parameters, whereby areas with shallower silicate bending bands also have shallower 1.05 μm and 2.74 μm bands and lower albedo with higher nanophase troilite abundances. These correlations, combined with analysis of the silicate bending band in laboratory experiments of space weathered and mildly heated carbonaceous chondrites, suggests that decreased silicate bending band depths signify decomposition of phyllosilicates, likely Fe-bearing, due to space weathering or mild heating (<600 °C) via solar radiation during Bennu's time in near-Earth space. There is a strong association of larger silicate bending band depths in areas dominated by small rocks and unresolved material and in areas with small (≤ 25 m) craters identified as the spectrally reddest on Bennu, suggesting that this material has been more recently exposed due to impact and/or mass wasting processes. The shallowest silicate bending depths are associated with larger rocks and boulders that appear to have the longest surface exposure history, although there is band depth variation among them suggesting either initial composition variation that resulted in different responses to space weathering or heating, or varied exposure history of individual boulders themselves. We predict that any grains returned from Bennu with a significant surface exposure history will be characterized by shallower 22.7 μm, 1.05 μm and 2.7 μm band depths and increased sulfide (troilite) abundance, as well as textural and chemical evidence for phyllosilicate dehydration.

Asteroids, surfaces↗
NASA NTRS2022

GRO 95577 (CR1) as a mineralogical analogue for asteroid (101955) Bennu

Orbital spectra of asteroid (101955) Bennu collected by NASA's Origins, Spectral Interpretation, Resource Identification, Security–Regolith Explorer (OSIRIS–REx) spacecraft have identified ungrouped C, CI, and CM meteorites having petrologic types 1, 1/2, and 2 as the best mineralogical analogues to Bennu to date. Here we present spectral evidence that Grosvenor Mountains (GRO) 95577, a CR1, is a better analogue for Bennu's bulk surface mineralogy. CR-like parent bodies are targets of interest because they contain some of the most pristine materials from the solar nebula and can contain substantial amounts of H2O and OH− in addition to exotic organics. Unfortunately, terrestrial weathering makes constraining their indigenous mineralogy and organics challenging. Analysis of samples retrieved directly from an asteroid would help us disentangle the effects of terrestrial weathering and asteroidal aqueous alteration and hence whether some of the exotic organics and large populations of presolar grains were affected by terrestrial processes in meteorites. If Bennu is comprised of CR1(−like) material, in whole or in part, the OSIRIS–REx returned sample represents a tremendous opportunity to explore in depth what is currently a unique material among carbonaceous chondrites.

Asteroids↗
NASA NTRS2021

Spectrophotometric Modeling and Mapping of (101955) Bennu

Using hyperspectral data collected by OVIRS, the visible and infrared spectrometer onboard the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) spacecraft, we modeled the global average spectrophotometric properties of the carbonaceous asteroid (101955) Bennu and mapped their variations. We restricted our analysis to 0.4–2.5 µm to avoid the wavelengths where thermal emission from the asteroid dominates (>2.5 µm). Bennu has global photometric properties typical of dark asteroids; we found a geometric albedo of 0.046 ± 0.007 and a linear phase slope of 0.024 ± 0.007 mag deg–1 at 0.55 µm. The average spectral slope of Bennu’s normal albedo is –0.0030 µm–1, and the phase reddening parameter is 4.3´10–4 µm–1 deg–1, both over the spectral range of 0.5–2.0 µm. We produced normal albedo maps and phase slope maps at all spectral channels, from which we derived spectral slope and phase reddening maps. Correlation analysis suggests that phase slope variations on Bennu are likely due to photometric roughness variation. A correlation between photometric roughness and thermal roughness is evident, implying that the roughness of Bennu is self-similar on scales from tens of microns to meters. Our analysis reveals latitudinal trends in the spectral color slope and phase reddening on Bennu. The equatorial region appears to be redder than the global average, and the spectral slope decreases towards higher latitudes. Phase reddening on Bennu is relatively weak in the equatorial region and shows an asymmetry between the northern and southern hemispheres. We attributed the latitudinal trend to the geophysical conditions on Bennu that result in a global pattern of mass flow towards the equator.

Jian-Yang Li (李荐扬)↗
NASA NTRS2021

The Role of Hydrated Minerals and Space Weathering Products in the Bluing of Carbonaceous Asteroids

The surfaces of airless bodies such as lunar and S-type asteroids typically become spectrally redder in visible to near-infrared reflectance with longer exposures to space weathering. However, some carbonaceous asteroids instead become spectrally bluer. Space weathering experiments on carbonaceous meteorites have provided some clues as to the space weathering products that could produce spectral bluing. We applied these experimental results to our Hapke radiative transfer model, with which we modeled spectral data from the OSIRIS-REx mission in order to determine whether these space weathering products—specifically, nanophase and microphase metallic iron, troilite, and magnetite—could explain the globally blue spectrum of the carbonaceous asteroid (101955) Bennu. The model suggests that the surface of Bennu has microphase iron, nanophase magnetite, and nanophase and microphase troilite. Considering previous space weathering experiments together with our spectral modeling of Bennu, we posit that the presence of nanophase magnetite is what causes a carbonaceous asteroid to become spectrally bluer with exposure time. Nanophase magnetite can form on asteroids that have Fe-bearing hydrated minerals (phyllosilicates). On anhydrous carbonaceous asteroids, nanophase iron forms instead of magnetite, leading to spectral reddening. We therefore predict that samples returned by the OSIRIS-REx mission from Bennu will have more nanophase magnetite than nanophase iron with nanophase and microphase sulfides, whereas samples returned by the Hayabusa2 mission from the carbonaceous asteroid (162173) Ryugu, which is spectrally red, will contain nanophase and microphase sulfides as well as more nanophase iron than nanophase magnetite.

David Trang↗
NASA NTRS2020

Widespread carbon-bearing materials on near-Earth asteroid (101955) Bennu

(101955) Bennu is a dark, Earth orbit-crossing, asteroid thought to be assembled from the fragments of an ancient collision. Spatially-resolved visible and near-infrared spectra of Bennu provide details about its surface properties and composition. In addition to a hydrated phyllosilicate band, we detect a ubiquitous 3.4-micron absorption feature, which we attribute to a mix of organic and carbonate materials. The shape and depth of this absorption feature vary across Bennu’s surface, spanning the range seen among similar main-belt asteroids. Its distribution does not correlate with the temperature, reflectance, spectral slope, or hydrated minerals although those characteristics correlate with each other in some cases. The deepest 3.4-micron absorptions occur on individual boulders. The variations may be due to differences in abundance, recent exposure, or space weathering.

Amy A. Simon↗
NASA NTRS2020

Visible-near Infrared Spectral Indices for Mapping Mineralogy and Chemistry with OSIRIS-Rex

The primary objective of the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission is to return to Eartha pristine sample of carbonaceous material from the primitive asteroid (101955) Bennu. To support compositional mapping of Bennu as part of sample site selection and characterization, we tested 95 spectral indices on visible to near infrared laboratory reflectance data from minerals and carbonaceous meteorites. Our aim was to determine which indices reliably identify spectral features of interest. Most spectral indices had high positive detection rates when applied to spectra of pure, single-component materials. The meteorite spectra have fewer and weaker absorption features and, as a result, fewer detections with the spectral indices. Indices targeting absorptions at 0.7 and 2.7–3lm, which are attributable to hydrated minerals, were most successful for the meteorites. Based on these results, we identified a set of 17 indices that are most likely to be useful at Bennu. These indices detect olivines, pyroxenes, carbonates, water/OH-bearing minerals, serpentines, ferric minerals, and organics. Particle size and albedo are known to affect band depth but had a negligible impact on interpretive success with spectral indices. Preliminary analysis of the disk-integrated Bennu spectrum with these indices is consistent with expectations given the observed absorption near 3lm. Our study prioritizes spectral indices to be used for OSIRIS-REx spectral analysis and mapping and informs the reliability of all index-derived data products, including a science value map for sample site selection.

Hannah H. Kaplan↗
NASA NTRS2020

Visible–near infrared spectral indices for mapping mineralogy and chemistry with OSIRIS-REx

The primary objective of the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission is to return to Earth a pristine sample of carbonaceous material from the primitive asteroid (101955) Bennu. To support compositional mapping of Bennu as part of sample site selection and characterization, we tested 95 spectral indices on visible to near infrared laboratory reflectance data from minerals and carbonaceous meteorites. Our aim was to determine which indices reliably identify spectral features of interest. Most spectral indices had high positive detection rates when applied to spectra of pure, single-component materials. The meteorite spectra have fewer and weaker absorption features and, as a result, fewer detections with the spectral indices. Indices targeting absorptions at 0.7 and 2.7–3 µm, which are attributable to hydrated minerals, were most successful for the meteorites. Based on these results, we identified a set of 17 indices that are most likely to be useful at Bennu. These indices detect olivines, pyroxenes, carbonates, water/OH-bearing minerals, serpentines, ferric minerals, and organics. Particle size and albedo are known to affect band depth but had a negligible impact on interpretive success with spectral indices. Preliminary analysis of the disk-integrated Bennu spectrum with these indices is consistent with expectations given the observed absorption near 3 lm. Our study prioritizes spectral indices to be used for OSIRIS-REx spectral analysis and mapping and informs the reliability of all index-derived data products, including a science value map for sample site selection.

Hannah H. Kaplan↗