Lunar-style space weathering is well understood, but cannot be extended to asteroids in general. The two best studied Asteroids (433 Eros and 243 Ida) exhibit quite different space weathering styles, ...and neither exhibits lunar-style space weathering. It must be concluded that at this time the diversity and mechanisms of asteroid space weathering are poorly understood. This introduces a significant unconstrained variable into the problem of analyzing asteroid spectral data. The sensitivity of asteroid surface material characterizations to space weathering effects – whatever their nature – is strongly dependent upon the choice of remote sensing methodology. The effects of space weathering on some methodologies such as curve matching are potentially devastating and at the present time essentially unmitigated. On other methodologies such as parametric analysis (e.g., analyses based on band centers and band area ratios) the effects are minimal. By choosing the appropriate methodology(ies) applied to high quality spectral data, robust characterizations of asteroid surface mineralogy can be obtained almost irrespective of space weathering. This permits sophisticated assessments of the geologic history of the asteroid parent bodies and of their relationships to the meteorites. Investigations of the diversity of space weathering processes on asteroid surfaces should be a fruitful area for future efforts.
The Dawn mission has provided new evidence strengthening the identification of asteroid Vesta as the parent body of the howardite, eucrite, and diogenite (HED) meteorites. The evidence includes ...Vesta's petrologic complexity, detailed spectroscopic characteristics, unique space weathering, diagnostic geochemical abundances and neutron absorption characteristics, chronology of surface units and impact history, occurrence of exogenous carbonaceous chondritic materials in the regolith, and dimensions of the core, all of which are consistent with HED observations and constraints. Global mapping of the distributions of HED lithologies by Dawn cameras and spectrometers provides the missing geologic context for these meteorites, thereby allowing tests of petrogenetic models and increasing their scientific value.
► Dark material on Vesta observed by Dawn is remnant carbonaceous chondrite impactor. ► Global abundance of carbonaceous chondrite material <6vol.% similar to howardites. ► Distribution of dark ...material suggests delivery during Veneneia basin formation event. ► Suggests extensive volatile delivery onto Vesta by carbonaceous impactors. ► First direct link between HED meteorites and surface of Vesta.
NASA’s Dawn spacecraft observations of Asteroid (4) Vesta reveal a surface with the highest albedo and color variation of any asteroid we have observed so far. Terrains rich in low albedo dark material (DM) have been identified using Dawn Framing Camera (FC) 0.75μm filter images in several geologic settings: associated with impact craters (in the ejecta blanket material and/or on the crater walls and rims); as flow-like deposits or rays commonly associated with topographic highs; and as dark spots (likely secondary impacts) nearby impact craters. This DM could be a relic of ancient volcanic activity or exogenic in origin. We report that the majority of the spectra of DM are similar to carbonaceous chondrite meteorites mixed with materials indigenous to Vesta. Using high-resolution seven color images we compared DM color properties (albedo, band depth) with laboratory measurements of possible analog materials. Band depth and albedo of DM are identical to those of carbonaceous chondrite xenolith-rich howardite Mt. Pratt (PRA) 04401. Laboratory mixtures of Murchison CM2 carbonaceous chondrite and basaltic eucrite Millbillillie also show band depth and albedo affinity to DM. Modeling of carbonaceous chondrite abundance in DM (1–6vol.%) is consistent with howardite meteorites. We find no evidence for large-scale volcanism (exposed dikes/pyroclastic falls) as the source of DM. Our modeling efforts using impact crater scaling laws and numerical models of ejecta reaccretion suggest the delivery and emplacement of this DM on Vesta during the formation of the ∼400km Veneneia basin by a low-velocity (<2km/s) carbonaceous impactor. This discovery is important because it strengthens the long-held idea that primitive bodies are the source of carbon and probably volatiles in the early Solar System.
► We examine reflectance spectra of 39 CM carbonaceous chondrites. ► We search for spectral variability and relationships to petrography. ► Spectral slopes range from blue to red. ► CM spectra ...exhibit nearly ubiquitous absorption band near 0.7, 0.9, and 1.1
μm due to phyllosilicates. ► Band depths range up to 10% and absolute reflectance ranges from 2.6% to 13% in the visible region.
We have examined the spectral reflectance properties and available modal mineralogies of 39 CM carbonaceous chondrites to determine their range of spectral variability and to diagnose their spectral features. We have also reviewed the published literature on CM mineralogy and subclassification, surveyed the published spectral literature and added new measurements of CM chondrites and relevant end members and mineral mixtures, and measured 11 parameters and searched pair-wise for correlations between all quantities. CM spectra are characterized by overall slopes that can range from modestly blue-sloped to red-sloped, with brighter spectra being generally more red-sloped. Spectral slopes, as measured by the 2.4:0.56
μm and 2.4
μm:visible region peak reflectance ratios, range from 0.90 to 2.32, and 0.81 to 2.24, respectively, with values <1 indicating blue-sloped spectra. Matrix-enriched CM spectra can be even more blue-sloped than bulk samples, with ratios as low as 0.85. There is no apparent correlation between spectral slope and grain size for CM chondrite spectra – both fine-grained powders and chips can exhibit blue-sloped spectra. Maximum reflectance across the 0.3–2.5
μm interval ranges from 2.9% to 20.0%, and from 2.8% to 14.0% at 0.56
μm. Matrix-enriched CM spectra can be darker than bulk samples, with maximum reflectance as low as 2.1%. CM spectra exhibit nearly ubiquitous absorption bands near 0.7, 0.9, and 1.1
μm, with depths up to 12%, and, less commonly, absorption bands in other wavelength regions (e.g., 0.4–0.5, 0.65, 2.2
μm). The depths of the 0.7, 0.9, and 1.1
μm absorption features vary largely in tandem, suggesting a single cause, specifically serpentine-group phyllosilicates. The generally high Fe content, high phyllosilicate abundance relative to mafic silicates, and dual Fe valence state in CM phyllosilicates, all suggest that the phyllosilicates will exhibit strong absorption bands in the 0.7
μm region (due to Fe
3+–Fe
2+ charge transfers), and the 0.9–1.2
μm region (due to Fe
2+ crystal field transitions), and generally dominate over mafic silicates. CM petrologic subtypes exhibit a positive correlation between degree of aqueous alteration and depth of the 0.7
μm absorption band. This is consistent with the decrease in fine-grained opaques that accompanies aqueous alteration. There is no consistent relationship between degree of aqueous alteration and evidence for a 0.65
μm region saponite-group phyllosilicate absorption band. Spectra of different subsamples of a single CM can show large variations in absolute reflectance and overall slope. This is probably due to petrologic variations that likely exist within a single CM chondrite, as duplicate spectra for a single subsample show much less spectral variability. When the full suite of available CM spectra is considered, few clear spectral–compositional trends emerge. This indicates that multiple compositional and physical factors affect absolute reflectance, absorption band depths, and absorption band wavelength positions. Asteroids with reflectance spectra that exhibit absorption features consistent with CM spectra (i.e., absorption bands near 0.7 and 0.9
μm) include members from multiple taxonomic groups. This suggests that on CM parent bodies, aqueous alteration resulted in the consistent production of serpentine-group phyllosilicates, however resulting absolute reflectances and spectral shapes seen in CM reflectance spectra are highly variable, accounting for the presence of phyllosilicate features in reflectance spectra of asteroids across diverse taxonomic groups.
Color and Albedo Heterogeneity of Vesta from Dawn Reddy, Vishnu; Nathues, Andreas; Le Corre, Lucille ...
Science (American Association for the Advancement of Science),
05/2012, Letnik:
336, Številka:
6082
Journal Article
Recenzirano
Multispectral images (0.44 to 0.98 μm) of asteroid (4) Vesta obtained by the Dawn Framing Cameras reveal global color variations that uncover and help understand the north-south hemispherical ...dichotomy. The signature of deep lithologies excavated during the formation of the Rheasilvia basin on the south pole has been preserved on the surface. Color variations (band depth, spectral slope, and eucrite-diogenite abundance) clearly correlate with distinct compositional units. Vesta displays the greatest variation of geometric albedo (0.10 to 0.67) of any asteroid yet observed. Four distinct color units are recognized that chronicle processes—including impact excavation, mass wasting, and space weathering—that shaped the asteroid's surface. Vesta's color and photometric diversity are indicative of its status as a preserved, differentiated protoplanet.
► Most CI reflectance spectra exhibit mineral-associated absorption bands. ► Magnetite seems the likeliest explanation for blue-sloped CI spectra. ► Larger grain size samples generally have the ...bluest and darkest spectra. ► Phase angle can cause CI spectra to become redder or bluer. ► Underdense CI samples have lower reflectance than regularly packed CI samples.
Existing reflectance spectra of CI chondrites (18 spectra of 3 CIs) have been augmented with new (18 spectra of 2 CIs) reflectance spectra to ascertain the spectral variability of this meteorite class and provide insights into their spectral properties as a function of grain size, composition, particle packing, and viewing geometry. Particle packing and viewing geometry effects have not previously been examined for CI chondrites. The current analysis is focused on the 0.3–2.5μm interval, as this region is available for the largest number of CI spectra. Reflectance spectra of powdered CI1 chondrites are uniformly dark (<10% maximum reflectance) but otherwise exhibit a high degree of spectral variability. Overall spectral slopes range from red (increasing reflectance with increasing wavelength) to blue (decreasing reflectance with increasing wavelength). A number of the CI spectra exhibit weak (<5% deep) absorption bands that can be attributed to both phyllosilicates and magnetite. Very weak absorption bands attributable to other CI phases, such as carbonates, sulfates, and organic matter may be present in one or a few spectra, but their identification is not robust. We found that darker spectra are generally correlated with bluer spectral slopes: a behavior most consistent with an increasing abundance of fine-grained magnetite and/or insoluble organic material (IOM), as no other CI opaque phase appears able to produce concurrent darkening and bluing. Magnetite can also explain the presence of an absorption feature near 1μm in some CI spectra. The most blue-sloped spectra are generally associated with the larger grain size samples. For incidence and emission angles <60°, increasing phase angle results in darker and redder spectra, particularly below ∼1μm. At high incidence angles (60°), increasing emission angle results in brighter and redder spectra. More densely packed samples and underdense (fluffed) samples show lower overall reflectance than normally packed and flat-surface powdered samples. Some B-class asteroids exhibit selected spectral properties consistent with CI chondrites, although perfect spectral matches have not been found. Because many CI chondrite spectra exhibit absorption features that can be related to specific mineral phases, the search for CI parent bodies can fruitfully be conducted using such parameters.
► We have derived photometric, spectral phase and temperature functions for Vesta. ► Spectral phase functions affect band parameters i.e. band depth and Band Area Ratio. ► Temperature affects the ...Band I and II centers of the pyroxene absorption band. ► These effects need to be corrected for before mineralogical characterization.
Phase angle and temperature are two important parameters that affect the photometric and spectral behavior of planetary surfaces in telescopic and spacecraft data. We have derived photometric and spectral phase functions for the Asteroid 4 Vesta, the first target of the Dawn mission, using ground-based telescopes operating at visible and near-infrared wavelengths (0.4–2.5
μm). Photometric lightcurve observations of Vesta were conducted on 15 nights at a phase angle range of 3.8–25.7° using duplicates of the seven narrowband Dawn Framing Camera filters (0.4–1.0
μm). Rotationally resolved visible (0.4–0.7
μm) and near-IR spectral observations (0.7–2.5
μm) were obtained on four nights over a similar phase angle range. Our Vesta photometric observations suggest the phase slope is between 0.019 and 0.029
mag/deg. The G parameter ranges from 0.22 to 0.37 consistent with previous results (e.g., Lagerkvist, C.-I., Magnusson, P., Williams, I.P., Buontempo, M.E., Argyle, R.W., Morrison, L.V. 1992. Astron. Astrophys. Suppl. Ser. 94, 43–71; Piironen, J., Magnusson, P., Lagerkvist, C.-I., Williams, I.P., Buontempo, M.E., Morrison, L.V. 1997. Astron. Astrophys. Suppl. Ser. 121, 489–497; Hasegawa, S. et al. 2009. Lunar Planet. Sci. 40. ID 1503) within the uncertainty. We found that in the phase angle range of 0°
<
α
⩽
25° for every 10° increase in phase angle Vesta’s visible slope (0.5–0.7
μm) increases 20%, Band I and Band II depths increase 2.35% and 1.5% respectively, and the BAR value increase 0.30. Phase angle spectral measurements of the eucrite Moama in the lab show a decrease in Band I and Band II depths and BAR from the lowest phase angle 13° to 30°, followed by possible small increases up to 90°, and then a dramatic drop between 90° and 120° phase angle. Temperature-induced spectral effects shift the Band I and II centers of the pyroxene bands to longer wavelengths with increasing temperature. We have derived new correction equations using a temperature series (80–400
K) of HED meteorite spectra that will enable interpretation of telescopic and spacecraft spectral data using laboratory calibrations at room temperature (300
K).
► Near-IR spectrum of near-Earth Asteroid 1999 TA10 suggests a composition similar to diogenite meteorites from the mantle of Asteroid Vesta. ► This is the first fragment of Vesta’s mantle detected ...in near-Earth space. ► The presence of mantle fragment provides us with a constraint on the on the thickness of Vesta’s mantle, assuming it came from the south pole impact basin. ► Based on the thickness ratio of Vesta’s crust, mantle and core, we think Vesta’s parent material was more oxidized.
We report the first detection of a fragment of Asteroid (4) Vesta’s mantle in the near-Earth and main belt Vestoid populations. The near-infrared (NIR) spectrum of near-Earth Asteroid (237442) 1999 TA10 shows band parameters, and inferred surface mineralogy, and pyroxene chemistry, similar to diogenite meteorites, which are believed to be fragments of (4) Vesta’s upper mantle. This strongly indicates that the impact that led to the excavation of 1999 TA10 was deep enough to sample Vesta’s upper mantle and provides constraints on Vesta’s internal structure that can be verified by the Dawn mission.
•Chelyabinsk meteorite source region in the main asteroid belt is the Flora family.•Shock/impact melt can explain spectral properties of Flora and Baptistina families.•Shock/impact melt presence ...could lead to ambiguous taxonomic classification.
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. Our work confirms previous studies (e.g., Vernazza et al. 2008. Nature 454, 858–860; de León, J., Licandro, J., Serra-Ricart, M., Pinilla-Alonso, N., Campins, H. 2010. Astron. Astrophys. 517, A23; Dunn, T.L., Burbine, T.H., Bottke, W.F., Clark, J.P. 2013. Icarus 222, 273–282), linking LL chondrites to 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 11 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 conceptual problem with part of this scenario is that impact melts are very rare within ordinary chondrites. Of the ∼42,000 ordinary chondrites, less than 0.5% (203) of them contain impact melts. A major reason that impact melts are rare in meteorites is that high impact velocities (V>10km/s) are needed to generate the necessary shock pressures and temperatures (e.g., Pierazzo, E., Melosh, H.J. 1998. Hydrocode modeling of oblique impacts: The fate of the projectile. In: Origin of the Earth and Moon, Proceedings of the Conference. LPI Contribution No. 957) unless the target material is highly porous. Nearly all asteroid impacts within the main belt are at ∼5km/s (Bottke, W.F., Nolan, M.C., Greenberg, R., Kolvoord, R.A. 1994. Collisional lifetimes and impact statistics of near-Earth asteroids. In: Tucson, Gehrels T. (Ed.), Hazards Due to Comets and Asteroids. The University of Arizona Press, Arizona, pp. 337–357), which prevents them from producing much impact melt unless they are highly porous. However, shock darkening is an equally efficient process that takes place at much lower impact velocities (∼2km/s) and can cause the observed spectral effects. Spectral effects of shock darkening and impact melt are identical. 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. Collisional mixing of shock blackened/impact melt and LL5 chondritic material could have created the Baptistina Asteroid Family with composition identical to those of the Floras, but with subdued absorption bands. Shock darkening and impact melt play an important role in altering the spectral and albedo properties of ordinary chondrites and our work confirms earlier work by Britt and Pieters (Britt, D.T., Pieters, C.M. 1994. Geochimica et Cosmochimica Acta 58, 3905–3919).
•A mineralogical assessment of 14 asteroids was completed.•Two asteroids with L-chondrite surface assemblages were identified.•Three asteroids’ compositions are consistent with ...olivine-orthopyroxenitic diogenites.•Three asteroids’ compositions are consistent with harzburgitic diogenites.
The research is an integrated effort beginning with telescopic observations and extending through detailed mineralogical characterizations to provide constraints on the composition and meteorite affinities of a subset of fourteen asteroids in/near the 3:1 Kirkwood Gap. Eight asteroids were identified as having either one or two absorption features, while six were deemed featureless. The compositional analysis of Asteroids (355) Gabriella and (1447) Utra reveal Fs and Fa values which are consistent with values for the L-type ordinary chondrites (Fs19–22 and Fa22–26). The location of these two bodies with respect to each other and to the previously identified L-chondrite parent body Asteroid (1722) Goffin, suggests a small L-chondrite genetic family. These results support the model that the L-chondrites come from an asteroid family rather than from a single object. Asteroids (1368) Numidia, (1587) Kahrstadt, (1854) Skvortsov, (2497) Kulikovskij, and (5676) Voltaire were analyzed and determined to have “basaltic” silicate mineralogies similar to those of the HED (howardite–eucrite–diogenite) meteorite group. In particular, we found that the compositions of (1368), (1587) and (1854) are consistent with olivine-orthopyroxenitic diogenites, while (2497) and (5676)’s compositions are consistent with harzburgitic diogenites. The Band I and Band II absorption feature depths are much shallower than seen in diogenite spectra, typically ∼70% depth (Burbine, T.H. et al. 2000. Forging asteroid–meteorite relationships through reflectance spectroscopy. Lunar Planet. Sci. XXXI. Abstract 1844). The nature of the weak features seen in the asteroid spectra when compared to measured band depths of in situ diogenite samples indicate an additional mechanism(s) acting to weaken the features, most likely space weathering. The aforementioned five asteroids are plausible sources for the olivine-orthopyroxenitic diogenites and harzburgitic diogenites, and very well may be fragments of Vesta. Asteroid (46) Hestia is an interesting object whose surface minerals may be consistent with a CR2 chondrite; however, the unique spectrum deserves further study in the future. Featureless Asteroids (248) Lameia, (1960) Guisan, (3345) Tarkovskij and (6212) 1993 MS1 surface materials are likely organic assemblages consistent with the Type 1 or 2 carbonaceous chondrite meteorite class; however specific terrestrial meteorite analog could not be identified. The spectra of Asteroids (3228) Pire and (3999) Aristarchus are consistent with each other and have been assigned to the Eulalia by Walsh et al. (Walsh, K.J. et al. 2013. Icarus 225, 283–297). Spectrally they are similar to (495) in terms of blue-slope and albedo (Fieber-Beyer, S.K., et al. 2012. Icarus 221, 593–602), thus increasing our confidence the three bodies are truly related dynamically and genetically. By extrapolation and due to their location adjacent to the 3:1 Kirkwood Gap, (3228) and (3999) are plausible sources of the CV3OXB carbonaceous chondrites.
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