► 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.
Context: The Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission detected an infrared absorption at 3.4μm on near-Earth asteroid(101955) ...Bennu. This absorption is indicative of carbon species, including organics, on the sur-face.
Aims: We aim to describe the composition of the organic matter on Bennu by investigating the spectral features in detail.
Methods: We use a curated set of spectra acquired by the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) that have features near 3.4μm (3.2 to 3.6μm) attributed to organics. We assess the shapes and strengths of these absorptions in the context of laboratory spectra of extraterrestrial organics and analogs.
Results: We find spectral evidence of aromatic and aliphatic CH bonds. The absorptions are broadly consistent in shape and depth with those associated with insoluble organic matter in meteorites. Given the thermal and space weathering environments on Bennu, it is likely that the organics have not been exposed for long enough to substantially decrease the H/C and destroy all aliphatic molecules.
► 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.
Spacecraft missions have observed regolith blankets of unconsolidated subcentimetre particles on stony asteroids
. Telescopic data have suggested the presence of regolith blankets also on ...carbonaceous asteroids, including (101955) Bennu
and (162173) Ryugu
. However, despite observations of processes that are capable of comminuting boulders into unconsolidated materials, such as meteoroid bombardment
and thermal cracking
, Bennu and Ryugu lack extensive areas covered in subcentimetre particles
. Here we report an inverse correlation between the local abundance of subcentimetre particles and the porosity of rocks on Bennu. We interpret this finding to mean that accumulation of unconsolidated subcentimetre particles is frustrated where the rocks are highly porous, which appears to be most of the surface
. The highly porous rocks are compressed rather than fragmented by meteoroid impacts, consistent with laboratory experiments
, and thermal cracking proceeds more slowly than in denser rocks. We infer that regolith blankets are uncommon on carbonaceous asteroids, which are the most numerous type of asteroid
. By contrast, these terrains should be common on stony asteroids, which have less porous rocks and are the second-most populous group by composition
. The higher porosity of carbonaceous asteroid materials may have aided in their compaction and cementation to form breccias, which dominate the carbonaceous chondrite meteorites
.
Context. The NASA Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission has obtained thousands of spectra of asteroid (101955) Bennu with the ...OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS).
Aims. We present a spectral search for minor absorption bands and determine compositional variations on the surface of Bennu.
Methods. Reflectance spectra with low and high spatial resolutions were analyzed for evidence of weak absorption bands. Spectra were also divided by a global average spectrum to isolate unique spectral features, and variations in the strongest band depths were mapped on a surface shape model. The global visible to near-IR spectrum of Bennu shows evidence of several weak absorption bands with depths of a few percent.
Results. Several observed bands are consistent with phyllosilicates, and their distribution correlates with the stronger 2.74-μm hydration band. A 0.55-μm band is consistent with iron and is deepest in the spectrally reddest areas on Bennu. The presence of hydrated phyllosilicates and iron oxides indicates substantial aqueous alteration in Bennu’s past.
Conclusions. Bennu’s spectra are not identical to a limited set of carbonaceous chondrite spectra, possibly due to compositional properties and spatial scale differences; however, returned samples should contain a mixture of common chondrite materials.
The dwarf planet (1) Ceres, the largest object in the main asteroid belt with a mean diameter of about 950 kilometres, is located at a mean distance from the Sun of about 2.8 astronomical units (one ...astronomical unit is the Earth-Sun distance). Thermal evolution models suggest that it is a differentiated body with potential geological activity. Unlike on the icy satellites of Jupiter and Saturn, where tidal forces are responsible for spewing briny water into space, no tidal forces are acting on Ceres. In the absence of such forces, most objects in the main asteroid belt are expected to be geologically inert. The recent discovery of water vapour absorption near Ceres and previous detection of bound water and OH near and on Ceres (refs 5-7) have raised interest in the possible presence of surface ice. Here we report the presence of localized bright areas on Ceres from an orbiting imager. These unusual areas are consistent with hydrated magnesium sulfates mixed with dark background material, although other compositions are possible. Of particular interest is a bright pit on the floor of crater Occator that exhibits probable sublimation of water ice, producing haze clouds inside the crater that appear and disappear with a diurnal rhythm. Slow-moving condensed-ice or dust particles may explain this haze. We conclude that Ceres must have accreted material from beyond the 'snow line', which is the distance from the Sun at which water molecules condense.
The dwarf planet Ceres (diameter 939 km) is the largest object in the main asteroid belt. Recent investigations suggest that Ceres is a thermally evolved, volatile-rich body with potential geological ...activity, a body which was never completely molten but possibly differentiated into a rocky core, an ice-rich mantle, and which may contain remnant internal liquid water. Thermal alteration and exogenic material infall contribute to producing a (dark) carbonaceous chondritic-like surface containing ammoniated phyllosilicates. Here we report imaging and spectroscopic analyses of Occator crater derived from the Framing Camera and the Visible and Infrared Spectrometer onboard Dawn. We found that the central bright spot (Cerealia Facula) of Occator is ∼30 Myr younger than the crater itself. The central spot is located in a central pit which contains a dome that is spectrally homogenous, exhibiting absorption features that are consistent with carbonates. Multiple radial fractures across the dome indicate an extrusive formation process. Our results lead us to conclude that the floor region was subject to past endogenic activity. Dome and bright material in its vicinity formed likely due to a long-lasting, periodic, or episodic ascent of bright material from a subsurface reservoir rich in carbonates. Originally triggered by an impact event, gases, possibly dissolved from a subsurface water/brine layer, enabled material rich in carbonates to ascend through fractures and be deposited onto the surface.
Context.
The NASA mission OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) has been observing near-Earth asteroid (101955) Bennu in close ...proximity since December 2018. In October 2020, the spacecraft collected a sample of surface material from Bennu to return to Earth.
Aims.
In this work, we investigate spectral phase reddening – that is, the variation of spectral slope with phase angle – on Bennu using spectra acquired by the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) covering a phase angle range of 8–130°. We investigate this process at the global scale and for some localized regions of interest (ROIs), including boulders, craters, and the designated sample collection sites of the OSIRIS-REx mission.
Methods.
Spectra were wavelength- and flux-calibrated, then corrected for the out-of-band contribution and thermal emission, resampled, and finally converted into radiance factor per standard OVIRS processing. Spectral slopes were computed in multiple wavelength ranges from spectra normalized at 0.55
μ
m.
Results.
Bennu has a globally negative spectra slope, which is typical of B-type asteroids. The spectral slope gently increases in a linear way up to a phase angle of 90°, where it approaches zero. The spectral phase reddening is monotonic and wavelength-dependent with highest values in the visible range. Its coefficient is 0.00044
μ
m
−1
deg
−1
in the 0.55–2.5
μ
m range. For observations of Bennu acquired at high phase angle (130°), phase reddening increases exponentially, and the spectral slope becomes positive. Similar behavior was reported in the literature for the carbonaceous chondrite Mukundpura in spectra acquired at extreme geometries. Some ROIs, including the sample collection site, Nightingale, have a steeper phase reddening coefficient than the global average, potentially indicating a surface covered by fine material with high micro-roughness.
Conclusions.
The gentle spectral phase reddening effect on Bennu is similar to that observed in ground-based measurements of other B-type asteroids, but much lower than that observed for other low-albedo bodies such as Ceres or comet 67P/Churyumov-Gerasimenko. Monotonic reddening may be associated with the presence of fine particles at micron scales and/or of particles with fractal structure that introduce micro- and sub-micro roughness across the surface of Bennu.
► We measured reflectance spectra of 26 carbonaceous chondrites that have been aqueously altered and thermally metamorphosed. ► Some variations in their reflectance spectra can be related to ...temperature of thermal metamorphism. ► Temperatures experienced by these meteorites were not high enough to result in widespread olivine formation. ► Progressive thermal metamorphism results in loss of phyllosilicate absorption bands. ► Samples heated to approximately 500–700°C show the lowest overall reflectance and weakest silicate absorption bands.
We examined the spectral reflectance properties of 26 carbonaceous chondrites (CCs) that show evidence of aqueous alteration and subsequent thermal metamorphism (termed ATCCs). We also reviewed the thermal and aqueous alteration history of these meteorites and searched for trends between spectral parameters and temperature histories in order to uncover spectral–compositional relationships. Aqueous alteration results in the production of phyllosilicates from anhydrous silicate precursors – largely serpentine group phyllosilicates, and increasing amounts of saponite group phyllosilicates with increasing aqueous alteration. Thermal metamorphism results in dehydration of these phyllosilicates and production of abundant amorphous material except at the highest temperatures (≳900°C), as well as alteration of carbonaceous components. ATCCs are a spectrally diverse group in almost all respects. Spectral slopes, as measured by the ratio of reflectance at 2.4μm to the local peak or inflection in the 0.5–0.8μm region and 2.4/1.5μm ratios range from 0.78 to 1.48, and 0.93 to 1.24, respectively (blue-sloped spectra have ratio values of <1). ATCC powder spectra (<75, <100, or <125μm) are generally dark, with maximum reflectance at the local peak or inflection in the 0.5–0.8μm region, or maximum reflectance at any wavelength ranging from 2.6% to 8.9%, and 3.5% to 10.3%, respectively. All ATCC spectra exhibit an absorption feature in the ∼0.8–1.3μm region, with band depths ranging from ∼1% to 8%. This feature is diverse in terms of number of apparent absorption bands. The presence of mixed valence Fe2+–Fe3+ phyllosilicates, as evidenced by an absorption band near 0.7μm with a depth of up to 5%, and Mg-bearing phyllosilicates, as evidenced by an MgOH combination band in the 2.3–2.4μm region, are seen in many of the least thermally metamorphosed ATCC spectra. The depth of the 0.7μm band generally decreases with increasing temperature. Olivine-associated absorption bands in the 0.8–1.3μm region seem to be more prevalent in the more metamorphosed ATCC spectra. However clearly-resolvable olivine absorption bands are not present in ATCC spectra, suggesting that thermal metamorphism did not lead to the production of widespread crystalline Fe2+-bearing olivine. The reddest ATCC powder spectra are generally the darkest, and C content is correlated with decreasing overall reflectance and weakly correlated with spectral slope. When the degree of thermal metamorphism was compared to various spectral measures of slope, band depth, and overall reflectance, no strong correlations emerged. However, it does appear that the most thermally metamorphosed ATCCs have generally flatter spectral slopes. ATCC chip spectra are brighter and less red-sloped than powder spectra, but band depths are generally comparable. Laboratory-heated CIs and CMs generally exhibit the same types of spectral changes seen in naturally thermally metamorphosed ATCCs. For laboratory-heated CM and CI chondrites, and ATCCs for which temperature estimates are available, reflectance generally decreases with increasing temperature to ∼500°C, and then increases to higher temperatures. Silicate absorption band depths are generally least for temperatures of ∼600–800°C. Below this temperature interval, ATCC spectra show more phyllosilicate-like absorption bands. ATCC spectra generally become flatter with increasing temperature above ∼400°C. Temperatures in excess of those experienced by the ATCCs (∼900°C) are required for the appearance of well-resolved olivine absorption bands.
► We examine the spectral reflectance properties of 14 CR and CR-like carbonaceous chondrites. ► CR spectra have a weak phyllosilicate absorption feature near 1.1
μm and red spectral slopes. ► The ...phyllosilicate absorption band becomes more resolvable with increasing aqueous alteration. ► CR spectra do not exhibit a phyllosilicate ferrous-ferric charge transfer band near 0.7
μm. ► The spectra of CR finds are all affected by terrestrial weathering products.
Powdered samples of a suite of 14 CR and CR-like chondrites, ranging from petrologic grade 1 to 3, were spectrally characterized over the 0.3–2.5
μm interval as part of a larger study of carbonaceous chondrite reflectance spectra. Spectral analysis was complicated by absorption bands due to Fe oxyhydroxides near 0.9
μm, resulting from terrestrial weathering. This absorption feature masks expected absorption bands due to constituent silicates in this region. In spite of this interference, most of the CR spectra exhibit absorption bands attributable to silicates, in particular an absorption feature due to Fe
2+-bearing phyllosilicates near 1.1
μm. Mafic silicate absorption bands are weak to nonexistent due to a number of factors, including low Fe content, low degree of silicate crystallinity in some cases, and presence of fine-grained, finely dispersed opaques. With increasing aqueous alteration, phyllosilicate: mafic silicate ratios increase, resulting in more resolvable phyllosilicate absorption bands in the 1.1
μm region. In the most phyllosilicate-rich CR chondrite, GRO 95577 (CR1), an additional possible phyllosilicate absorption band is seen at 2.38
μm. In contrast to CM spectra, CR spectra generally do not exhibit an absorption band in the 0.65–0.7
μm region, which is attributable to Fe
3+–Fe
2+ charge transfers, suggesting that CR phyllosilicates are not as Fe
3+-rich as CM phyllosilicates. CR2 and CR3 spectra are uniformly red-sloped, likely due to the presence of abundant Fe–Ni metal. Absolute reflectance seems to decrease with increasing degree of aqueous alteration, perhaps due to the formation of fine-grained opaques from pre-existing metal. Overall, CR spectra are characterized by widely varying reflectance (4–21% maximum reflectance), weak silicate absorption bands in the 0.9–1.3
μm region, overall red slopes, and the lack of an Fe
3+–Fe
2+ charge transfer absorption band in the 0.65–0.7
μm region.
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