Context.
The Hayabusa2 spacecraft launched by Japan Aerospace Exploration Agency has been conducting observations of the asteroid (162173) Ryugu since June 2018. The Telescopic Optical Navigation ...Camera (ONC-T) onboard Hayabusa2 has obtained thousands of images under a variety of illumination and viewing conditions.
Aims.
Our objective is to examine and validate the camera calibration, derive a photometric correction for creating global albedo maps, and to interpret the photometric modeling results to characterize the surface of Ryugu.
Methods.
We observed (162173) Ryugu with the Gemini-South telescope, and combined these measurements with other published ground-based observations of the asteroid. The ground-based observations were compared with the data obtained by ONC-T in order to validate the radiometric calibration mutually. We used a combination of the Hapke disk-integrated and disk-resolved model equations to simultaneously analyze the combined ground- and spacecraft-based data.
Results.
The average spectrum of Ryugu was classified as Cb-type following the SMASSII taxonomy and C/F-type following the Tholen taxonomy based on spacecraft observations. We derived Hapke model parameters for all seven color filters, which allowed us to photometrically correct images to within an error of <10% for ~80% of the image pixels used in the modeling effort. Using this model, we derived a geometric albedo of 4.0 ± 0.5% (
v
band) for Ryugu. The average reflectance factor at the standard illumination condition was 1.87 ± 0.14% in the
v
band. Moreover we measured a phase reddening of (2.0 ± 0.7) × 10
−3
μ
m
−1
deg
−1
for Ryugu, similar to that observed for the asteroid (101955) Bennu.
Conclusions.
The global color map showed that the general trend was for darker regions to also be redder regions, however there were some distinct exceptions to this trend. For example, Otohime Saxum was bright and red while Kibidango crater was dark and blue. The darkness and flatness of Ryugu’s reflectance might be caused by a high abundance of organic materials.
Context.
Space weathering (SW) is crucial to improve the understanding of the evolution of optical characteristics on airless bodies. The classical view based on research of the Moon suggests that SW ...decreases albedo (darkening) and steepens spectral slope (reddening) in visible to near-infrared (VIS-NIR) wavelengths, producing nanophase iron (npFe
0
). However, this conclusion is not perfectly applicable to asteroids.
Aims.
In this study, we focus on investigating the space weathering spectral alteration effects (SWSAE) and the causes of spectral alteration on various types of asteroids after long-term continuous micrometeoroid bombardments.
Methods.
We used a pulsed laser to irradiate eight meteorites at the same energy, namely, of 28 mJ, in ten shots, including ordinary chondrites (OCs), aubrite (Aub), enstatite chondrites (ECs), CO, CV, and CM carbonaceous chondrites. Then we measured and compared the virgin and irradiated VIS-NIR reflectance spectra of these meteorites. We further surveyed the causes of spectral alteration through a scanning electron microscope and transmission electron microscope.
Results.
Three different SWSAE are shown: (1) darkening and reddening on OCs, Aub, CO, and CV chondrites; (2) brightening and reddening on ECs; (3) brightening and bluing on CM chondrite. After irradiation, npFe
0
and nanophase iron-nickel particles were respectively found in CV and CO chondrites; thick amorphous layers without any nanophase particles were found in Aub; melting and sputtering of metal were observed in ECs; a great deal of vesicles or bubbles without any nanophase particles were found in CM chondrite.
Conclusions.
The long-term SW via micrometeoroid bombardments can spectrally remodel asteroid surfaces in different ways: darken and redden anhydrous silicate asteroids (e.g., S-, E-, and K-types); brighten and redden metal-rich M-type objects. The SWSAE of volatiles-rich carbonaceous asteroids (e.g., Ch-, Cgh-, and D-types) is related to SW degree: darkening and bluing at low degree then brightening and continue bluing as the SW degree increases. The various spectral units on Ryugu, Bennu, and Phobos can be created by the heterogeneity of the degree of SW.
► 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 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.
We present a new global survey of the purest anorthosite (PAN) rock using the Spectral Profiler onboard Kaguya. We found that PAN rocks are widely distributed over the Moon, including the Feldspathic ...Highland Terrain and the south and north polar regions. All PAN sites are associated with huge impact structures with diameters larger than 100 km. Based on the global distributions of PAN and olivine‐rich sites, we propose the existence of a massive PAN layer with a thickness of ∼50 km below an uppermost mafic‐rich mixed layer with a thickness of ∼10 km. Below the PAN layer, a lower crustal layer with olivine‐rich materials may be present on the nearside, but not on the far side of the Moon. The existence of a PAN layer with a thickness of ∼50 km suggests an Al2O3 abundance of 33 to 34 wt.% in the lunar crust, which is higher than previous estimates of <32 wt.%. Our data indicate the massive production event of PAN during the early stage of the formation of the Moon, supporting the lunar magma ocean scenario.
Key Points
The existence of a massive layer of pure anorthosite on the Moon
Massive production of pure anorthosite produced during the lunar magma ocean
Dichotomy of lunar lower crust
ABSTRACT CM chondrites are the most common type of hydrated meteorites, making up ∼1.5% of all falls. Whereas most CM chondrites experienced only low-temperature (∼0°C-120°C) aqueous alteration, the ...existence of a small fraction of CM chondrites that suffered both hydration and heating complicates our understanding of the early thermal evolution of the CM parent body(ies). Here, we provide new constraints on the collisional and thermal history of CM-like bodies from a comparison between newly acquired spectral measurements of main-belt Ch/Cgh-type asteroids (70 objects) and existing laboratory spectral measurements of CM chondrites. It first appears that the spectral variation observed among CM-like bodies is essentially due to variations in the average regolith grain size. Second, the spectral properties of the vast majority (unheated) of CM chondrites resemble both the surfaces and the interiors of CM-like bodies, implying a "low" temperature (<300°C) thermal evolution of the CM parent body(ies). It follows that an impact origin is the likely explanation for the existence of heated CM chondrites. Finally, similarly to S-type asteroids and (2) Pallas, the surfaces of large (D > 100 km)-supposedly primordial-Ch/Cgh-type main-belt asteroids likely expose the interiors of the primordial CM parent bodies, a possible consequence of impacts by small asteroids (D < 10 km) in the early solar system.
ABSTRACT
The JAXA Hayabusa2 mission accomplished the formation of an artificial crater on the asteroid Ryugu. The aim of this work is to analyse the area surrounding the artificial crater and reveal ...spectral variability compared to the same region before the crater formation, to mineralogically and physically characterize the subsurface exposed material. The crater’s investigation focused on the analysis of two regions corresponding to the inner part of crater (the pit and the crater wall/floor), two areas related to ejecta deposited close to the crater, two areas of ejecta moved far from the crater, and two external areas. Each area was investigated both before and after the crater formation, by the study of the photometrically corrected spectral parameters: the 1.9 $\mu$m reflectance, the near-infrared spectral slope, and the depth of the bands at 2.7 and 2.8 $\mu$m. The subsurface material of the post-crater areas shows deeper absorption bands, a decrease in reflectance, and a reddening in spectral slope with respect to the surface material of pre-crater areas. The subsurface regolith could have experienced a lower OH devolatilization due to space weathering and/or could be composed of finer dark grains than the surface layer. The ejecta reached distances of $\sim $20 m from the impact point, mainly moving in the northern direction; nevertheless, a few ejecta also reached the south-eastern part of crater.
The Near-Earth Asteroid 162173 Ryugu (1999 JU3) was investigated by the JAXA Hayabusa2 mission from June 2018 to November 2019. The data acquired by NIRS3 spectrometer revealed a dark surface with a ...positive near-infrared spectral slope. In this work we investigated the spectral slope variations across the Ryugu surface, providing information about physical/chemical properties of the surface.
We analysed the calibrated, thermally and photometrically corrected NIRS3 data, and we evaluated the spectral slope between 1.9 μm and 2.5 μm, whose values extend from 0.11 to 0.28 and the mean value corresponds to 0.163±0.022. Starting from the mean value of slope and moving in step of 1 standard deviation (0.022), we defined 9 “slope families”, the Low-Red-Slope families (LR1, LR2 and LR3) and the High-Red-Sloped families (HR1, HR2, HR3, HR4, HR5, HR6). The mean values of some spectral parameters were estimated for each family, such as the reflectance factor at 1.9 μm, the spectral slope, the depth of bands at 2.7 μm and at 2.8 μm. A progressive spectral reddening, darkening and weakening/narrowing of OH bands is observed moving from the LR families to the HR families.
We concluded that the spectral variability observed among families is the result of the thermal metamorphism experienced by Ryugu after the catastrophic disruption of its parent body and space weathering processes that occurred on airless bodies as Ryugu, such as impact cratering and solar wind irradiation. As a consequence, the HR1, LR1, LR2 and LR3 families, corresponding to equatorial ridge and crater rims, are the less altered regions on Ryugu surface, which experienced the minor alteration and OH devolatilization; the HR2, HR3, HR4, HR5 families, coincident with floors and walls of impact craters, are the most altered areas, result of the three processes occurring on Ryugu. The strong reddening of the HR6 family (coincident with Ejima Saxum) is likely due to the fine-sized material covering the large boulder.
•We performed VIS–NIR spectro-goniometric measurements on multiple granular samples.•Photometric parameters were derived from the Hapke model over the VIS–NIR range.•The phase function and the ...macroscopic roughness are wavelength-dependent.•The phase function is affected by the structure and the absorptivity of the grains.•Hapke macroscopic roughness is to first order correlated with the grains absorptivity.
Surface scattered sunlight carries important information about the composition and microtexture of surface materials, thus enabling tracing back the geological and climatic processes that occurred on the planetary body. Here we perform laboratory spectro-goniometric measurements of granular samples (45–75μm fraction) with different composition and physical properties over the VIS–NIR spectral range (0.4–2.5μm). To quantify the evolution of the scattering properties over the VIS–NIR, we use an inversion procedure based on a Bayesian approach to estimate photometric parameters from the Hapke radiative transfer model. The granular samples are also carefully characterized by optical and SEM techniques in order to link these scattering variations with the grains’ physical properties. Results show that the scattering properties are wavelength-dependent and can vary significantly over the VIS–NIR spectral range. In particular, the phase function of a granular material is affected by both the absorptivity and the external and internal structure of the grains, from the millimeter scale down to the wavelength scale. Our results also confirm that the macroscopic roughness parameter, as defined by Hapke, is to first order correlated with the absorptivity of the particles, through multiple scattering effects, and thus mostly corresponds to a measurement of the particles shadowing. Photometric datasets, typically obtained at a given wavelength that can vary from one study to another, should therefore be compared and interpreted with caution when extrapolating across wavelengths. Our results also suggest that multi-wavelength photometry could potentially provide a much richer signature than with single-wavelength photometry, opening new perspectives into the characterization of surface materials.
► 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.
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