► We studied the phase reddening on near-Earth asteroids and lab samples spectra. ► Phase reddening has no significant impact on the mineralogical analysis. ► Phase reddening is comparable to certain ...degree of space weathering. ► Phase reddening can lead to an ambiguous taxonomic classification.
Phase reddening is an effect that produces an increase of the spectral slope and variations in the strength of the absorption bands as the phase angle increases. In order to understand its effect on spectroscopic observations of asteroids, we have analyzed the visible and near-infrared spectra (0.45–2.5μm) of 12 near-Earth asteroids observed at different phase angles. All these asteroids are classified as either S-complex or Q-type asteroids. In addition, we have acquired laboratory spectra of three different types of ordinary chondrites at phase angles ranging from 13° to 120°. We have found that both, asteroid and meteorite spectra show an increase in band depths with increasing phase angle. In the case of the asteroids the Band I depth increases in the range of ∼2°<g<70° and the Band II depth increases in the range of ∼2°<g<55°. Using this information we have derived equations that can be used to correct the effect of phase reddening in the band depths. Of the three meteorite samples, the (olivine-rich) LL6 ordinary chondrite is the most affected by phase reddening. The studied ordinary chondrites have their maximum spectral contrast of Band I depths at a phase angle of ∼60°, followed by a decrease between 60° and 120° phase angle. The Band II depths of these samples have their maximum spectral contrast at phase angles of 30–60° which then gradually decreases to 120° phase angle. The spectral slope of the ordinary chondrites spectra shows a significant increase with increasing phase angle for g>30°. Variations in band centers and band area ratio (BAR) values were also found, however they seems to have no significant impact on the mineralogical analysis. Our study showed that the increase in spectral slope caused by phase reddening is comparable to certain degree of space weathering. In particular, an increase in phase angle in the range of 30–120° will produce a reddening of the reflectance spectra equivalent to exposure times of ∼0.1×106–1.3×106years at about 1AU from the Sun. This increase in spectral slope due to phase reddening is also comparable to the effects caused by the addition of different fractions of SMFe. Furthermore, we found that under some circumstances phase reddening could lead to an ambiguous taxonomic classification of asteroids.
•The Murchison CM2 chondrite has been comprehensively characterized by reflectance spectroscopy.•Reflectance generally decreases with increasing grain size and spectral slope becomes ...bluer.•Decreasing porosity leads to decreasing absorption band depths.•Increasing phase angle generally results in decreasing band depth, redder spectral slope, and decreasing reflectance.
Carbonaceous chondrites (CCs) are important materials for understanding the early evolution of the solar system and delivery of volatiles and organic material to the early Earth. Presumed CC-like asteroids are also the targets of two current sample return missions: OSIRIS-REx to asteroid Bennu and Hayabusa-2 to asteroid Ryugu, and the Dawn orbital mission at asteroid Ceres. To improve our ability to identify and characterize CM2 CC-type parent bodies, we have examined how factors such as particle size, particle packing, and viewing geometry affect reflectance spectra of the Murchison CM2 CC. The derived relationships have implications for disc-resolved examinations of dark asteroids and sampleability. It has been found that reflectance spectra of slabs are more blue-sloped (reflectance decreasing toward longer wavelengths as measured by the 1.8/0.6 µm reflectance ratio), and generally darker, than powdered sample spectra. Decreasing the maximum grain size of a powdered sample results in progressively brighter and more red-sloped spectra. Decreasing the average grain size of a powdered sample results in a decrease in diagnostic absorption band depths, and redder and brighter spectra. Decreasing porosity of powders and variations in surface texture result in spectral changes that may be different as a function of viewing geometry. Increasing thickness of loose dust on a denser powdered substrate leads to a decrease in absorption band depths. Changes in viewing geometry lead to different changes in spectral metrics depending on whether the spectra are acquired in backscatter or forward-scatter geometries. In backscattered geometry, increasing phase angle leads to an initial increase and then decrease in spectral slope, and a general decrease in visible region reflectance and absorption band depths, and frequent decreases in absorption band minima positions. In forward scattering geometry, increasing phase angle leads to small non-systematic changes in spectral slope, and general decreases in visible region reflectance, and absorption band depths. The highest albedos and larger band depths are generally seen in the lowest phase angle backscattering geometry spectra. The reddest spectra are generally seen in the lowest phase angle backscatter geometry spectra. For the same phase angle, spectra acquired in forward scatter geometry are generally redder and darker and have shallower absorption bands than those acquired in backscatter geometry. Overall, backscatter geometry-acquired spectra are flatter, brighter, and have deeper 0.7 µm region absorption band depths than forward scatter geometry-acquired spectra. It was also found that the 0.7, 0.9, and 1.1 µm absorption bands in Murchison spectra, which are attributable to various Fe electronic processes, are ubiquitous and can be used to recognize CM2 chondrites regardless of the physical properties of the meteorite and viewing geometry.
► 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.
•Photometric characterization of (101955) Bennu, target of the OSIRIS-REx mission, was conducted between the years 2005–2012.•The rotation period, ECAS colors, BVRI colors and phase function were ...determined.•Color photometry is consistent with being a primitive B-type carbonaceous asteroid.•A linear phase slope of 0.040±0.003 magnitudes per degree of phase angle is consistent with an albedo of 0.03–0.04.
The NASA OSIRIS-REx mission will retrieve a sample of the carbonaceous near-Earth Asteroid (101955) Bennu and return it to Earth in 2023. Photometry in the Eight Color Asteroid Survey (ECAS) filter system and Johnson–Cousins V and R filters were conducted during the two most recent apparitions in 2005/2006 and 2011/2012. Lightcurve observations over the nights of September 14–17, 2005 yielded a synodic rotation period of 4.2905±0.0065h, which is consistent with the results of Nolan et al. (2013). ECAS color measurements made during the same nights confirm the B-type classification of Clark et al. (Clark, B.E., Binzel, R.P., Howell, E.S., Cloutis, E.A., Ockert-Bell, M., Christensen, P., Barucci, M.A., DeMeo, F., Lauretta, D.S., Connolly, H., Soderberg, A., Hergenrother, C., Lim, L., Emery, J., Mueller, M. 2011. Icarus 216, 462–475). A search for the 0.7μm hydration feature using the method of Vilas (Vilas, F. 1994. Icarus 111, 456–467) did not reveal its presence. Photometry was obtained over a range of phase angles from 15° to 96° between 2005 and 2012. The resulting phase function slope of 0.040 magnitudes per degree is consistent with the phase slopes of other low albedo near-Earth asteroids (Belskaya, I.N., Shevchenko, V.G. 2000. Icarus 147, 94–105).
Abstract
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 on Mauna Kea, Hawaii, on 2016 July 13 to constrain the object's surface composition, meteorite analogues, and link to other asteroids in the main belt and NEA populations. We also modelled 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. 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. 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. The best possible meteorite analogues for our spectrum of Ryugu are two CM2 carbonaceous chondrites, Mighei and ALH83100.
The exploration of the surface geology of Venus has been hampered by its inhospitable conditions and thick and opaque atmosphere. Fundamental properties, such as crustal composition and heterogeneity ...remain poorly constrained. Multiple analytical techniques are required to better understand its geology. A spectroscopy‐based laboratory study of the emissivity properties of Venus‐relevant igneous rocks, measured at 440°C by Dyar, Helbert, Maturilli, et al. (2020; https://doi.org/10.1029/2020GL090497) shows that the use of multiple atmospheric windows in the 1‐µm region can provide strong constraints on the FeO content of Venus‐relevant igneous rocks, and by extension, the type of igneous rock. These results will help improve our ability to map the surface geology of Venus remotely.
Plain Language Summary
The extreme conditions of Venus’ atmosphere and surface make exploration by optical techniques difficult. A few successful landed missions and radar observations have helped to understand its surface, which appears to be volcanic in nature. In spite of Venus’ global shroud of clouds, some spectral “windows” exist, which are selected wavelengths where the atmosphere and clouds become more transparent. These windows allow us to measure radiation coming off the surface, and differences in the intensity of this radiation can be related to variations in the iron (FeO) content of different rocks, which is also correlated with different types of volcanic rocks.
Key points
Venus surface mapping can be advanced using near‐infrared atmospheric windows
Machine learning and laboratory spectra can help to quantify surface composition
While phyllosilicates were the first water-bearing minerals inferred to be present on Mars from analysis of Earth-based telescopic spectra (McCord et al. 1982), it took recent spacecraft missions to ...enable mapping the global distribution and determining the types of specific species that are present on the surface and their geological context.
New 40Ar/39Ar dating of impact-melted K-feldspars and impact melt rock from the ∼40 km Lake Saint Martin impact structure in Manitoba, Canada, yielded three plateau ages and one mini-plateau age in ...agreement with inverse isochron ages for the K-feldspar melt aliquots and a minimum age for a whole-rock impact melt sample. A combination of two plateau ages and one isochron age, with a weighted mean of 227.8±0.9 Ma ±1.1 Ma; including all sources of uncertainty (2σ; MSWD = 0.52; P=0.59), is considered to represent the best-estimate age for the impact. The concordant 40Ar/39Ar ages for the melted K-feldspars, derived from impact melt rocks in the eastern crater moat domain and the partially melted Proterozoic central uplift granite, suggest that the new dates accurately reflect the Lake Saint Martin impact event in the Carnian stage of the Late Triassic. With a relative error of ±0.4% on the 40Ar/39Ar age, the Lake Saint Martin impact structure counts among the most precisely dated impact structures on Earth. The new isotopic age for Lake Saint Martin significantly improves upon earlier Rb/Sr and (U–Th)/He results for this impact structure and contradicts the hypothesis that planet Earth experienced the formation of a giant ‘impact crater chain’ during a major Late Triassic multiple impact event.
•The large Lake Saint Martin impact structure was dated using 40Ar/39Ar step-heating.•40Ar/39Ar dating produced a weighted mean age of 227.8±1.1 Ma (2σ) for the impact.•The new age for Lake Saint Martin contradicts a Late Triassic multiple impact event.•High-temperature geochronometers are recommended for the accurate dating of impacts.
Abstract The Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instrument onboard the Mars 2020 Perseverance rover detected so far some of the most ...intense fluorescence signals in association with sulfates analyzing abraded patches of rocks at Jezero crater, Mars. To assess the plausibility of an organic origin of these signals, it is key to understand if organics can survive exposure to ambient Martian UV after exposure by the Perseverance abrasion tool and prior to analysis by SHERLOC. In this work, we investigated the stability of organo-sulfate assemblages under Martian-like UV irradiation and we observed that the spectroscopic features of phthalic and mellitic acid embedded into hydrated magnesium sulfate do not change for UV exposures corresponding to at least 48 Martian sols and, thus, should still be detectable in fluorescence when the SHERLOC analysis takes place, thanks to the photoprotective properties of magnesium sulfate. In addition, different photoproduct bands diagnostic of the parent carboxylic acid molecules could be observed. The photoprotective behavior of hydrated magnesium sulfate corroborates the hypothesis that sulfates might have played a key role in the preservation of organics on Mars, and that the fluorescence signals detected by SHERLOC in association with sulfates could potentially arise from organic compounds.
► 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.
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