Space Weathering Research Articles

Study on surface characteristics of Chang’E-5 fine grained lunar soil

<p indent="0mm">Chang’E-5 lunar soil is the youngest lunar soil sample returned by human beings so far, and the shovel samples record the space weathering history of the lunar surface in sampling area. This study found that on the surface of the fine-grained lunar soil particles of Chang’E-5 are widely distributed typical morphological features, such as microcraters, mineral cleavage surfaces, gasification deposition, and covered glass, which reflect that in the space dominated by meteorites and micrometeorites bombardment, the effects of impact fragmentation, melting sputtering, gasification deposition, etc., have transformed the morphology and characteristics of lunar soil particles during weathering transfomation. The results reveal the transformation mechanism of the extreme environment of the lunar surface on the mineral morphology, thus helping scholars better understand the weathering process in space.

Regolith mineral detection and abundance estimation based on the LMS spectral data of Chang&amp;rsquo;E-5 lander

<p indent="0mm">Determining the mineral components of the lunar regolith is key to understanding the Moon’s genesis and evolution, and it is one of the important scientific objectives of the Chang’E-5 Mission. The <italic>in-situ</italic> spectra collected by the lunar mineralogical spectrometer (LMS) of Chang’E-5 provide valuable data for interpreting the mineral composition of the landing site. However, little work has been performed to analyze the LMS data, and the traditional lunar mineral detection method based on the radiation transfer model cannot simultaneously obtain information on the mineral end-member, abundance, and particle size. To achieve the multiple goals, we propose a sparse unmixing method based on the spectral library of various particle-size minerals. The mineral detection problem is transformed into a sparse optimization problem and solved by the alternating direction algorithm. We have analyzed eight sets of LMS reflectance spectra and found that the lunar soil in the sampling area is composed of agglutinates and glass (49.36%), pyroxene (14.08%), plagioclase (18.58%), ilmenite (16.11%), and olivine (1.86%). The detected mineral composition is consistent with the composition of young lunar mare basalts. In addition, 79.83% of the detected minerals have a small particle size of less than 75 μm, of which the average particle size of olivine, plagioclase, pyroxene, ilmenite, agglutinates, and glass is 48, 205, 59, 37.4, and 9.7 μm, respectively. Differences in mineral particle size suggest that the effects of space weathering on the lunar surface vary by the component minerals. Moreover, the LMS data provide <italic>in-situ</italic> spectral evidence of water existence on the lunar surface, i.e., the spectral absorption at <sc>2850 nm.</sc> We estimate the water content of the sampling area using the effective single-particle absorption-thickness (ESPAT) value and the temperature derived from an improved lunar thermal model that has been cross-validated with the temperature records of several thermal sensors on the Chang’E-5 lander. The result shows that rocks are more hydrous than the regolith at the Cheng’E-5 landing site. The water content of the rock is as high as <sc>201 ppm,</sc> while the average water content of the sampling area is <sc>67 ppm.</sc>

Quantifying the effects of submicroscopic metallic iron on VIS–NIR spectra of lunar soils

Metallic iron (Fe0) particles with sizes ranging from a few nanometers to the submicroscopic scale and formed by space weathering are specific components of lunar soil. Previous studies have suggested that the iron significantly alters the optical properties of lunar soil. For example, nanophase metallic iron (npFe0) causes both reddening and darkening of the lunar soil spectrum, and submicroscopic metallic iron (SMFe) only causes darkening. Here, we prepared SMFe particles with an average size of ∼180 nm embedded within melt glasses through carbothermal reduction experiments to analogize agglutinated glasses in the lunar soil. We evaluated the effect of SMFe content on visible and near-infrared (VIS–NIR) reflectance spectra of these lunar soil samples simulants. The spectral data show that SMFe content plays a key role in the optical properties of samples, including the average reflectance in the VIS-NIR range (400–2150 nm), and the absorption depth at ∼2 μm. A small amount (0.05 wt%) of SMFe mainly causes significant spectral darkening, and the average reflectance is reduced by 50% when the SMFe content rises to 0.36 wt%. Both the average reflectance and the absorption depth at ∼2 μm show a negative correlation with the SMFe content. We developed a quantitative model relating the spectral characteristics and the SMFe abundance based on experimental results. Thus, the SMFe contents play a key role in altering spectral characteristics of airless bodies during remote sensing spectroscopic detection.

Optimized sintering strategy for lunar regolith simulant particles bound via vat photopolymerization

In deep space exploration, additive manufacturing (AM) of lunar regolith is extremely attractive by using Moon's in-situ resources with the potential to reduce spacecraft launching costs and alleviate Earth's resource limit. However, previous studies are limited to feasibility investigation by fabricating samples using lunar regolith simulant (LRS) and have not considered real lunar environmental conditions such as space weathering, temperature cycles, and exosphere, leaving a big unknown on how to obtain reliable samples via AM using really lunar regolith on the Moon. Hence, the influence of several environmental conditions on LRS-based vat photopolymerization (VP) is comprehensively studied in this work, to obtain better mechanical properties for practical application. First, the effects of different particle size distributions in the lunar regolith caused by space weathering on VP are examined. An optimal middle D50 of 2.37 μm is proved to be beneficial for great stability, high resolution (30 μm), and mechanical properties. Secondly, the strategy of debinding in the nitrogen followed by sintering in the air is proven to be more beneficial for improving mechanical properties and reducing oxygen consumption, preventing samples from reduced durability caused by the wide temperature cycle. Overall, this study provides an initial step for developing in-situ resource utilization on the Moon using the AM method by considering the lunar environment conditions.

Lunar glass

Lunar glass, a significant component of lunar soil, is produced by non-equilibrium processes on the moon, such as volcanic eruptions, meteorite impacts, solar wind, and cosmic radiation. Lunar glass of different origins has ability to record historical information of the formation and evolution of the moon. This article presents a comprehensive review of the research progress of lunar glasses found within the CE-5 lunar soil. Delving into their fundamental physical properties and microstructure, we explore the specific mechanisms behind the formation of lunar glasses. Furthermore, this article focuses on the various roles that lunar glasses play in studies of lunar evolution, such as acting as a “natural camera” that captures the moon's internal and surface changes over different epochs, encompassing lunar origin, magma activity, impact events, space weathering, and the origin of water. The ultra-stable lunar glass with disordered atomic structure can sustainably preserve lunar resources. It is estimated that lunar glasses have reserved approximately 260000 tons of &lt;sup&gt;3&lt;/sup&gt;He, and 27 billion tons of water. Moreover, lunar glasses serve as an invaluable lunar chronometer, providing a reliable temporal framework to data volcanic activity and impact events. This temporal framework, in turn, serves as a vital tool for investigating the evolution of lunar water, magnetic fields and reconstructing the extensive billion-year history of lunar impacts.

Neural network for determining an asteroid mineral composition from reflectance spectra

Context.Chemical and mineral compositions of asteroids reflect the formation and history of our Solar System. This knowledge is also important for planetary defence and in-space resource utilisation. In the next years, space missions will generate extensive spectral datasets from asteroids or planets with spectra that will need to be processed in real time.Aims.We aim to develop a fast and robust neural-network-based method for deriving the mineral modal and chemical compositions of silicate materials from their visible and near-infrared spectra. The method should be able to process raw spectra without significant pre-processing.Methods.We designed a convolutional neural network with two hidden layers for the analysis of the spectra, and trained it using labelled reflectance spectra. For the training, we used a dataset that consisted of reflectance spectra of real silicate samples stored in the RELAB and C-Tape databases, namely olivine, orthopyroxene, clinopyroxene, their mixtures, and olivine-pyroxene-rich meteorites.Results.We used the model on two datasets. First, we evaluated the model reliability on a test dataset where we compared the model classification with known compositional reference values. The individual classification results are mostly within 10 percentage-point intervals around the correct values. Second, we classified the reflectance spectra of S-complex (Q-type and V-type, also including A-type) asteroids with known Bus–DeMeo taxonomy classes. The predicted mineral chemical composition of S-type and Q-type asteroids agree with the chemical composition of ordinary chondrites. The modal abundances of V-type and A-type asteroids show a dominant contribution of orthopyroxene and olivine, respectively. Additionally, our predictions of the mineral modal composition of S-type and Q-type asteroids show an apparent depletion of olivine related to the attenuation of its diagnostic absorptions with space weathering. This trend is consistent with previous results of the slower pyroxene response to space weathering relative to olivine.Conclusions.The neural network trained with real silicate samples and their mixtures is applicable for a quantitative mineral evaluation of spectra of asteroids that are rich in dry silicates. The modal abundances and mineral chemistry of common silicates (olivine and pyroxene) can be derived with an accuracy better than 10 percentage points. The classification is fast and has a relatively small computer-memory footprint. Therefore, our code is suitable for processing large spectral datasets in real time.

Review of the Spectral Effects of Space Weathering on C-type Asteroids

Review of the Spectral Effects of Space Weathering on C-type Asteroids

Detection of ferric iron in an exsolved lunar pyroxene using electron energy loss spectroscopy (EELS): Implications for space weathering and redox conditions on the Moon

AbstractTo shed light on the mechanism of formation of nanophase iron particles (npFe) in space‐weathered materials from airless bodies, we analyzed exsolved and unexsolved space‐weathered lunar pyroxenes from Apollo 17 sample 71501. The exsolved pyroxene allowed for the observation of the effects of space weathering on similar mineral phases with variable composition. Using coordinated scanning transmission electron microscopy with energy‐dispersive X‐ray spectroscopy and electron energy loss spectroscopy (EELS), we determined that two coexisting pyroxenes in the exsolved grain showed systematic variations in response to space weathering, despite equivalent exposure conditions. The npFe in the space‐weathered rim of augite lamellae were smaller and fewer than the npFe in the rim of pigeonite lamellae. EELS spectrum imaging revealed the presence and heterogeneous distribution of Fe0, Fe2+, and Fe3+ in the exsolved pyroxene. Metallic iron occurred in the npFe, a mixture of Fe2+ and Fe3+ occurred in the pigeonite lamellae, and the augite lamellae contained virtually all Fe3+. Approximately 50% of the total Fe measured in the exsolved pyroxene grain was ferric. Partitioning of Fe2+ and Fe3+ among the lamellae is invoked to explain the difference in npFe development in pigeonite and augite. The results of this study, the first to identify Fe3+ in a crystalline lunar ferromagnesian silicate, have implications for our understanding of how space weathering might proceed in oxidized phases. Furthermore, the discovery of an Fe3+‐rich pyroxene also supports attribution of the 0.7 μm absorption feature observed in Galileo Solid State Imager data to oxidized Fe in clinopyroxenes.

Photometry of LROC NAC resolved rock-rich regions on the Moon

The study and investigation of meter and sub-meter scale geological features, especially boulders and boulder fields, on the surface of airless bodies can provide insight into the evolution of the regolith and the contribution of various processes to its formation. Prior studies have examined the photometric properties of the lunar regolith surrounding young craters using image ratios. We extend this methodology to extracting surface properties, in particular the roughness characteristics, exclusive to boulder fields and the boulders that constitute them around impact craters. Our understanding of the response of boulders to space weathering, micrometeorite abrasion, thermal fatigue, and consequently their evolution into regolith can be improved by characterizing the surface roughness of the uppermost layer of boulders. In this study, rock-rich regions on the Moon are investigated using photometric roughness by employing a normalised logarithmic phase ratio difference metric to measure and compare the slope of the phase curve (reflectance versus phase angle) of a rock-rich field to a rock-free field. We compare the photometric roughness of rock-rich fields on simulated images with the photometric roughness of rock-rich fields on LROC NAC images (0.5 m/pixel). The simulated terrains are constructed with a set of geologically informed rock properties, i.e., rock size, shape, and size frequency distribution. The artificial terrains are then converted to reflectance images using the Hapke AMSA (Hapke, 2002) reflectance model. Using this technique, we determine that rock-rich surfaces are not necessarily photometrically rougher than rock-free areas. Additionally, we find the roughness of resolved rock fields to indicate the presence of diverse sub-mm scale rock roughness (microtopography) and, possibly, variable rock single scattering albedo. These latter properties are likely controlled by rock petrology and material response to weathering and erosion. Spatial clustering of photometrically smooth and rough boulder fields in the downrange and uprange of two craters is observed, reflecting ejecta asymmetry and possibly indicating asymmetric modification of ejecta rock surfaces during the impact excavation process.

A dehydrated space-weathered skin cloaking the hydrated interior of Ryugu

Without a protective atmosphere, space-exposed surfaces of airless Solar System bodies gradually experience an alteration in composition, structure and optical properties through a collective process called space weathering. The return of samples from near-Earth asteroid (162173) Ryugu by Hayabusa2 provides the first opportunity for laboratory study of space-weathering signatures on the most abundant type of inner solar system body: a C-type asteroid, composed of materials largely unchanged since the formation of the Solar System. Weathered Ryugu grains show areas of surface amorphization and partial melting of phyllosilicates, in which reduction from Fe3+ to Fe2+ and dehydration developed. Space weathering probably contributed to dehydration by dehydroxylation of Ryugu surface phyllosilicates that had already lost interlayer water molecules and to weakening of the 2.7 µm hydroxyl (–OH) band in reflectance spectra. For C-type asteroids in general, this indicates that a weak 2.7 µm band can signify space-weathering-induced surface dehydration, rather than bulk volatile loss.

Mature lunar soils from Fe-rich and young mare basalts in the Chang’e-5 regolith samples

Space weathering on airless bodies produces metallic iron (Fe0) particles in the rims of mineral grains, which affect visible and near-infrared spectra and complicate the identification of surface materials. The Chang’e-5 mission provides an opportunity to couple information gained from its returned samples with in situ observations and orbital monitoring to gain insight on the details of space weathering on extremely Fe-rich basalts. By putting together all these data, we could extract a soil maturity index (Is/FeO) at the Chang’e-5 landing site of ~66 ± 3.2, indicative of a formation age for the Xu Guangqi crater, whose ejecta dominate the site, of 240–300 Myr ago. In addition, abundant large Fe0 particles were found in the sample, indicating that both the inherited Fe0 particles from late-stage mare basalts and the dense clustering of oversaturated Fe0 in extremely FeO-rich (>17 wt%) basalts contribute to observed Fe0 abundances. We suggest that space weathering of Fe-richer basalt generates Fe0 particles with a larger grain size and faster production rate.

Review of meteorite irradiation tests to support next C-type asteroid missions

ABSTRACT Effect of space weathering of airless asteroids could be better understood by artificial irradiation tests on meteorites in laboratories. This work surveys the infrared and Raman analysis based interpretation of simulated charged particle irradiation tests in order to better understand near-future observational possibilities of asteroid visiting missions and also to support the planning of next missions and directions of detector improvement. Recent works properly targeted different meteors and meteor relevant minerals, evaluating bulk meteorite spectra, during the irradiation tests. He+, (Ne+, Kr+), and Ar+ ions were used with fluxes characteristic for inner planetary system solar wind, considering 1–10 million yr exposure durations. Although main meteorite minerals were irradiated and analysed, one missing aspect is that only bulk analysis have been done, not minerals separately in their original embedded context. Some Earth based mineral references were also analysed; however, they might not necessarily behave similar to the same type of reference minerals and irradiation effect is poorly known for feldspar, troilite, and magnetite. Darkening should be also further analysed for separate minerals too, together with the record of peak shape and position changes. Infrared ATR analysis might still provide such data in the future using the recently emerged technology, as well as Raman analysis – however for flyby missions’ infrared is the useful method while Raman can be used only at in situ missions. The overview including the tables to support the identification of specific missing information related gaps in our current knowledge and directions for future research.

Dark spots on Mercury show no signs of weathering during 30 Earth months

Dark spots on Mercury are recently-formed thin and low-reflectance materials that are related with volatile activity and supposed to be much shorter-lived than their central hollows. Containing unique information about the possible building blocks of Mercury, dark spots have unresolved darkening phases, formation mechanism, and lifetime. Here we investigated reflectance spectra, sub-resolution roughness, and temporal changes of dark spots using the full-mission dataset of MESSENGER. We find that dark spots have the highest concentration of graphite and an intense outgassing origin. Temporal imaging for dark spots reveals no detectable reflectance changes in 30 Earth months, and possibly over 40 Earth years. These observations demonstrate that thermally unstable sulfides such as MgS and CaS are not the major components in dark spots. Possible reflectance changes of dark spots may occur at much longer time scales, providing a reference for modeling the spectral behavior of graphite caused by space weathering on Mercury.

Simulation of surface regolith gardening and impact associated melt layer production under ns-pulsed laser ablation

The effect of surface regolith gardening and melt layer production produced by space weathering (SW) (owing to micrometeorite bombardment) of surficial regolith layers of airless planetary surfaces was investigated in an experimental setup by using laser-induced ablation of powdered analog material (synthetic Fo100) under vacuum with a ns-pulsed infrared laser. The investigated analog pellets were prepared from the fine fraction (< 1 μm) up to a grain size of 280 μm, which resembles the uppermost regolith surface of many airless planetary bodies. The Fo-powder was pressed into shape to form a pellet. We focused here on nanometer-sized structural modifications that are induced in the relocated grains, sputtered off ejecta material and melt sprinkles that formed away from the craters caused by laser irradiation of the pressed pellet surface. The ejecta particles were redistributed over the entire pellet surface and beyond. The forming sputter film, melt sprinkles and ballistically ejected grains were caught on carbon film grids positioned nearby the craters. The grids were investigated with a transmission electron microscope (TEM) to discern between the distinct deposition types that were formed by ejecta condensate and partially molten ejected nanometer-size analog grains. Apart from a heavily modified pellet surface, we found that deposited droplets are mostly amorphous with minor nanocrystalline subdomains. Eight out of ten droplets show distinct incipient crystallization stages. This indicates at a relatively high amount of amorphous regolith material at the incipient stage of SW for airless bodies, if the regolith is altered via micrometeorite bombardment.

Far‐Ultraviolet Photometric Characteristics of JSC‐1A and LMS‐1 Lunar Regolith Simulants: Comparative Investigations With Apollo 10084

AbstractWe have characterized the far‐ultraviolet (FUV) spectro‐photometric response of lunar soil simulants JSC‐1A and LMS‐1, reporting notable differences from our previous results for Apollo soil 10084 (Raut et al., 2018, https://doi.org/10.1029/2018JE005567). While JSC‐1A and LMS‐1 were designed to emulate the geotechnical and compositional properties of a low‐Ti and high‐Ti mare soil respectively, these terrestrial simulants lack “space weathering” attributes such as the nanophase iron present in the weathered rims of Apollo grains and glassy agglutinates. Photometric analyses of the JSC‐1A phase curves reveal a ∼3–4 fold increase in single scattering albedo (SSA) and a forward scattering behavior compared to 10084. LMS‐1 is shown to have SSA nearly twice that of 10084 and a near isotropic reflectance. Additionally, both JSC‐1A and LMS‐1 spectra present a blue slope in the FUV, with the JSC‐1A slope ∼10× larger than that reported for the 10084 soil. Our analyses imply that low‐Ti content corroborated using energy dispersive x‐ray spectroscopy, correlates to brighter FUV reflectance and a greater spectral blue slope for JSC‐1A, while space weathering components likely contribute to the backscattering of FUV light by the Apollo soil relative to both simulants. Further work with an extended set of Apollo soils is warranted to deconvolute the relative contributions of weathering and composition to their FUV spectro‐photometric response.

Estimation of the space weathering timescale on (25143) Itokawa: Implications on its rejuvenation process

Context. The space weathering timescales of near-Earth S-type asteroids have been investigated using several approaches (i.e., experiments, sample analyses, and theoretical approaches), yet there are orders of magnitude differences. Aims. We aim to examine the space weathering timescale on a near-Earth S-type asteroid, Itokawa, using Hayabusa/AMICA images, and further investigate the evolutional process of the asteroid. Methods. We focused on bright mottles on the boulder surfaces generated via impacts with interplanetary dust particles (IDPs). We compared the bright mottle size distribution with an IDP flux model to determine the space weathering timescale. Results. We found that the space weathering timescale on Itokawa’s boulder surfaces is 103 yr (in the range of 102–104 yr), which is consistent with the timescales of space weathering by light ions from the solar wind. Conclusions. From this result, we conclude that Itokawa’s surface has been weathered rapidly in 103 yr but portions of the surface are exposed due to seismic shaking that was triggered by a recent impact, which created the Kamoi crater.

Spectral Evolution of Dark Asteroid Surfaces Induced by Space Weathering over a Decade

The surfaces of airless bodies like asteroids in the solar system are known to be affected by space weathering. Experiments simulating space weathering are essential for studying the effects of this process on meteorite samples, but the problem is that the time spent to reproduce space weathering in these experiments is billions of times shorter than the actual phenomenon. In 2010 December, the T-type asteroid 596 Scheila underwent a collision with an impactor a few tens of meters in size. A decade later, there is an opportunity to study how the surface layer of this asteroid is being altered by space weathering after the impact. To do so, we performed visible spectrophotometric and near-infrared spectroscopic observations of 596 Scheila. The acquired spectrum is consistent with those observed shortly after the 2010 impact event within the observational uncertainty range. This indicates that the surface color of dark asteroids is not noticeably changed by space weathering over a 10 yr period. This study is the first to investigate color changes due to space weathering on an actual asteroid surface in the solar system. Considering that fresh layers are regularly created on asteroid surfaces by collisions, we suggest a genetic link between D/T-type and dark (low albedo) X-complex asteroids and very red objects such as 269 Justitia, 732 Tjilaki, and 203 Pompeja. New observations show that 203 Pompeja has an X-type-like surface, with some local surface areas exhibiting a very red spectrum.

Sub-surface alteration and related change in reflectance spectra of space-weathered materials

Context. Airless planetary bodies are studied mainly by remote sensing methods. Reflectance spectroscopy is often used to derive their compositions. One of the main complications for the interpretation of reflectance spectra is surface alteration by space weathering caused by irradiation by solar wind and micrometeoroid particles. Aims. We aim to evaluate the damage to the samples from H+ and laser irradiation and relate it to the observed alteration in the spectra. Methods. We used olivine (OL) and pyroxene (OPX) pellets irradiated by 5 keV H+ ions and individual femtosecond laser pulses and measured their visible (VIS) and near-infrared (NIR) spectra. We observed the pellets with scanning and transmission electron microscopy. We studied structural, mineralogical, and chemical modifications in the samples. Finally, we connected the material observations to changes in the reflectance spectra. Results. In both minerals, H+ irradiation induces partially amorphous sub-surface layers containing small vesicles. In OL pellets, these vesicles are more tightly packed than in OPX ones. Any related spectral change is mainly in the VIS spectral slope. Changes due to laser irradiation are mostly dependent on the material’s melting temperature. Of all the samples, only the laser-irradiated OL contains nanophase Fe particles, which induce detectable spectral slope change throughout the measured spectral range. Our results suggest that spectral changes at VIS-NIR wavelengths are mainly dependent on the thickness of (partially) amorphous sub-surface layers. Furthermore, amorphisation smooths micro-roughness, increasing the contribution of volume scattering and absorption over surface scattering. Conclusions. Soon after exposure to the space environment, the appearance of partially amorphous sub-surface layers results in rapid changes in the VIS spectral slope. In later stages (onset of micrometeoroid bombardment), we expect an emergence of nanoparticles to also mildly affect the NIR spectral slope. An increase in the dimensions of amorphous layers and vesicles in the more space-weathered material will only cause band-depth variation and darkening.

Phobos and Deimos surface composition: search for spectroscopic analogues

ABSTRACT Phobos and Deimos, the two satellites of Mars, were largely studied in the past using ground-based telescope and spacecraft data, although most of the data were obtained by opportunity observations performed by Mars dedicated orbiters. Despite the data available so far, the main composition of the two moons is not yet fully understood. The possible presence of hydrated minerals along with mafic minerals olivine and pyroxene seems to be the most plausible interpretation, but more investigations are needed. MIRS spectrometer on-board the future JAXA MMX sample return mission will help to unveil the open question on the composition of Phobos and Deimos. In this work, we review past spectroscopic observations of the Martian moons, both from ground observatories and spacecraft data set, aiming at better understanding the constraints in interpreting the Mars satellites composition and at identifying the best spectroscopic analogues. We also present new laboratory measurements on mineral mixing and meteorites to match the satellites spectral behaviour. New measurements were acquired at INAF-Astrophysical Observatory of Arcetri and IPAG laboratories at room conditions exploring different geometries and the results obtained set new constraints for future laboratory measurements. Our preliminary results confirm that the surface of Phobos and Deimos can be associated with samples characterized by a higher presence of dark components (e.g. amorphous carbon) or minerals produced by space weathering (e.g. Fe0 and FeS-bearing materials). Presence of dark component could also be totally responsible for the reduced hydrated band observed on the moons without invoking dehydration or OH-implantation on anhydrous surface.

A new approach to feature-based asteroid taxonomy in 3D color space

The taxonomic classification of asteroids has been mostly based on spectroscopic observations with wavelengths spanning from the visible (VIS) to the near-infrared (NIR). VIS-NIR spectra of ~2500 asteroids have been obtained since the 1970s; the Sloan Digital Sky Survey (SDSS) Moving Object Catalog 4 (MOC 4) was released with ~4 × 105 measurements of asteroid positions and colors in the early 2000s. A number of works then devised methods to classify these data within the framework of existing taxonomic systems. Some of these works, however, used 2D parameter space (e.g., gri slope vs. z-i color) that displayed a continuous distribution of clouds of data points resulting in boundaries that were artificially defined. We introduce here a more advanced method to classify asteroids based on existing systems. This approach is simply represented by a triplet of SDSS colors. The distributions and memberships of each taxonomic type are determined by machine learning methods in the form of both unsupervised and semi-supervised learning. We apply our scheme to MOC 4 calibrated with VIS-NIR reflectance spectra. We successfully separate seven different taxonomy classifications (C, D, K, L, S, V, and X) with which we have a sufficient number of spectroscopic datasets. We found the overlapping regions of taxonomic types in a 2D plane were separated with relatively clear boundaries in the 3D space newly defined in this work. Our scheme explicitly discriminates between different taxonomic types (e.g., K and X types), which is an improvement over existing systems. This new method for taxonomic classification has a great deal of scalability for asteroid research, such as space weathering in the S-complex, and the origin and evolution of asteroid families. We present the structure of the asteroid belt, and describe the orbital distribution based on our newly assigned taxonomic classifications. It is also possible to extend the methods presented here to other photometric systems, such as the Johnson-Cousins and LSST filter systems.