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A&A
Volume 672, April 2023
Article Number A189
Number of page(s) 15
Section Planets and planetary systems
DOI https://doi.org/10.1051/0004-6361/202244499
Published online 20 April 2023

© The Authors 2023

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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1 Introduction

The current state of the asteroid distribution is the result of many incidents that have occurred over their lifetimes: the radial transference triggered by the movements of gas giants, the catastrophic disruptions among asteroids, mixing of asteroids, solar-radiation-induced radial scattering, and gravitational interferences through resonances of planets. It is currently widely accepted that asteroids were formed in different places from their current locations and were subsequently displaced by the migration of gas and ice giants (e.g., Morbidelli et al. 2005; Walsh et al. 2011). This hypothesis of drastic repositioning of asteroids is supported by recent laboratory analyses of meteorites, which led to the finding of a bimodal distribution of the isotopic ratio between noncarbonaceous and carbonaceous chondrites (e.g., Warren 2011; Kruijer et al. 2020; Bermingham et al. 2020). The current distribution of asteroids provides an important constraint for the models that bridge and depict the evolution from the beginning to the current states of the Solar System. Hydrated minerals on minor bodies are especially important when discussing the origin of volatiles on Earth (Owen & Bar-Nun 1995). Primitive dark minor bodies, which inherit many hydrated minerals from the primordial Solar System, are the most plausible sources of the water accreted by Earth later on (Morbidelli et al. 2000). Moreover, if the water on Earth was delivered from outside, carbonaceous chondritic asteroids are more plausible than comets (Altwegg et al. 2015; Marty et al. 2017). These open questions motivated the recent sample-return missions to the primitive asteroids (162173) Ryugu and (101955) Bennu, namely Hayabusa2 by Japanese Aerospace Exploration Agency (JAXA; Watanabe et al. 2019) and OSIRIS-REx by the National Aeronautics and Space Administration (NASA; Lauretta et al. 2019), respectively.

Hydrated asteroids are mainly constituted by phyllosilicates, which are a product of aqueous alteration on anhydrous silicate rocks in hydrothermal systems. The presence of phyllosilicates constrains the environmental parameters of the early Solar System, such as the water/rock ratio and the accretion timing of the planetesimals. Asteroids are usually classified according to their spectral shape and albedo (e.g., Tholen 1984; Bus & Binzel 2002; DeMeo et al. 2009; Mahlke et al. 2022). The first well-established taxonomy was defined using principal component analysis (PCA) in the near-ultraviolet (NUV) to visible (VIS) range with albedo by Tholen (1984). He found three major clusters in the principal component space, namely S, C-X, and D. Those clusters are well defined and might have formed in different regions in the early Solar System. Primitive asteroids often refer to the C-X and D classes. However, the origin of C-X and D classes might be different.

The D-class asteroids are especially dominant in the Jupiter Trojans (DeMeo & Carry 2014). They are hypothesized to be planetesimals formed in trans-Neptunian orbit and captured into co-orbital motion with Jupiter during the time when the giant planets migrated by removing neighboring objects (Morbidelli et al. 2005; Levison et al. 2009). Moreover, D-class asteroids are known to exhibit very red spectra compatible with trans-Neptunian objects and cometary nuclei in the visible wavelength region (Jewitt & Luu 1990; Luu et al. 1994; Emery & Brown 2003; Fornasier et al. 2004; Campins et al. 2006; Licandro et al. 2008, 2018; DeMeo & Binzel 2008).

More recently, the silicate spectral features found in Spitzer mid-infrared (MIR) spectra of the comet nuclei matched those of D-class Trojans (Kelley et al. 2017). Therefore, the formation region and materials for C-X and D classes could be fundamentally different. However, to date, the observations and especially samples from D class asteroids are insufficient to conclude whether or not there is a fundamental difference between C-X and D classes. We would like to leave this question for the future sample-return and/or in situ remote-sensing spacecraft missions, such as Martian Moons eXplorer (Campagnola et al. 2018). On the other hand, the differences between C and X complexes are not obvious in the principal component space defined by the NUV to VIS asteroid spectra, and could be continuous. X-class asteroids can be categorized into P, M, and E according to albedo value. The P-class asteroids have low albedos, similar to C-class ones. This suggests possible continuous compositional variation between C and P classes.

Among the C complex, Tholen (1984) defined F, B, and G subclasses. The F class is characterized by blue to flat visible spectra, which are the end member of the C complex. The class was defined by the distance from the central cluster of the C complex in the PCA space. In particular, the members of F classes show less drop-off to the NUV wavelength. The B class shows a blue visible spectral slope and drop-off into the IR. Also, it is known that the albedo of the B class (e.g., (2) Pallas) is marginally higher than that of the typical C class. The G class is assigned to the objects with exceptionally deep UV absorption. These subclasses are not well separated from the C complex, and should be considered as end members of the C complex.

Near-ultraviolet absorption is observed in spectra of both hydrous meteorites and primitive asteroids. The steep absorption shortward of 0.4 μm is attributable to an Fe2+-Fe3+ charge transfer band (Gaffey & McCord 1979). Hydrated layer-lattice silicate or clay mineral grains comprise the bulk of CI-CM assemblages. Hydrated mineral grains contain both Fe2+ and Fe3+, giving rise to an intense charge-transfer absorption in the blue, which is evident even with very-low-albedo asteroids. Other iron-rich silicates such as the olivine and pyroxene in CV and CO meteorites also produce NUV absorption. However, these minerals have lower optical densities than hydrated silicates, which means their UV absorption will be more effectively suppressed by the presence of opaque minerals, and high-optical-density hydrated silicates can dominate the NUV spectral reflectance (Feierberg et al. 1981). The strong correlation between the NUV and 3 μm was first suggested by Feierberg et al. (1985), who measured the reflectance at 2.92 μm instead of the OH-band itself. Most hydrous meteorites, that is CI, CM, and CR, exhibit a strong absorption in the NUV (Cloutis et al. 2011a,b, 2012; Hiroi et al. 2021). Hiroi et al. (1996b,a) showed a correlation between NUV and OH-band for hydrous meteorites, including the heated Murchison (CM2) and Ivuna (CI1) meteorites. More specifically, once meteorites are heated and dehydrated, both the NUV and OH-band absorptions are weakened. These authors also pointed out that some naturally thermally metamorphosed CI and CM meteorites (ATCC) exhibit much weaker NUV absorptions.

Even though the laboratory measurements suggest the possibility of hydrated silicate measurements by the NUV absorption, the quantitative NUV absorption distribution among asteroids has not been discussed. This might be because of the difficulty in obtaining NUV reflectance observations. One reason for this difficulty is the low sensitivity in the NUV for CCDs and the rapid decrease in solar photon flux at shorter wavelengths. Moreover, because the Rayleigh scattering by the atmosphere has a greater effect on shorter wavelengths, the observable photon flux will be much lower and the signal-to-noise ratio (S/N) will also be lower for the NUV region from the ground. Another reason is the scarcity of well-characterized solar analogs. Spectroscopic measurements of the reflectance of asteroids usually use a solar-analog flux observed in similar sky conditions to divide an asteroid flux. However, there are very few well-characterized solar analogs in the NUV (Hardorp 1978; Tedesco et al. 1982; Tatsumi et al. 2022). Therefore, for quantitative spectroscopic measurements in the NUV, the solar analogs need to be investigated first. There have been a few attempts to do this with ground-based spectroscopic studies in the NUV (Tatsumi et al. 2022). However, the possible uncertainties or errors have not been adequately investigated. On the other hand, the photometric studies are more reliable in the NUV, because the photometric filters are well studied and characterized by numerous standard stars and a high S/N is possible thanks to the broad bands. Therefore, the well-defined asteroid reflectance colors in the NUV photometric surveys are suitable datasets for our investigation. In this study, we investigate the NUV distribution among asteroids in the main asteroid belt out to the Cybele and Hilda regions, and discuss the distribution of hydrated asteroids. Finally, we discuss the formation of primitive asteroids and planetesimals. Our study can act as a stepping stone toward future NUV investigations of asteroid surveys such as the Gaia DR3 spectroscopic data (Gaia Collaboration 2023; Tanga et al. 2023) and the J-PLUS data (Morate et al. 2021).

2 Diagnostics of hydrated minerals in other wavelength regions

Hydrated minerals among asteroids have been investigated by reflectance spectroscopy. Direct indications of hydrated minerals can be found around the 3 μm wavelength range. Lebofsky (1978) first detected the 3 μm absorption on (1) Ceres using ground-based observations, which was later confirmed by in situ observations made by the Dawn spacecraft (de Sanctis et al. 2015; Ammannito et al. 2016). Later, more primitive asteroids were found to have the 3 μm absorption (indeed the large majority of the C and P classes observed in this region have it). There is variation in the depth, center, and shape of this band (Lebofsky 1980; Lebofsky et al. 1990; Feierberg et al. 1985; Jones et al. 1990; Rivkin et al. 1995, 2002, 2015, 2019, 2022; Takir & Emery 2012; Takir et al. 2015). The 3 μm band is a broad and complex absorption of metal-OH, interlayer water, water ice, NH3-bearing phases, carbonates, and organics. Metal-bearing phyllosilicates, such as serpentine and saponite, exhibit a sharp absorption centered at 2.7-2.8 μm; we refer to this range as the OH band hereafter for convenience. The peak wavelength can evolve from the weakly altered CMs (maximum depth at ~2.8 μm) to the extensively altered CMs and CIs (maximum depth at ~2.7 μm) because of variations in the chemistry of the phyllosilicate phases from Fe-rich to Mg-rich (Beck et al. 2010). The OH-band overlaps with the atmospheric water band and cannot be directly observed using ground-based telescopes. The AKARI space IR telescope directly investigated this region for 66 asteroids (Usui et al. 2019) and found that 17 out of 22 (77%) C-complex asteroids and 5 out of 8 (63%) low-albedo (pV < 0.11) X-complex asteroids have significant OH-band absorption (Usui et al. 2019). Only two out of 17 S-complex asteroids show a possible OH-band absorption. One T-class asteroid, (308) Polyxo, shows the OH-band absorption, while none of the three D-class asteroids do (although only one T-class was observed by AKARI). It should be noted that, so far, a possible OH-band was detected on only one D-type asteroid, (773) Irmintraud, in ground-based observations (Kanno et al. 2003). However, (773) Irmintraud was later observed by AKARI and the OH-band was not confirmed (Usui et al. 2019).

The shallow absorption feature around 0.7 μm is also known to be associated with Fe-bearing phyllosilicates, and is caused by Fe2+-Fe3+ intervalence charge transfer. Given the abundance of Fe-bearing phyllosilicates in CMs, many of these objects exhibit this feature. The 0.7 μm band was observed on asteroids as well (Vilas & Gaffey 1989; Vilas et al. 1993; Vilas 1994; Barucci et al. 1998). The strong correlation between the 0.7 μm band and the OH band was pointed out by various studies (Vilas 1994; Howell et al. 2011; Rivkin et al. 2015). AKARI confirmed that the 0.7 μm feature always associates with the OH-band (Usui et al. 2019). However, the absence of the 0.7 μm feature does not necessarily mean anhydrous objects (Rivkin et al. 2015; Usui et al. 2019). Therefore, the presence of the 0.7 μm band on asteroids is more likely to be a proxy of CM-like mineral composition but not a proxy of phyllosilicates in general. The 0.7 μm band central wavelength of asteroids is shorter than that of CM meteorites. Fornasier et al. (2014) suggested CM2 meteorites could be only a subset of those asteroids with the 0.7 μm band based on the comparison of the central wavelength between CM2 meteorites and primitive asteroids. Alternatively, based on laboratory reflectance spectra, Vilas (1994) proposed that the difference in the central wavelength of the 0.7 μm absorption feature observed between CM2 meteorites and primitive asteroids is a function of temperature on the asteroid surfaces. It is also possible that the 0.7 μm band is shallower compared with the depth of the OH band, meaning that there could be observational bias due to the low S/N for the 0.7 μm band. However, this band is modeled as detectable in a visible spectrum with an S/N of 10 (Vilas et al. 1997).

thumbnail Fig. 1

Absolute-magnitude distribution of asteroids. The distributions of primitive asteroids from SDSS and ECAS datasets used in this study are indicated by orange and blue hatches. The integrated primitive asteroid dataset of SDSS and ECAS is shown by the green line. The black dashed line is the distribution of all asteroids in the database (accessed in December 2022). The x-axis at the top corresponds to approximated diameter assuming an albedo value of pV = 0.06 using Eq. (7), which is typical of C types.

3 Asteroid spectrophotometry dataset

The number of asteroid observational datasets down to the NUV wavelength range is relatively limited so far. Three photometric surveys, Sloan Digital Sky Survey (SDSS), Eight Color Asteroid Survey (ECAS), and J-PLUS were conducted down to wave-lengths of < 0.4 μm. We did not include the J-PLUS data in our analysis because most of the objects are included in the SDSS and ECAS. ECAS obtained photometric information using eight Alters: s (0.337 μm), u (0.359 μm), b (0.437 μm), v (0.550 μm), w (0.701 μm), x (0.853 μm), p (0.948 μm), and z (1.041 μm) (Zellner et al. 1985), and SDSS using five filters: u (0.3557 μm), g (0.4825 μm), r (0.6261 μm), i (0.7672 μm), and z (0.9097 μm) (Fukugita et al. 1996). The SDSS and ECAS surveys are complementary in terms of the absolute magnitude of the objects, meaning that SDSS mostly observed 8 < ℋ < 20 and the objects in ECAS are mostly ℋ < 10 (Fig. 1). This is because SDSS has a unique attribute in that it is not able to measure the largest, brightest asteroids. Thus, we used these two datasets to cover a wide size range of asteroids. In addition, we also used the NIR spectra obtained by the AKARI space telescope, and the NIR spectrophotometry obtained by MOVIS.

3.1 Eight Color Asteroid Survey

ECAS1 is the photometric survey of 589 asteroids, among which 405 were chosen as high-quality data. The authors derived the color indices of the asteroids to give mean color indices of zero for four well-characterized solar analogs (Tedesco et al. 1982). Therefore, spectral reflectance Rλ can be obtained by log(Rλ) = ±0.4cλ, where cλ is the tabulated color index and the negative sign is chosen for wavelengths shorter than v band.

Tholen (1984) developed a taxonomic classification based on cluster analysis in the principal component space applied to the ECAS dataset with known geometric albedos, which were mainly provided by the Tuscon Revised Index of Asteroid Data (TRIAD; Morrison & Zellner 1979). We used the albedo pV, absolute magnitude ℋ, and diameter d values adapted by the AcuA dataset by AKARI (Usui et al. 2011) and the NEO-WISE dataset (Mainzer et al. 2019), which are the most recent datasets and cover a greater number of asteroids, while the original Tholen taxonomy used the albedo from TRIAD (Bender et al. 1978). A comparison of the AKARI, IRAS, and NEO-WISE surveys suggests that NEOWISE might overestimate the albedo for large asteroids, possibly owing to detector saturation (Usui et al. 2014). While AKARI completed the albedo survey for the asteroids ℋ < 9, WISE measured many small asteroids, which peak at ℋ ~ 15. By updating albedo using these two albedo catalogs, we were able to exploit the albedo of 536 asteroids out of 589 asteroids in ECAS. Furthermore, we find that there is a significant discrepancy in the albedo values between AKARI+NEOWISE and TRIAD.

Figure 2 shows the albedo of the same asteroids in AKARI+NEOWISE and TRIAD, suggesting that the TRIAD dataset may underestimate albedo. Linear fitting of this dataset indicates that the AKARI+NEOWISE values are 1.4 times the TRIAD values. This affects the classification of E (high albedo), M (medium albedo), and P (low albedo) classes in the X complex using albedo. Although originally the threshold of P class was pV < 0.08, using the AKARI+NEOWISE dataset the threshold is better set at pV < 0.11. Moreover, the bimodal histogram distribution of X class shows the minimum to be around pV ~ 0.11 (Usui et al. 2013). We therefore use the pV < 0.11 for classifying P class in this study.

We used high-quality 212 asteroids classified into C-complex, P (low-albedo X), and D classes based on the Tholen taxonomy with albedo values adopted from AKARI+NEOWISE. We calculated the reflectance at the wavelengths of SDSS by interpolation of two reflectance values at the nearest wavelengths from ECAS, and then calculated spectral slopes using the following equations: (1) (2)

where R is the reflectance at the SDSS filter interpolated from the two nearest filters of the ECAS filter system, and λ is the effective filter wavelength of the SDSS filter. In addition, we defined the NUV absorption strength as SNUV − SVIS. We also measured the indication of a 0.7 μm band absorption in the following two ways: (3) (4)

The positive values of these parameters may indicate the stronger absorption at the 0.7 μm band. The albedo, spectral slope, NUV absorption, and HYD for each taxonomic class are summarized in Table 1. Some asteroids overlap in multiple classes, and therefore the total number of samples in all taxonomic classes exceeds 212. This result suggests that the VIS spectral slope SVIS and NUV absorption strength can classify those primitive asteroid classes. The G class exhibits the deepest NUV absorption by definition. The C and B classes have similar NUV absorption strengths but their VIS spectral slopes are different. There are also large differences in albedo, and G and B classes are especially bright among primitive asteroids. Moreover, although the wavelengths of the SDSS filter system are not optimized for measuring the depth of the 0.7 μm band, the good correlation between HYDECAS and HYDSDSS is shown in Table 1. This agrees with the previous work by Rivkin (2012) showing that the Ch and Cgh asteroids (in SMASS taxonomy), which exhibit the 0.7 μm band absorption, have positive values of HYD. This suggests that HYDSDSS could be a good indicator of the 0.7 μm band. Nevertheless, the high HYD value for D types might be because of the concave spectral shape of their visible reflectance.

thumbnail Fig. 2

Albedo values in TRIAD (data from Morrison & Zellner 2020) compared with the AKARI+NEOWISE dataset. The solid line indicates a linear fit y = 1.4x and the dashed line indicates y = x.

3.2 Sloan Digital Sky Survey

The SDSS is the nontargeted broadband photometry survey. There have been several exploitations of the Solar System objects from SDSS so far. The SDSS Moving Object Catalog (MOC) is a series of photometric surveys of moving targets, mainly asteroids (Ivezić et al. 2002). The fourth data release of SDSS MOC was published in 2008 (Ivezić et al. 2010). This contained more than 100,000 known Solar System objects at that time, although it contains only a fraction of the entire SDSS dataset. More recently, Sergeyev & Carry (2021) conducted an exhaustive search for moving objects in SDSS images including the entire observational period. This catalog includes ~380 000 known Solar System objects and its completeness is estimated to be about 95%. We limited the semi-major axis from 2 to 5.2 au to obtain the main belt to the Cybele, Hilda, and Jupiter Trojan zone asteroids. In the same way as for the ECAS dataset, the albedo values were adopted from the AKARI and NEOWISE data. In our analysis, we did not include the asteroids without albedo values. First, good-quality data were selected based on the photometry flag and the errors in observations. Sergeyev & Carry (2021) defined the photometry flag to discriminate poor data. Moreover, as the u-band observations tend to have larger errors, we used objects with errors of smaller than 0.1 mag. Second, to separate primitive asteroids from the whole dataset, we applied the following thresholds based on the color boundaries of B, C, X, and D complexes calculated by Sergeyev & Carry (2021): the bluer group (g – r) < 0.55 and (i – z) > −0.15 for the C complex, and the redder group (g − r) ≥ 0.55 and pV < 0.11 for P and D classes. This is because the B class is known to sometimes contain high albedo members (e.g., Tholen 1984; Usui et al. 2013; Alí-Lagoa et al. 2013). The primitive asteroids selected from the whole dataset are shown in Fig. 3. Also, we did not use faint samples with absolute magnitude ℋ < 17.5 for main belt asteroids in order to avoid observational bias in which small, dim objects at large distances are less observable. These selections gave us the final photometric spectra of 8,956 objects. As for the ECAS dataset, the VIS and NUV spectral slopes were calculated using the following equations: (5) (6)

where λ is the effective filter wavelength of the filter and the solar colors are (i – g)⊙ = −0.57, (g – u)⊙ = −1.40 from Holmberg et al. (2006).

Table 1

Spectral slopes and albedo for each of Tholen’s primitive taxonomy classes.

3.3 Comparison between SDSS and ECAS

Using the SDSS and ECAS dataset, we obtained the spectrophotometry of 9,168 objects in total. We estimated the diameter from the absolute magnitude. The transformation between the asteroid absolute magnitude ℋ and its effective diameter d [km] requires knowledge of the albedo pV (Harris & Harris 1997), (7)

Among the asteroids that have been observed multiple times with SDSS, there are 132 objects common to both SDSS and ECAS (including all taxonomy classes). We therefore compare the VIS and NUV spectral slopes in both catalogs. We find slight offsets in between the values from the two catalogs (Fig. 4). This might be because of uncertainty in the solar color in the broad-band filters, and the interpolation of spectra to match wavelengths of ECAS to SDSS. We corrected this offset by adding the median values of differences in the spectral slopes between SDSS and ECAS, −0.27 μm−1 and −0.08 μm−1 from the NUV and VIS slope values calculated from SDSS, respectively.

thumbnail Fig. 3

Primitive asteroids (blue) separated from other classes (orange) based on our criteria in the color plots: (a) (g – r) vs. (i – z); (b) (g – r) vs. (g – i); and (c) (u – g) vs. (g – i).
thumbnail Fig. 4

Differences in spectral slope values of the common objects between ECAS and SDSS. We measured 132 asteroids common to the two catalogs and compared the values. We see a shift of −0.08 (μm−1) for VIS slope and −0.27 (μm−1) for NUV slope in the SDSS data.

4 Relation between the OH-band and the NUV absorption

In this section, we present the results of our investigation of the correlation between the OH-band and the NUV absorption, which was previously pointed out by Feierberg et al. (1985); Hiroi et al. (1993, 1996b,a). We used recent datasets from meteorites and asteroids to check whether or not the NUV absorption could be used as a proxy for the existence of hydrated minerals among the primitive asteroids.

4.1 Meteorites

Primitive asteroids are linked to carbonaceous chondrites. In particular, the hydration of meteorites and asteroids is thought to occur through aqueous alteration of precursor anhydrous minerals. CM, CI, and CR meteorites are reported to have phyllosilicate abundances of 70–90 vol%, 81–84 vol%, and 1–70 vol%, respectively (Bland et al. 2004; Howard et al. 2011, 2015; King et al. 2015). Some hydrous carbonaceous chondrites experienced subsequent thermal metamorphism (ATCCs; Ikeda 1992; Nakamura 2005; Tonui et al. 2014). The abundance of phyllosilicate in ATCCs varies from 0 to >80 vol% depending on the degree of heating (King et al. 2021). ATCCs in heating stage IV (heating temperature of >750 °C) do not contain phyllosilicates due to decomposition, while those in heating stages I and II (300–500°C) show similar phyllosilicate abundances to unheated CM/CIs (King et al. 2021).

In this study, we focus on the hydrous carbonaceous chondrites and ATCCs to illustrate the spectral characteristics related to hydration and dehydration states. Hiroi et al. (2021) recently measured the reflectance spectra of 148 carbonaceous chondrites selected from the Antarctic meteorite collections of National Institute of Polar Research in Japan, NASA Johnson Space Center, and Smithsonian Institute National Museum of Natural History. Together with the previous data from the RELAB database (see Table 3 in Hiroi et al. 2021), 78 spot spectra from 77 hydrous meteorites (CI, CM, CR, Tagish Lake, and ATCCs) were obtained covering the wavelength range of 0.3–4 μm under ambient air. Hiroi et al. (2021) conducted sixth-order Gaussian fitting to evaluate the atmospheric water contamination. Water absorption is a broad and round-shaped absorption typically around 3.1 μm. Here, we use the Gaussian fitting bands – which have a central wavelength of shorter than 2.8 μm – so as to evaluate a real OH-band absorption.

Hiroi et al. (1993) showed that naturally heated CI/CM meteorites (ATCCs) and Murchison samples heated in the laboratory have a similar UV to NIR spectral shape. Their experimental and natural CI/CM spectra show the strong correlation between NUV and OH-band absorption strengths. However, at the time of this latter publication, the absorbed water was not taken into account for measurement of the OH-band and a broad absorption around 3 μm can be observed in their spectra. Therefore, using the given sixth-order Gaussian fitting by Hiroi et al. (2021), we remove the contamination of terrestrial absorbed water by taking into account the Gaussian components that have a central wavelength of shorter than 2.8 μm. Figure 5 shows the NUV and OH-band absorption strengths of hydrous meteorites. We find a similar correlation to that in Hiroi et al. (1993): the more hydrated meteorites show deeper NUV absorption. ATCC samples (CM(D)/CM(U)/CI(D)) closely follow the trends seen in heating experiments. This is because the phyllosilicates are decomposed by heating. Another important point here is that powder samples (open symbols) tend to show deeper absorptions than chip samples. When compared with other hydrated carbonaceous chondrites, Tagish Lake exhibits lower NUV absorption, possibly because absorption features are masked by high carbon contents, ~5.4 wt% (Brown et al. 2000). Tagish Lake has extremely low reflectance and does not show any feature in the wavelength range of <2.5 μm. Tagish Lake is also known to have a very red visible reflectance spectrum (Hiroi et al. 2001). Tagish Lake’s dark-red spectrum, without the strong NUV absorption, suggests similarity with P and D classes. Some P-class asteroids exhibit OH-band absorption, suggesting that Tagish Lake could be the meteorite analog. Although a D-class asteroid with a clear OH-band absorption has not been found, the sample size is relatively small and the analog meteorites of P or D classes require further investigation in the OH-band region. Our result shows a strong correlation between the NUV and the OH-band absorption strength in hydrous meteorite spectra. In particular, CMs and CIs exhibit up to 50% of OH-band and 3–4 μm−1 NUV absorption strengths. CMs without 0.7 μm band absorption (blue triangles) exhibit relatively shallow absorptions in both the OH-band and NUV.

4.2 Asteroids

AKARI is the IR space telescope developed by JAXA (Murakami et al. 2007). The Infrared Camera (IRC) onboard AKARI was used to obtain IR reflectance spectra from 2.5 to 5 μm for 66 asteroids, including 34 successfully observed primitive asteroids (C-complex, P, and D classes) (Usui et al. 2019); this was the first large survey directly observing the 3 μm region. All the primitive asteroids observed by AKARI are larger than 80 km in diameter. Even though the correlation between OH-band and NUV absorption was suggested by Feierberg et al. (1985) and Hiroi et al. (1996b,a), at that time the OH-band (2.7 μm band) of asteroids was not directly observed and the depth at 2.9–3.0 μm was measured instead. Therefore, here we compare the depth of the 2.7 μm band from the AKARI data and corresponding NUV absorption from the ECAS data. AKARI and ECAS observed the same set of 32 primitive asteroids. The 0.7 μm absorption uses three bands of ECAS data, v, w, and x. Here, a positive value for the 0.7 μm absorption means that the spectral shape is concave and shows absorption, while a negative value means that the spectral shape is convex and shows no clear absorption. Figure 6 shows the positive correlation between OH-band depth and NUV absorption. Asteroids with NUV absorption > 2 μm−1 show >30% absorption in OH-band and >1% absorption in the 0.7 μm band. Moreover, most asteroids showing the 0.7 μm band with a depth of >1% have NUV absorption >1 μm−1. It should be noted that even asteroids without the 0.7 μm band but the depth >−1% (pink and purple circles) tend to show an offset towards deeper NUV absorption than those with depth <−l% (yellow circles). This might be because their higher contents of Fe-rich phyllosilicates cause the 0.7 μm region to appear flatter rather than convex. This is a reasonable assumption because both the 0.7 μm and NUV absorptions are possibly caused by iron in the phyllosilicates. Those shown as pink and purple circles in Fig. 6 may contain more Fe-rich phyllosilicates than those shown by yellow circles. Based on the correlation of OH-band and NUV absorptions observed in both asteroids and meteorites, we can estimate the degree of hydration based on NUV absorption.

thumbnail Fig. 5

OH-band depth and NUV absorption of hydrated carbonaceous chondritic meteorites. The meteorite sample spectra are from Hiroi et al. (2021) (available at the RELAB database). Open symbols indicate the powder samples and filled symbols indicate chip samples. CMs are classified into those with (circle) and without (triangle) the 0.7 μm band absorption. Squares with lines show the heating experiments of Ivuna (CI1) and Murchison (CM2) conducted in Hiroi et al. (1996b,a). Unheated Ivuna and Murchison are shown by squares with circles inside. CM(D), CM(U), and CI(D) are ATCCs.

5 Distribution of NUV absorption in the asteroid main belt

The asteroid main belt has been divided into three zones: inner (IMB; 2.06–2.50 au), middle (MMB; 2.50–2.82 au), and outer (OMB; 2.82–3.28 au). The Cybele and Hilda zones are located beyond the asteroid main belt at 3.3–3.5 au (between 2:1 and 5:3 mean-motion resonances or MMR with Jupiter) and 4 au (the 3:2 MMR with Jupiter), respectively. The Jupiter Trojans are trapped in Jupiter’s L4 and L5 Lagrangian regions at 5 au, and are therefore considered to be dynamically stable. We also divided the asteroids into different size ranges by diameter d: very large (d > 100 km), large (50 < d < 100 km), medium (10 < d < 50 km), small (5 < d < 10 km), and very small (d < 5 km). The NUV absorption strength distribution for each region with a different size range is shown in Fig. 7. The median value and interquartile range for each group is shown in Table 2. To evaluate the difference in the distributions among IMB, MMB, and OMB asteroids, we calculated the p-values for each combination. First, we applied the Shapiro-Wilk test to test the normality for each group. If the p-value for a group is less than 0.05, indicating a nonparametric distribution, we used the Mann-Whitney U test to calculate the probability that the two groups of populations are equal. If the p-value of the Shapiro-Wilk test for a group is larger than 0.05, indicating a normal distribution, we used the Student’s t-test. For the large-diameter populations (d > 50 km), the NUV absorption strengths are not significantly different (p > 0.05 for all combinations IMB-MMB, IMB-OMB, and MMB-OMB). This suggests that the NUV distributions of the large-diameter populations are similar through the main asteroid belt. On the other hand, the distributions of the small populations (d < 10 km) are different for IMB-MMB and MMB-OMB with p ≪ 0.01, while the difference between IMB and OMB is not significant, p > 0.05. Based on the statistical test, the IMB and OMB populations with d < 10 km show lower NUV absorption strengths (0.1 −0.4 μm−1) compared with the MMB population (0.4–0.7 μm−1). The distributions of the medium and small populations in IMB and OMB are slightly different, p < 0.05. The MMB distribution is similar between medium and small populations. Overall, MMB shows the deepest NUV absorption for all size groups except for the very large d > 100 km group, suggesting that the MMB population is the most hydrated among the different populations of the main asteroid belt. And the IMB population is the least hydrated for d < 10 km.

In order to visualize the taxonomic distribution, we classified the primitive asteroids into F, C, B, G, P, and D using the values in Table 3. The thresholds for the classifications were defined based on the ECAS taxonomy. Figure 8 shows the distribution of our samples in the NUV absorption versus VIS slope space. We depict the ECAS samples as colored circles in order to demonstrate the correspondence of our classification and the ECAS taxonomy. The line dividing blue, F, and B classes from red, C, G, and P classes (L1) is defined by the direction of the largest dispersion of the dataset. The average spectrophotometry in the SDSS filters of the taxonomy classified by Table 3 is shown in Fig. 9. Using this classification, we are able to measure the number fraction of each taxonomy as a function of size and region (Fig. 10 and Table 4). DeMeo & Carry (2014) investigated a similar distribution using only the VIS wavelength. Thus, they were not able to distinguish between C, F, and G, while our dataset, which includes the NUV region, enables us to do this and to exploit more compositional information (see Sect. 7).

The C-type asteroids are dominant for asteroids with d > 50 km, and are relatively minor for smaller sizes. The F types are common throughout the main asteroid belt, even out to the smaller asteroids of the Cybele region. Components of the main belt are completely different between d > 50 km and d < 10 km and F-type asteroids are the major component of d < 10 km in IMB. All the classes are almost evenly distributed for MMB and OMB for small asteroids d < 10 km, although the enhancement of F class is found for asteroids with d < 10 km. We also note that P-class asteroids represent more than 20% throughout most of the main asteroid belt regions and size ranges, which is comparable to other types, suggesting that P types are not a minor component of the main asteroid belt. Although P types are dominant for the Cybele and Hilda regions, if P types originated from the outer Solar System, they have significantly contaminated the main asteroid belt. Nevertheless the abundance of P types throughout the main asteroid belt suggests that these objects were formed in the same reservoir as the C-complex asteroids. In contrast, in most main asteroid belt zones and size groups, the D-type asteroids are minor, representing ≤ 10%, while they are a major component for the large-size Jupiter Trojans, and the middle-size Hilda and Cybele zones. These latter two zones contain completely different taxonomic populations from size to size, which agrees with the findings of DeMeo & Carry (2014). For example, in the Cybele zone, P and D types are dominating for population d > 10 km, whereas F types are the largest population in d < 10 km.

Our distribution can be compared with that found by Tholen (1984). As most asteroids in the sample of Tholen (1984) are larger than 50 km, we can simply compare the distribution of d > 50 km. These latter authors found that the C types are dominant among primitive asteroids in the main belt: ~70% for IMB, ~80% for MMB, and ~70% for OMB. Our result also suggests a dominant distribution of C types for large asteroids, but less dominant than that found by Tholen (1984). Instead, we find more P types distributed throughout the main belt, namely ~20%, which is consistent with the findings of DeMeo & Carry (2014). This is may be because P types have relatively low albedo and the number of P types could be underestimated by the discovery bias. Similarly, we also find several D types in the main belt. The distributions we find for the IMB are quite different from those presented by Tholen (1984), which may be because of the small sample size of these authors, which was less than 10.

As was suggested by Tholen (1984), there is a rapid decrease in C-type asteroids beyond the main belt. In the Cybele zone, Tholen (1984) found that ~35% are either C or P type and ~20% are D type, while our results suggest a greater abundance of P type in large asteroids and that D type represent only 10%. Our results are more consistent with those of DeMeo & Carry (2014), and again this could be due to bias in the sampling method. In the Hilda and Jupiter Trojan zones, our results are relatively consistent with those of Tholen (1984) because these populations are dominated by P and D types, both of which show comparably low albedos.

thumbnail Fig. 6

OH-band depth and NUV absorption of primitive asteroids. Symbol color indicates the 0.7 μm band depth (HYDECAS). Asteroids with deep 0.7 μm band absorptions show relatively deep OH-band and NUV absorptions. Even asteroids with the 0.7 μm band depth of −1 % to 1% exhibit slightly higher NUV absorptions than those with a shallower 0.7 μm band.
thumbnail Fig. 7

Distribution of spectral slopes among the main asteroid belt. Left column: NUV slope. Middle column: VIS slope. Right column: NUV absorption strength (SNUV – SVIS). The Y axis indicates the number of asteroids in a bin. The blue line is the inner main belt, the yellow line is the middle main belt, and the red line is the outer main belt. The asteroids were divided according to diameter; first row, d > 100 km; second row, 50 < d < 100 km; third row, 10 < d < 50 km; fourth row, 5 < d < 10 km. The vertical lines are the median value for each group.
Table 2

NUV absorption strength for each size range and region in the main asteroid belt.

Table 3

Thresholds for taxonomic classification used in this study to classify asteroids in a similar fashion to the Tholen taxonomy.

thumbnail Fig. 8

Spectral characteristics of all primitive asteroids using the ECAS and SDSS datasets (black circles). The colored circles indicate the ECAS dataset with the taxonomy defined by Tholen (1984). The dashed line shows the direction of the largest dispersion of the black circles (y = −0.15x + 0.05). The taxonomic classifications (Table 3) are also visualized.
thumbnail Fig. 9

Average spectrophotometry of taxonomic classes through the SDSS filter set classified in Table 3. The error bars show the interquartile range in the taxonomic classes. (Left) Spectra for the entire wave-length of SDSS filters. (Right) Same spectra as in the left panel but within the NUV range.

6 Near-infrared characteristics of the primitive asteroids

The MOVIS catalogs (Popescu et al. 2016, 2018) include the surveys of Solar System objects observed in a serendipitous manner by the VISTA-VHS (Visible and Infrared Survey Telescope for Astronomy – VISTA Hemisphere Survey) program (McMahon et al. 2013; Sutherland et al. 2015). This survey covered the entire sky seen from the southern hemisphere using the NIR broad-band filters Y (centered at 1.020 μm), J (1.252 μm), H (1.645 μm), and Ks (2.147 μm). The data retrieved for Solar System objects include: the detections catalog (MOVIS-D), the magnitudes catalog (MOVIS-M), and the colors catalog (MOVIS-C). The number of measured colors for each Solar System object varies according to the observing strategy and to the limiting magnitude of each filter. The average time interval between the measurements (which affect the color uncertainty) done with Y and J filters is 8.47 ± 5.99 min, while for J and Ks filters is 7.40 ± 6.56 min. This time interval is negligible compared with the typical rotation period of main belt asteroids, which is on the order of several hours (Warner et al. 2009), and therefore the errors introduced by the light-curve variations can be ignored.

We used the latest version of the MOVIS-C catalog (Popescu et al. 2018) to compare with the results we obtained based on the SDSS and ECAS dataset. We find 242 objects in common with accurate (Y – J) and (J – Ks) colors, where the errors follow the conditions (Y – J)err ≤ 0.05 and (J – Ks)err ≤ 0.05, respectively. These selected samples include 39 objects classified as D type, 79 P types, 40 C types, 21 B types, 31 F types, and 32 G types based on our classification in Sect. 5. Table 5 shows some statistical parameters of (Y – J) and (J – Ks) colors for each of these classes.

The differences between C, P, and D types are outlined in both (Y – J) and (J – Ks) colors and follow the reddening trend from C to P to D. The average (J – Ks) color for P types is (J – Ks)P = 0.485 ± 0.120 mag, which is almost 1σ larger than the average of C types, of namely (J – Ks)C = 0.421 ± 0.076 mag. In this wavelength region, that is, (1.25–2.2) μm, P types show a broad spectral behavior (outlined by the value of the dispersion of their colors). The (Y – J) color distributions of C and P types are slightly different, although the average values are comparable: (Y – J)C = 0.258 ± 0.040 and (Y – J)P = 0.278 ± 0.060. The marginally red spectrophotometry (over 1.020–1.252 μm of these two types can be inferred by comparing these average values with the median for the solar analogs (Y – J)G2V = 0.219 (Popescu et al. 2018).

D types have a well-separated distribution for both (Y – J) and (J – Ks) color (Fig. 11). The average values are very red: (Y – J)D = 0.360 ± 0.135 mag (a value of about 2σ larger than the other primitive classes), and (J – Ks)D = 0.537 ± 0.102 mag. The wide spread of (J – Ks) may indicate compositional variations inside this group.

The (Y – J) color distribution is similar for the B, C, F, and G classes (Table 5). The color values are smaller than those of P and D classes. We find that B types exhibit a red spectral spectrophotometric slope, which is indicated by their (J – Ks) color (Table 5). This shows potential differences in NIR. This spectral “turn up” at longer wavelengths of 1.2–2.1 μm might be similar to the trend in the Themis group classified by Clark et al. (2010) and the G1 and G2 groups presented by de León et al. (2012). These authors found a good match between the B-type asteroids with this NIR upturn and CM, CI, and thermally metamorphosed CI/CM. We note that the B types defined by DeMeo et al. (2009) should have negative NIR slopes, which means that (Y – J) and (J – Ks) colors should have a lower value than the colors of solar analogs, which are (Y – J)⊙ = 0.219 mag, (J – Ks)⊙ = 0.336 mag (Popescu et al. 2018). However, our classification is made using only the NUV-VIS colors provided by the SDSS filters applying the Tholen taxonomy.

Figure 11 outlines that less than ≈ 10% of our C, B, F, and G types are below this threshold, meaning DeMeo’s B type. Thus, the majority of the B, C, F, and G types found in this sample have positive spectral slopes in the 1.02–2.2 μm spectral intervals. This can be explained by considering the spectral variety presented by Clark et al. (2010) and de León et al. (2012). These authors found that asteroids classified as B types according to their visible spectra show abroad variation of NIR spectral slopes, ranging from negative, blue slopes to positive, moderately red slopes. The strong correlation between the NUV-VIS and NIR wavelengths is not found in the C-complex classes, that is, B, C, F, and G classes, while B types may exhibit a potential difference in NIR.

thumbnail Fig. 10

Taxonomic distribution of primitive asteroids as a function of size and dynamical populations. The total number in each bin is shown in the center of the pie charts.

7 Discussion

7.1 Aqueous alteration through the asteroid belt

It is important to reveal the distribution of the aqueous alteration state through the current asteroid belt to constrain the boundary conditions of Solar System evolution calculations, such as the Grand-Tack model (e.g., Walsh et al. 2011) and the Nice model (e.g., Morbidelli et al. 2005). Here, we discuss the aqueous alteration state as a function of the semi-major axis based on NUV, 0.7 μm band, and 3 μm features.

Based on the OH-band depth by indirect measurements of ground-based observations of a reflectance drop from 2.5 to 2.9 μm, Jones et al. (1990) found that the hydrated silicates slightly decline in abundance with distance from the Sun from 2.5 to 3.5 au because of the P- and D-type populations in the outer main belt. However, Rivkin et al. (2015) more recently argued that when focusing on the Ch asteroids, OH-band depth is poorly correlated with the semi-major axis, suggesting that the population is well mixed in the asteroid belt. Direct measurements of the OH-band depth by the AKARI space telescope did not find a correlation between OH-band depth and the current semi-major axis of asteroids, either (Usui et al. 2019). We note that the asteroids observed in this region are quite large in size, namely of more than several tens of kilometers. Therefore, by looking at the large members, the hydrated state does not show a clear correlation with the semi-major axis through the main belt. This is consistent with the similar NUV absorption strength through the main asteroid belt (Fig. 7).

Nevertheless, based on the band shape, the broad and rounded 3.1 μm band – which is possibly due to ice frost – was found in semi-major axes beyond 3.1 au coupled with the P and D types, while the sharp 3.1 μm band was found in a very wide range of 2.5–4 au (Takir & Emery 2012). This may suggest that the outer main belt preserves the composition of anhydrous silicates, water ice, and possibly complex organic materials originating from the outer Solar System.

The 0.7 μm band and the OH-band features are highly correlated, suggesting that the presence of the 0.7 μm band indicates phyllosilicates resulting from the aqueous alteration process (Vilas 1994; Howell et al. 2011; Fornasier et al. 2014; Usui et al. 2019). Using the ECAS dataset, the distribution of 0.7 μm absorption was investigated by Vilas (1994), who found that the percentage of objects showing 0.7 μm absorption in different spectral types decreases in the following order: G > C > B > F > P > D. Fornasier et al. (2014) came to the same conclusion based on spectroscopic observations. Asteroids with the 0.7μm band were proposed to dominate a zone from 2.6 to 3.5 au by Vilas (1994) and later on this was updated to a zone from 2.3 to 3.1 au by Fornasier et al. (2014). Furthermore, Fornasier et al. (2014) found that more than half of the objects in the IMB show the 0.7 μm band, although MMB is the main region of hydrated objects in terms of number among the main asteroid belt. Rivkin (2012) tried to measure the percentage of objects with the 0.7 μm band using the SDSS photometry data, finding 30% to 40% through the main asteroid belt, although there are slight differences: OMB is the lowest, MMB is the highest. In summary, observations show that the MMB has the highest percentage of objects with the 0.7 μm band absorption.

The NUV absorption strength decreases in the following order, G > C/B > F > P > D, which is in good agreement with the percentage of objects showing the 0.7 μm band found by Vilas (1994) and Fornasier et al. (2014). The G-type asteroids, the closest analogs of the CM chondrites and characterized by NUV and 0.7 μm band absorptions, are distributed in the MMB with highest ratio. The B types are possibly analogs of CI or CM considering the turning up in NIR. Most B types are distributed in the MMB and OMB. The C types show moderate NUV absorption and almost flat reflectance spectra at VIS-NIR. The C types are prominent in the large members of the main belt, suggesting possible primordial bodies that have survived since the beginning of Solar System. Therefore, the C-type spectra might show the composition of the outermost surface, such as NH4-bearing phyllosilicates observed on the surface of (1) Ceres (Ammannito et al. 2016). The P/D types are also abundant in MMB, OMB, and beyond, although the domination in the Cybele, Hilda, and Jupiter Trojan zones is remarkable. However, P types are more abundant than D types in the main belt region. P types might be an end member of the C-complex asteroids, which agrees with the findings of previous studies (Vernazza et al. 2017, 2021; Mahlke et al. 2022). It has been hypothesized that D types were implanted in the Cybele and Hilda regions later than the formation of the main belt (Levison et al. 2009).

In contrast, the IMB is dominated by F types – especially its small members –, which have the lowest NUV absorption strength among C complexes, which could be composed of Fe-poor phyllosilicates, such as CI chondrites or thermally metamorphosed carbonaceous chondritic materials. Previously, the F types were thought to be thermally metamorphosed carbonaceous chondrites because of their low NUV absorptions (Hiroi et al. 1993). If this is indeed the case, the asteroids in the IMB might have experienced high temperatures when formed; for example, rapid formation and/or large-size planetesimals (e.g., Grimm & McSween Jr 1993; Neumann et al. 2020). It also should be noted that small asteroids could be fragments of catastrophic disruptions and have rubble-pile structure, and therefore they may exhibit the internal compositions. This suggests the possibility that asteroids currently in the IMB had a relatively less heated crust and highly heated internal composition. Alternatively, they could be CI chondritic material (see also Sect. 7.3). This CI chondritic material could originate from fragments of the cores of large d > 100 km IDP-like bodies, as hypothesized by Vernazza et al. (2017). In that case, they likely experienced low heating and might have formed in the outer Solar System, which means a slow formation pathway and/or a high water-rock ratio (e.g., Nakamura et al. 2022). These two hypotheses for the F-type composition imply totally different Solar Systemformation histories. However, we do not have clear evidence to make a robust conclude. Therefore, more sample-return missions will be of great importance. If the second case is found to reflect reality, the unique distribution of F types suggests that they were implanted in their current positions by mechanisms different from those responsible for the other primitive asteroids, or were formed in a distinctive place in the early Solar System.

In summary, the characteristics of aqueous alteration through the main belt to the Cybele and Hilda zones can be depicted as (1) Fe-poor phyllosilicates or thermally metamorphosed carbonaceous chondrites in asteroids of the IMB with d < 10 km, (2) Fe-rich phyllosilicates in asteroids of the MMB and OMB with d > 10 km, (3) slightly aqueously altered or unaltered comet-like materials with water frost in the Cybele, Hilda, and Jupiter Trojan zones, and (4) partially aqueously altered materials for all regions.

Table 4

Primitive asteroid distribution over taxonomic classes and dynamical populations for different asteroid size bins.

Table 5

(Y – J) and (J – Ks) of each taxonomic class discussed in this work.

thumbnail Fig. 11

Normalized cumulative distribution of (Y – J) (top panels), and of (J – Ks) (bottom panels) for the sample of 242 objects taxonomically classified in this work using the ECAS and SDSS dataset.

7.2 Grain size and surface physical condition

It is known that the physical surface conditions affect the spectral slopes. Fine-grain samples show a redder spectral slope over VIS to NIR wavelengths (e.g., Cloutis et al. 2018). Grain sizes on the surfaces of asteroids can be estimated through thermal inertia derived by thermal IR observations. Thermal inertia is indicative of surface roughness at a larger scale than the typical diurnal heat propagation distance. Thermal inertia correlates with surface grain size: high thermal inertia can be interpreted as indicating coarse or rigid rock surface and low thermal inertia is interpreted as indicating fine grain regolith (Gundlach & Blum 2013). Based on the thermal IR observations, increasing thermal inertia with decreasing asteroid diameter was found by Delbo’ et al. (2007) and Hanuš et al. (2018). Specifically, many of the asteroids with d > 10 km show low thermal inertia of <100 J m−2 s−1/2 K−1 (Hanuš et al. 2018). If the grain size plays a significant role on the asteroids, we would find the variation in spectral slope among different size groups. However, we did not find a significant difference in visible spectral slope between the d < 10 km group and the d > 10 km group (Fig. 7). This might suggest that grain size is not the primary cause of the change in spectral slope on asteroid surfaces.

Polarimetry is also a powerful tool for estimating the surface conditions of an asteroid. The linear polarization degree can be measured as a function of phase angle (see Belskaya et al. 2015, as a review). The linear polarization degree of asteroids reaches a minimum value (Pmin) at a certain phase angle (αmin). The primitive bodies, such as D-class asteroids and the dark side of Iapetus, exhibit the smallest |Pmin| (Hasegawa et al. 2021), while asteroids with the 0.7 μm band (Ch,G) show the largest αmin and |Pmin| (Belskaya et al. 2017). F types have values in between these two end members, and are characterized by relatively small αmin and |Pmin| compared with other C-complex asteroids (Belskaya et al. 2005, 2017; Gil-Hutton & Cañada-Assandri 2012; Hasegawa et al. 2021). Asteroids classified as C and P types show similar inversion angles (the angle at which the linear polarization changes its sign) but different depths of negative polarization (Belskaya et al. 2017). It should be noted that spectral taxonomic classes of asteroids defined by the reflectance in NUV-VIS space were well separated in polarization space (Belskaya et al. 2017). Although F and B types sometimes show similar negative visible slopes, they are well separated in polarization space. If the grain size plays a dominant role in determining the polarization feature, taxonomic classes should align in the same order according to the value of the slope of their visible spectra. However, this is not the case, which suggests that the polarization features are related to differences in composition rather than particle sizes or physical conditions.

The interpretation of Pmin and the depth of the negative branch are not yet fully understood. The coherent back-scatter enhancement has been considered by several authors as the most plausible cause of negative polarization (Shkuratov 1985, 1991; Shkuratov et al. 1994; Muinonen 1990; Mishchenko 1993). More recent laboratory and theoretical studies showed a considerable role of single-particle scattering in the formation of negative polarization (Ovcharenko et al. 2001; Shkuratov et al. 2004). A few experimental studies suggested that adding small amounts of a bright material to a dark material might enhance the negative polarization branch, when bright particles have more prominent negative polarization than the dark absorbing particles (Shkuratov et al. 2004; Belskaya et al. 2005). The relatively small values of the parameters αmin and |Pmin| for D- and F-type asteroids can be treated as a diagnostic of optical homogeneity and the darkness of the regolith microstructure on scales of the order of the wavelength (Belskaya et al. 2005), while large values of αmin and |Pmin| for Ch- and G-type asteroids can be explained as inhomogeneity caused by bright inclusions such as calcium-aluminum-rich inclusions and chondrules (Devogèle et al. 2017). The peculiar polarimetric characteristics of the F type could also contribute to the NUV reflectance behavior. It is therefore important to take into account polarimetric behavior when classifying asteroids in the future.

thumbnail Fig. 12

Taxonomic distribution of primitive asteroids through the main belt to the Cybele and Hilda zones: distributions of C, F, G, and B types (top) and C, P, and D types (bottom).

7.3 F-type asteroids: Relation with (162173) Ryugu and (101955) Bennu

F types can only be classified using the NUV wavelengths; they are sometimes confused with B- and C-types in the visible wave-length range. The F-type asteroids have a peculiar distribution compared with other types of primitive asteroids (Fig. 12): they concentrate in the IMB. Moreover, the Hayabusa2 spacecraft recently visited asteroid (162173) Ryugu, which is an F-type asteroid (Tatsumi et al. 2022). Ryugu was considered to originate from the IMB (Campins et al. 2010) and more specifically from the Polana-Eulalia family (Campins et al. 2013; Sugita et al. 2019; Tatsumi et al. 2021b). Therefore, the Ryugu samples could be a mineralogical representative of the F-type asteroids. From the analysis of the returned samples, Ryugu, which shows a strong and sharp OH absorption at 2.72 μm, has a composition similar to that of CI chondrites (Yada et al. 2021; Pilorget et al. 2022; Yokoyama et al. 2022; Nakamura et al. 2022).

Based on the remote-sensing spectra, Ryugu was presumed to be a dehydrated carbonaceous chondrite due to its very flat and shallow absorption features in VIS-NIR (Sugita et al. 2019; Kitazato et al. 2019; Tatsumi et al. 2021b). The case of Ryugu and CI chondrites led us to misinterpret the remote-sensing spectra because the terrestrial contamination of CI chondrites could occur and also Ryugu has been affected by the space weathering. This is a good lesson to learn and suggests that most of our carbonaceous chondrite meteorites might be contaminated and do not show exactly the right spectra to compare with asteroid surfaces. Nevertheless, CI chondrites are composed mainly of Mg-rich phyllosilicates and lack chondrules and CAIs (Cloutis et al. 2011a). A lower abundance of Fe-rich pyllosilicates or the presence of magnetite might be the cause of the flat NUV reflectance in this case. Alternatively, the space weathering after the catastrophic disruption of the parent body might be causing the shallower absorption features of phyllosilicates (Noguchi et al. 2023). These geochemical characteristics match the distant formation from the Sun. Moreover, the measurement of Cr isotopic heterogeneities suggests that CI chondrite Orgueil was formed farther from the Sun than Murchison (CM), Allende (CK), or Tagish Lake (Fukai & Arakawa 2021). If F types were formed at a great distance from the Sun, they were implanted relatively recently into their current positions, mainly in the IMB. Another noticeable feature of F types is that they were relatively small in size upon formation. This might be why they were able to be easily moved by disturbances due to the giant planets. The mechanism of displacement needs to be investigated further by dynamical calculations.

Furthermore, the asteroid (101955) Bennu, visited and sampled by the OSIRIS-REx spacecraft, is also classified as an F type (Hergenrother et al. 2013). Bennu could also originate from the Polana-Eulalia family in the IMB (Campins et al. 2010; Bottke et al. 2015; Tatsumi et al. 2021a). The returned samples from Bennu and Ryugu will add more geochemical information to our understanding of the F-type asteroids.

8 Conclusions

Phyllosilicates on primitive asteroids are the result of an aqueous alteration process in the early history of the Solar System. They enable us to constrain the formation conditions of the primitive asteroids and planetesimals. The NUV absorption has been proposed to be a good proxy for phyllosilicates, but this has not been investigated well. For this reason, we explored the NUV region for dark primitive asteroids using two spectrophotometric surveys, SDSS and ECAS. Photometric surveys are more suitable for investigations of the sensitive NUV region than spectroscopic observations from the ground, which can be greatly affected by atmospheric conditions and the solar analogs. Using two surveys we can cover 9,168 asteroids with ℋ < 17.5.

First, we investigated the correlation between the NUV absorption and the OH-band (2.7 μm) absorption for asteroids and meteorites. We find a close correlation between the NUV and OH-band absorptions, which was originally discussed by Feierberg et al. (1985) and Hiroi et al. (1993). We also find that grain size may contribute to the deeper band absorptions. Furthermore, Fe-bearing phyllosilicates, which also show the 0.7 μm band absorption, may contribute to the deeper NUV absorption. Based on these correlations, we confirm that the NUV absorption is a good proxy for the phyllosilicate abundance on asteroids.

The Tholen (1984) taxonomic classification of asteroids takes into account the NUV absorption. Following the Tholen taxonomy, we classified the asteroids and investigated their distribution in the main belt to the Cybele, Hilda, and Jupiter Trojan zones. We find that large asteroids with d > 50 km and small asteroids with d < 10 km have a completely different taxonomic distribution. Large asteroids show a relatively similar distribution through the main belt, while small asteroids exhibit a different distribution in the inner main belt compared to the middle and outer main belt. The inner main belt region shows a significantly small value for the NUV absorption and is dominated by F types. On the contrary, the major constituent of the middle and outer main belt regions is G type asteroids, which have the largest NUV absorption. The Cybele, Hilda, and Jupiter Trojan zones show distinctive distributions, being dominated by the red members, P and D types. F types are found to also be abundant in the small members of the Cybele zone.

We find that the distribution of F types is unique: they concentrate in the small members of the inner main belt region. There is still little constraint on the composition of F types. Recent sample-return missions, namely Hayabusa2 to (162173) Ryugu and OSIRIS-REx to (101955) Bennu, will also add fundamental chemical information on F types. So far, the Ryugu samples have been found to be compositionally similar to CI chondrites. Therefore, the main constituent of F types may be CI-like Mg-rich phyllosilicates. Furthermore, the Gaia space telescope has obtained data from spectroscopic observations for numerous asteroids in this missing NUV wavelength. Gaia will open a new horizon to reveal the compositional distribution of Solar System using the NUV wavelength range in further detail.

Acknowledgements

The authors thank the anonymous reviewer for the constructive and elaborated comments and suggestions. E.T., F.T.R., J.dL., and J.L. acknowledge support from the Agencia Estatal de Investigación del Ministerio de Ciencia e Innovación (AEI-MCINN) under grant “Hydrated minerals and organic compounds in primitive asteroids” (PID2020-120464GB-I00/doi:https://doi.org/10.13039/501100011033). J.dL. also acknowledges financial support from the Spanish Ministry of Science and Innovation (MICINN) through the Spanish State Research Agency, under Severo Ochoa Program 2020-2023 (CEX2019-000920-S). M.P. was supported by the grant of the Romanian National Authority for Scientific Research – UEFISCDI, project no. PN-III-P1-1.1-TE-2019-1504. S.H. was supported by the Hypervelocity Impact Facility (former name: The Space Plasma Laboratory), ISAS, JAXA.

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All Tables

Table 1

Spectral slopes and albedo for each of Tholen’s primitive taxonomy classes.

In the text
Table 2

NUV absorption strength for each size range and region in the main asteroid belt.

In the text
Table 3

Thresholds for taxonomic classification used in this study to classify asteroids in a similar fashion to the Tholen taxonomy.

In the text
Table 4

Primitive asteroid distribution over taxonomic classes and dynamical populations for different asteroid size bins.

In the text
Table 5

(Y – J) and (J – Ks) of each taxonomic class discussed in this work.

In the text

All Figures

thumbnail Fig. 1

Absolute-magnitude distribution of asteroids. The distributions of primitive asteroids from SDSS and ECAS datasets used in this study are indicated by orange and blue hatches. The integrated primitive asteroid dataset of SDSS and ECAS is shown by the green line. The black dashed line is the distribution of all asteroids in the database (accessed in December 2022). The x-axis at the top corresponds to approximated diameter assuming an albedo value of pV = 0.06 using Eq. (7), which is typical of C types.
In the text
thumbnail Fig. 2

Albedo values in TRIAD (data from Morrison & Zellner 2020) compared with the AKARI+NEOWISE dataset. The solid line indicates a linear fit y = 1.4x and the dashed line indicates y = x.
In the text
thumbnail Fig. 3

Primitive asteroids (blue) separated from other classes (orange) based on our criteria in the color plots: (a) (g – r) vs. (i – z); (b) (g – r) vs. (g – i); and (c) (u – g) vs. (g – i).
In the text
thumbnail Fig. 4

Differences in spectral slope values of the common objects between ECAS and SDSS. We measured 132 asteroids common to the two catalogs and compared the values. We see a shift of −0.08 (μm−1) for VIS slope and −0.27 (μm−1) for NUV slope in the SDSS data.
In the text
thumbnail Fig. 5

OH-band depth and NUV absorption of hydrated carbonaceous chondritic meteorites. The meteorite sample spectra are from Hiroi et al. (2021) (available at the RELAB database). Open symbols indicate the powder samples and filled symbols indicate chip samples. CMs are classified into those with (circle) and without (triangle) the 0.7 μm band absorption. Squares with lines show the heating experiments of Ivuna (CI1) and Murchison (CM2) conducted in Hiroi et al. (1996b,a). Unheated Ivuna and Murchison are shown by squares with circles inside. CM(D), CM(U), and CI(D) are ATCCs.
In the text
thumbnail Fig. 6

OH-band depth and NUV absorption of primitive asteroids. Symbol color indicates the 0.7 μm band depth (HYDECAS). Asteroids with deep 0.7 μm band absorptions show relatively deep OH-band and NUV absorptions. Even asteroids with the 0.7 μm band depth of −1 % to 1% exhibit slightly higher NUV absorptions than those with a shallower 0.7 μm band.
In the text
thumbnail Fig. 7

Distribution of spectral slopes among the main asteroid belt. Left column: NUV slope. Middle column: VIS slope. Right column: NUV absorption strength (SNUV – SVIS). The Y axis indicates the number of asteroids in a bin. The blue line is the inner main belt, the yellow line is the middle main belt, and the red line is the outer main belt. The asteroids were divided according to diameter; first row, d > 100 km; second row, 50 < d < 100 km; third row, 10 < d < 50 km; fourth row, 5 < d < 10 km. The vertical lines are the median value for each group.
In the text
thumbnail Fig. 8

Spectral characteristics of all primitive asteroids using the ECAS and SDSS datasets (black circles). The colored circles indicate the ECAS dataset with the taxonomy defined by Tholen (1984). The dashed line shows the direction of the largest dispersion of the black circles (y = −0.15x + 0.05). The taxonomic classifications (Table 3) are also visualized.
In the text
thumbnail Fig. 9

Average spectrophotometry of taxonomic classes through the SDSS filter set classified in Table 3. The error bars show the interquartile range in the taxonomic classes. (Left) Spectra for the entire wave-length of SDSS filters. (Right) Same spectra as in the left panel but within the NUV range.
In the text
thumbnail Fig. 10

Taxonomic distribution of primitive asteroids as a function of size and dynamical populations. The total number in each bin is shown in the center of the pie charts.
In the text
thumbnail Fig. 11

Normalized cumulative distribution of (Y – J) (top panels), and of (J – Ks) (bottom panels) for the sample of 242 objects taxonomically classified in this work using the ECAS and SDSS dataset.
In the text
thumbnail Fig. 12

Taxonomic distribution of primitive asteroids through the main belt to the Cybele and Hilda zones: distributions of C, F, G, and B types (top) and C, P, and D types (bottom).
In the text

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