Spectral evolution of dark asteroid surfaces induced by space weathering over a decade

It is known that the surface layers of airless bodies in the Solar System are usually not fresh, having been exposed to space for long periods of time. The effect responsible for altering the surfaces of these objects and affecting their colors and reflectance spectra is called space weathering. This process was first evidenced by the distinct colors of the youngest craters on the Moon (Gold, 1955). Analysis of lunar soil samples then revealed that space weathering on the lunar surface is caused by the impact of micrometeoroids and implantation of solar wind (Hapke et al., 1975).

The lack of main-belt asteroids with spectra similar to those of ordinary chondrites suggested that space weathering also affected the surface layers of S-complex asteroids (e.g., Pieters & McFadden, 1994). Telescopic observations of S-complex and Q-type asteroids (Binzel et al., 1996) and rendezvous observations of S-complex asteroids by spacecraft (e.g., Veverka et al., 2000; Hiroi et al., 2006) later confirmed that space weathering causes changes in the reflectance spectra of the surfaces of S-complex asteroids. In particular, the samples returned from the S-complex asteroid 25143 Itokawa were compositionally similar to ordinary chondrites (e.g., Nakamura et al., 2011; Yurimoto et al., 2011). Nanophase iron particles, which had been found on the lunar soil (Keller & McKay, 1993), were also found in the Itokawa samples (Noguchi et al., 2011), thereby proving that the reflectance spectra of S-complex asteroids are reddened by space weathering.

For a long time, space weathering of dark asteroids (albedo values less than 0.1) such as C-complex and D-type asteroids did not receive much attention because many dark asteroids had been found to exhibit spectra consistent with carbonaceous chondrites (e.g., Clark et al., 2002). Nesvorný et al. (2005) was the first to show a trend in space weathering of C-complex asteroids based on correlations between family ages and spectral slopes. Lantz et al. (2018) further showed trends in spectral slope due to space weathering for each spectral type of dark asteroids observed from telescopic and laboratory study data.

Laboratory simulation experiments of space weathering using minerals and meteorites are very important for understanding spectral changes in response to this process on the surface of asteroids (e.g., Moroz et al., 1996; Yamada et al., 1999; Dukes et al., 1999). In particular, the results of simulated space weathering experiments on carbonaceous chondrites, which are considered to be the meteorite counterparts of dark asteroids, are essential to understand the spectral changes induced by space weathering on this type of object (e.g., Vernazza et al., 2013; Matsuoka et al., 2015; Lantz et al., 2017). While laboratory experiments are important research methods for understanding space weathering, there is one obstacle that cannot be overcome: time simulated experiment alter the spectra of the exposed samples over milli-year timescales, whereas the spectral changes induced by space weathering on actual asteroids act over thousand-year timescales or more. Although the irradiating fluxes are increased billions of times in intensity with respect to actual space weathering during the experiments, it cannot be guaranteed that this properly reproduces what is happening in nature over such long timescales. If experiments could be performed on meteorites such that they match the space-weathered age of the actual observed asteroid and if the resulting irradiated meteorite and the asteroid spectra could be compared, this problem would be resolved.

Another point of interest is the characteristic time over which space weathering significantly affects the spectra. Studies combining asteroid spectra with the age of their respective collisional families showed spectral changes in response to space weathering over million- to billion-year timescales (Nesvorný et al., 2005; Vernazza et al., 2009). In addition, space weathering has been reported to be efficient over thousand year timescales, although this was reported for ordinary chondrites instead of carbonaceous chondrites (Noguchi et al., 2014; Jin & Ishiguro, in press). However, it remains unknown how spectra change due to space weathering over shorter time periods. In particular, it would be very interesting to know if spectral slope changes linearly or logarithmically with time over periods shorter than a thousand to a million years.

The surface of the T-type asteroid 596 Scheila with an albedo of 0.038¹¹1Mean values obtained from IRAS (Tedesco et al., 2002), AKARI (Usui et al., 2011), and WISE (Masiero et al., 2011). was refreshed during an impact with a few-tens-of-meters projectile in 2010 (Ishiguro et al., 2011a, b). In this collision event, the spectral type of 596 turned redder, from T type to D type (Hasegawa et al., 2022). Assuming that the surface layer of 596 was space-weathered before the impact, and that a fresh surface appeared after the event, it was concluded that objects compositionally similar to 596 become bluer in spectral slope due to space weathering. This year marks approximately 10 years since the collisional event. This means that the refreshed surface of 596 has been exposed to space weathering for a period of approximately 10 years. To test whether the surface layer of 596 and its reflectance spectrum changed during that period of time, we performed spectral observations of 596 in 2022, more than a decade after the collisional event.

In this study, we investigate the spectral changes caused by space weathering over a short timescale of 10 years, which is a much shorter timescale than the formation age of the solar system. A comparable study of asteroids over a short timescale of 300 years was recently reported by Fatka et al. (2022). These authors found that two near-Earth asteroids (NEAs) 2019 ${\mathrm{PR_{2}}}$ and 2019 ${\mathrm{QR_{6}}}$ may have formed as a pair about three hundred years ago. The authors also showed that the two asteroids have spectral slopes that are equal to or redder than those of D-type asteroids. This suggests that the fresh surface layers of D-type asteroids before space weathering effects become apparent may be similar to very red objects (VROs) found by Hasegawa et al. (2021), which have a redder spectral slope than the average D-type asteroid (DeMeo et al., 2009; Hasegawa et al., 2022). Figure 1 shows that the very red NEAs, which may be very young, have a similar slope in visible spectra to the VROs 203 Pompeja with an albedo of 0.045¹ and 269 Justitia with an albedo of 0.080¹. This indicates that the surface ages of 203 and 269 may be as young as a few hundred years. Therefore, we also conducted observations of 203 and 269 to investigate if a slope change occurred in their spectra over time. Note that one further VRO: 732 Tjilaki with an albedo of 0.068²²2Mean values obtained from IRAS (Tedesco et al., 2002) and AKARI (Usui et al., 2011). was discovered in Bourdelle de Micas et al. (in press). We also investigated if a slope change of occurred in 732 spectra over time using literature.

New near-infrared (NIR) observations of 596 Scheila were carried out with the SpeX NIR spectrograph (Rayner et al., 2003) on the 3.2 m NASA Infrared Telescope Facility (IRTF) located on Maunakea, Hawaii on 2022 April 4. Both Landolt 102-1081 and Landolt 107-998 were used as solar analogues for the NIR observation (Marsset et al., 2020). Reduction of NIR spectral images was conducted using the Image Reduction and Analysis Facility (IRAF; Tody 1993) and the Interactive Data Language (IDL). Close in time, visible observations were conducted in the synchronous imaging camera with $g^{\prime}$-, $R_{\rm C}$-, and $I_{\rm C}$-bands (Yanagisawa et al., 2010) on the 1.05 m Murikabushi Telescope located on Ishigakijima, Japan on 2022 Febrary 8. HD142801 was used as a calibration star for the visible observation. Data reduction for photometry was performed using the IRAF. The procedures for the NIR and visible observations and reduction are as described in Binzel et al. (2019) and Hasegawa et al. (2021), respectively.

NIR spectrum of 269 Justitia was collected in 2009 June 15 using SpeX/IRTF (Emery private comm.). HD 124901 was used as a solar analogue star. Spectral images were reduced by the IDL. The procedures for the NIR observations and reduction of 269 are as described in Emery et al. (2011).

NIR spectra of 203 Pompeja were obtained in 2020 December 1 and 2022 June 10 and 27. As in previous observations, the observations were made using SpeX/IRTF. BD+00 2717, HD 249566, and HD 106116 were used as a solar analogue star on 2020 December 1, 2022 June 10, and 2022 June 27, respectively. Reduction of the spectroscopic images was performed with the IDL. The procedures for the NIR observations and reduction of 203 are as described in Takir & Emery (2012) and Avdellidou et al. (2021). Visible spectra of 203 Pompeja were obtained on 2022 June 18, 19, 29, and 30. Colorimetric observations were acquired with the robotic telescope TRAPPIST-South (TS) (Jehin et al., 2011) located, at the ESO La Silla Observatory in Chile. TS is a 0.6 m Ritchey-Chrétien telescope operating at f/8 and equipped with a thermoelectrically-cooled FLI ProLine 3041-BB CCD camera. The data were reduced using the PHOTOMETRYPIPELINE (Mommert, 2017) and calibrated to the band they were observed using the Pan-STARRS catalogue.

The visible spectra of 269 Justitia measured on 1994 November 05 and 2010 January 11 were retrieved from Bus & Binzel (2002) and Cellino et al. (2020), respectively. Most data collected on 2018 July 16 were retrieved from Sergeyev et al. (2022). The mean visible spectra was also obtained somewhere between 2014 August 05 and 2017 May 28 (Gaia Collaboration et al., in press)³. These visible spectra of 269 at 0.45–0.85 $\mu$m are consistent with one another (Figure 5). This indicates that the visible spectrum of 269 at 0.45–0.85 $\mu$m did not change over 22 years. Two NIR spectra of 269 were acquired on 2002 September 28 (Hasegawa et al., 2021) and 2005 May 11 (DeMeo et al., 2009). Moreover, the NIR spectrum of 269 was obtained on 2009 June 15 (Emery, private comm.). The slopes of the three NIR data between 0.8–0.95 $\mu$m are consistent with the slope of the visible data from Sergeyev et al. (2022) (Figure 5).

The slope of the visible spectrum of 269 between 0.45–0.95 $\mu$m is found to have remained unchanged for at least about 16 years. Although the observed wavelengths are different, the results of unchanged spectra are the same for 269 and 596.

Visible spectra of 732 Tjilaki were obtained on 1992 March 04 (Xu et al., 1995), 1997 March 07 (Lazzaro et al., 2004), and 2021 October 01 (Bourdelle de Micas et al., in press). The mean visible spectra was also acquire between 2014 August 05 and 2017 May 28 (Gaia Collaboration et al., in press)³. NIR spectrum was taken on 2021 October 23 were retrieved from Bourdelle de Micas et al. (in press). These visible spectra of 732 at 0.6–0.9 $\mu$m are consistent with one another (Figure 5).

The slope of the visible spectrum between 0.6–0.9 $\mu$m of 732 has not changed for at least about 29 years. We conclude that the spectra of 732 have not changed within the observed time interval, which is consistent with our results for 269 or 596.

For 203 Pompeja, a visible spectrum, visible spectrophotometric color, and NIR spectrum were taken on 2021 January 29, Febrary 4, and January 14, respectively (Hasegawa et al., 2021). Meanwhile, most of the data from Sergeyev et al. (2022) for 203 were acquired from 2018 April 16 to May 15. The visible spectra of 203, which were acquired by Sawyer (1991); Gaia Collaboration et al. (in press)³, match the one of Sergeyev et al. (2022). The slope of the spectrum of 203 obtained by Hasegawa et al. (2021) is redder than that of a typical D type, but the slope of the spectrum obtained by Sergeyev et al. (2022) is comparable to that of a T type (Figure 5). In addition, new visible and NIR spectra of 203 were obtained on 2022 June 18, 19, 29, and 30, and 2020 December 1 and 2022 June 10 and 27, respectively and classified as X types. The above indicate that there is a large slope variation of the visible and NIR spectra of 203 acquired in Hasegawa et al. (2021) with respect to the other years. Moreover, the latitude and longitude of the Sub-Earth point at the time of both observations by IRTF coincide (see Table 2). Table 2 was based on the shape model of 203 from Appendix A.

The fact that the NIR spectra of other asteroids obtained using the same solar analogue stars on the same date as Hasegawa et al. (2021) are consistent with previous spectra gives credit to this NIR spectrum of 203 (see Appendix B). Additional visible data for three other asteroids obtained by the same surveys as 203 are highly consistent with the data from Sergeyev et al. (2022) and Gaia Collaboration et al. (in press) (see Appendix C). Therefore, it appears that the visible spectrum and spectrophotometric color reported in Hasegawa et al. (2021) are valid. Furthermore, the slope of the measured visible spectrum (0.7–0.8 $\mu$m) is consistent with the slope in the NIR spectrum (0.8–0.9 $\mu$m), which further validates the observational result of Hasegawa et al. (2021).

The first possible cause of the spectral differences is cometary activity, as reported on 596. Therefore, the radial profiles of 203 around the observation period of Hasegawa et al. (2021) were investigated by comparing the radial profiles with field stars to detect a faint comet-like coma. However, no coma is detected within the background detection limit of 25.1 mag arcsec^-2 in the $r$ band (see Appendix D). This evidence suggests that the spectral differences may not be triggered by an unforeseen cometary activity.

The only difference between the IRTF observations is the latitude of the sub-solar point. Sub-solar points were at relatively high latitudes⁴⁴4This is applied to the first shape/pole model, whereas the second shape/pole solution has the northern and southern hemispheres reversed. in 2021 observations (Table 2). The existence of regions with a VRO-like spectrum in high latitudes is necessary to explain the spectral differences. There may be a giant crater, that is shaded under low sub-solar point and whose areas are not visible, such as the giant crater on asteroid 253 Mathilde (Veverka et al., 1997), for example. Since the VRO-like spectra are much brighter in the NIR than the X-type spectra, even if the area of the crater illuminated by sunlight is small, the feature may appear in the spectrum. An asteroid with surface heterogeneity was also reported in Avdellidou et al. (2021). These authors suggested that the very red spectrum may correspond to a fresh crater formed by an impact as their studied asteroid 223 Rosa, also showed a very red spectrum when observed at different epochs. Finding craters with a VROs-like color on asteroids with an X-type surface implies that asteroids with a VROs surface layer evolved to an X-type-like surface layer by space weathering. If solar wind is the main cause of space weathering, the time of sunlight exposure at the bottom of craters such as this one, where sunlight is less directly illuminated, is much shorter than on other surfaces, so that the time for sunlight penetration is relatively longer and the spectral features may not disappear. Considering that VROs have the same spectral properties as RR and IR class trans-Neptunian objects (Fulchignoni et al., 2008; Fraser et al., submitted), this supports the proposal of (Vernazza et al., 2021; Rivkin et al., 2022) that D/T-type and dark X-complex asteroids originated from the same transplanetary and/or transneptunian planetesimal region.

A comparative study of the spectra of the T/D-type asteroid 596 Scheila before, immediately after, and 10 years after the 2010 impact event reveals the following:

• •

  Fresh surfaces on dark asteroids with reddish spectra like 596 do not undergo any noticeable spectral changes induced by space weathering over a 10-year timescale.

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  This is the first time that the absence of spectral changes due to space weathering has been measured over an accurate time interval for an actual asteroid, as opposed to meteorite samples measured in laboratory simulations.

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  During the first decade of space weathering following a resurfacing event, the spectral change progression of dark and reddish asteroids does not appear to progress logarithmically but linearly.

Applying the findings of our study to VROs: 203 Pompeja, 269 Justitia, and 732 Tjilaki, we can suggest the following:

• •

  The spectrum of 203 Pompeja was found to be a VRO in the 2021 observations, but an X type in the other observational epoch. The existence of a giant crater with a VRO spectrum in one of the polar regions is suggested to explain this spectral difference. This would imply that the spectra of VROs evolve into dark X types as space weathering progresses over time.

• •

  Combining the results from the study of Fatka et al. (2022) on asteroid pairs and our own results, it is possible that the spectral slope of VROs remains mostly unchanged over a timescale of 300 years.

We would like to thank the referee for the careful review and constructive suggestions, which helped us to improve the manuscript significantly. We are grateful to Dr. Joshua P. Emery for sharing valuable 269 data and supporting 203 observation. We greatly appreciate Dr. Petr Fatka and Dr. Nicholas A. Moskovitz for useful information about their observed NEAs. This work is based on observations collected at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract 80HQTR19D0030 with the National Aeronautics and Space Administration, Ishigakijima Astronomical Observatory, National Astronomical Observatory of Japan, and at Seoul National University Astronomical Observatory. The authors acknowledge the sacred nature of Mauna kea and appreciate the opportunity to observe from the mountain. The instrumentation at IAO was supported by a Grant-in-Aid for Scientific Research on Priority Areas (19047003). TRAPPIST is a project funded by the Belgian Fonds (National) de la Recherche Scientique (F.R.S.-FNRS) under grant PDR T.0120.21. We thank the Las Cumbres Observatory and their staff for its continuing support of the ASAS-SN project. ASAS-SN is supported by the Gordon and Betty Moore Foundation through grant GBMF5490 to the Ohio State University, and funded in part by the Alfred P. Sloan Foundation grant G-2021-14192 and NSF grant AST-1908570. Development of ASAS-SN has been supported by NSF grant AST-0908816, the Mt. Cuba Astronomical Foundation, the Center for Cosmology and AstroParticle Physics at the Ohio State University, the Chinese Academy of Sciences South America Center for Astronomy (CAS-SACA), the Villum Foundation, and George Skestos. This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. ATLAS is primarily funded to search for near earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen’s University Belfast, the Space Telescope Science Institute, the South African Astronomical Observatory (SAAO), and the Millennium Institute of Astrophysics (MAS), Chile. ZTF data were based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under GrantNo. AST-2034437 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center atStockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, the TANGOConsortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore NationalLaboratories, and IN2P3, France. Operations are conducted by COO, IPAC, and UW. This study has utilized the SIMBAD database, operated at CDS, Strasbourg, France, and the JPL HORIZONS ephemeris generator system, operated at JPL, Pasadena, USA. This work is based on data provided by the Minor Planet Physical Properties Catalogue (MP3C) of the Observatoire de la Côte d’Azur. F.D. and M.M. were supported by the National Aeronautics and Space Administration under grant No. 80NSSC18K0849 and 80NSSC18K1004 issued through the Planetary Astronomy Program. Any opinions, findings, and conclusions or recommendations expressed in this letter are those of the authors and do not necessarily reflect the views of the National Aeronautics and Space Administration. The work of J.H. has been supported by the Czech Science Foundation through grant 20-08218S. C.A. and M.D. acknowledge support from ANR ”ORIGINS” (ANR-18-CE31-0014). C.A. and M.D. were Visiting Astronomers at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract 80HQTR19D0030 with the National Aeronautics and Space Administration. E.J. is FNRS Senior Research Associate. M.Im acknowledges the support from the Korea Astronomy and Space Science Institute grant under the R&D program (Project No.2020-1-600-05) supervised by the Ministry of Science and Technology and ICT (MSIT), and the National Research Foundation of Korea (NRF) grant, No. 2020R1A2C3011091, funded by MSIT. M.I. was supported by the NRF grant No. 2018R1D1A1A09084105. This study was supported by JSPS KAKENHI (grant nos. JP20K04055, JP21H01140, JP21H01148, and JP22H00179) and by the Hypervelocity Impact Facility (former facility name: the Space Plasma Laboratory), ISAS, JAXA.