Europlanet Science Congress 2021
Virtual meeting
13 – 24 September 2021
Europlanet Science Congress 2021
Virtual meeting
13 September – 24 September 2021
Imagery, photometry and spectroscopy of small bodies and planetary surfaces


Imagery, photometry and spectroscopy of small bodies and planetary surfaces
Convener: Frédéric Schmidt | Co-conveners: Stéphane Erard, Maria Gritsevich, Antti Penttilä
Fri, 17 Sep, 15:10–15:55 (CEST)

Session assets

Discussion on Slack

Oral and Poster presentations and abstracts

Chairpersons: Frédéric Schmidt, Stéphane Erard, Antti Penttilä
Small bodies observations
Lucie Riu, Rosario Brunetto, John Carter, Brigitte Gondet, Vincent Hamm, Kentaro Hatakeda, Yves Langevin, Cateline Lantz, Tania Le Pivert-Jolivet, Damien Loizeau, Aiko Nakatoh, Tatsuaki Okada, Cédric Pilorget, François Poulet, Tomohiro Usui, Toru Yada, Kasumi Yogata, Aurélie Moussi-Soffys, and Jean-Pierre Bibring

Introduction: On December 6, 2020, the Hayabusa2 mission successfully returned to Earth ~ 5.4 g of samples collected at the surface of the C-type asteroid Ruygu [1,2]. Its surface was first sampled on February 22, 2019, then on July 12, 2019, close to a 10-meter large artificial crater, so as to possibly access sub-surface material [3]. The collected samples are now kept at the Extraterrestrial Samples Curation Center of JAXA at ISAS in Sagamihara, Japan, for a first round of preliminary analyses, with the objective to characterize in a non-destructive manner both the bulk samples and a few hundreds of grains extracted from them [4]. In particular, the objective is 1) to support their further detailed characterization by the international initial analysis teams, which will start their activity in July 2021, and 2) to catalog the grains, accessible to the international community through AO selection, starting mid-2022.

The preliminary characterization of these samples is being conducted with a visible microscope with four color filters, a FTIR spectrometer operating in the 1-5 µm range and MicrOmega, a hyperspectral NIR microscope developed at Institut d'Astrophysique Spatiale (Université Paris-Saclay/CNRS, Orsay, France), operating in the near-infrared range (0.99-3.65 µm) [5]. It is noteworthy that never before have the preliminary analyses of returned extraterrestrial samples included the characterization by a NIR hyperspectral microscope.

Results: Preliminary outcomes of the analyses performed with MicrOmega will be presented at the conference. In particular, the question of the representativity of the samples collected by the Hayabusa2 spacecraft will be addressed thanks to the comparison of the spectra obtained by MicrOmega and the NIRS3 remote sensing IR spectrometer [6] which performed a spectral characterization (1.8-3.2 µm) of Ryugu's surface, including the sites of the samples' collection [7,8]. A preliminary analysis of the spatial compositional heterogeneity will be presented. Specific signatures, detected in grains typically present in <1% of the pixels, but of high relevance regarding the processes determining Ryugu formation and evolution, will also be discussed.

References: [1] Binzel R. P. et al. (2002), Physical Properties of Near-Earth Objects. pp. 255-271, [2] Vilas F. (2008) The Astronomical Journal 135 (4), 1101-1105, [3] Morota et al. (2020) Science 368, Issue 6491, pp. 654-659, [4] Yada T. et al., in preparation, [5] Bibring J.-P. et al. (2017) Astrobiology 17, Issue 6-7, pp.621-626, [6] Iwata T. et al. (2017) Space Science Reviews 208 (1-4), 317-337, [7] Kitazato K. et al. (2019) Science 364 (6437), 272-275, [8] Kitazato K. et al. (2020) Nature Astronomy, Volume 5, p. 246-250.

How to cite: Riu, L., Brunetto, R., Carter, J., Gondet, B., Hamm, V., Hatakeda, K., Langevin, Y., Lantz, C., Le Pivert-Jolivet, T., Loizeau, D., Nakatoh, A., Okada, T., Pilorget, C., Poulet, F., Usui, T., Yada, T., Yogata, K., Moussi-Soffys, A., and Bibring, J.-P.: Hayabusa2 Returned Samples: First Results From the MicrOmega Investigation Within the ISAS Curation Facility, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-564,, 2021.

Samuel Jackson, Ben Rozitis, Simon Green, Ulrich Kolb, Aaron Andrews, Lord Dover, and Stephen Lowry

1. Background

The nature of asteroid phase curves is typically explained solely by the scattering properties of atmosphereless particulate surfaces. Models such as the H, G [1] and H, G1, G2 [2] systems aim to approximate the behaviour of these scattering properties without any assumption of the physical processes involved. However, these and other models are unable to account for changes in disk-integrated brightness of asteroids due to changing viewing aspect geometry during and between apparitions. If the viewing geometry of an asteroid changes rapidly, then the change in the cross-sectional area visible by the observer can impart modulations to the phase curve, becoming more significant for elongated asteroids [3]. These effects are likely to be significant for asteroids that undergo large changes to viewing aspect during apparitions, i.e. the near-Earth Asteroids (NEAs). Shape-induced modulations to phase curves limit the taxonomic information that can be extracted from parameters such as G, or G1 & G2. In this work we are aiming to characterise the magnitude of these shape effects, and investigate the potential error introduced into interpretation of individual phase curves.

2. Modelling Phase Curve Variability

To study this effect, we have simulated asteroid phase curves for a variety of shapes over different geometries. Figure 1 describes one of these geometries that has the Earth kept fixed, with the asteroid sweeping past from south to north during a close approach with the Earth. We create ellipsoidal models, with a spherical model as a control, with differing pole orientations. The rotationally averaged reduced magnitudes for points along the orbit are simulated using a Hapke scattering model [4], using assumed S-type parameters based on (433) Eros [5].

We note significant deviations from the spherical model for elongated objects over these geometries and observe clear separation of pre- and post-opposition phase curves for certain pole orientations (Figure 2). The large differences between these phase curves and the spherical model at high phase angles indicate that taxonomic information may be far more limited in individual phase curves of NEAs than previously thought. Using a single parameter taxonomic fitting method [6] we obtain a classification for the pre-opposition phase curve in Figure 2(c) as C-type, and for the post-opposition phase curve we obtain an E-type classification. The taxonomic classification using this method changes entirely based on when the asteroid is observed. The separation of the phase curve in Figure 2(c) is significantly larger at higher phase angles, underscoring the importance of this effect in NEA phase curves.

3. Observational Detection of Phase Curve Variability

Phase curve variability has been detected in rotationally averaged calibrated photometry from the PIRATE telescope of NEA (159402) 1999 AP10 over a single apparition from 2020-07-29 to 2021-01-24 (Figure 3). PIRATE is a 0.43m aperture telescope located at Observatorio del Teide, Tenerife. These observations were conducted as part of an ongoing NEA observation and facility characterisation project [7]. A convex shape model is constructed from the uncalibrated PIRATE photometric data and additional archival data from ALCDEF [8]. Using simulations of the phase curve over this apparition using the convex model and assumed S-type Hapke parameters we verify that the variability in the phase curve is a result of aspect changes during the apparition.

4. Future Work

Further work will investigate the potential impact of shape modulations to phase curves in the formulation and optimisation of the basis functions in phase curve models. Methods for extracting taxonomic information from phase curves with potential shape modifications will need to be developed to continue to aid the classification of NEAs from photometric data in existing and upcoming datasets (e.g. LSST).


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[3] Rozitis et al. 2020, Bulletin of the AAS, 52(6)

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[6] Penttilä et al. 2016, Planetary and Space Science, 123, 117

[7] Jackson et al. 2021, submitted to PASP

[8] Stephens et al. 2018, AAS, DPS Meeting #50, id 417.03.

How to cite: Jackson, S., Rozitis, B., Green, S., Kolb, U., Andrews, A., Dover, L., and Lowry, S.: Phase Curve Variability of near-Earth Asteroids, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-255,, 2021.

Kritti Sharma, Harsh Kumar, Bryce Bolin, Varun Bhalerao, Gadiyara Anupama, and Sudhanshu Barway

The discovery, characterisation, and monitoring of Near-Earth Objects (NEOs) are critical for understanding and potentially mitigating the long-term threats to our civilisation from Potential Hazardous Asteroids (PHAs). Current survey telescopes have a sensitivity of around 20–21 magnitude, which means PHAs, with absolute magnitude H > 22, are typically discovered at distances of a fraction of an astronomical unit (Jedicke et al. 2016). This has two consequences: first, the angular speeds of these objects are often tens of arc-seconds per minute in discovery data, smearing out the faintest objects and making the discovery even more challenging. Second, uncertainties in preliminary orbits calculated from discovery data coupled with the relatively short distance from Earth lead to large uncertainties in the sky positions of these objects: ranging from tens of arc-seconds to a fraction of a degree just half a day after discovery. To confirm such faint objects and refine their orbits, one needs meter class telescopes with relatively wide fields of view.

GROWTH-India Telescope (GIT1, MPC Observatory Code: N51) is a 70-cm robotic telescope located at the Indian Astronomical Observatory (IAO) site at Hanle, Ladakh. Equipped with a 4k back-illuminated CCD, GIT has a 0.7° field of view at 0.67" resolution and reaches a depth of r' ≈ 21 in 5-minute exposures. GIT was set up as part of the Global Relay of Observatories Watching Transients Happen (Kasliwal et al. 2019)2, an international collaboration with NEO studies as one of its key science goals.

GIT began solar system observations in September 2020 and has been used to observe 75 solar system objects as of May 2021 (Figure 1). GIT is broadly used for three types of solar system studies. First, we study NEOs to confirm the discoveries and refine their orbits. The geographic location of GIT — on the opposite side of Earth to the major NEO discovery engines like ZTF (Bellm et al. 2018), CSS (Christensen et al. 2018), ATLAS (Tonry et al. 2018) and PanSTARRs (Chambers et al. 2016) — allows us to observe NEOs before the positional uncertainties blow up to unmanageable scales. Secondly, we undertake coordinated observations of solar system objects with other observatories to obtain continuous coverage over long time spans. An excellent example is the joint study of the episodically active asteroid (6478) Gault (Purdum et al. 2021) where continuous observations were key to reveal the 2.5-hour rotational period. Lastly, we also undertake observations of cometary outbursts - 29P/Schwassmann-Wachmann 1 (Kelley et al. 2021d), 22P/Kopff (Kelley et al. 2021a), C/2020 R4 (ATLAS) (Kelley et al. 2021b) and 99P/Kowal 1 (Kelley et al. 2021c).

Calculating accurate asteroid positions and magnitudes from non-sidereally tracked images poses a unique challenge. All the star images appear as streaks, and standard astrometry software are ill-suited for detecting and extracting such elongated sources. A common approach is to use the start (end) point of each streak as markers for the star positions at the start (end) of the exposure. But this approach needs significant human intervention. We have developed a new automated pipeline to process such images and obtain asteroid positions and photometry. Our pipeline detects streaks in the image and creates a synthetic image with point-like sources that can be solved for astrometry with any software. The final astrometric solutions can be transferred back to the original image, giving a coordinate system for the mid-point of observations. Testing our pipeline on sources with angular speeds as high as 2'/min and stellar streaks up to 5' in length, we have consistently obtained asteroid positions with arc-second residuals (Figure 2). This pipeline is built using open-source python packages like photutils (Bradley et al. 2020), and will soon be released publicly.

The GROWTH-India Telescope (GIT), set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay with support from the Indo-US Science and Technology Forum (IUSSTF) and the Science and Engineering Research Board (SERB) of the Department of Science and Technology (DST), Government of India. It is located at IAO (Hanle), operated by IIA.

Figure 1: An (a, e, i) orbital distribution of all solar system objects observed with GIT, including 36 NEOs, 15 Main-Belt Asteroids (MBAs), 1 Jupiter Trojan, 7 Periodic Comets and 16 Non-Periodic Comets. The region between perihelion q = 1.3 AU and aphelion Q = 0.983 AU demarcates the NEO regime. We have observed 5 Atens (NEOs with Q ≥ 0.983 AU and a < 1 AU), 21 Apollos (NEOs with a ≥ 1 AU and q ≤ 1.0167 AU) and 9 Amors (NEOs with 1.0167 AU < q ≤ 1.3 AU). The orbital parameters of periodic comets with a > 4 AU and non-periodic comets are not included in this figure. We have also observed 5 Inner MBAs (a ≤ 2.5 AU and q ≥ 1.66 AU), 5 Central MBAs (2.5 < a ≤ 2.8 AU and q ≥ 1.66 AU) and 7 Outer MBAs (a > 2.8 AU and q ≥ 1.66 AU).

Figure 2. Observed-minus-Computed (O-C) residuals in the X and Y directions of all positions of NEOs observed with GIT. The NEOs tracked had a proper motion in the range of 4 − 120"/min. The exposures of 40 − 500 s were taken, resulting in streaking reference stars of lengths in the range 22 − 310". Astrometry on this data was performed using the GROWTH-India Astrometry Pipeline with a standard deviation of 0.57", which is fairly good for faint and fast-moving NEOs.


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How to cite: Sharma, K., Kumar, H., Bolin, B., Bhalerao, V., Anupama, G., and Barway, S.: GROWTH-India Observations of Solar System Objects, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-378,, 2021.

Pamela Cambianica

Pamela Cambianica1, Gabriele Cremonese1, Giovanni Munaretto1,2, Maria Teresa Capria3,Marco Fulle4, Walter Boschin5,6,7, Luca Di Fabrizio5, Avet Harutyunyan5

1.INAF Astronomical observatory of Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy (email:

2. Department of Physics and Astronomy "Galileo Galilei", University of Padova, Vicolo dell'Osservatorio 3, 35141 Padova, Italy

3. INAF-IAPS, Via Fosso del Cavaliere 100, Tor Vergata, 00133 Roma, Italy

4. INAF Astronomical Observatory of Trieste, via Tiepolo 11, 38121 Trieste, Italy

5. Fundación Galileo Galilei-INAF, Rambla José Ana Fernandez Pérez 7, E-38712 Breña Baja, TF, Spain

6. Instituto de Astrofisica de Canarias, C/Via Lactea s/n, E-38205 La Laguna (Tenerife), Spain

7. Departamento de Astrofisica, Univ. de La Laguna, Av. del Astrofisico Francisco Sánchez s/n, E-38205 La Laguna (Tenerife), Spain


Comets are primitive bodies left over from the formation of the Solar System. Formed at large distance from the Sun, comets have been preserved at low temperatures since their birth. Therefore, the study of comets offers a unique opportunity to investigate the physical and chemical processes that occurred during the early stages of the formation and evolution of our Sun and Solar System. Most of the species in the coma are the products of physical and chemical processes acting on the parent molecules in the nucleus or in the inner coma. The study of their emission lines could then provide a large information on the physical phenomena occurring in the coma and on the composition of the nucleus. Comet C/2020 F3 (NEOWISE) is considered as the brightest comet in the northern hemisphere since comet C/2005 O1 (Hale-Bopp) in 1997. From Earth, comet NEOWISE was observable within elongations less than 20 degrees from the Sun between 11 June and 9 July, 2020. The perihelion occurred on 3 July, 2020, at a small heliocentric distance of 0.29 AU. Observations from the Comet OBServation Database (COBS)1 of the Minor Planet Center (MPC)2 show that comet NEOWISE had brightened from a visual magnitude of about 8 at the beginning of June to 0 early July.



1. Observation and emission line identification

We obtained two high resolution optical spectra of comet NEOWISE on 26 July and 5 August, 2020, by using the High Accuracy Radial velocity Planet Searcher (HARPS-North) echelle spectrograph installed on the 360-cm Telescopio Nazionale Galileo (TNG). This instrument covers the wavelength range between 383 and 693 nm, with a resolving power of 115000. The unique passage and brightness of comet NEOWISE yielded spectra with a large number of emission lines, providing information on the coma composition and the physical and chemical processes occurring on the nucleus. The spectra have been used to generate a catalog of emission lines to be used for future studies of comets, since there are no catalogs in the literature with this spectral resolution. The spectral extraction has been performed automatically by the HARPS-N Data Reduction Pipeline. To compile the high-resolution catalog, we collected and digitized several laboratory molecular line lists covering the same wavelength range of our spectra. Once the molecular line lists were collected, we analyzed our spectra with the aim of finding wavelength coincidences of emission lines in the line lists and emission features in our spectra. We included all catalog lines matching our observed emissions within the spectral resolution, i.e. +/- Δλ= λ/R, where R is the resolving power. To verify the reliability of our identification, we used spectra from both nights. Finally, to further validate the final identification, we compared our catalog with other atlas resulting from the spectral analysis of other comets [1,2,3,4,5,6,7,8,9,10,11,12].  

2. Results

We catalogued more than 4000 comet emissions. We found cometary lines due to C2, C3, CH, CN, CO+, H2O+, N2+ ,NaI, NH2, and [OI]. In particular, we found 82 CN lines, some of which belong to the violet system (B2Σ+ --> X2Σ+ band). The spectral resolution of our spectra allows to reveal definite structure of the 3883 Å sequence of the CN bands, and the P-branch is resolved into individual lines. Our spectra also reveal the presence of sodium. The identified emissions belong to the sodium doublet at 5889.95 and 5895.92 Å. We identified the emission of the forbidden green oxygen line at 5577.31 Å, and the strong red doublet emission of [OI] at 6300.31 and 6363.78 Å. C2 and C3 neutral radicals are also present in the spectra of comet NEOWISE. In particular, we found C3 emission lines in the 4050-Å Group both in the spectrum of 26 July, and in the spectrum of 5 August, 2020.We also identified NH2 and CH emissions, and observed three ionic species (H2O+, N2+, and CO+).


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How to cite: Cambianica, P.: A high spectral resolution catalog of emission lines in the visible spectrum of comet C/2020 F3 (NEOWISE), Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-523,, 2021.

Alvaro Alvarez-Candal, Paula Benavidez, Adriano Campo Bagatin, Toni Santana-Ros, and Santiago Jimenez Corral

Phase curves of minor bodies describe their brightness change with phase angle, once distance effects have been removed. Using phase curves it is possible to obtain absolute magnitudes, useful parameters as they can be used as a proxy of sizes, with limitations due to albedo. In particular, in this work, we present phase curves of several thousands of minor objects in the filter system of the SLOAN Digital Sky Survey (SDSS).

We obtained the phase curves using the Moving Object Catalog (MOC) of the SDSS including in the final uncertainties those of the input magnitudes and also the uncertainty due to the likely change in magnitude due to rotational variation of the objects. The final products are the absolute magnitudes Hλ and G12λ, where λ indicates any of the five central wavelengths of the SDSS filter system. We computed colors at zero phase angle, or absolute colors, that are not affected by phase effects and could be used as a benchmark for future studies. We also analyze the behavior at small phase angles (<7.5 degrees) where the opposition effect dominates.

How to cite: Alvarez-Candal, A., Benavidez, P., Campo Bagatin, A., Santana-Ros, T., and Jimenez Corral, S.: The multi-wavelength phase curves of minor bodies from the SLOAN Moving Objects Catalog, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-38,, 2021.

Alessandra Migliorini, Tatiana A. Michtchenko, Daniela Lazzaro, Maria Cristina De Sanctis, Monica Lazzarin, Fiorangela La Forgia, Mauro Barbieri, and Dino Mesa


Basaltic asteroids, known as V-types, are distributed throughout the main belt region.

The greatest group of these objects is located in the inner main belt, constituting the Vesta family. However, in the last 20 years, many objects showing a basaltic composition have been identified also beyond 2.5 AU. These seem not to be related to (4) Vesta and require further investigation to better constrain their nature.

We present the spectroscopic and dynamical investigation of 23 asteroids, 15 of which located in the middle and outer main belt regions. Results show that asteroids in these regions, confirmed as basaltic based on their spectra in the visible and near infrared, are more likely related to other asteroidal families, possibly differentiated.


  • Introduction

Basaltic material is reckoned as the result of an extensive geochemical differentiation, resulting in a body with a dense metallic core, a mantle of lighter olivine-rich material and an even lighter basaltic surface. This process should occur only on large-size objects, like (4) Vesta for instance, due to the heat needed to melt the chondritic material. The spectrum of (4) Vesta presents two deep absorption bands, at 0.92-0.94 μm and at 2.0μm, which are representative of pyroxenes. It also shows a very peculiar absorption band at 506 nm due to a forbidden transition of Fe2+. Most of the basaltic asteroids, identified through photometric measurements and then spectroscopically confirmed, are located in the inner main belt region (2.15-2.5AU) and mostly constitute the so-called Vesta family. This family counts more than 4500 members, only a small percentage of which have been spectrally characterised so far. These asteroids are indeed dynamically linked to Vesta (Milani et al. 2014) and share similar spectroscopic properties (Duffard et al. 2006; Moskovitz et al. 2010; De Sanctis et al. 2011; Migliorini et al. 2017). However, recent measurements allowed the identification of basaltic asteroids located beyond 2.5 AU (Lazzaro et al. 2000; Hardersen et al. 2004, 2018; Roig et al. 2008; De Sanctis et al. 2011; Solontoi et al. 2012; Ieva et al. 2018, Leith et al. 2017; Medeiros et al. 2019; Migliorini et al. 2017, 2021), which are unlikely related to Vesta, based on dynamical considerations. In addition, their surface composition shows some differences with respect to the V-types in the Vesta family region (see Jasmim et al. 2013).

In the present work, we summarise the properties of basaltic asteroids, especially located beyond 2.5 AU, that were observed at Telescopio Nazionale Galileo (TNG) and ESO facilities during several observing runs. We also discuss their dynamical evolution, in relation to asteroid families that seem to be differentiated.


  • Spectral analysis

We have obtained vis and nir spectra of 23 asteroids, 15 of which located in the middle and outer main belt, selected as putative V-types according to vis and nir photometric surveys (Roig and Gil-Hutton 2006; Carvano et al. 2010; Licandro et al. 2017). Eight more asteroids in the inner main belt, non-belonging to the Vesta family, were observed for comparison reasons. Figure 1 shows the distribution of the observed asteroids in the proper semimajor axis-proper inclination plane. Sixteen asteroids show the pyroxene bands at 1 and 2 μm, confirming their basaltic nature. In addition, these all show the faint band at 506 nm, strengthening the composition of the so-called V-type asteroids. Of the asteroids in the middle and outer belt, 9 are confirmed as basaltic asteroids, while 3 are more compatible with Q- or S- complex.

Fig. 1. Distribution of the observed asteroids in the inner, middle and outer main belt region. Observed asteroids are marked with black dots (in the inner main belt region), yellow diamonds (in the middle) and white squares (in the outer).

  • Dynamical results

The presence of basaltic asteroids far from the Vesta family region raises the question whether the V-type asteroids, observed all around the main belt, are indeed basaltic, like Vesta, or from some other bodies. In our work, the dynamics of a total of 14 asteroids in the middle main belt and 14 in the outer main belt (following Michtchenko et al. 2016), all confirmed as V-types was investigated. Among these objects, 7 were finally identified as either members or located nearby some families (Migliorini et al. 2021), whose parent body was probably differentiated or partially differentiated. This finding is in agreement with the hypothesis that V-types in the middle and outer regions have not been originated from (4) Vesta. In addition, this suggests that the number of differentiated objects in the middle and outer main belt must be much larger than previously assumed.

Our work has contributed to enlarge the number of asteroids in the middle and outer main belt region, confirmed as basaltic, and successfully identified asteroidal families, which might contain basaltic asteroids.



Based on observations made with the VLT/X-shooter of the European Southern Observatory. This work is based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fondazione Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.

This research was partly supported by the Italian Space Agency (grant DAWN-VIR n. I/004/12/0).




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How to cite: Migliorini, A., Michtchenko, T. A., Lazzaro, D., De Sanctis, M. C., Lazzarin, M., La Forgia, F., Barbieri, M., and Mesa, D.: Properties of basaltic asteroids in the middle and outer main belt, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-117,, 2021.

Eri Tatsumi, Julia de León, Marcel Popescu, Javier Licandro, and Fernando Tinaut

Polana, Eulalia, and Themis are known to have negative spectral slope (blue) in visible wavelengths (VIS). We revisited our observations of those asteroid family members from 2010 to 2012 with the 3.56-m Telescopio Nazionale Galileo (El Roque de Los Muchachos Observatory, La Palma, Spain), in order to characterize their reflectance in the near-ultraviolet region (NUV, 0.35-0.55 µm). These spectrally blue asteroids are gaining great attention because they can be related to the sample return mission targets, (162173) Ryugu (Watanabe et al. 2019) and (101955) Bennu (Lauretta et al. 2019). These two asteroids show flat or upturn in NUV (Tatsumi et al. 2020, DellaGiustina et al. 2020). This peculiar characteristic may be a hint to seek their parent bodies. Tholen et al. (1984) suggested that spectrally blue asteroids can be categorized into two types: F and B types. Especially F types are characterized by a flat NUV reflectance. We aim to characterize the reflectance spectra of 20 members from these families and discuss the possible link to two mission targets.

In asteroid spectroscopy, to remove solar colors from the observed asteroid spectra, measurements of solar analogs (instead of the Sun itself) are commonly used. Although solar analogs are known to be spectrally similar to the Sun over the visible wavelength range, they are not very well characterized in NUV region. The stars listed in Landolt (1983) are commonly used as solar analogs. However, we found that they are not the case for NUV due to the difference in CN (~0.385 µm)  absorption and metallically (Fig. 1). To avoid this problem, we use only Hyades 64 (HD 28099) to obtain the reflectance spectra. This star was well characterized in NUV by Hardorp (1978) and was found spectrally closest analog to the Sun. While the spectral differences caused by the incorrect solar analog are acceptable in visible wavelength, on the NUV region this produces erroneous results. Actually, even if we use the same data, we obtain very different NUV spectra from those presented de Leon et al. (2016), as they used Hyades 64 and several Landolt stars. This is showing how important are the solar analogs. Similarly, the Eight-Color Asteroid Survey (ECAS, Zellner et al. 1985) used only four well characterized solar analogs in the NUV to obtain their spectrophotometry from 0.32 to 1.05 µm. Thus, we collect the family member spectra also from ECAS in addition to our observations.

We measured the NUV and VIS (0.50-0.80 µm) spectral slopes for characterizing the family members (Fig. 2). The results show two groups, Themis and Eulalia-Polana. Both groups follow the trend from redder in VIS with more upturn in NUV to bluer in VIS with more downturn in NUV. The downturn in NUV typically starts at < 0.44 µm. Themis group is offset to more NUV absorption than the Eulalia-Polana group. For comparison, Ryugu and Bennu are both on the trend of the Eulalia-Polana group, which suggests two space mission targets were originated possibly from these families. Inside of each group, there is a large variation in both VIS and UV slopes, which may be caused by the hydration or by the space weathering alteration. Hiroi et al. (1996) showed that NUV absorption correlates with 3 µm absorption band based on the laboratory measurements of carbonaceous chondrites. Meaning that, more NUV downturn may indicate more hydration. Despite, a recent study (Hendrix and Vilas, 2019) showed that space weathering also affects the NUV region, suggesting that more UV upturn is proportional with the space weathering degree. If Ryugu and Bennu are originated from the same family, the degree of hydration is more plausible for explaining the NUV variation than space weathering because they show different degree of OH-band absorptions (Kitazato et al. 2019, Hamilton et al. 2019). The relation between Ryugu and Bennu will be revealed by the sample analyses in near future.

We confirmed that Eulalia-Polana members are mostly F types and Themis members are mostly B types, which is originally argued by Tholen (1984). Moreover, both Ryugu and Bennu are classified into F types in Tholen’s taxonomy. We found that there is a concentration of F types in the inner main belt region. Thus, our spectral analysis strongly suggests that both Ryugu and Bennu are originated from the inner main belt based on spectroscopy. This is consistent with probability calculation by Campins et al. (2010, 2013).