Currently there are over 1 million asteroids discovered. Each month over 1 mln astrometric and photometric observations are reported to the Minor Planet Center permitting dynamical and physical studies. Owing to large ground- and space-based surveys (such as Gaia, SDSS, ATLAS) hundreds of thousands of bodies can be at least partly physically characterised.
Those numerous, sparse and often incidental asteroid observations are balanced by relatively small in numbers but dense, targeted and accurate ground-based measurements. Those allow for a more detailed, tailored analysis, both in terms of observing techniques currently not available in survey mode (e.g. polarimetry, spectroscopy) and time spent on a single object.
The aim of this session is to open the discussion about the contribution of traditional vs./and survey-like data to our understanding of the origin and evolution of individual asteroids and populations. Furthermore what do they bring-in in terms of answering the big questions in planetary science such as formation and evolutions of the Solar System, planets and other planetary systems.
We invite all contributions concerning physical and/or dynamical modeling of small bodies based on traditional observations as well as survey-like data.
Tue, 21 Sep, 15:10–15:55
Chairpersons: Irina Belskaya, Agnieszka Kryszczyńska, Dagmara Oszkiewicz
The Data Release 3 by the Gaia mission (ESA) will not only multiply by a large factor the volume of observations, but will also add more quality and complexity. With respect to DR2, that appeared in 2018. The number of asteroids with astrometry and photometry will be multiplied by a factor >10, and data will span a longer time interval. Also, for the first time a set of reflectance spectra for several thousand asteroids will be released. Some planetary satellites and candidate new asteroids will also be included. The improvement in volume, accuracy and variety of data will add new dimensions to the contribution of Gaia to asteroid science as this will probably be the most extensive and self consistent set of visible spectra available up to know.
In this talk, we will mainly focus on the general properties of the asteroid data in DR3 (statistics on the sample) and on the improvement in astrometry with respect to DR2. Based on the results obtained from the exploitation of DR2, we will review the expected impact of DR3 in terms of improved orbits, Yarkovsky determination, prediction of asteroid occultations. The properties of asteroid spectra in DR3 will be presented in another contribution to this meeting by M. Delbo'.
How to cite: Tanga, P.: The solar system data in the forthcoming Data Release 3 by the Gaia mission of ESA: a preview, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-263, https://doi.org/10.5194/epsc2021-263, 2021.
How to cite: Delbo, M., Galluccio, L., De Angeli, F., Tanga, P., Cellino, A., Pauwels, T., and Mignard, F.: Gaia spectroscopic view of the asteroid main belt and beyond, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-112, https://doi.org/10.5194/epsc2021-112, 2021.
How to cite: Dziadura, K., Oszkiewicz, D., Spoto, F., and Bartczak, P.: Yarkovsky drift detectability using the Gaia DR2 asteroid astrometry , Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-132, https://doi.org/10.5194/epsc2021-132, 2021.
Asteroids, along with other small bodies are what is left of the original planetesimal disk from the planet-formation era. But not all asteroids that we observe today are planetesimals. Many of those planetesimals, during the history of the solar system, were destroyed by impacts, creating families of smaller asteroid fragments, which make the vast majority of the current main belt population. However, very few of these collisional fragments are actually linked to known families. The smaller are the sizes of the family members, the more numerous they are. Moreover, due to non-gravitational forces, known as the Yarkovsky effect, the members of these families move away from the original location with a velocity proportional to 1/D, where D is the asteroid diameter. These two facts result in that the smaller are the studied asteroids the more confusing is the picture of the original compositional distribution.
Our team developed a novel methodology to identify the planetesimals in the main belt (Bolin et al., 2017, Delbo' et al., 2017, 2019, 2021), based on finding asteroid families from correlations between their 1/Diameter and their semi-major axis (this is the so-called V-shape of asteroid families). By removing all asteroids that are inside these V-shapes and thus belong to families, we revealed the planetesimal population. This new analysis has now been completed in the Inner Main Belt (IMB) between 2.1 and 2.5 au with the identification of 71 planetesimals.
We started a spectroscopic survey of the identified IMB planetesimals, aiming at constraining their composition and mineralogy, information that is of paramount importance for defining the original compositional gradient of the main belt, including that of materials of high exobiological interest, such as hydrated minerals and carbonaceous compounds.
The survey was mainly carried out at the 1.82m Copernico Telescopio (Asiago, Italy) for spectroscopy in the visible range, and at the 4.2m Lowell Discovery Telescope (Flagstaff, USA) and 3.2m NASA Infrared Telescope Facility (Mauna Kea, USA) for the near infrared range (0.95-2.3 micron). Few data come from unpublished observations at the 3.6m New Technology Telescope (La Silla, Chile) and the 1.22m telescope in Asiago made previously by our team. The new data presented here come from 14 distinct observing runs spread out between 1999 and 2020.
To complete the survey of the IMB planetesimals, we also used spectra in the visible and NIR range published in the literature. In fact, several of the identified planetesimals are relatively large and bright, and already studied in spectroscopy.
After standard spectral reduction procedures, we merged the visible and near-infrared spectra (when available) of a given target to obtain a full VIS+NIR spectrum and we perform the taxonomic classification following the Bus-DeMeo taxonomy (Bus et al., 2002; DeMeo et al., 2009) using the M4AST tool (http://m4ast.imcce.fr) (Popescu et al., 2012). A visual inspection to identified the presence of absorption bands characteristic of some classes was performed to strength the taxonomic classification. In addition, we compute for each planetesimal several spectral parameters, such as spectral slopes, and center, depth and minimum of absorption bands, when present. Finally, we used the RELAB database (Pieters, 1983), to look for meteorite analogues of each planetesimal.
As expected, we found that the majority of the IMB planetesimals belongs to the S-complex (about 50 %, Fig. 1). The population also includes 20% of X-complex, 18 % to C-complex and 12% of end members (D, K, L and V-type). Interestingly, our survey reveals that more than 60% of the IMB carbonaceous-rich planetesimals belong to the Ch/Cgh types, as they show features associated to hydrated materials, indicating the presence of water ice at relatively small heliocentric distances. Surprisingly, about 4% of the planetesimals belong to the D-type, which are usually located in the outer Main Belt and in the Jupiter Trojan swarms. Finally, no olivine-rich A-type planetesimal is found. A-types are supposed to be formed by the collisional exposure of a mantle of a differentiated parent body. The fact that they are absent in the IMB planetesimal population supports the aforementioned theory of the A-type origin.
Here we will present the spectroscopic and compositional results of the IMB planetesimals as well as the implications for planetary formation models.
Figure 1 : The spectroscopic distribution of the studied planetesimals using the Bus-DeMeo taxonomy.
References : Bryce T. Bolin et al., Icarus, Volume 282, 2017, Pages 290-312 ; Marco Delbo et al., Science : 1026-1029, 2017 ; Marco Delbo et al., A&A 624 A69 (2019) ; Schelte J. Bus et al., Icarus, Volume 158, Issue 1, 2002, Pages 146-177 ; Francesca E. DeMeo et al., Icarus, Volume 202, Issue 1, 2009, Pages 160-180 ; M. Popescu et al., A&A 544 A130 (2012) ; Pieters, C. M. (1983), J. Geophys. Res., 88( B11), 9534– 9544,
How to cite: Bourdelle de Micas, J., Fornasier, S., Delbo, M., Avdellidou, C., and Van Belle, G.: A survey of Inner Main Belt planetesimals : composition and mineralogy, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-198, https://doi.org/10.5194/epsc2021-198, 2021.
Asteroids are the remnant blocks of the early stages of the formation of our Solar System. In particular, those classified as “primitive” are believed to contain the most pristine and almost unprocessed materials (water-bearing minerals, carbon compounds, and organics), and therefore they provide unique information on the formation and evolution of our planetary system, including how water appeared on Earth. Among these objects, primitive near-Earth asteroids (NEAs) are of particular interest. Due to their proximity they are impact hazards to Earth, but they are also the ideal targets for space missions. That is the case of primitive NEAs (101955) Bennu and (162173) Ryugu, primary targets of NASA’s OSIRIS-REx and JAXA’s Hayabusa 2 sample return missions, respectively, currently on their way to encounter the two asteroids. The main asteroid belt, located between the orbits of Mars and Jupiter (2.1-5.2 AU), and in particular collisional families, are currently considered the principal source of NEAs (Bottke et al. 2002; Bottke et al. 2005). In the case of the two primitive NEAs mentioned above, several studies have shown that the most likely source is the Polana collisional family (Campins et al. 2010, 2013), a primitive family located in the inner belt. Other large primitive families in that region are Erigone, Sulamitis, and Clarissa. Smaller primitives families like Klio, Chaldaea, Svea and Chimaera can also be found in the same region (Nesvorny et al. 2015).
With the main objective of supporting the science return of OSIRIS-Rex and Hayabusa 2, in 2010 our group started a coordinated effort to characterize the surface composition of primitive asteroids not only in the collisional families of the inner belt, but in the central and outer belt: our PRIMitive Asteroids Spectroscopic Survey (PRIMASS) includes both visible and near-infrared spectra. Up to now, in the frame of PRIMASS, our group has studied several primitive families wihtin the inner main belt: the Polana-Eulalia complex (de León et al. 2016; Pinilla-Alonso et al. 2016), Erigone (Morate et al. 2016), Sulamitis and Clarissa (Morate et al. 2018a), and Klio, Chaldaea, Chimaera, and Svea (Morate et al. 2019). One interesting result was that Erigone. Sulamitis, Klio, Chaldaea, and Chimaera, presented different percentages of asteroids with an absorption band centered at 0.7μm and associated to hydrated silicates, while the Polana, Clarissa, and Svea families showed no signs of hydration. This result remarks the need for spectral characterization as even the families classified all a priori as primitive can show compositional differences.
Continuing with our PRIMASS survey, we started the characterization of the families in the central part of the belt (2.50-2.82 AU). According to Nesvorný et al. (2015), there are at least 5 primitive families in that region, and for the present work we have focused on three of them: Padua, Nemesis, and Hoffmeister. As it can be seen in Fig. 1A, they overlap in the (a, i) orbital parameter space, and two of them overlap even in the (a, e) space. This might be indicative of a common origin and interestingly, the three families show a similar age. They also overlap in the (a, H) space (Fig. 1B), which make them an ideal case to see if we can discriminate between members from each family using spectroscopy. According to the taxonomical classification of their largest member using visible spectra, Hoffmeister is classified as a CF type family (neutral to blue spectral slope), Nemesis is a C-type family (neutral slope), and Padua is an X-type (redder slope). The distribution of WISE albedos (Mainzer et al. 2011) of Hoffmeister is rather different from what is seen on Nemesis and Padua (Fig. 1C), also indicative of different composition. Only spectra will help to compositionally characterize these families and to search for the presence of the 0.7 μm absorption band associated to hydration. This will allow us to compare the level of hydration in families from the inner to the outer belt (De Prá et al. 2017) and map the water inventory of the asteroid belt to constrain evolutionary models.
Figure 1: A) Distribution of the members of the three primitive collisional families in semimajor axis (a) vs. Eccentricity (top panel) and sine of inclination (bottom panel). The three families clearly overlap in the (a,i) space. B) Distribution of the three families in the absolute magnitude (H) - a space. There are clear overlapping regions where we can test if members of each family can be identified using spectra. C) Distribution of the albedos measured by WISE for the members of the three families.
In order to study these three families, we obtained visible spectra for a total of 124 asteroids (44 within the Nemesis and Hoffmeister families, and 36 within the Padua family) using the OSIRIS spectrograph at the 10.4m GTC, located at the Observatorio Del Roque de Los Muchachos (La Palma, Spain). In this work, we will present the first spectroscopic study of the Nemesis, Hoffmeister, and Padua families, and we will compare the results with those obtained for the families located in the inner main belt.
How to cite: Morate, D., de León, J., Licandro, J., de Prá, M., Pinilla-Alonso, N., Campins, H., and Cabrera-Lavers, A.: PRIMASS visits the primitive collisional families in the central asteroid belt:Nemesis, Hoffmeister, and Padua, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-424, https://doi.org/10.5194/epsc2021-424, 2021.
The Javalambre Photometric Local Universe Survey (J-PLUS) is an observational campaign that aims to obtain photometry in 12 ultraviolet-visible filters (0.3–1 μm) of ∼8 500 deg2 of the sky observable from Javalambre (Teruel, Spain). Due to its characteristics and strategy of observation, this survey will let us analyze a great number of Solar System small bodies, with improved spectrophotometric resolution with respect to previous large-area photometric surveys in optical wavelengths.
The main goal of this work is to present here the first catalog of magnitudes and colors of minor bodies of the Solar System compiled using the first data release (DR1) of the J-PLUS observational campaign: the Moving Objects Observed from Javalambre (MOOJa) catalog.
Using the compiled photometric data we obtained very-low-resolution reflectance (photospectra) spectra of the asteroids. We first used a σ-clipping algorithm in order to remove outliers and clean the data. We then devised a method to select the optimal solar colors in the J-PLUS photometric system. These solar colors were computed using two different approaches: on one hand, we used different spectra of the Sun, convolved with the filter transmissions of the J-PLUS system, and on the other, we selected a group of solar-type stars in the J-PLUS DR1, according to their computed stellar parameters. Finally, we used the solar colors to obtain the reflectance spectra of the asteroids.
We present photometric data in the J-PLUS filters for a total of 3 122 minor bodies (3 666 before outlier removal), and we discuss the main issues of the data, as well as some guidelines to solve them.
How to cite: Morate, D., Marcio Carvano, J., Alvarez-Candal, A., De Prá, M., Licandro, J., Galarza, A., Mahlke, M., and Solano-Márquez, E.: J-PLUS: A first glimpse at spectrophotometry of asteroids – The MOOJa catalog, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-425, https://doi.org/10.5194/epsc2021-425, 2021.
The letter M (metal) to classify several asteroids was proposed for the first time by Zellner and Gradie  because of similarity of their polarimetric and spectral properties to iron meteorites. However, the possible meteorite analogues according to spectral data of these asteroids included not only iron meteorites but also some types of enstatite chondrites . M-class was one of seven major classes in Tholen’s taxonomy well separated from other classes by featureless spectra and moderate surface albedo . In the recent classifications based on spectral data alone [4,5] M-class is a part of X-complex which includes all asteroids with featureless spectra regardless of their albedo.
M-asteroids have caused a great interest to their study because it was believed that these asteroids could be the remnants of the metal cores of differentiated planetesimals. The largest M-type asteroid (16) Psyche has been selected as a target of the forthcoming NASA space mission. However, numerous observations of M-type asteroids by different techniques revealed that they can have diverse composition. In [6,7] the spectroscopic and radar observations were analyzed together to clarify the composition of M-type asteroids. According to their estimates, only about a third of M-type asteroids can be metal-dominated asteroids [6,7] although there is some inconsistency in compositional predictions between spectroscopic and radar observations .
Our goal is to consider polarimetric observations of M-type asteroids as complimentary technique to spectral and radar data and explore how polarimetry can improve our understanding of the nature and diversity of M-type asteroids. Here we present the results of new polarimetric observations of M-type asteroids and their analysis using all available data.
For observations we selected targets from the list of asteroids classified as M-type in  or X-complex asteroids in [4,5] with moderate surface albedo from 0.1 to 0.35. The main aim of our observations was the reliable determination of the values of polarimetric parameters characterizing the negative branch of polarization, i.e. the depth of polarization minimum Pmin and the inversion angle. Observations were started in 2020 and involved three telescopes: the 2.6-m telescope of the Crimean Astrophysical Observatory, the 2-m telescope of the Bulgarian National Astronomical Observatory in Rozhen and the remotely controlled Tohoku 60-cm telescope at Haleakala Observatory, Hawaii. Observations were made using CCD polarimeters in V or R filters at the 2-m and 2.6-m telescopes, and simultaneously in BVR filters at the 60-cm telescope. In total, polarimetric observations of 18 asteroids have been carried out from August 2020 to May 2021.
We have analyzed the new observational data together with the available literature data on the polarimetry of M-type asteroids. The number of M-type asteroids for which it is possible to determine at least one of the polarimetric parameters (Pmin or the inversion angle) has increased to 45 objects. This is more than 70% of all main belt asteroids with diameters over 40 km that can be attributed to the M-type. The previous analysis by Gil-Hutton  included a data-sample of 26 M-type asteroids. We searched for possible relationships of polarimetric parameters with visible and infrared spectral slopes and broadband colors as well as infrared and radar albedos. We found that polarimetric parameters are diagnostic on asteroid’s composition and can be used to improve our current understanding of the composition of M-type asteroids based on spectral and radar data.
Polarimetric observations of M-type asteroids, analyzed together with their spectral and radar data, provide a better understanding of the composition and nature of M-type asteroids.
Ukrainian team is supported by the National Research Foundation of Ukraine (grant N 2020.02/0371 “Metallic asteroids: search for parent bodies of iron meteorites, sources of extraterrestrial resources”).
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How to cite: Belskaya, I., Berdyugin, A., Krugly, Y., Rumyantsev, V., Donchev, Z., Sergeyev, A., and Mykhailova, S.: What can the polarimetric properties of M-type asteroids tell us about their composition?, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-359, https://doi.org/10.5194/epsc2021-359, 2021.
We present first results from a polarimetric survey of asteroids in the near-infrared J and H bands (1.25 and 1.65 microns, respectively). This survey has been enabled by the newly commissioned WIRC-Pol instrument on the Palomar 5 m telescope. WIRC-Pol simultaneously senses the four linear polarization components from the target, while using a half-wave plate to beam swap between them. This setup allows us to obtain bandpass polarimetric accuracies better than 0.1% for our targets. WIRC-Pol also obtains low resolution spectra of each Stokes component, allowing us to investigate the spectropolarimetric properties of our targets as a function of phase. We show polarimetric phase curves for objects that have been sampled at multiple phases, our initial spectropolarimetric findings, and discuss the results from modeling these observations.
How to cite: Masiero, J., Muinonen, K., Tinyanont, S., and Millar-Blanchaer, M.: Polarimetric properties of asteroids in the near-infrared, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-47, https://doi.org/10.5194/epsc2021-47, 2021.
Organic materials are crucial for understanding the processes that took place at the early stages of Solar System formation and can bring some inputs on the origin of life on Earth. Organics is widely present in various groups of carbonaceous chondrite meteorites that are believed to be originated from dark primitive asteroids . The absorption bands at 3.38, 3.42, and 3.47 µm are assigned to symmetric and asymmetric modes of CH3 and CH2 groups . Thus, the identification of organic materials on the surface of low-albedo objects is rather difficult due to the fact that organic compounds have characteristic features outside the most accessible spectral region (0.4-2.5 µm). Furthermore, there is an overlap between organic and carbonate absorption bands [e.g., 3]. Up to now, the presence of organic band was detected only for a handful of objects, such as (1) Ceres , (24) Themis , (52) Europa , (65) Cybele , (121) Hermione , and (704) Interamnia . The presence of organic features was also detected on the surface of the comet 67P/Churyumov-Gerasimenko .
In this work we studied the available spectra of low-albedo asteroids in order to find the signs of an absorption feature around 3.4 µm band and to examine the occurrence of organic matter on asteroid surfaces. We found 122 published spectra for 92 low-albedo asteroids which cover the range of 3-4 µm. Following spectra classification by , the majority of objects in the sample belong to the C-complex group. The rest of the objects belong to X-group, D-group, and T-group. We reduced our sample to 41 objects for which good-quality spectra were available. The presence of an absorption feature at 3.4 µm is detected for 20 asteroids (Fig. 1). As could be seen from the figure, the organic band is found for all asteroids in the sample that are larger than ~250 km, which is most probably related to a higher S/N ratio.
The band parameters such as central position and depth were calculated by fitting a 3.4 µm band with an Exponentially Modified Gaussian (EMG) following the method described in . We found no correlation between the depth and position of the 3.4 µm band and the orbital elements of the asteroids.
Fig. 1. Diameter vs. S/N ratio in the 3.3-3.5 μm wavelength range for dark asteroids in our ample.
Spectral types are not distributed evenly among objects with and without a band around 3.4 µm: all the spectra, except (121) Hermione, not showing the 3.4 µm band belong to the Ch or Chg classes, whereas asteroids with a detected band mostly belong to C, B, and P types. However, only two Ch/Chg asteroids in the sample, (51) Nemausa and (78) Diana, have high S/N spectra. Thus, the absence of the organic band for Ch and Cgh type asteroids can be related to the generally lower S/N ratio and/or a shallower organic band for these groups. Furthermore, there is a tendency for asteroids with the 3.4 µm band to have redder J-K colors and more neutral U-V colors (Fig. 2, left). Additionally, there is a trend for asteroids with a detected 3.4 µm band to have lower albedo (Fig. 2, right).
Fig. 2. U-V vs. J-K colors (left) and U-V vs. albedo value taken from the AKARI survey (right). The largest asteroids in the sample (1) Ceres and (2) Pallas are not shown in the plots.
Acknowledgements. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 870403.
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How to cite: Hromakina, T., Barucci, M. A., Belskaya, I., Fornasier, S., Merlin, F., and Praet, A.: Investigation of the 3.4 µm absorption band in the spectra of low-albedo asteroids, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-199, https://doi.org/10.5194/epsc2021-199, 2021.
The aim of the project is the classification of asteroids according to the most commonly used asteroid taxonomy (Bus-Demeo et al. 2009) with the use of various machine learning methods like Logistic Regression, Naive Bayes, Support Vector Machines, Gradient Boosting and Multilayer Perceptrons. Different parameter sets are used for classification in order to compare the quality of prediction with limited amount of data, namely the difference in performance between using the 0.45mu to 2.45mu spectral range and multiple spectral features, as well as performing the Prinicpal Component Analysis to reduce the dimensions of the spectral data.
This work has been supported by grant No. 2017/25/B/ST9/00740 from the National Science Centre, Poland.
How to cite: Klimczak, H., Kotłowski, W., Oszkiewicz, D., DeMeo, F., Kryszczyńska, A., Kwiatkowski, T., and Wilawer, E.: The impact of different parameter sets on the classification of asteroid types, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-807, https://doi.org/10.5194/epsc2021-807, 2021.
In this work, we present the experimental phase function and degree of linear polarization of two
sets of samples consisting of forsterite and spinel particles. The size distributions of the studied
samples span over a broad range in the scattering size parameter domain. This work is part of
an ongoing experimental project devoted to understand photopolarimetric observations of asteroids
and comets. In particular, we study the effect of the size on the scattering matrix elements, finding
a strong dependence of characteristic parameters, e.g. maximum of polarization and inversion angle,
on particles size.
Polarimetric observations of dust clouds are a powerful tool in planetary science. They allow us
investigating the nature and properties of solar system bodies and planetary systems in different
stages of evolution, e.g. asteroids, comets, and protoplanetary disks. For example, they can be
used as a reference to refine the taxonomic classification of asteroids [1, 2] or they can help in the
discrimination of objects with cometary origin . Some dust materials, like olivine and spinel,
are remarkably interesting for the investigation and characterization of solar system small bodies.
In particular, olivine is an extensively diffuse silicate mineral and spinel, a magnesium/aluminum
mineral, is a characteristic component of the unusual class of presumably ancient Barbarian asteroids
as well as an important component of Calcium Aluminium rich Inclusions (CAI) found in primitive
meteorites [6, 7]. Physical and optical properties of the dust, such as their refractive index, size,
composition, and structure define their ability to scatter the light. Therefore, in order to study these
materials, we need to experimentally characterize their photopolarimetric curves.
We analyze six samples of olivine and spinel with different sizes. The samples denoted as Pebble
consist of millimeter-sized grains and lie in the geometrical optics regime. Further, two size
distributions consisting of particles smaller than 30 and 100 micrometers are produced out of the
olivine and spinel bulk samples. The measurements have been performed at the IAA Cosmic Dust
Laboratory (CODULAB), Granada, Spain . The instrument allows to measure the scattering
matrix of a cloud of particles and can be set also to retrieve the scattering matrix of single mm-sized
particles . The measurements have been obtained at 520 nm for the mm-sized grains and at 514
nm for the micron-sized samples. The scattering angle covers the range from 3° to 177°.
Figures 1 and 2 show the phase function and degree of linear polarization (DLP) respectively of
olivine and of spinel samples.
The phase function curves show a strong dependence on particle size. We see that the micron-sized
samples have lower values with a rather flat trend at side- and back-scattering regions and a strong
increase in the forward direction. In contrast, the pebbles show u-shaped phase functions. The slope
of the phase function at side- and back-scattering regions is stronger in the case of the spinel.
The DLP curves also show a dependence on the size. They have the typical bell shape with a
negative branch at low phase angles. Spinel Pebble shows the higher maximum of polarization. The
three spinel samples show a well-defined negative polarization branch with an inversion angle located
around 28° regardless of the particle size. It is interesting to note in the case of the olivine samples
the inversion angle is highly dependent on particle size. The high inversion angle of Barbarian
asteroids polarization curves could be related to the presence of spinel in the form of millimeter
grains of regolith.
Figure 1: Phase function curves (left) and degree of linear polarization (right) for the three olivine
Figure 2: Phase function curves (left) and degree of linear polarization (right) for the three spinel
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How to cite: Frattin, E., Muñoz, O., Jardiel, T., Gómez Martín, J. C., Moreno, F., Peiteado, M., Tanga, P., Libourel, G., and Cellino, A.: Experimental phase function and degree of linear polarization of mm-sized and micron-sized olivine and spinel particles., Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-236, https://doi.org/10.5194/epsc2021-236, 2021.
Two asteroid sample return missions studied, in-situ, two primitive asteroid targets to unravel their physical and chemical properties as well as obtain regolith samples for return to Earth. We describe remote observations from OSIRIS-REx and Hayabusa2 to determine the hydration content of these primitive asteroid surfaces and implications for their aqueous alteration histories.
The NASA mission—Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer—OSIRIS-REx  studied the asteroid (101955) Bennu for two and a half years starting on its arrival at the asteroid on December 2018. The sample collection of surface regolith occurred on October 20th 2020 followed by the spacecraft departure from the asteroid on May 10th 2021 to begin its return cruise to deliver the sample to Earth in September 2023.
The JAXA mission Hayabusa2  studied the asteroid (162173) Ryugu for a year and a half (June 2018 to November 2019), and the twice-collected regolith samples with the re-entry capsule landed on Earth on December 5th 2020. These samples are currently being analyzed in Earth laboratories.
Both missions had a near-infrared spectrometer onboard, amongst other instruments, which are the OVIRS spectrometer (OSIRIS-REx Visible and InfraRed Spectrometer)  and the NIRS3 spectrometer (Near-Infrared Spectrometer) .
The analysis of the asteroid surface reflectance spectra revealed the presence of an absorption band associated with OH/H2O centered near 2.74 microns  for asteroid Bennu and 2.72 microns for asteroid Ryugu . This absorption band is caused by hydrated phyllosilicates across both asteroid surfaces. The absorption band, however, differs in center, shape and strength between the two asteroids with a weak and narrow band in the case of Ryugu and a wide asymmetric band for Bennu. This leads to the diagnoses of OH-bearing phyllosilicates on Ryugu  while H2O- and OH-bearing phyllosilicates on Bennu .
A similar absorption band has been observed in laboratory spectra of carbonaceous chondrite meteorites [7, 8]. Separately, the meteorite H content for many of these meteorites was measured by Alexander et al. [9, 10]. Correlations between spectral parameters computed on the hydrated phyllosilicate absorption band of clay minerals and their laboratory-measured water content was found by Milliken et al. [11, 12, 13] and absolute water estimation of Mars regolith was performed by .
As described in Praet et al. [15, 16], the normalized optical path length (NOPL) and effective single-scattering albedo (ESPAT) spectral parameters have been applied to estimate the hydrated phyllosilicates water and hydroxyl group hydrogen content (hereafter H content) of each asteroid global average surface. The estimation of the global mean H content of Bennu is 0.71 ± 0.28 wt.% and 0.52 −0.21+0.16 wt.% for Ryugu.
In the case of Bennu, the H content surface distribution shows a correlation with the geomorphology with higher values in the high latitudes and lower values in the equatorial band (between –20° and 20° latitudes). Whereas no such correlation is evident in the case of Ryugu as the NOPL and ESPAT parameter computed across its surface do not display any correlation with its surface geomorphological structures. These estimates and spatial trends will be updated as new information is derived from the returned samples (e.g., with enhanced thermal tail removal).
The estimated global H content value for Bennu is consistent with the H content range of aqueously altered meteorites such as heated CMs and C2 Tagish Lake, which is in agreement with [5, 16, 18]. As for Ryugu, its global H content is most similar to more strongly heated CMs, which is coherent with the best meteorite analogs for Ryugu near-infrared spectra (thermally metamorphosed CIs and shocked CMs) .
Our estimates of phyllosilicate water and hydroxyl group hydrogen content on Bennu and Ryugu, if confirmed by laboratory analysis on both returned samples, will allow the application of the same method to other asteroids, observed from the ground, and from space-telescopes. For asteroids with spectra exhibiting hydrated phyllosilicate absorption bands, such as the ones collected by the AKARI spectral survey  for example, estimation of their global mean H content will be possible.
The study of water and hydroxyl abundance on primitive asteroids is important for understanding the origin of terrestrial water and to constrain dynamical models and evolutionary processes to better understand the origin and evolution of our Solar System.
We are grateful to the entire OSIRIS-REx Team for making the encounter with Bennu possible. This material is based upon work supported by NASA under Contract NNM10AA11C issued through the New Frontiers Program. We also thank the Hayabusa2 JAXA teams for their efforts in making the mission successful. AP and MAB acknowledge funding support from CNES.
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How to cite: Praet, A., Barucci, M. A., Clark, B. E., Kaplan, H., Simon, A., Hamilton, V., Kitazato, K., and Matsuoka, M.: Comparison of the mean surface hydrogen content estimation of the asteroids (101955) Bennu and (162173) Ryugu and perspective for other asteroids., Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-288, https://doi.org/10.5194/epsc2021-288, 2021.
Asteroids with an average heliocentric distance of 1 au present special challenges to surveys. Because of the very slow Earth-relative net motion from one orbital revolution to the next, they can remain far from our planet and close to the Sun's location in the sky (Fig 1). For this reason, the likelihood of discovering objects for this type should be generally lower than for other near-Earth asteroids (NEAs). Observational incompleteness for these Earth coorbitals was quantified by Tricarico (2017) and is now readily apparent in the NEA inventory as a deficit of asteroids with a semimajor axis of 1 au (Fig 2).
Figure 1: Illustration of Earth-relative motion of an asteroid with a = 1.001 au and initially 180o away in orbital mean longitude. The asteroid is initially located behind the sun as seen from Earth and drifts towards the Earth’s location at a rate of 0.5o/year. After 180 yr, solar elongation has increased from 0 to 45o.