Oral and Poster presentations and abstracts
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 Few data come from unpublished observations at the 3.6m New Technology Telescope (La Silla, Chile) and the 1.22m telescope in Asiago made previously by our team.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.
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 [1]. The absorption bands at 3.38, 3.42, and 3.47 µm are assigned to symmetric and asymmetric modes of CH3 and CH2 groups [2]. 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 [4], (24) Themis [5], (52) Europa [6], (65) Cybele [7], (121) Hermione [8], and (704) Interamnia [9]. The presence of organic features was also detected on the surface of the comet 67P/Churyumov-Gerasimenko [10].
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 [11], 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 [12]. 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.
References
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[6] Takir, D., Emery, J. P. Icarus, 219, 641, 2014.
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[8] Hargrove, K. D., Kelley, M. S., Campins, H., Licandro, J., Emery, J. Icarus, 221, 453, 2012.
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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.
Abstract
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.
Introduction
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 [3]. 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.
Measurements
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 [4]. 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 [5]. 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°.
Results
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
samples.
Figure 2: Phase function curves (left) and degree of linear polarization (right) for the three spinel
samples.
References
[1] Belskaya I.N. et al., Refining the asteroid taxonomy by polarimetric observations. ICARUS, Vol.
284, pp. 30-42, 2017.
[2] López-Sisterna C. et al., Polarimetric survey of main-belt asteroids. VII. New results for 82
main-belt objects. A&A, Vol. 626, A42, 2019.
[3] Cellino A. et al., Unusual polarimetric properties of (101955) Bennu: similarities with F-class
asteroids and cometary bodies. MNRAS, Vol.481, pp.L49-L53, 2018.
[4] Muñoz O. et al., Experimental determination of scattering matrices of dust particles at visible
wavelengths: The IAA light scattering apparatus. JQSRT, Vol. 111, 187 196, 2009.
[5] Muñoz O. et al., Experimental Phase Function and Degree of Linear Polarization Curves of
Millimeter sized Cosmic Dust Analogs ApJSS, Vol. 247, pp.19, 2020.
[6] Cellino A. et al., A successful search for hidden Barbarians in the Watsonia asteroid family.
MNRAS, Vol. 439, L75-L79, 2014.
[7] Devogéle M. et al., New polarimetric and spectroscopic evidence of anomalous enrichment in
spinel-bearing calcium-aluminium-rich inclusions among L-type asteroids. ICARUS, Vol. 304,
pp. 31-57. 2018.
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.
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.
Figure 2: Number of NEAs in the NEODYS database (https://newton.spacedys.com/neodys/) on 16 May 2021 as a function of semimajor axis counted with a bin size of 0.02 au. Only asteroids with 1-sigma semimajor axis uncertainty of 0.001 au or better were included. The error bars correspond to Poisson counting statistics for each bin. Note the lack of asteroids with a semimajor axis of 1 au.
We have constructed a simple survey simulator to understand how this bias behaves for different asteroid orbits. The relative longitude λ - λEarth between the Earth and the asteroid is a key parameter determining whether an asteroid is discovered or not, affecting even bright asteroids that should otherwise be easy to discover even far from the Earth. Figure 3 shows the observational completeness for H=14 (D=4 km for pv=0.25) asteroids on a 40-yr synthetic survey down to a limiting magnitude of V=20.7 and a solar elongation cutoff of 70o. At a=1 au - the exact co-orbital condition - the asteroid is stationary as seen from the Earth and completeness is determined solely by the fraction of the orbit that resides within the solar elongation limit. Orbits away from 1 au gradually drift away from the anti-solar point and eventually exit the ``blind spot’’ caused by the solar elongation limit, allowing their detection. An important implication is that the gap in Fig 2 could contain undiscovered km-sized or larger asteroids which may be potentially hazardous (PHAs). To eliminate or significantly reduce the gap in a few decades or sooner, it would be necessary either to operate a survey with a much reduced solar elongation limit or move the detector away from the Earth.
Figure 3: Observational completeness for asteroids with semimajor axis within 0.01 au of Earth’s for a synthetic 40-year survey with a solar elongation cutoff of 70o and a limiting magnitude V=20.7. The asteroid parameters were H=14, e=0.1 and I=5o. Completeness takes values from 0 to 1 and is linearly proportional to greyscale intensity. The region of near-zero completeness centred at a = 1.000 au and Δλ = 180o is caused by the asteroid not exceeding the elongation cutoff for the duration of the survey.
At the meeting we will show model results to demonstrate how the observational completeness for co-orbital asteroids depends on the elements of the orbit: eccentricity, inclination, periapsis and node. An estimate for the number of as-yet-undiscovered co-orbital asteroids as a function of size will also be provided. In addition, we will be applying the simulator to known asteroids presently observed in different libration modes of the 1:1 resonance: Trojans (2010 TK7; Connors et al, 2011), horseshoes (419624 2010 SO16; Christou & Asher, 2011) and quasi-satellites (469219 Kamoʻoalewa; Chodas, 2016) and aim to report the results at the conference.
Acknowledgements: This work was supported via grant ST/R000573/1 from the UK Science and Technology Facilities Council (STFC).
References:
Chodas, P., 2016, The orbit and future motion of Earth quasi-satellite 2016 HO3, DPS meeting #48, id.311.04.
Christou, A. A., Asher, D. J., 2011, A long-lived horseshoe companion to the Earth, MNRAS, 414, 2965-2969.
Connors, M., Wiegert, P., Veillet, Ch., 2011, Earth’s Trojan asteroid, Nature, 475, 481-483.
Tricarico, P., 2017, The near-Earth asteroid population from two decades of observations, Icarus, 284, 416-423.
How to cite: Christou, A., Nedelchev, B., Borisov, G., Dell’Oro, A., and Cellino, A.: Earth's blind spot: A closer look at observational biases for Earth coorbital asteroids, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-92, https://doi.org/10.5194/epsc2021-92, 2021.