Europlanet Science Congress 2020
Virtual meeting
21 September – 9 October 2020
Europlanet Science Congress 2020
Virtual meeting
21 September – 9 October 2020

Oral presentations and abstracts

SB10

The on-going missions to small bodies have provided invaluable observations regarding the properties of primitive small body surfaces in different places of the Solar System, their cratering record, as well as the signatures of other processes (e.g. thermal).

The aim of this session is to open the discussion regarding the impact process on small body surfaces, the role of their physical properties and in particular their surface materials. We welcome contributions regarding:

- Studies on the latest advances in observational (e.g. spectroscopy) and experimental techniques (e.g. production of analogue materials) to characterise small bodies and their surface materials.

- Studies on laboratory impact experiments and theoretical modelling of impacts; planetary space missions which, by imaging small bodies and other planetary surfaces, allow the investigation of the outcome of collisional events (Rosetta, New Horizons, Dawn, Hayabusa2 and OSIRIS-REx); asteroid families that are consequence of the collisional break-up of their parent bodies; collisions among asteroids of different compositions that can lead to surface contamination and material mixing. Observational and experimental studies on other processes that occur on the surfaces of small bodies such as thermal cycling etc.

Convener: Chrysa Avdellidou | Co-conveners: Ron Ballouz, Dayl Martin, Sabina D Raducan

Session assets

Session summary

Chairperson: Chrysa Avdellidou
EPSC2020-102
Asteroid masses obtained with INPOP planetary ephemerides
(withdrawn)
Agnes Fienga, Chrysa Avdellidou, and Josef Hanus
EPSC2020-23
Luc Lajaunie, Manish N. Sanghani, William D.A. Rickard, José. J. Calvino, Kuljeet K. Marhas, and Martin Bizzarro

Introduction Primitive extraterrestrial materials like carbonaceous chondrite matrices and interplanetary dust particles contain tiny dust grains that were formed in the winds of red giant branch, or asymptotic giant branch stars (AGB) and in the ejecta of novae and supernovae (SNe) explosions before the formation of our solar system. Following their formation, these tiny stardust grains of submicron size traversed through the interstellar medium before being incorporated into the cloud of gas and dust that collapsed and created our solar system. Presolar grains survived the high energy processes that created our solar system and, in their isotopic compositions, preserved the fingerprints that are the nucleosynthetic signatures of the parent stellar sources of the grains.1 Correlating isotopic data of individual presolar silicates with microstructural and chemical analyses obtained by (S)TEM, provides a unique opportunity to provide better insights into physiochemical conditions of grain formation in stellar environments, grain alteration in the interstellar and parent body processes and also helps constraining various astrophysical grain condensation models. In this work, isotopic, structural and chemical analysis of nine presolar silicate grains from the CH3/CBb3 chondrite Isheyevo and CR2 chondrite NWA801 are reported. The grains studied here are found within the lithic clasts in Isheyevo and fine grained chondrule rims in NWA801 that have experienced lower amount of parent body alteration and hence the chemical compositions of presolar grains studied here are minimally altered.

Experimental Presolar oxygen anomalous grain search using oxygen isotope imaging was done in-situ using NanoSIMS50 ion microprobe and five grains from AGB and four grains from SNe, were selected for (S)TEM investigations. The TEM lamellas were prepared using a TESCAN LYRA3 FIB-SEM at Curtin University. Structural and chemical analysis of presolar grains were performed by combining high-resolution scanning TEM imaging, spatially-resolved electron energy-loss spectroscopy (EELS) and spatially-resolved energy-dispersive X-ray spectroscopy (EDS) by using a FEI Titan Cubed Themis 60-300 microscope at the University of Cádiz which was operated at 200 kV. EDS quantification was corrected by using a standard reference sample of known composition and density and by taking into account the thickness of the probed area as determined by using low-loss EELS. EELS spectrum images for fine structures (mostly, O-K, Si-L2,3 and Fe-L2,3 edges) analyses were acquired with the monochromator excited allowing an energy resolution of about 0.4 eV. After denoising using principal components analysis and removal of the multiple scattering, we were able to map the heterogeneities related to the Fe oxidation states and to the oxygen local chemical environment. For the chemical mapping of the Fe3+/ ∑Fe ratios, we have used a home-made Python routine based on the determination of the modified white-lines ratio.2 It allowed us to compare the degree of aqueous alteration of the grain with the surrounding rim and matrix grains.

Results TEM and STEM data have revealed a strong heterogeneity and a broad range of structural and chemical compositions of the grains that enabled us to compare the stellar grain condensation environments (e.g. AGB stars and SNe), and suggest widely varying formation conditions for the presolar silicates identified in this study. Only one of the grains originally condensed as an amorphous grain has shown preferential sputtering of Mg, indicating that Mg-rich amorphous grains are not preferentially destroyed. Several grains are found with signatures that represent interstellar, nebular and parent body alteration. An oldhamite-like grain (Figure 1) within a presolar enstatite grain is probably the first observation of an oldhamite grain as a seed grain for the condensation of an enstatite grain in stellar atmospheres. This grain corresponds to a local increase in Mg and a local decrease in Fe with respect to the surrounding matrix. The surface of the grain surface of up to ~40 nm is composed of higher amounts of Ca and S.  Below the surface of the grain and on the left side, diffusion streaks rich in Ca can be observed up to the lower boundary of the grain (on about 250 nm). The diffusion of Ca could be related to thermal processes and/or aqueous alterations undergone by the grain. Figure 2 shows typical EELS chemical maps acquired on the same grain. A very thin Fe-rich rim is also seen around the enstatite grain with a Fe3+/ΣFe ratio of about 0.6-0.7. The presence of several spherical nodules of Fe and Ni sulfide can also be highlighted in the matrix around the grain in the EDS and EELS chemical maps (red arrows in Figures 8 and 9). They have a diameter of about 30-45 nm and are similar to GEMS-like materials. Interestingly, they present a core/shell structure All these results, which will be discussed in detail, point out the importance of coordinated isotopic, microstructural and chemical studies of presolar silicates to investigate the processes that may have played a role in shaping our solar system.

 

Figure 1. a) STEM-HAADF micrograph of the grain Isheyevo_9. The red dashed lines line highlights the boundary of the grain. b) Superposition of HAADF micrograph and Ca chemical maps derived from EDS analysis. Chemical maps derived from EDS analyses corresponding to c) Fe, d) Ni, e) Mg and f) S elements. The red arrows highlight the presence of GEMS-like materials.

 

Figure 2. Chemical maps derived from the EELS analysis for the grain Isheyevo_9 and corresponding to a) Fe, b) O, c) the O-K pre-peak, d) F and e) the Fe+3/ΣFe ratio. The black scale bar represents 0.1 µm. The red arrows highlight the presence of GEMS-like materials.

 

How to cite: Lajaunie, L., Sanghani, M. N., Rickard, W. D. A., Calvino, J. J., Marhas, K. K., and Bizzarro, M.: Combined multi-isotopic and (S)TEM study of pre-solar silicates to probe the solar system’s prenatal history, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-23, https://doi.org/10.5194/epsc2020-23, 2020.

EPSC2020-126ECP
Stefano Rubino, Cateline Lantz, Donia Baklouti, Hugues Leroux, Ferenc Borondics, and Rosario Brunetto

Sample return missions Hayabusa2 (JAXA) and OSIRIS-REx (NASA) found evidence of hydrated silicates on the surface of C and B-type asteroids Ryugu [1] and Bennu [2]. This detection relied on the study of the Near-IR spectra from remote sensing observations of the asteroids' surfaces. Specifically, the  feature is responsible for the OH-stretching mode in hydrated silicates. This feature’s position is related to the composition and structure of minerals [3]. However, atmosphere-less bodies in our Solar System, such as Ryugu and Bennu, are affected by space weathering (SpWe). SpWe might alter the structure and composition of the mineral, thus affecting the IR band profile, depth and position, and complicating the interpretation of remote sensing data [4, 5].

We performed ion bombardment experiments on two serpentines and one saponite, to better understand how SpWe affects the remote sensing of hydrated silicates. These two classes of phyllosilicates are particularly abundant in hydrated carbonaceous chondrites [6], which have been used as standards for the surface materials on primitive asteroids [7, 8]. The ion-bombardment experiments were conducted at room temperature in a vacuum chamber (10-7mbar) on pellets made from our phyllosilicate samples. We used He+ at 40 keV and fluences of 1*1016, 3*1016 and 6*1016 ions/cm2.

We studied the in-situ behaviour of the 2.7 µm band as a function of ion fluence. We found that the evolution of the OH-stretching feature in phyllosilicates depends on the phillosilicate’s nature. For the saponite sample, the feature’s intensity seems to decrease as the band broadens slightly, without changing position. For both serpentine samples, the feature shifts toward longer wavelengths, while peak intensity and width are not strongly affected.

The observed diversity may be explained by the different crystal structure among our two phyllosilicate classes. The observation of a band shift for one of our sample’s classes indicates that space weathering can introduce a bias in the interpretation of NIR remote sensing observations of hydrated minerals. The extent of this shift is detectable by the instruments onboard Hayabusa2 and OSIRIS-REx [11, 12].

References.

[1] Watanabe, S.-I., Tsuda, Y., Yoshikawa, M., et al. 2017, Space Sci Rev, 208, 3; [2] Lauretta, D. S., Balram-Knutson, S. S., Beshore, E., et al. 2017, Space Sci Rev, 212, 925; [3] Besson, G., & Drits, V. A. 1997, Clays Clay Miner, 45, 158; [4] Lantz, C., Brunetto, R., Barucci, M. A., et al. 2017, Icarus, 285, 43; [5] Brunetto, R., Lantz, C., Nakamura, T., et al. 2020, Icarus, 345, 113722; [6] King, A. J., Schofield, P. F., Howard, K. T., & Russell, S. S. 2015, Geochim Cosmochim Acta, 165, 148; [7] Kitazato, K., Milliken, R. E., Iwata, T., et al. 2019, Science, 364, 272; [8] Hamilton, V. E., Simon, A. A., Christensen, P. R., et al. 2019, Nat Astron, 3, 332 ; [9] Mitra, S., Prabhudesai, S. A., Chakrabarty, D., et al. 2013, Phys Rev E Stat Nonlin Soft Matter Phys, 87, 062317 ; [10] Auzende, A.-L. 2003, Evolution des microstructures des serpentinites en contexte convergent: effet du degré de métamorphisme et de la déformation; [11] Iwata, T., Kitazato, K., Abe, M., et al. 2017, Hayabusa2, 317, ; [12] Christensen, P. R., et al. 2019, in 82nd Annual Meeting of The Meteoritical Society, Vol. 2157

How to cite: Rubino, S., Lantz, C., Baklouti, D., Leroux, H., Borondics, F., and Brunetto, R.: NIR remote identification of phyllosilicates and space weathering, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-126, https://doi.org/10.5194/epsc2020-126, 2020.

EPSC2020-128
Tomas Kohout, Evgeniya Petrova, Grigoriy Yakovlev, Victor Grokhovsky, Antti Penttilä, Alessandro Maturilli, Juulia-Gabrielle Moreau, Stepan Berzin, Joonas Wasiljeff, Irina Danielko, Dmitry Zamyatin, Razilia Muftakhetdinova, and Mikko Heikkilä

Introduction

Shock-induced changes in planetary materials related to impacts or planetary collisions are known to be capable of altering their optical properties. One such example is observed in ordinary chondrite meteorites. The highly shocked silicate-rich ordinary chondrite material is optically darkened and its typical S-complex-like asteroid spectrum is altered toward a darker, featureless spectrum resembling the C/X complex asteroids. Thus, one can hypothesize that a significant portion of the ordinary chondrite material may be hidden within the observed C/X asteroid population.

The exact pressure-temperature conditions of the shock-induced darkening are, however, not well constrained and due to this gap in knowledge, it is not possible to correctly assess the significance of the shock darkening within the asteroid population. In order to address this shortcoming, we experimentally investigate the gradual changes in the chondrite material optical properties together with the associated mineral and textural features as a function of the shock pressure. For this purpose, we use a Chelyabinsk meteorite (LL5 chondrite), which is subjected to a spherical shock experiment. The spherical shock experiment geometry allows for a gradual increase in the shock pressure within a single spherically shaped sample from 15 GPa at its rim toward hundreds of gigapascals in the center.

Results

Four distinct zones were observed with an increasing shock load (Fig. 1). We number the zones in the direction of increasing shock from the outside toward the center as zones I–IV The optical changes in zone I are minimal up to ~50 GPa. In the region of ~50–60 GPa corresponding to zone II, shock darkening occurs due to the troilite melt infusion into silicates. This process abruptly ceases at pressures of ~60 GPa in zone III due to an onset of silicate melting and immiscibility of troilite and silicate melts. Silicate melt coats residual silicate grains and prevents troilite from further penetration into cracks. At pressures higher than ~150 GPa (zone IV), complete recrystallization occurs and is associated with a second-stage shock darkening due to fine troilite-metal eutectic grains.

The order of the spectral curves in the UV-VIS-NIR (ultraviolet – visible – near-infrared) region follows the visual brightness in which zone I is the brightest, followed by zones III and II, and zone IV is the darkest one (Fig. 2). The MIR reflectance (Fig. 3) follows the same albedo order as UV-VIS-NIR up to the primary Christiansen feature at 8.7 µm. At higher wavelengths in the Si-O reststrahlen bands region, the reflectance order changes with zones II and III, which are brighter than zones I and IV. The comparison of the powdered sample spectra to the one obtained from the rough saw-cut surface reveals the following trends. The overall reflectance of the powdered sample is an order of magnitude lower compared to the rough surface one. The reststrahlen bands in both samples show similar positions at approximately 9.1, 9.5–9.6, 10.3, 10.8, 11.3, and 11.8–12 µm. They are dominated by olivine with possible presence of orthopyroxene. The amplitudes of the reststrahlen bands are higher in the rough surface sample. The transparency feature at 12.7 µm is only observed in the powdered sample. The primary Christiansen feature at 8.7 µm is more pronounced in the powdered sample, while the secondary one at 15.6 µm is of a low amplitude in both samples.

Conclusions

The important finding is the presence of the two distinct shock darkening mechanisms in ordinary chondrite material with characteristic material fabric and distinct pressure regions. These two regions are separated by a pressure interval where no darkening occurs. Thus, the volume of the darkened material produced during asteroid collisions may be somewhat lower than assumed from a continuous darkening process. While the darkening mainly affects the UV-VIS-NIR region and 1 and 2-µm silicate absorption bands, it does not significantly affect the silicate spectral features in the MIR region. These are more affected by material roughness. MIR observations have the potential to identify darkened ordinary chondrite material with an otherwise featureless UV-VIS-NIR spectrum.

How to cite: Kohout, T., Petrova, E., Yakovlev, G., Grokhovsky, V., Penttilä, A., Maturilli, A., Moreau, J.-G., Berzin, S., Wasiljeff, J., Danielko, I., Zamyatin, D., Muftakhetdinova, R., and Heikkilä, M.: Experimental constraints on the ordinary chondrite shock darkening caused by asteroid collisions, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-128, https://doi.org/10.5194/epsc2020-128, 2020.

EPSC2020-906ECP
Zoe Landsman and Daniel Britt

The Exolith Lab at The University of Central Florida’s Center for Lunar and Asteroid Surface Science (CLASS) produces high-mineralogical fidelity asteroid and planetary regolith simulants (several are pictured in Figure 1). Regolith simulants are used as analogs in scientific studies and in hardware development and testing for space exploration and in-situ resource utilization (ISRU) efforts. It is crucial that such simulants accurately replicate the mineral, chemical and physical properties of the simulated regoliths. Mineralogical fidelity is the foundation of our approach. For each simulant, we source individual minerals from industrial suppliers to reproduce the mineralogy and bulk chemistry of a reference material. We also control for particle size distribution, crushing the regolith simulant to a power law distribution or sieving to a specified distribution as requested.

Asteroid and Phobos Simulants

We offer a CI meteorite-based primitive asteroid simulant and have also developed CM and CR based simulants [1]. Volatile-rich asteroids are high-priority targets for ISRU efforts. The CI simulant scores highly in fidelity against reference materials in its mineralogy and bulk chemistry, grain and bulk density, magnetic properties, mechanical cobble strength, and patterns of volatile release [2]. Simulating the organic component of planetary materials is a challenge, as this component contains hazardous polycyclic aromatic hydrocarbons (PAHs). In our simulants, we instead use sub-bituminous coal as the organic component. Sub-bituminous coal has similar aromaticity and chemical composition to PAHs but is significantly less hazardous. Another challenge is sourcing the phyllosilicate components. CM mineralogies in particular contain the Fe-serpentine cronstedtite, which does not occur naturally on Earth in the commercial quantities needed for large scale simulant production. We substitute other phyllosilicates, including Mg-serpentines, opting for the lizardite polymorph rather than hazardous chrysotile.

We have recently developed two Phobos simulants for testing of hardware for the upcoming JAXA Martian Moons eXploration (MMX) mission, which includes a Phobos sample return objective. These simulants are PGI-1 (Phobos Giant Impact), based on the hypothesis that Phobos was formed by a giant impact with Mars, and PCA-1 (Phobos Captured Asteroid), based on the captured asteroid origin hypothesis. The PCA-1 recipe is adapted from our CI asteroid recipe. PGI-1 is CI mixed with a simplified Mars crust and mantle. In addition to our standard tests of bulk chemistry and particle size distribution, we measured the abrasivity, angle of repose, compressive strength, shear strength, cohesion, flowability, and hardness for both Phobos simulants. A paper on these results is in review. This is part of a larger effort to systematically characterize the physical properties of our simulants, including geotechnical properties that are of interest for spacecraft hardware development. We will make these results available to the community in the peer-reviewed literature.

Other Simulants

Exolith Lab also offers the following simulants:

  • Lunar Highlands Simulant (LHS-1): Generic highlands soil with a particle size distribution matched to Apollo 16 samples.
  • Lunar Mare Simulant (LMS-1): Generic mare soil with a particle size distribution matched to Apollo samples.
  • Mars Global Simulant (MGS-1) and variations: Global basaltic Mars soil simulant based on Rocknest soil. Two variants (a sulfate-enriched version, MGS-1S, and a clay-enriched version, MGS-1C) were also developed for water-extraction studies.
  • Jezero Delta Simulant (JEZ-1): Developed based on remote sensing of the Jezero delta soils that will be the target of the Mars 2020 Perseverance rover.
  • DUST-Y: Physical silicate dust simulant with ~1-10 micron particles. Useful for fine dust mitigation studies.

About Exolith Lab

Exolith Lab is a not for profit extension of CLASS. Our objective is to provide high-mineralogical fidelity, safe planetary regolith simulants to the community at the cost of materials, labor, and facilities. We publicly document the development and recipes for the simulants so that they can serve as an open standard, for use and modification as needed.  To order Exolith simulants or for more information, visit our website at https://sciences.ucf.edu/class/exolithlab/. There is also a Planetary Simulant Database available at https://simulantdb.com/.

References

How to cite: Landsman, Z. and Britt, D.: Simulated Asteroid and Planetary Materials at the CLASS Exolith Lab, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-906, https://doi.org/10.5194/epsc2020-906, 2020.

EPSC2020-489ECP
Bryce Bolin, Christoffer Fremling, Timothy Holt, Matthew Hankins, Shreya Anand, Kishalay De, Kunal Deshmukh, Mansi Kasliwal, Alessandro Morbidelli, Josiah Purdum, and Robert Quimby

Keck Time-resolved Spectrophotometry of Temporarily-Captured Minimoon 2020 CD3

Bryce T. Bolin (1,2), Christoffer Fremling (1), Timothy R. Holt (3,4), Matthew J. Hankins (1), Shreya Anand (1), Kishalay De (1), Kunal Deshmukh (5), Mansi M. Kasliwal (1), Alessandro Morbidelli (6), Josiah Purdum(7), Robert Quimby (7,8)

(1) Division of Physics, Mathematics and Astronomy, Caltech, Pasadena, CA 91125, USA (bbolin@caltech.edu)

(2) IPAC, Mail Code 100-22, Caltech, 1200 E. California Blvd., Pasadena, CA 91125, USA

(3) Centre for Astrophysics, University of Southern Queensland, Queensland, Australia

(4) Southwest Research Institute, Department of Space Studies, Boulder, CO. USA.

(5) Dept. of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India

(6) Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Boulevard de l’Observatoire, CS 34229, F-06304 Nice cedex 4, France

(7) Dept. of Astronomy, San Diego State University, 5500 Campanile Dr, San Diego, CA 92182, U.S.A.

(8) Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan

We report on rotationally-averaged visible spectrophotometry of minimoon 2020 CD3, the second known asteroid to be temporarily gravitationally captured by the Earth-Moon system, taken with Keck I/LRIS  between 434 nm and 912 nm in B, g, V, R, I and RG850 bands as it was leaving the Earth-Moon system on 2020 March 23 UTC. We find that the broad-band spectrum of 2020 CD3 most closely resembles a V-type asteroid and some lunar rock samples with a reddish surface with a slope of 18 +/- 3 % between 434 nm and 761 nm corresponding to colors of g-r = 0.62 +/-0.08, r-i = 0.21 +/- 0.06 and an absorption band at ~900 nm corresponding to i-z = -0.54 +/- 0.10 (Isaacson et al. 2011, DeMeo & Carry 2013) as seen in the left panel of Fig. 1. Assuming typical albedos of V-type asteroids, we constrain 2020 CD3's size to be ~1.0 +/- 0.1 m in diameter when combined with our measured H magnitude of 31.74 +/- 0.14.  In our time-series data, we detect periodic lightcurve variations corresponding to a rotation period of ~573 s and a Lomb-Scargle false alarm probability of <10-4 (Fig. 2, left panel) with a lightcurve amplitude of ~1 magnitude implying a b/a axial ratio of ~2.5 (Fig. 2, right panel), though this is atypical for meter-scale asteroids which are thought to have rotation periods ~60 s (Bolin et al. 2014). In addition, we refine the orbit of 2020 CD3 with our observations extending the observational arc to 37 days between 2020 February 15 and 2020 March 23 constraining its duration of capture of 2020 CD3 to be ~1.2 years a typical capture lifetime for minimoons (Granvik et al. 2012) as seen in the right panel of Fig. 1. We also constrain the effect of radiation pressure on its orbit with an estimated area-to-mass ratio of 6.9 +/- 2.1 10-4 m2/kg*. Using our diameter and area-to-mass ratio estimates, we calculate a density of 1.6-3.3 g/cm3, broadly compatible with the densities of other small asteroids with densities constrained by radiation pressure measurements (Micheli et al. 2012) and lunar rock (~2.4 g/cm3, Kiefer et al. 2012) somewhat less dense than the typical ~3.5g/cm3 density of V-type asteroids (Carry 2012). We searched for pre-discovery observations of 2020 CD3 in the ZTF archive (Masci et al. 2019) but were unable to locate any positive detections.

References:  Bolin et al. 2014, Icarus, 241, 280-297, Carry 2012, PSS, 73, 98-118, DeMeo & Carry 2013, Icarus, 226, 723-741, Kiefer et al. 2012, GRL, 39, 7, Isaacson et al. 2011, M&PS, 46, 228-251, Granvik et al. 2012, Icarus, 218, 262-277, Masci et al. 2019. PASP, 131, 018003, Micheli et al. 2012, NA, 17, 446-452, Rein & Liu. 2012. A&A, 537, A128

*calculated using find_orb by Bill Gray: https://www.projectpluto.com/find_orb.htm

 

Figure 1. Left panel: reflectance photometric spectrum of 2020 CD3 consisting of B, g, V, R, I and RG850 observations of 2020 CD3 on 2020 March 23 UTC. The 1 σ uncertainties of the normalized reflectivity are indicated by the error bars on the datapoints. The spectrum has been normalized to unity at 550 nm indicated by the red cross. The spectral range of S, V and C-type asteroids from the Bus-DeMeo asteroid taxonomic catalogue (DeMeo et al. 2009) are over-plotted with the V-type spectrum most closely resembling the spectra of 2020 CD3. The average spectrum of coarse bulk lunar rock samples is plotted for reference (Isaacson et al. 2011). Right panel: Orbital evolution of 2020 CD3 based on the orbit determined by publicly available observations of 2020 CD3 available from the Minor Planet Canter dated from 2020 February 15 UTC to our observations taken on 2020 March 23 UTC, performed using REBOUND n-body integrator (Rein & Liu. 2012). Sub-right panel top left: x vs. y Earth-centred co-rotating frame showing the orbital path of 2020 CD3. The blue data points show the orbital path before 2020 March 23 and the red data points show the orbital path after March 23. Sub-right panel top right: x vs. z Earth-centred co-rotating frame showing the orbital path of 2020 CD3. Sub-right panel bottom left: time vs semi-major axis showing the orbital evolution of 2020 CD3. Sub-right panel bottom right: time vs. geocentric distance in au of 2020 CD3.

 

Figure 2. Left panel: Lomb-Scargle periodogram of lightcurve period vs spectral power using the B, g and R lightcurve data from our 2020 March 23 UTC Keck I/LRIS observations. A peak in the power is located at single-peaked rotation period of 573.4 s with a false alarm probability of <10-4. Right panel: phased lightcurve using data converting the B, g and R into H magnitudes corresponding to a single-peak lightcurve period of 574 s and a peak-to-trough amplitude of ~1 magnitude. The 1 σ uncertainties of the magnitudes are indicated by the error bars on the datapoints.

How to cite: Bolin, B., Fremling, C., Holt, T., Hankins, M., Anand, S., De, K., Deshmukh, K., Kasliwal, M., Morbidelli, A., Purdum, J., and Quimby, R.: Keck Time-resolved Spectrophotometry of Temporarily-Captured Minimoon 2020 CD3, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-489, https://doi.org/10.5194/epsc2020-489, 2020.

EPSC2020-137ECP
Antara Sen, Beth E. Clark, Edward A. Cloutis, Amanda R. Hendrix, Daniella N. DellaGiustina, Daniel M. Applin, Stephanie Connell, Alexis Parkinson, and Salvatore Ferrone

OSIRIS-REx is a sample return mission to near-Earth Asteroid (101955) Bennu (Lauretta et al. 2017). The asteroid is spectrally classified as a B-type (Clark et al. 2011), and phyllosilicates similar to those found in carbonaceous chondrites have been detected on its surface (Hamilton et al. 2019).  Bennu has a relatively flat (and blue) reflectance spectrum in the 0.4 to 3.7 micron spectral range and has a low albedo of ~4.5% (Golish et al. 2020).

Bennu has a rough and rocky surface. Imaging data from the OSIRIS-REx Camera Suite (OCAMS; Rizk et al. 2018) reveals that boulders in the size range from 1 to 10 meters dominate the surface (Lauretta et al. 2019; DellaGiustina and Emery et al. 2019). Observed boulder textures range from smooth to hummocky and breccia-like (Walsh et al. 2019; DellaGiustina and Emery et al. 2019). The smoother rocks appear to be smaller, brighter, and more angular, while the rougher rocks appear to be larger, darker, and highly textured.  Because spectral variations on Bennu are subtle and associated with albedo (Clark et al. 2019), the question arises: Could the observed color variations be due to texture variations alone, or are space-weathering variations required to explain the observations?  To isolate the spectral effects of texture on the spectral properties of Bennu, we first simulate Bennu’s spectrum using a two-component mixture, then we check to see whether texture changes in this analog can account for the observed color and albedo trends.  

Simulated Bennu Spectral Analog: We synthesized physical mixtures of saponite (SAP105 with ~25 wt.% dolomite) with two forms of carbon: <0.56-micron particles of graphite (GRP102) and <0.021-micron particles of carbon lampblack (LCA101) (Figure 1). These mixtures have the consistency of fine silt and clay mud before they are dried and baked in an oven at ~200˚ Celsius, hence we call them “mudpies”.  The spectrum of each mudpie was measured at four textures — textured rock (broken surface), sanded slab, coarse grain, and fine grain — using an ASDI LabSpec 4 Hi-Res in the wavelength range of 0.35–2.5 microns (Figure 1). We also modelled the spectra of intimate mixtures of serpentine with magnetite at the same four textures. 

Bennu Spectral Data: We compare the measured and modelled mudpie spectra with Bennu’s reflectance spectra acquired by the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS, from 0.4 to 2.5 microns; Reuter et al., 2018), and with multispectral image data from the MapCam instrument (at 473, 550, 698, and 847 nm). We selected OVIRS spectra representing the full range of spectral properties of Bennu from the 12:30pm (local solar time) Equatorial Station 3 of the Detailed Survey campaign (Lauretta et al., 2017), at a ground sample distance of ~20 m per spectrum. We selected MapCam 4-point spectra similarly representing a  full range of color properties of Bennu from the early afternoon Baseball Diamond Flyby 2 of the Detailed Survey campaign, at ~30 cm per pixel.  With these datasets, we can constrain the best possible mudpie analogs for the surface texture and opaque abundance on Bennu. We find that the best analog mudpie for Bennu contains 10% carbon lampblack and 90% saponite.

In Figure 2, we show that the color variations observed on Bennu at 470 and 550 nm can be explained better with our best-analog mudpie ground to grain sizes between 45 and 1000 microns than with mudpies measured at two different surface textures (sanded slab and textured rock).  Based on our color trend comparisons with the successful analogs, we conclude that mudpie simulated textures can fully reproduce the range of color trends observed on Bennu, and because texture effects have high-amplitude spectral effects, it is not necessary to invoke space weathering to explain the different color and albedo trends across Bennu’s surface.

However, according to DellaGiustina et al. (in revision), the geologic context of color trends on Bennu implies that space weathering effects may be required to explain the spectral bluing of craters over time.  Hence, spectral variations on Bennu could be a combination of texture effects and opaque abundance variations caused by space weathering.

References

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DellaGiustina, D. N., et al. Diverse Color and Reflectance of Asteroid (101955) Bennu. Submitted to Science (in revision).

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