Oral presentations and abstracts

JAXA’s Hayabusa2 is a sample-return mission was launched on Dec. 3, 2014 for bringing back first samples from a C-complex asteroid [1,2]. It arrived at asteroid Ryugu on June 27, 2018 and left for Earth on Nov. 13, 2019 after conducting global remote-sensing observations, two touchdown sampling operations, rover deployments, and an artificial impact experiment. We review our science results and update the mission status of Hayabusa2 in this presentation. 

The global observations revealed that Ryugu has a top-shaped body with very low density (1.19±0.02 g/cc) [3], spatially uniform Cb-type spectra without strong Fe-rich serpentine absorption at 0.7-um [4], and a weak but significant OH absorption at 2.7 um [5]. Based on these observations, we proposed that Ryugu materials may have experienced aqueous alteration and subsequent thermal metamorphism due to radiogenic heating [4]. However, other scenarios, such as impact-induced thermal metamorphism and extremely primitive carbonaceous materials before extensive alteration, were also considered because there were many new properties of Ryugu whose origins are unclear. Also, numerical calculations show that impact heating can raise the temperatures high enough to dehydrate serpentine at typical collision speed in the asteroid main belt [6].  

Further analysis using high-resolution data obtained at low-altitude descents for both rehearsal and actual touchdown operations as well as the artificial impact experiment by small carryon impactor (SCI) and landers observations the Ryugu surface on allowed us to find out what caused the properties of Ryugu. For example, subtle but distinct latitudinal variation of spectral slope in optical wavelengths found in the initial observations [4] turned out be caused by solar heating or space weathering during orbital excursion toward the Sun and subsequent erosion of the equatorial ridge owing to slowdown in Ryugu’s spin rate [7]. The SCI impact created a very large (~17 m in crest diameter) crater consistent with gravity-controlled scaling showing that Ryugu surface has very low intra-boulder cohesion and the Ryugu surface is very young and well mixed [8].

Furthermore, the MASCOT lander also showed that typical boulders on Ryugu is not covered with a layer of fine regolith [9] and yet possess very low thermal inertia (282+93/-35 MKS) consistent with highly porous structure [10]. This value is consistent with the global values or Ryugu [4, 11], suggesting that the vast majority of boulders on Ryugu are very porous. However, thermal infrared imager (TIR) also found that Ryugu has a number of “dense boulders” with high thermal inertia (>600 MKS) consistent with typical carbonaceous chondrites, showing that Ryugu’s parent body must have had a large enough gravity and pressure to compress the constituent materials [11]. This observation supports that Ryugu originated from a large parent body, such as proto-Polana and proto-Eulalia, which are estimated to be ~100 km in diameter.

Some of the dense boulders were also covered by multi-band images of optical navigation camera (ONC-T) and turned out to have C-type spectra with albedos much higher than the Ryugu average [12]. These spectra and albedos are similar to carbonaceous chondrites heated at low temperatures. Although the total mass of these high-albedo boulders on Ryugu is estimated to be very small (< 1%), the spectral and albedo varieties are much greater than the bulk Ryugu surface and approximately follow the dehydration track of carbonaceous chondrites [12]. These spectral match supports that Ryugu materials experienced aqueous alteration and subsequent thermal metamorphism. The dominance of a high-temperature component and scarcity of lower temperature components are consistent with radiogenic heating in a relatively large parent body because large bodies would have only thin low-temperature thermal skin and large volume of high-temperature interior. 

If radiogenic heating is really responsible for Ryugu’s moderate dehydration, this may place a very important constraint on the timing of the formation of Ryugu’s parent body. Because the radiogenic heat source for most meteorite parent bodies are likely extinct species, such as 26Al, the peak temperature is chiefly controlled by the timing of accretion [13]. Thus, high metamorphism temperatures (several hundred degrees in Celsius) of Ryugu’s bulk materials inferred from spectral comparison with laboratory heated CM and CI meteorites [4, 12] require Ryugu’s parent body formed early in the Solar System. Because Ryugu’s parent body contained substantial amount of water at the time of formation, it must have been formed outside the snowline. Thus, the birth place of Ryugu’s parent body would be a high-accretion-rate location outside the snowline.

Recent high-precision measurements of stable isotopes of meteorites have found that there is a major dichotomy between carbonaceous chondrites (CCs) and some iron meteorites, which formed outside Jupiter’s orbit, and non-carbonaceous meteorites (NCs), which formed inside Jupiter’s orbit [e.g., 14]. If Ryugu belongs to CCs, then Ryugu materials could be form near Jupiter, where accretion could occur early. Thus, measurements of stable isotopes of elements, such as Cr, Ti and Mo, of Ryugu samples to be returned to Earth by the end of 2020 would be highly valuable for constraining the original locations of Polana or Eulalia, among the largest C-complex asteroids in the inner main belt. 

Acknowledgements: This study was supported by JSPS Core-to-Core program “International Network of Planetary Sciences”, CNES, and Univ. Co?te d’Azur. 

References:  [1] Watanabe et al., SSR, 208, 3-16, 2017. [2] Tsuda et at., Acta Astronaut. 91, 356-363, 2013. [3] Watanabe et al., Science, 364, 268-272, 2019. [4] Sugita et al., Science, 364, eaaw0422, 2019. [5] Kitazato et al., Science, 364, 272-275, 2019. [6] Michel et al., Nature Comm., 11, 5184, 2020. [7] Morota et al., Science, 368, 654-659, 2020. [8] Akarawa et al. Science, 368, 67-671, 2020. [9] Jaumann et al. Science, 365, 817-820, 2019.  [10] Grott et al., Nature Astron. 3, 971-976, 2019.  [11] Okada et al., Nature, 579, 518-522, 2020. [12] Sugimoto et al. 51st LPSC, #1770, 2020.  [13] Grimm and McSween, Science, 259, 653-655, 1993.  [14] Kruijer et al., PNAS, 114, 6712-6716, 2017. 

How to cite: Sugita, S., Honda, R., Morota, T., Kameda, S., Tatsumi, E., Tachibana, S., Kitazato, K., Okada, T., Namiki, N., Arakawa, M., Michel, P., Domingue, D., Tanaka, S., Yoshikawa, M., Watanabe, S., and Tsuda, Y.: Mission Status of Hayabusa2, Science Highlights, and Outlook for Sample Analyses, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-995, https://doi.org/10.5194/epsc2020-995, 2020.

Thermal imaging, or thermography, has revealed the surface physical state of the C-type near-Earth asteroid 162173 Ryugu (Okada et al., 2020). The asteroid is the target body of JAXA Hayabsua2 asteroid sample return mission, and it has been characterized through remote sensing and surface experiments, and will be deeply and accurately investigated by analysis of returned sample. Thermal observations are among such multi-scale observations, providing a new insight into understanding planetary evolution process.

Thermal infrared imager TIR (Okada et al., 2017; 2020) was used to take one-rotation global thermal images of Ryugu at every 6° step, from the home position (20 km altitude) or from the Mid-Altitude (5 km altitude). There were two big surprises contrary to the predictions before arrival at Ryugu: i) flat diurnal temperature profiles compared to the case of non-rough surface, and ii) non-cold spots identified for most of boulders. The flat diurnal temperature profiles and its maximum temperature in a day indicate that Ryugu must have very rough surfaces made of highly porous materials, derived from the thermal inertia of 300 ± 100 J K^-1s^-0.5m^-2 (hereafter, tiu). Non-cold boulders indicate that boulders are less consolidated or compacted than typical carbonaceous chondrite meteorites, and shows the same thermophysical properties as the surroundings. TIR was also used to take close-up thermal images during the descent operations, and to have proven that the surface of asteroid is covered with fragments of porous rocks, larger than several centimeters in diameter. The typical size of fragments larger than thermal skin depth (~35 mm) results in similar thermal properties between the boulders and their surroundings. We also consider the surface roughness effect (Shimaki et al., 2020) to obtain the maps of thermal inertia ( 225 ± 45 tiu) and the roughness (0.41 ± 0.05) at the same time, corresponding to very rough surfaces made of highly-porous materials. This thermal inertia is basically consistent with the value (282 +93/-35 tiu) by in situ measurement using a thermal radiometer MARA on MASCOT lander (Grott et al., 2019). Furthermore, in the close-up thermal images, there were found boulders colder by 20 °C or more, indicating the thermal inertia of typical carbonaceous chondrite meteorites.

Considering these results, we proposed a formation scenario of Ryugu: fluffy cosmic dusts gathered to form porous planetesimals, and then much larger sized but still porous bodies. A low degree of consolidation and alteration has occurred at most of the body, while a higher degree of consolidation or alteration proceeded at the deep interior. Huge meteoritic impacts destroyed and fragmented the bodies, and part of those fragments were re-accreted to form the next generation, rubble-pile bodies (asteroids). Boulders found on Ryugu might have originated from the deep interior of parent bodies, so that most of them are very porous and less consolidated but some of them are relatively dense materials similar to carbonaceous chondrites, which might have originated from the interior. Due to YORP effect, the rotation rate decreased to current one, and the current shape of a spinning top-shape were formed. Analysis of returned sample will make progress in our knowledge of the planetary formation process.

How to cite: Okada, T., Tanaka, S., Shimaki, Y., Sakatani, N., Arai, T., Senshu, H., Demura, H., Kouyama, T., Sekiguchi, T., and Fukuhara, T. and the Hayabusa2 TIR Team: Highly porous nature of C-type asteroid 162173 Ryugu revealed by thermal imaging from Hayabusa2, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-12, https://doi.org/10.5194/epsc2020-12, 2020.

Thermophysical analyses of planetary bodies such as the Moon, Mars, and numerous asteroids have allowed for remote estimates of regolith physical properties, such as particle size and packing density, as well as the relative spatial abundance of boulders.  Here we define “regolith” as a particulate assemblage where most particles are comparable to or smaller than the length scale of the diurnal skin depth (the e-folding depth of the diurnal thermal wave).

Until recently, regolith and boulders were believed to be thermally quite distinct; regolith on the Moon, Mars, and most asteroids was usually known or suspected to be fine (i.e., ranging from fine dust to sand), meaning that it was known or assumed to have a thermal inertia much lower than that of boulders and bedrock. Upon the arrival of NASA OSIRIS-REx at asteroid Bennu and JAXA Hayabusa2 at asteroid Ryugu and the subsequent thermophysical analyses of the respective asteroid surfaces, this preconceived notion of thermophysically distinct regolith and boulders/bedrock was found to be flawed (DellaGiustina and Emery et al., 2019; Sugita et al., 2019). Boulders cover the vast majority of both asteroids’ surfaces, yet the thermal inertia values determined for these boulder-rich surfaces fall within a range that was previously believed to exclusively represent coarse, sand-to-pebble–sized regolith. Recent work has been devoted to the analysis of the boulders’ thermal inertia and the physical interpretation thereof (Grott et al., 2019; Rozitis et al., in revision); the general conclusion so far is that the boulders have very low thermal conductivity and density owing to the presence of numerous pores and fractures, more so than most (or perhaps all) carbonaceous chondrite meteorites in Earth’s collections. As such, these boulders are likely to be structurally distinct from all known meteorite specimens.

Although fine-particulate regolith is rare on the surface of Bennu, it is present in some locations. It is still of great interest to probe the physical properties of this regolith by means of thermal analysis so as to make predictions about the properties of the samples that will be returned by OSIRIS-REx and to learn about the evolution of the asteroid surface and the mechanisms by which regolith is produced and lost or destroyed. There remains the enigmatic question: although we suspect that the boulders on Bennu are distinct from the meteorite collection, will the returned samples of particulate material share these distinct properties? At what scales can we define the physical thermally relevant properties of  boulders and regolith particles on Bennu? In other words, if the boulders are indeed highly fractured and porous, are the regolith particles also fractured and porous or are their dimensions below the relevant length scales?

It is very challenging to estimate the physical properties of the regolith on Bennu using thermal data, even in regions where it appears to be abundant, due to the coarseness of the regolith. Commonly used planetary thermophysical models rely on the assumption that the material on the surface, be it regolith or rock, can be approximated as a continuous, non-discretized material with physical properties that are either constant with depth or are allowed to vary with depth in some well-defined way (e.g. an exponential density increase, or up to a few layers of physically distinct material, such as regolith on bedrock or dust coatings). This assumption of material continuity is valid when regolith particles are smaller, or perhaps even much smaller, than the diurnal skin depth of the thermal wave. However, high-resolution images of the Nightingale Crater on Bennu, which is the OSIRIS-REx mission’s primary sample collection site, revealed a particle size frequency distribution (SFD) that crosses this threshold; i.e., there are particles present that are smaller than, comparable to, and larger than the diurnal skin depth (~1­–5 cm) present within a single, meters-wide observation footprint. The thermophysical behavior of such a regolith configuration has never  been comprehensively studied and likely cannot be properly approximated with standard 1D thermal modeling methods.

We will present preliminary results using a 3D regolith model where we render hundreds of regolith particles and rocks, approximated as spheres, in a finite element mesh framework. The model is heated diurnally with a solar source to study the thermal response of such skin depth–crossing SFDs under Bennu surface-like conditions. The SFD of the particles is informed by particle size counts in Nightingale Crater from high-resolution visible images. With the SFD partially constrained, we are able to focus our efforts on exploring the material property parameter space, namely to estimate the thermal conductivity and density of individual regolith particles. Given the coarseness of the regolith on Bennu, we find that the model is more sensitive than one might expect to the thermal conductivity of the individual regolith particles owing to the effects of particle non-isothermality (Ryan et al., 2020). Although this present modeling work is focused on analyzing thermal emission data of Bennu obtained by the OSIRIS-REx Thermal Emission Spectrometer (OTES, Christensen et al., 2018), we aim to expand our efforts to study the more general thermal behavior of coarse regoliths and regoliths with wide SFDs under a range of solar heating conditions.

Acknowledgements

This material is based upon work supported by NASA under Contract NNM10AA11C issued through the New Frontiers Program. We are grateful to the entire OSIRIS-REx Team for making the encounter with Bennu possible.

References:

Christensen, P.R. et al. (2018). The OSIRIS-REx Thermal Emission Spectrometer (OTES) Instrument. Space Science Reviews 214, 87.

DellaGiustina, D.N., Emery, J.P., et al. (2019) Properties of rubble-pile asteroid (101955) Bennu from OSIRIS-REx imaging and thermal analysis, Nature Astronomy, DOI:10.1038/s41550-019-0731-1.

Grott, M., Knollenberg, J., et al. (2019) Low thermal conductivity boulder with high porosity identified on C-type asteroid (162173) Ryugu, Nature Astronomy, DOI:10.1038/s41550-019-0832-x.

Rozitis, B., Ryan, A.J., Emery, J.P., et al. (in revision) Asteroid (101955) Bennu’s Weak Boulders and Thermally Anomalous Equator, Science Advances, submitted March 2020.

Ryan, A., Pino Muñoz, D., Bernacki, M., Delbo, M. (2020) Full-Field Modeling of Heat Transfer in Asteroid Regolith: Radiative Thermal Conductivity of Polydisperse Particulates, JGR:Planets. DOI:10.1029/2019JE006100.

Sugita, S., Honda, R., et al. (2019) The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes, Science. DOI:10.1126/science.aaw0422

How to cite: Ryan, A., Pino Muñoz, D., Rozitis, B., Bernacki, M., Delbo, M., Emery, J., Burke, K., Bennett, C., Siegler, M., Cambioni, S., Hamilton, V., Christensen, P., and Lauretta, D.: Thermophysical Analysis of Regolith on (101955) Bennu: The Coarse Regolith Conundrum, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-172, https://doi.org/10.5194/epsc2020-172, 2020.

Introduction:  The Japanese spacecraft Hayabusa2 made the rendezvous with a C/F-type asteroid Ryugu from June 2018 to November 2019 [1]. After the global mapping and the first touch-down operation, Small Carry-on Impactor (SCI) operation was successfully conducted on 5 April 2019 to make an artificial crater (SCI crater) [2]. This operation aimed to reveal cratering mechanism on asteroids with very small gravity. Mass of the impactor is ~2 kg and the impact speed was estimated as ~2 km/s. Moreover, this operation gave us the precious opportunity to uncover the internal material of Ryugu and observe it by the remote-sensing instruments. The observation of SCI crater might give us the insights of space weathering, heating, and layering on Ryugu. Hayabusa2 is equipped with a telescopic multiband camera, ONC-T with seven color filters in UV to NIR wavelength; ul: 0.40 µm, b: 0.48 µm, v: 0.55 µm, Na: 0.59 µm, w: 0.70 µm, x: 0.86 µm, p: 0.95 µm [3].

We investigate spectral slope, UV down/up-turn, and 0.7-µm band absorption around the SCI crater. In the global observation, the spectral slope was the most prominent variation in visible spectra [3]. The bluer regions, such as the equatorial ridge and the pole regions, of Ryugu correspond to the higher gravitational potential regions, suggesting the mass wasting from higher to lower potential regions leads to expose the fresh bluer subsurface material on Ryugu [3]. Recently more detailed mapping on the pole regions were conducted and possible 0.7-µm band absorption and UV less upturn (flat UV) were suggested [4]. These two features could be indicative of hydrated minerals, such as Fe-bearing phyllosilicates. This observational result leaded a scenario of space weathering by solar wind in the past [4]. The observational facts from the SCI crater could be also strong constraints on space weathering effect on Ryugu. In this study, we report the visible color observation results of the SCI crater and discuss the processes occurred on the surface of Ryugu in the context of both the global observation and the SCI crater observation.

Data processing:  Before (8 March 2019) and after (16 May 2019) the SCI operation, ONC-T multi-band observations at the SCI target site were conducted. The spatial resolution of before and after the SCI observations are 13 and 5.5 cm/pixel, and the phase angles are 17˚ and 30˚, respectively. After the conversion to radiance factor (I/F) [5,6] and co-registration to v-band images, we measured the spectral indexes, such as spectral slope (0.48 - 0.86 µm), 0.7-µm band depth, and UV-index. UV-index measures the excess from the spectral slope.

Color of the SCI crater:  Figure 1 shows the spectral slope map for the SCI crater. The floor of the SCI crater is found to be bluer than surrounding region. The clear color difference is observed between inside and outside of the SCI crater. The floor material might be excavated sublayer of the impact site. It should be also noted that the possible impact site between two boulders, Mobile boulder and Stable boulder, are redder in comparison with the global average. We also see the relatively red part on the eastern wall of the crater. Moreover, after the SCI operation, Mobile boulder was excavated and we see the bluer color for the part which had been lied underground. The UV-index value of inside the SCI crater is slightly smaller than surrounding area. However, the 0.7-µm band depth seems to be similar for inside and outside of the SCI crater.

Discussions: Based on the global observation, we had expected to see blue material inside of the SCI crater. Even we see the relatively bluer material inside of the SCI crater than its vicinity, the degree of blueness of the SCI crater is similar to the equatorial ridge and is far redder than the material on both north and south poles which has negative value of spectral slope. Relatively blue material inside of the SCI crater supports the hypothesis of the fresh bluer material under the red surface material as suggested by [3,7]. However, since we see the bluest material on the pole regions, the sublayer of the SCI target region might have experienced space weathering process in the past. Moreover, absence of 0.7-µm band feature also suggests that the space weathering had affected not only the outermost surface but also meter-scaled substrates. Thus, we hypothesize mass wasting by the YORP spin-up on the formation of the equatorial ridge [8], gardening by impacts, or granular convection [9] could form the space weathered subsurface layer.

Another interesting color feature is the redder material in the middle of the SCI crater, at the impact point. We can see the similar feature for some of the natural craters of several tens meter scale. This could be explained by the metamorphism by high pressure and temperature process [10], breaking up of material just at the impact site which can be sometimes observed by the impact experiments, or excavating another layer underneath.

Acknowledgments: We are grateful to the entire Hayabusa2 team for making the encounter with Ryugu possible. This study is supported by the JSPS core-to-core program “International Network of Planetary Sciences”. ET acknowledges financial support from the project ProID2017010112 under the Operational Programs of the European Regional Development Fund and the European Social Fund of the Canary Islands (OP-ERDF-ESF), and the Canarian Agency for Research, Innovation and Information Society (ACIISI). 

References: [1] Watanabe, S. et al. (2019) Science 364, 268. [2] Arakawa, M. et al.(2020) Science 368, 67. [3] Sugita, S. et al. (2019) Science 364, eaaw0422. [4] Tatsumi, E. et al. (2019) Asteroid Science in the Age of Hayabusa2 and OSIRIS-Rex, #2091. [5] Tatsumi, E. et al. (2019) Icarus 325, 153. [6] Tatsumi, E. et al. (2019) LPSC 50, Abstract #1745. [7] Morota, T. et al. (2020), Science 368, 654. [8] Sugiura, K. et al. (2019) Asteroid Science in the Age of Hayabusa2 and OSIRIS-Rex, #2012. [9] Tsuchiyama, A. et al. (2011) Science 333, 1125. [10] Hiroi, T. et al. (2020) LPSC 51.

How to cite: Tatsumi, E. and the Hayabusa2 team: Visible color of the artificial crater on Ryugu created by Small Carry-on Impactor, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-331, https://doi.org/10.5194/epsc2020-331, 2020.

Abstract

In September 2019, the OSIRIS-REx Camera Suite (OCAMS; Rizk et al., 2018) globally mapped asteroid (101955) Bennu in four broadband filters (b′, v, w, x) covering 0.44 to 0.89 microns. The multispectral images show complex relationships between color and morphology on Bennu’s surface. These data indicate that Bennu’s color has been influenced by primordial heterogeneity and space weathering.

Introduction

To evaluate relationships between color and morphology on Bennu, we radiometrically and photometrically corrected OCAMS images acquired by the MapCam instrument (Golish et al., 2020a, Golish et al., 2020b). Calibrated images were subsequently mosaicked to develop band-ratio and principal component analysis (PCA) maps. To establish statistically meaningful trends between color, reflectance, and morphological features, we mapped ~1600 boulders and ~700 craters, and extracted their average MapCam color indices.

Bennu’s global average spectrum is blue (–1% per 0.1 μm) in MapCam data (0.44 to 0.89 μm), but spectral slopes can vary from blue (negative) to red (positive) at the meter scale. The diverse color and reflectance of boulders suggests primordial heterogeneity inherited from Bennu’s parent body and exogenic impactors (DellaGiustina and Kaplan et al., accepted). Spectral changes in craters as a function of radius indicate that color may also be influenced by space weathering on Bennu.  

Boulders

On the basis of reflectance and color, we categorized Bennu’s boulders into four types: 1) High-reflectance boulders (>4.9% normal albedo) are brighter than units having the average color of Bennu, texturally smooth, exhibit angular morphology, and have blue spectral slopes across the MapCam v to x bands. 2) Dark boulders (≤4.9% normal albedo) are subangular and have rougher, more undulating surface textures compared to the bright boulders. They encompass a wide range of sizes and include the largest boulders on the asteroid (25 to 100 m in diameter). 3) Boulders with very high reflectance (up to 0.26; ~1% in number) show evidence of an absorption at 1 μm (downturn in the x band). These boulders were spectroscopically identified to contain pyroxene using data acquired by the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS; Reuter et al., 2018). 4) About 2% of boulders surveyed have an absorption feature that is detectable above the radiometric uncertainty of OCAMS at 0.7 μm (absorption depth of 2 to 10%). This absorption has been observed in spectra of primitive asteroids and carbonaceous meteorites and has been attributed to the Fe²⁺-Fe³⁺ intervalence charge transfer associated with hydrated clay-bearing phyllosilicates.

Craters

The color of the largest craters (>100 m) on Bennu is indistinguishable from that of the average terrain. However, many small (≤25 m) craters are redder than average across MapCam b′ to x filters, resulting in neutral to red spectral slopes. The size distribution of these small reddish craters implies that they are the youngest component of the global crater population.

References

Lauretta, D. S., et al. “OSIRIS-REx: sample return from asteroid (101955) Bennu.” Space Science Reviews 212.1-2 (2017): 925-984.

Rizk, B., et al. “OCAMS: the OSIRIS-REx camera suite.” Space Science Reviews 214.1 (2018): 26.

Golish, D. R., et al. “Ground and in-flight calibration of the OSIRIS-REx camera suite.” Space Science Reviews 216.1 (2020): 12.

Golish, D. R., et al. “Disk-resolved photometric modeling and properties of asteroid (101955) Bennu.” Icarus (2020): 113724.

DellaGiustina, D. N., and Kaplan, H. H., et al. “Exogenic basalt on asteroid (101955) Bennu”. Nature Astronomy, in revision.

Reuter, D.C., et al.  “The OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS): Spectral Maps of the Asteroid Bennu”. Space Science Reviews 214.1 (2018): 54. 

How to cite: DellaGiustina, D. and the OSIRIS-REx color mapping and interpretation team: Relationships Between Color & Morphology on Bennu, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-481, https://doi.org/10.5194/epsc2020-481, 2020.

1. Introduction

We present visible to near infrared (VNIR) and thermal infrared (TIR) spectral data for asteroid (101955) Bennu collected by the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) [1, 2] and the OSIRIS-REx Thermal Emission Spectrometer (OTES) [3]. The data discussed here were collected during the 12:30 pm Equatorial Station of the Detailed Survey mission phase and Reconnaissance A (varying local times). Constraints applied to the selection of data are described by [4-6].

2. OVIRS results

Early results [7] revealed that an unambiguous “3-µm” band is present, consistent with the presence of hydrated (phyllo-)silicates. The specific position of this band in OVIRS spectra, 2.74 µm ± 0.01, is consistent with the positions observed in low petrologic subtype CM2 meteorites [8]. The global distribution of this feature is described by [9].

Since the acquisition of global mapping data at ~20-30 m/spot, we have identified a complex of features in the 3.2–3.6 µm region that we attribute to the presence of C-bearing compounds (organics and carbonate minerals) [4, 5]. Absorption band positions, widths, and relative strengths appear to be associated with a variable mixture of organics and multiple carbonate minerals. The varying shape and depth of a 3.4-µm absorption feature across Bennu’s surface spans the range seen among disk-averaged spectra of main-belt carbonaceous asteroids. Bennu’s distribution of carbon-bearing materials does not correlate with the distribution of hydrated minerals, surface brightness, or geologic features. Carbonate features identified in this spectral region are interpreted as having a variety of cation compositions. The organic features are consistent with aromatic and aliphatic C-H bonds like those of insoluble organic matter in meteorites and other primitive objects [10, 11]. The deepest 3.4-micron absorptions occur on individual boulders, and surface variation may be attributable to differences in abundance, fresh exposure by processes such as thermal fracturing, or differences in space weathering. There is no definitive spectral evidence of either organics or carbonates outside of the 3.2–3.6 μm region.

Several weak absorption bands also have been observed [4]. These are consistent with phyllosilicates (e.g., the 1.4-µm region), Fe-bearing phases (e.g., 1.05-µm region), and magnetite (0.55 µm).

3. OTES results

OTES spectra acquired during the Preliminary Survey mission phase are broadly consistent with carbonaceous chondrites (CCs) in the CI/CM groups [7]. Potential evidence of magnetite is present in features at 555 and 346 cm^–1 [1] and is consistent with aqueous alteration. Detailed Survey measurements at ~40 m/spot exhibit spectral variability, primarily in the shape of the silicate stretching feature and the depth of the silicate bending feature. These variations can be described by two endmember spectral types, T1 and T2, which appear to primarily represent differences in the amount of fine particulate (<~65–100 µm) dust across the surface of Bennu [6]. The dust appears to be up to ~10–15 microns thick and does not exhibit the spectral characteristics of typical fine-particulate materials. Thermal inertia data also constrain the thickness of any dust layer to <50 µm [12]. The locations of T2 spectra, inferred to have a slightly greater proportion of dust cover, tend to correspond with those of boulders having rough surfaces, suggesting that dust may be trapped preferentially on these rocks.

Analysis of the shape of the silicate bending feature complex centered at ~440 cm^–1 (~22.7 µm) indicates that anhydrous silicates are likely to comprise less than ~10 vol.% of the bulk silicate mineralogy.

A CC (C1) clast, 91A_1, from the Almahata Sitta meteorite (Ur-ung) exhibits a strong similarity to the T1 spectrum, particularly in the silicate stretching region. This sample exhibits signs of mild heating [13], with ~10 vol% recrystallized olivine.

We cannot rule out that portions of Bennu’s surface lithology(-ies) have been heated but such heating is likely to be limited to temperatures <400°C [e.g., 14].

4. Implications for the returned sample

The material collected from Bennu’s surface will be constrained by the design of the sampling mechanism to particles less than ~2 cm in diameter [15]. Imaging [16] and spectral data suggest that particles smaller than the maximum sampleable size are present and should be collected. It is probable that the sample will be dominated volumetrically by phyllosilicate minerals, with anhydrous silicates unlikely to comprise more than ~10 vol.% of the silicate mineralogy. Magnetite is expected to be in the returned sample, as are carbonates and organics. VNIR and TIR spectral features imply that at least some of Bennu’s surface materials may differ from those of typical CC meteorites.

Acknowledgements

This material is based upon work supported by NASA under Contract NNM10AA11C issued through the New Frontiers Program. We are grateful to the entire OSIRIS-REx Team for making the encounter with Bennu possible.

References

[1] Reuter, D. C. et al. Space Science Reviews, 214, 54, 2018.

[2] Simon A. A. et al. Remote Sensing, 10, 1486, 2018.

[3] Christensen, P. R. et al. Space Science Reviews, 214, 87, 2018.

[4] Simon, A. A. et al. LPSC 51, #1046, 2020.

[5] Kaplan, H. H. et al. LPSC 51, #1050, 2020.

[6] Hamilton, V. E. et al.: Evidence for limited compositional and particle size variation on asteroid (101955) Bennu from thermal infrared spectroscopy, in prep.

[7] Hamilton, V. E. et al. Nature Astronomy, 3, 332-340, 2019.

[8] Takir, D. et al. Meteoritics & Planetary Science, 48, 1618-1637, 2013.

[9] Praet, A. et al., this meeting.

[10] Kaplan, H. H. et al., Composition of organics on asteroid (101955) Bennu, in prep.

[11] Simon, A. A. et al.: Global Mineralogy of (101995) Bennu from the OSIRIS-REx Visible and InfraRed Spectrometer, in prep.

[12] Rozitis, B. et al.: Asteroid (101955) Bennu’s weak boulders and thermally anomalous equator, Science Advances, in revision.

[13] Goodrich, C. A. et al. Meteoritics & Planetary Science, 54, 2769 – 2813, 2019.

[14] Hanna, R. D. et al., Icarus, 346, 113760, 2020.

[15] Bierhaus, E. B. et al. Space Science Reviews, 214, 107, 2018.

[16] DellaGiustina, D. N. and J. P. Emery et al. Nature Astronomy, 3, 341-351, 2019.

How to cite: Hamilton, V., Simon, A., Kaplan, H., Goodrich, C., Lauretta, D., and Spectral Analysis Working Group, T. O.-R.: VNIR-TIR spectroscopy of (101955) Bennu, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-166, https://doi.org/10.5194/epsc2020-166, 2020.

In early December 2018, NASA’S Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) [1] spacecraft arrived at the Near-Earth asteroid (101955) Bennu, [2,3] with the objective of globally characterizing the asteroid and returning a sample to Earth in 2023. Given this context, we probe the spectral nature of craters on Bennu, using spectral data acquired by the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) [4].

     2. Data and methods

We selected 46 craters from the crater catalogue of Bennu [5], depending on the availability of spectral data. The spectral data are calibrated as detailed in [6,7] and follow the photometric correction of [8], which results in reflectance factor (REFF) spectra corrected to the geometry of i=30°, e=0°, α =30°. We then derive an average spectrum of a crater from all the spectra whose footprints fall in its interior. From this average spectrum, we compute the following four spectral parameters to characterize the spectral properties of each crater.

1) Depth of the absorption band at 2.7 μm

2) Position of the 2.7-μm band minimum

3) Normalized spectral slope from 0.55 to 2.0 μm

4) Photometrically corrected REFF at 0.55 μm

For comparison with the crater spectra, we derive an average global spectrum from the Equatorial Station 3 (EQ3) data set [1] (acquired at phase angle α ∼ 8°), which has an almost complete spatial coverage of Bennu.

     3. Results and Discussion

• Detection of a shortward shift of the 2.7-μm absorption band minimum

The minimum position of the 2.7-μm absorption band in some craters shifts towards wavelengths shortward of 2.74 μm, the measured band position in Bennu’s global average spectrum [9]. This absorption band is associated with hydrated phyllosilicates, resulting from aqueous alteration of Bennu in the past. The minimum position of this band is related to Mg/Fe ratio of the phyllosilicates [10,11], which depends on the extent of the alteration. This minimum position is therefore an indication of the alteration the material experienced. We measure this shift to be up to 25 nm. The presence of such a shift of this band can be translated into the presence of fresher and/or less altered material inside the craters, in the context of space weathering for C-complex asteroids [12, 13].

• Anti-correlation between spectral slopes and REFF suggests the presence of fine grains

The relation between spectral slopes and REFF varies across craters. Particularly, it varies even among different scans of the same crater. To better understand this variation, Pearson’s correlation coefficient (r) was estimated. We considered a series of spectra acquired as the spectrometer scans from the rim of a crater, across its center, and towards the rim again. The REFF and spectral slope of each spectrum of this scan were then compared using r. We find an anti-correlation between REFF and spectral slopes (increasing spectral slopes versus decreasing REFF) within some craters (Fig. 1). Laboratory experiments conducted on carbonaceous chondrite meteorites have shown that redder slopes are associated with finer grains [14]. We then checked image data of the corresponding footprints and found the presence of unresolved material at cm to sub-cm scales[LDS-(1] . As such, we suggest that the observed anti-correlation is an indication of the presence of fine grains inside craters.